ALSTOM HVDC For Beginners and Beyond
ALSTOM HVDC For Beginners and Beyond
ALSTOM HVDC For Beginners and Beyond
HVDC
GRID
FRANCE
CGEE Alsthom
USA
GE
GERmANy
AEG
(German HVDC Working Group; AEG, BBC, Siemens)
PREFACE
this booklets contents are intended to fill a gap in the available literature between the very basic introductory material generally available from suppliers and the more academic analysis of HVDC presented in text books. this booklet is therefore aimed at those who wish to gain a better understanding of the complex systems which are now forming an integral part of power transmission in the world today, a trend which will only increase. In recent years the technology of HVDC transmission using power transistors known as Voltage Source Converter (VSC) has been introduced into the market. Whilst sharing some commonality with Line Commutated Converter (LCC) HVDC in terms of the asynchronous nature of the interconnection and the benefits it can bring to the AC system the technology differs in several ways. In order to avoid any confusion with VSC technology this booklet focuses on LCC HVDC only. the first three chapters of the booklet provide an introductory overview of the subject of LCC HVDC, covering usage, configurations and basic operating principles. Chapter 4 contains more detailed examination of the main equipment of a HVDC converter station and chapter 5 discusses the layout of this equipment within the converter station. Chapters 6 and 7 review the operation of a HVDC converter and its control. Chapter 8 provides an introduction to static characteristics and introduces the concept of superposition of AC quantities onto the characteristics. An important design consideration of an LCC HVDC scheme relates to the reactive power loading that a HVDC converter station imposes on the network to which it is connected and this is reviewed in chapters 9 through to 13. Chapters 14 to 22 provide an explanation of the causes, effects and mitigation methods relating to converter generated harmonics, both AC and DC. A more detailed review of the control facilities available as standard on an LCC HVDC scheme are introduced in chapter 23, whilst chapters 24, 25 and 26 provide a more detailed technical discussion regarding HVDC converter valves, valve cooling and transformers. As a HVDC connection will always be a significant element within any power system its performance in terms of reliability, availability and losses are important considerations and these concepts are introduced in chapters 27 and 28. Special consideration has also been given to those in industry who may be in the position of having to prepare a specification for a HVDC converter scheme. Section 29 provides a description of the minimum studies normally performed as part of a turnkey HVDC project. Additionally, an Appendix is included at the end of this booklet which identifies the data needed for a budget quotation, that needed for tendering and the remaining data normally required during a contract. the data used in the creation of this booklet has come from many engineers within Alstom Grid UK PES and to all of them I am grateful. Any errors are mine.
Carl Barker
CONTENTS
Chapter
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
title
Introduction to HVDC HVDC configurations What is HVDC? A tour around the Single Line Diagram (SLD) of one end of a HVDC bipole converter Station layout How does a line commutated converter work Control of a HVDC link Static characteristics Reactive power in AC systems The reactive power load of a converter Reactive power sources within a converter station Controlling converter reactive power Voltage step changes Effects of harmonics in AC power systems Sources of harmonics in AC power systems How converters cause harmonics Pulse number and harmonic cancellation DC harmonics Characteristic and non-characteristic harmonics Harmonic filter design, types of filters AC harmonic performance and rating calculations DC harmonic performance and rating calculations Control facilities provided by HVDC schemes HVDC thyristor valves Thyristor valve cooling circuit HVDC converter transformers and their configurations Reliability and availability of a HVDC converter Losses in a converter station Contract stage studies for a HVDC contract References Appendix Data requirements for a HVDC scheme
Page
6 7 10 13 19 23 28 30 33 34 36 37 38 39 40 41 42 45 46 47 51 54 56 60 62 64 66 67 68 82 83
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1 INTRODUCTION TO HVDC
Electrical power is generated as an alternating current (AC). It is also transmitted and distributed as AC and, apart from certain traction and industrial drives and processes, it is consumed as AC. In many circumstances, however, it is economically and technically advantageous to introduce direct current (DC) links into the electrical supply system. In particular situations, it may be the only feasible method of power transmission. When two AC systems cannot be synchronised or when the distance by land or cable is too long for stable and/or economic AC transmission, DC transmission is used. At one converter station the AC is converted to DC, which is then transmitted to a second converter station, converted back to AC, and fed into another electrical network. In back-to-back HVDC schemes the two converter stations are brought under the same roof, reducing the DC transmission length to zero. HVDC transmission applications fall into four broad categories and any scheme usually involves a combination of two or more of these. the categories are: i) transmission of bulk power where AC would be uneconomical, impracticable or subject to environmental restrictions. ii) Interconnection between systems which operate at different frequencies, or between non-synchronised or isolated systems which, although they have the same nominal frequency, cannot be operated reliably in synchronism. iii) Addition of power infeed without significantly increasing the short circuit level of the receiving AC system. iv) Improvement of AC system performance by the fast and accurate control of HVDC power.
2 HVDC CONFIGURATIONS
2.1 monopolar HVDC Systems
Monopolar HVDC systems have either ground return or metallic return. A Monopolar HVDC System with Ground Return consists of one or more six-pulse converter units in series or parallel at each end, a single conductor and return through the earth or sea, as shown in Figure 2.1. It can be a cost-effective solution for a HVDC cable transmission and/or the first stage of a bipolar scheme [1]. At each end of the line, it requires an electrode line and a ground or sea electrode built for continuous operation. A Monopolar HVDC System with Metallic Return usually consists of one high voltage and one medium voltage conductor as shown in Figure 2.2. A monopolar configuration is used either as the first stage of a bipolar scheme, avoiding ground currents, or when construction of electrode lines and ground electrodes results in an uneconomical solution due to a short distance or high value of earth resistivity.
During an outage of one pole, the other could be operated continuously with ground return. For a pole outage, in case long-term ground current flow is undesirable, the bipolar system could be operated in monopolar metallic return mode, if appropriate DC arrangements are provided, as shown in Figure 2.4. transfer of the current to the metallic path and back without interruption requires a Metallic Return transfer Breaker (MRtB) and other specialpurpose switchgear in the ground path of one terminal. When a short interruption of power flow is permitted, such a breaker is not necessary. During maintenance of ground electrodes or electrode lines, operation is possible with connection of neutrals to the grounding grid of the terminals, with the imbalance current between the two poles held to a very low value. When one pole cannot be operated with full load current, the two poles of the bipolar scheme could be operated with different currents, as long as both ground electrodes are connected. In case of partial damage to DC line insulation, one or both poles could be continuously operated at reduced voltage.
Figure 2.4: Bipolar System with Monopolar Metallic Return for Pole Outage
In place of ground return, a third conductor can be added end-toend. this conductor carries unbalanced currents during bipolar operation and serves as the return path when a pole is out of service.
3 WHAT IS HVDC?
A simple representation of a HVDC interconnection is shown in Figure 3.1. AC power is fed to a converter operating as a rectifier. the output of this rectifier is DC power, which is independent of the AC supply frequency and phase. the DC power is transmitted through a conduction medium; be it an overhead line, a cable or a short length of busbar and applied to the DC terminals of a second converter. this second converter is operated as a line-commutated inverter and allows the DC power to flow into the receiving AC network. Conventional HVDC transmission utilises line-commutated thyristor technology. Figure 3.2 shows a simple thyristor circuit. When a gate pulse (ig) is applied while positive forward voltage is imposed between the anode and cathode (Vthy), the thyristor will conduct current (iL). Conduction continues without further gate pulses as long as current flows in the forward direction. thyristor turn-off takes place only when the current tries to reverse. Hence, a thyristor converter requires an existing alternating AC voltage (Vac) in order to operate as an inverter. this is why the thyristor-based converter topology used in HVDC is known as a line-commutated converter (LCC).
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11
AC Harmonic Filter (Double frequency) DC Reactor DC Voltage Measurement DC Harmonic Filter DC Pole 1 HV Line Connection
Valve Converter Pole Breaker HF Filter Quadri -valve DC Current Measurement N.B.S N.B.G.S
H F AC System
G.R.T.S
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4 A LOOK AT THE SINGLE LINE DIAGRAm (SLD) OF ONE END OF A HVDC BIPOLE CONVERTER
Figure 4.1 (opposite) shows a typical SLD of one end of a bipole overhead transmission line HVDC converter station. the following discussion reviews the major components which make up the converter station.
AC System Converter
4.1 AC Switchyard
the AC system connects to a HVDC converter station via a converter bus, which is simply the AC busbar to which the converter is connected. the AC connection(s), the HVDC connection(s) along with connections to AC harmonic filters and other possible loads such as auxiliary supply transformer, additional reactive power equipment, etc., can be arranged in several ways normally dictated by: reliability/redundancy requirements, protection and metering requirements, the number of separately switchable converters and local practice in AC substation design. Figure 4.2 shows a selection of AC connection arrangements that can be used in HVDC converter stations starting with (a) a simple, single, 3-phase busbar with one switchable connection to the AC system and the switchable AC harmonic filters connected directly to it. In such an arrangement it is not possible to use the AC harmonic filters for reactive power support of the AC system without having the converter energised (as the AC system connection is common). Figure 4.2(b) shows a scheme comprising two converters and includes an additional circuit breaker dedicated to each converter. In this arrangement the AC harmonic filters can be used for AC reactive power support without energising the converter. However, in common with Figure 4.2(a), a busbar fault will result in the complete outage of the converter station. to provide some additional redundancy a double busbar arrangement can be used as shown in Figure 4.2(c). In Figure 4.2(c) an AC busbar outage will result in those loads connected
Filter
Filter
Converter
Converter
AC System
Filter
Filter
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Converter
Converter AC System
Filter
Filter
to that busbar being disconnected until the disconnectors can be arranged to re-connect the load to the remaining, healthy busbar. Disconnector rearrangement will typically take in the order of ten seconds to complete and in some circumstances such an outage may not be acceptable, hence the arrangement shown in Figure 4.2(d) can be used, where each load is connected via a dedicated circuit breaker to each busbar, allowing for fast disconnection and reconnection in the event of a loss of a busbar (typically around 300 ms). A disadvantage of the arrangement shown in Figure 4.2(d) is the large number of AC circuit breakers required. In order to reduce the number of circuit breakers, the arrangement shown in Figure 4.2(e) can be used. In Figure 4.2(e) two loads can be individually switched between two three-phase busbars via three circuit breakers, hence, this configuration is commonly known as a breaker-and-a-half arrangement. Many other arrangements of AC switchyard configuration exist and have been used in association with existing HVDC schemes.
PLC signalling is a system which transmits a communication signal as an amplitude-modulated signal, superimposed on the fundamental frequency voltage signal of an AC power system. this system is used, in some power systems, as a communication system between AC system protection devices. However, the highfrequency interference generated by converter operation can overlap with the frequencies used for PLC communications (typically in the range of 40 kHz to 500 kHz). therefore, it is sometimes necessary to include a High Frequency (HF) filter (or PLC filter) in the connection between the converter bus and the converter in order to limit the interference that can propagate into the AC system. As with the AC harmonic filter, the HF filter comprises a high voltage connected capacitor bank, an air-core air-insulated reactor and an additional low voltage circuit composed of capacitors, reactors and resistors which are referred to as a tuning pack.
Converter
Converter
AC System
Filter
Filter
4.5 Converter
the converter provides the transformation from AC to DC or DC to AC as required. the basic building block of the converter is the six-pulse bridge; however, most HVDC converters are connected as twelve-pulse bridges. the twelve-pulse bridge is composed of 12 valves each
HVDC FOR BEGINNERS AND BEYOND 15
of which may contain many series-connected thyristors in order to achieve the DC rating of the HVDC scheme. For a HVDC power transmission scheme, the valves associated with each twelve-pulse bridge are normally contained within a purpose built building known as a valve hall. For back-to-back schemes, where both the sending and receiving end of the HVDC link are located on the same site, it is typical for the valves associated with both ends of the link to be located within the same valve hall.
AC System
Filter
Filter
Converter
4.7 DC Filter
Converter operation results in voltage harmonics being generated at the DC terminals of the converter, that is, there are sinusoidal AC harmonic components superimposed on the DC terminal voltage. this AC harmonic component of voltage will result in AC harmonic current flow in the DC circuit and the field generated by this AC harmonic current flow can link with adjacent conductors, such as open-wire telecommunication systems, and induce harmonic current flow in these other circuits. In a back-to-back scheme, these harmonics are contained within the valve hall with adequate shielding and, with a cable scheme, the cable screen typically provides adequate shielding. However, with open-wire DC transmission it may be necessary to provide DC filters to limit the amount of harmonic current flowing in
16 HVDC FOR BEGINNERS AND BEYOND
the DC line. the DC filter is physically similar to an AC filter in that it is connected to the high voltage potential via a capacitor bank; other capacitors along with reactors and resistors are then connected to the high voltage capacitor bank in order to provide the desired tuning and damping.
4.8 DC Switchgear
Switchgear on the DC side of the converter is typically limited to disconnectors and earth switches for scheme reconfiguration and safe maintenance operation. Interruption of fault events is done by the controlled action of the converter and therefore, with the exception of the NBS, does not require switchgear with current interruption capability. Where more than one HVDC Pole share a common transmission conductor (typically the neutral) it is advantageous to be able to commutate the DC current between transmission paths without interrupting the DC power flow. Figure 4.1 shows a typical Single Line Diagram (SLD) for a HVDC transmission scheme utilising DC side switchgear to transfer the DC current between different paths whilst on load. the following switches can be identified from Figure 4.1.
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the closing of the disconnector in order to put the HV conductor in parallel with the earth path. the GRtS is also used to commutate the load current from the HV conductor transferring the path to the earth (or ground return) path. Once current flow through the HV conductor is detected as having stopped, the disconnector can be opened, allowing the HV conductor to be re-energised at high voltage.
4.9 DC Transducers
DC connected transducers fall into two types, those measuring the DC voltage of the scheme and those measuring the DC current. DC voltage measurement is made by either a resistive DC voltage divider or an optical voltage divider. the resistive voltage divider comprises a series of connected resistors and a voltage measurement can be taken across a low voltage end resistor which will be proportional to the DC voltage applied across the whole resistive divider assembly. Optical voltage transducers detect the strength of the electric field around a busbar with the use of Pockel cells. DC current measurement for both control and protection requires an electronic processing system. Measurement can be achieved by generating a magnetic field within a measuring head which is sufficient to cancel the magnetic field around a busbar through the measuring head. the current required to generate the magnetic field in the measuring head is then proportional to the actual current flowing through the busbar. Devices using this method are commonly known as Zero Flux Current transducer (ZFCt). Optical current measurement makes use of, amongst others, the Faraday effect in which the phase of an optical signal in a fibre optic cable is influenced by the magnetic field of a busbar around which the cable is wound. By measuring the phase change between the generated signal and the signal reflected back from the busbar, the magnitude of the current can be found.
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5 STATION LAyOUT
the converter station is normally split into two areas:
the AC switchyard which incorporates the AC harmonic filters and HF filters the converter island which incorporates the valve hall(s), the control and services building, the converter transformers and the DC switchyard
An example of a converter station layout including the AC switchyard and the converter island is shown in Figure 5.1 with the actual site shown in Figure 5.2.
5.1 AC Switchyard
As with any AC switchyard, the complexity and therefore the space occupied varies, dependent upon the amount of both feeders and locally-switched elements to be interconnected. For a HVDC converter station, the AC switchyard may be part of a major node on the grid and therefore there may be a multiplicity of feeders, each with its associated towers, line end reactors, step-up/down transformers, etc. Conversely, the converter station could be located on the periphery of the network and therefore there may be only one or two feeders alongside the converter equipment. In both cases, however, the space occupied by these AC connections will be appropriate to the AC voltage level(s). typically, the main HVDC converter associated components located in the AC switchyard are the AC harmonic filters. these normally comprise ground-level mounted components located within a fenced-off compound. Compound access is only possible once the filters have been isolated and earthed. High frequency filter components, along with surge arresters, AC circuit breakers, disconnectors and earth switches are usually mounted on structures to allow walk-around access while the equipment is live.
Figure 5.2: Lindome, Sweden, Converter Station; Part of the 380 MW Konti-Skan HVDC Interconnection
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Figure 5.1: Lindome, Sweden, Converter Station Layout; Part of the 380 MW Konti-Skan HVDC Interconnection
420 kV AC busbars 285 kV DC lines Converter Transformers AC filters DC filters Thyristor Valves Valve Cooling System
Outdoor Valve Cooling System
Control Building
Converter transformers
Smoothing Reactor
AC Filters
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Within the valve hall, the thyristor valves are typically suspended from the roof of the building with the low voltage being closest to the roof and the high voltage being at the lowest point on the valve. An air gap between the bottom of the valve and the valve hall floor provides the high voltage insulation. the valve hall has an internal metal screen covering all walls, the roof and the floor. this screen creates a Faraday cage in order to contain the electromagnetic interference generated by the thyristor valve operation. the integrity of this screen is typically maintained by having the valve connection side converter transformer bushings protruding into the valve hall and connecting the bushing turrets to the building screen. the DC switchyard varies widely in complexity and physical arrangement between projects. For outdoor DC areas, the majority of the equipment (disconnectors, earth switches, transducers, etc.) is typically mounted on structures to create a walk-around area with only the DC filter, if present, ground mounted within a fenced-off area. However, where sound shielding is required around the DC reactor, this may be ground mounted with the sound shielding in the form of separate walls or an enclosure, also forming the safety barrier. When the DC area is located indoors, it is more common to have the majority of the equipment mounted at ground level in order to avoid an excessive height requirement for the building. In such circumstances, access to the whole, or parts of, the DC area is controlled by a fenced-off enclosure. the control and services building is also located on the converter island. this building generally contains equipment rooms such as:
Control room Cooling plant room Auxiliary supplies distribution Batteries Workshop Offices
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typical acoustic noise sources within a converter station (measured as sound power (P)) are:
DC smoothing reactor (110 dB(A) sound power) Converter transformer (105 dB(A) sound power) Valve cooling (air blast coolers) (100 dB(A) sound power) AC harmonic filter reactor (100 dB(A) sound power) transformer cooling (105 dB(A) sound power) AC harmonic filter capacitors (80 dB(A) sound power)
As an approximation, the acoustic noise sound pressure (L(A)) from any individual point source, at a distance from the component is calculated as follows: L() = P - 20 x Log10 - 8 Where: L() = the sound pressure at a distance (in metres). P = the acoustic sound power of the point source (dB(A)). = the distance from the point source at which the sound pressure is to be calculated (in metres). In order to meet the boundary, or nearest residence, acoustic noise limit, it may be necessary to add acoustic noise barriers or to modify the equipment itself. the barriers may take the form of walls or enclosures.
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Vd
1 Vc
Va
Vd
Vb 2
6.2 Commutation
In practice, the transfer of current from one diode to the next requires a finite time, since the current transfer is slowed down by the commutation reactance (made up of reactance in the converter transformer, the thyristor valve and a small amount in the HF filtering circuit). this produces an overlap between successive
23
periods of conduction in one half of the six-pulse bridge. Figure 6.4 shows that the mean direct voltage (Vd) has been reduced compared to Figure 6.2. Figure 6.4 also shows the valve current waveform during the commutation process, where current falls in one valve, while the current rises in the next valve in sequence. the time taken to commutate the current from one valve to the next is called the overlap angle, .
2.EL(RMS)
+/6 -/6
-/6
+/6
wt
1 3
Va
Vd
Vb 2
24
valve in sequence, and it is mathematically expressed as: = 180 - - It must be noted that the control of the output voltage of a six-pulse bridge is only achieved by the firing angle, . the extinction angle, , is a measure of the available turn-off time for the valve following the point in time where the valve is fired.
Ld La Lb Lc Ia Ib Lc T4 T6 T2 Id T1 T3 T5 Id
Vd
1 Vc 3
Direct Voltage
Va
Vd
Vb 2
25
1 C 3 C
6 2
Vd = 0 A A - Vd
26
AC Switchyard
27
28
Constant valve winding voltage control With this method of control, the converter transformer tapchanger is used to maintain the voltage applied to the AC terminals of each six-pulse bridge to a constant target value. Control of the current is then achieved by variation in converter operating angle. Constant firing angle range control With constant valve winding voltage control, the firing angle at lower power transmission levels can be large. to reduce the range over which the firing angle can operate in the steady state, the converter transformer tapchanger can be used to vary the applied AC voltage to the six-pulse bridge and hence limit the range over which the firing angle operates.
Constant valve winding voltage control this is the same as the equivalent rectifier control. Constant gamma angle range control this is similar to the rectifier constant firing angle range control but acts on the inverter extinction angle instead of the firing angle. Constant extinction angle control (CEA) With this method of control, the inverter DC voltage is allowed to vary in order to achieve a constant extinction angle with varying DC current. the inverter converter transformer tapchanger is used to adjust the applied AC terminal voltage in order to maintain the DC voltage to within a fixed, steady-state, range.
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8 STATIC CHARACTERISTICS
the static characteristics can be considered as the cerebral cortex of the converter as, in the same way as if you touch something hot with your hand you move it quickly away, without the involvement of higher brain functions, the static characteristics describe the way in which the converter responds to transients without involving higher control functions. the six-pulse bridge introduced in Section 4 can be simplified to a battery in series with a resistor as shown in Figure 8.1. Note that the resistor shown in Figure 8.1 is not an actual resistor but is simply included in the above circuit to simulate the voltage regulation effect of the impedance of the converter bridge connection. this resistor does not have any associated IR losses. Consider the circuit shown in Figure 8.1. As the DC current through the converter increases up to 1.0 pu, the voltage drop across the resistor increases, reducing the voltage at the DC terminal of the circuit as shown in Figure 8.2. Once at 1.0 pu DC current, the voltage can then be varied by increasing the firing angle. At a firing angle of 90, the DC voltage is zero but the DC current, if supplied from a separate source, remains at 1.0 pu. When in inverter mode, the converter will allow a DC current to flow through it supplied by a separate DC current source. As the firing angle increases (extinction angle decreases), the converter DC terminal voltage increases up to the minimum extinction angle at which point the DC current must be reduced to achieve further increases in DC terminal voltage, following a constant extinction angle line. By vertically flipping the inverter characteristic and plotting it on the same graph as the rectifier characteristic, the operating point, which is the point where the rectifier characteristic and the inverter characteristic cross, is found as shown in Figure 8.3. However, with these static characteristics, as can be seen in Figure 8.4, if the AC voltage applied to the rectifier falls then there are
Figure 8.2: Converter Operating Profile
Id Vdio x cos()
3 x x Lc
Vd
Vd = Vdio x cos() - 3 x x Lc Id
Figure 8.1: A Basic Six-Pulse Converter Model
30
multiple crossover points between the rectifier and the inverter. Hence, the operating point cannot be determined. to overcome this, the basic converter characteristics are modified in order to control the way that the converters respond during transient events. An example of a practical characteristic is shown in Figure 8.5. Note that in Figure 8.5 the constant current characteristic of the inverter is at a lower DC current than the constant current characteristic of the rectifier. Under normal operation, the inverter controls the DC voltage and the rectifier controls the DC current. However, if the AC terminal voltage at the rectifier falls such that the rectifier characteristic shown in Figure 8.5 crosses the inverter constant current characteristic, then the inverter will maintain the DC current at this level with the DC voltage being dictated by where the rectifier characteristic crosses the inverter constant current characteristic. the margin between the rectifier constant current characteristic and the inverter constant current characteristic is known as the current margin. Some dynamic characteristics can be superimposed on the static characteristic as shown in Figure 8.6. For example, a curve of constant real power can be superimposed indicating the required DC current for a given change in DC voltage to maintain the rectifier DC terminal power. Another characteristic that can be superimposed is one of constant reactive power. If the operating point were to be maintained along the reactive power curve, then at any point the reactive power absorbed by the converter would remain constant. Consequently, if there is a reduction in, for example, the rectifier AC system, then, by following an approximately constant reactive power curve, the change in reactive power at the inverter terminal is minimised, even though there is a change in real power. Consequently, the converter bus voltage at the inverter would remain approximately constant.
Figure 8.4: the Basic Static Characteristic of an HVDC Link with Reduced Rectifier AC terminal Voltage
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Figure 8.6: Constant Real and Reactive Power Characteristics Superimposed on the Static Characteristics
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Hence the reactive power absorption is approximately: Qdc = tan [cos-1()] x Pdc Where: Qdc = the reactive power absorption of the converter (pu), cos = the power factor of the converter (), Pdc = the real power of the converter station (pu). the reactive power absorption of a converter at rated load can be approximated as follows: Qdc0 = tan cos-1 (cos - p) 2
Id Id0 p
= the converter overlap angle (rad), = converter DC operating current (pu), = rated converter DC operating current (pu), = converter transformer leakage reactance (pu), = converter control angle, = alpha () for rectifier operation (rad), = gamma () for inverter operation (rad).
Where: Qdc0 = the reactive power absorption of the converter at rated DC current (pu).
From the overlap angle and the converter firing angle, the converter operating power factor can be approximately calculated as follows: cos = x [cos()+cos(+)]
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35
C1
L1
R1
C2
L2
R2
Q filter = f (C, V)
Figure 11.1: the Single Line Diagram of a typical AC Harmonic Filter
Lindome AC Filters
36
DC Power = DC Voltage x DC Current Hence, for a given DC power level the voltage can be reduced and the current proportionately increased at the expense of additional IR transmission losses. therefore, if the number of filters energised to meet AC harmonic filter performance exceeds the reactive power exchange limits, the converter operating conditions can be changed to increase the reactive power absorbed by the converter in order to achieve the desired exchange target between the converter station and the AC system. the change in DC conditions is achieved by lowering the DC voltage which requires the firing delay angle to be increased and with an increase in DC current, to maintain the DC power constant, the overlap angle increases, hence the reactive power absorbed by the converter increases. It must be noted that, as the DC side of the converter is common to the rectifier and inverter, changing the DC conditions will reduce, or increase, the reactive power load at both rectifier and inverter together. Figure 12.2 shows a typical operating range for the DC voltage on a back-to-back HVDC converter. In Figure 12.2 the upper limit is defined by the minimum allowable operating angles of the converter whilst the lower limit is defined by the maximum voltage transient that can be applied to the converter resulting from the firing voltage of a rectifier or recovery voltage of an inverter.
HVDC FOR BEGINNERS AND BEYOND
DC Power (%)
100
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= the change in AC voltage (p.u.) = the minimum Short Circuit Level of the AC system in which the switching operation is to take place (MVA) = the reactive power step to be imposed on the AC system (MVAr) = the total reactive power connected to the converter bus including the reactive power to be switched (MVAr)
Where the step change in AC voltage exceeds a defined limit, it is possible to increase the effective limit by imposing an opposite change in reactive power at the converter busbar. this opposite change can be achieved through converter action by applying a fast change to the DC voltage whilst maintaining the DC power as discussed in Section 12 above. As an example, consider switching in a filter onto an AC system that has a fundamental frequency VAr rating, which would exceed the AC voltage step change limit. By increasing the DC converter absorption at the same instant as the filter bank circuit breaker closes, the net reactive power exchanged with the AC system can be controlled and hence the step change in AC voltage.
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Overheating of capacitor banks Overheating of generators Instability of power electronic devices Interference with communication systems
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Power converters (HVDC, SVC, drives) Domestic electronics (television, video, personal computers, etc.) Non-linear devices - transformers - Voltage limiters Fluorescent lights Rotating Machines PWM converters
Planning Limit
Harmonic Distortion %
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28/06/2004 00:00
29/06/2004 00:00
30/06/2004 00:00
01/07/2004 00:00
02/07/2004 00:00
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05/07/2004 00:00
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[ [
It can be seen from equations (1) and (2) that each six-pulse bridge generates harmonic orders 6n 1, n = 1, 2, 3 ..., there are no triplen harmonics (3rd, 6th, 9th...) present and that for n = 1, 3, etc., the harmonics are phase shifted by 180. the idealised magnitudes of the six-pulse harmonics are shown in Table 17. By combining two six-pulse bridges with a 30 phase shift between them, i.e. by using Y/Y and Y/ transformers as shown in Figure 17.1 and summating equations (1) and (2), a twelve-pulse bridge is obtained. the idealised magnitudes of the twelve-pulse harmonics are shown in Table 17.2. the current waveforms shown in Figure 17.3 appear in the common connection to the transformers shown in Figure 17.1. If a Fourier analysis is performed on this idealised waveform, the following result is obtained: 3) I = 4 x 3 x Id x cos t - 1 cos 11 t + 1 cos 13 t - 1 cos 23 t + 1 cos 25 t ... 11 13 23 25
]
HVDC FOR BEGINNERS AND BEYOND
42
thus, in a twelve-pulse bridge, the harmonic orders 6n 1, n = 1, 3, 5 ... are effectively cancelled in the common supply leaving only the characteristic twelve-pulse harmonics i.e. 12n 1, n = 1, 2, 3, ... the idealised waveforms shown above will, in reality, be modified by the reactance of the supply system (mainly the transformer reactance). Due to this commutating reactance, the harmonic current magnitudes are reduced compared to those applicable to pure square wave pulses. the equations given above are based on the assumptions that, firstly, the DC current is linear, that is, the DC reactor is infinite and, secondly, the AC system voltage waveforms are sinusoidal. Because both of these assumptions are not valid for practical systems, more complex calculations are necessary and purpose built computer programs are used. the usual published formulae and graphs for these currents give magnitudes only. For special purposes (e.g. net harmonic contribution from two or more bridges of slightly different firing angles or reactances) both magnitude and phase (i.e. vector solutions) are required.
COMBINED
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(50 Hz) (250 Hz) (350 Hz) (550 Hz) (650 Hz) (850 Hz) (950 Hz) (1150 Hz) (1250 Hz) (n x 50 Hz)
(50 Hz) (250 Hz) (350 Hz) (550 Hz) (650 Hz) (850 Hz) (950 Hz) (1150 Hz) (1250 Hz) (n x 50 Hz)
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18 DC HARmONICS
the idealised voltage across an unloaded six-pulse converter is shown in Figure 18.1, and the idealised voltage across a twelvepulse converter is shown in Figure 18.2. the voltage is a mix of a direct voltage and harmonics. Table 18.1 shows the harmonics on the DC side produced by a six-pulse converter.
No-Load DC (Vd0)
6th 12th 18th 24th
(DC)
(300 Hz) (600 Hz) (900 Hz) (1200 Hz)
1.0000
0.0404 0.0099 0.0044 0.0025
table 18.1: Idealized DC Voltage Harmonics (RMS) at the terminals of a Six-Pulse Bridge
1 0.8 DC Voltage (p.u.) DC Voltage (p.u.) 0.6 0.4 0.2 0 0 100 200 Electrical Degrees 300
Figure 18.1: the Idealized Voltage Across the DC terminals of a Six-Pulse Bridge at no-load
Figure 18.2: the idealized voltage across the DC terminals of a twelvePulse Bridge at no-load
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46
1.0E+05 C1 1.0E+04
90
45
Magnitude
R1
-90
Harmonic Number
Figure 20.1: Single-tuned BandPass Filter Circuit Figure 20.2: Single-tuned Band-Pass Filter - Impedance Characteristic
Phase
L1
1.0E+03
47
the choice of the optimum filter solution is the responsibility of the contractor and will differ from project to project. the design will be influenced by a number of factors that may be specified by the customer:
Specified harmonic limits (voltage distortion, telephone interference factors, current injection), AC system conditions (supply voltage variation, frequency variation, negative phase sequence voltage, system harmonic impedance), Switched filter size (dictated by voltage step limit, reactive power balance, self-excitation limit of nearby synchronous machines, etc.), Environmental effects (ambient temperature range), Converter control strategy (voltage and overvoltage control, reactive power control), Site area (limited switch bays), Loss evaluation criteria, Availability and reliability requirements.
Different filter configurations will possess certain advantages and disadvantages when considering the above factors. As only the filter design and performance aspects are considered, additional equipment such as surge arresters, current transformers and voltage transformers are omitted from the circuits shown. In HV and EHV applications, surge arresters are normally used within the filters to grade the insulation levels of the equipment.
the tuned filter or band-pass filter which is sharply tuned to one or several harmonic frequencies.
these are filters tuned to a specific frequency, or frequencies. they are characterised by a relatively high q (quality) factor, i.e. they have low damping. the resistance of the filter may be in series with the capacitor and inductor (more often it is simply the loss of the inductor), or in parallel with the inductor, in which case the resistor is of high value. Examples of tuned filters include single (e.g. 11th) double (e.g. 11/13th) and triple (e.g. 3/11/13th) tuned types
the damped filter or high-pass filter offering a low impedance over a broad band of frequencies.
these are filters designed to damp more than one harmonic, for example a filter tuned at 24th harmonic will give low impedance for both 23rd and 25th harmonic, and even for most of the higher harmonics. Damped filters always include a resistor in parallel with the inductor which
48
produces a damped characteristic at frequencies above the tuning frequency. Examples of damped filters include single-tuned damped high-pass (e.g. HP12) and double-frequency damped high-pass (e.g. HP 12/24). the distinction between these two filter types may sometimes be almost lost depending on the choice of q-value for different filter frequencies. For a HVDC scheme with a twelve-pulse converter, the largest characteristic harmonics will be the following: 11th, 13th, 23rd, 25th, 35th, 37th, 47th, and 49th. As the level of the 11th and 13th harmonic are generally twice as high as for the rest of the harmonics, a common practice is to provide band-pass filters for the 11th and 13th harmonic and high-pass filters for the higher harmonics. Due consideration also has to be taken concerning the possible low-order resonance between the AC network and the filters and shunt banks. When a big HVDC scheme is to be installed in a weak AC system, a low-order harmonic filter (most often tuned to 3rd harmonic) may be also needed.
Simple connection with only two components, Optimum damping for one harmonic, Low losses, Low maintenance requirements.
Disadvantages: Multiple filter branches may be needed for different harmonics, Sensitive to detuning effects, May require possibility of adjusting reactors or capacitors.
1.0E+05 C1 1.0E+04
90
45
Magnitude
C2
L2
R2 1.0E+00 0 4 8 12 16 20 24 28 32 36 40 44 48 -90
Harmonic Number
Figure 20.3: Double-tuned BandPass Filter - circuit Figure 20.4: Double-tuned Band-Pass Filter - Impedance Characteristic
Phase
L1
R1
1.0E+03
49
Complex interconnection, with 4 or 5 C-L-R components, Requires two arresters to control insulation levels.
Optimum damping for two harmonics, Lower loss than for two single tuned branches, Only one HV capacitor and reactor needed to filter two harmonics, Mitigates minimum filter size problem for a low magnitude harmonic, Fewer branch types, facilitating filter redundancy.
Disadvantages: Sensitive to detuning effects, May require possibility of adjusting reactors or capacitors,
C1
1.0E+05 1.0E+04
90
L1
R1
45
Magnitude
C2
L2
R2
C3
L3
R3
-90
Harmonic Number
Figure 20.5: triple-tuned BandPass Filter - Circuit Figure 20.6: triple-tuned BandPass Filter tuned to 3rd, 11th and 24th Harmonic - Impedance Characteristic
50
Phase
1.0E+03
Isn
SYSTEM Zsn
Vsn
the current and voltage distortion can be calculated from the following expressions: 4) Isn 5) Vn = Zfn Z fn + Zsn Zfn x Zsn Zfn + Zsn x In
x In
In order to calculate harmonic performance and design the filters (i.e. Zfn), it is essential that detailed information be available on the harmonic currents generated by the HVDC converter (In) and the harmonic impedance of the supply system (Zsn).
51
52
53
54
An important consideration in the design of a DC filter, as opposed to an AC filter, is the main capacitor bank as, on the DC side, this will be subject to the applied DC voltage and hence the sharing of the DC voltage as well as the AC voltage must be controlled. this means that the resistive voltage distribution needs to be controlled in DC capacitors (Figure 22.3). For this reason it is common for DC filter capacitor banks to be constructed as one single tall bank as opposed to any form of split bank where the split banks would have post insulators between the capacitor racks and disturb the voltage distribution due to leakage currents across them.
55
the power transferred between the sending and receiving end of the HVDC link is controlled to meet an operator-set value at the point in the circuit where the DC power is defined, known as the compounding point. typically the compounding point is at the rectifier DC terminal but it can also be at the inverter DC terminal, the mid-point of the DC transmission conductors (e.g., at the border between two countries), the inverter AC terminal or the rectifier AC terminal. If the power demand is changed then the power order will ramp to the new power transfer level at a rate of change (known as the ramp rate) pre-selected by the operator. typically the maximum power limit is defined by an overload controller which is continuously calculating the thermal capability of the converter station equipment.
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57
58
23.5 DC Protection
A detailed description of the protections used within a HVDC station is beyond the scope of this document. However, it is worth noting that within a HVDC converter station the types of protection utilised fall into two categories:
AC connected equipment such as converter transformers and AC harmonic filter components, along with feeders and busbars, are protected using conventional AC protection relays. the converter, along with the DC circuit, is protected using hardware and software specifically purpose designed by Alstom Grid. typical DC specific protections include:
AC > DC AC Overcurrent DC Differential DC Overcurrent DC > AC AC Overvoltage Asymmetry AC Undervoltage Abnormal firing angle Low DC current DC Undervoltage
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Figure 24.2: Modern 8.5 kV 125 mm thyristor: Alloyed Silicon Slice (Left) and Complete Capsule (Right)
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Modern thyristor valves are relatively standardised, that is to say that the bulk of the real design work is carried out during the product development phase, hence, applying the valves to a particular project is a relatively straightforward matter. At its simplest, the work involved for a particular project may just involve adapting the number of series-connected thyristors according to the voltage rating requirements imposed by the overall system design. HVDC valves are almost never installed as individual units. Nearly always, several valves are combined together into a Multiple Valve Unit, or MVU. the MVU may either be mounted directly on the floor or, more commonly today, suspended from the ceiling. For economy of insulation, the valve design is often arranged so that the lower-voltage valves (usually those associated with the Deltaconnected six-pulse bridge) are used as part of the insulation on which the higher-voltage valves (usually those associated with the star-connected bridge) are mounted. Hence the low voltage end is the end at which the valve is attached to the floor or ceiling. the valves are typically stacked vertically into quadrivalve structures, with three such quadrivalves being required at each end of each pole. Figure 24.3 shows a typical suspended MVU. Careful attention has been paid to possible fire initiation processes within the modern thyristor valve. All components are generously rated, both thermally (to minimise the risk of overheating) and electrically (all other components in parallel with the thyristor are specified with voltage ratings in excess of those of the best thyristor which could be encountered). the damping capacitors, for example, are of oil-free construction. Hence, the potential spread of a fire throughout the valve can be virtually dismissed by the materials and components used.
HVDC
Figure 24.3: typical suspended MVU for HVDC for a 285 kVdc Application (dimensions approx 6 m x 4 m x 8 m tall)
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O Ring
Nylon nut
Figure 25.1: the Protective Electrode System Used in Alstom Grids Water-Cooled HVDC Valves
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Alstom Grids first water-cooled HVDC valves have now been in service since 1989 [6] and those of the Nelson River project have been in service since 1992, see Figure 25.2. Alstom has maintained the same design principles used on those projects, up to the present day, on all its HVDC valves and most of its FACtS converters. Alstom Grid has in excess of 70,000 small-diameter (15 mm) and 8,000 larger-diameter (50-75 mm) cooling connections installed in such converters around the world. Even with such a large installed base, there have been no reported problems caused by electrochemical erosion or deposition in any of Alstom Grids HVDC valves.
Figure 25.2: A valve hall from Valve Group VG13 of the Nelson River project in Canada, showing the large developed length used for the coolant pipework to span the distance between earth and the base of the valve stack at 330 kVdc.
Cooling plant
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Providing galvanic isolation between the AC and DC systems Providing the correct voltage to the converters Limiting effects of steady state AC voltage change on converter operating conditions (tapchanger) Providing fault-limiting impedance Providing the 30 phase shift required for twelve-pulse operation via star and delta windings
AC transformer insulation is designed to withstand AC voltage stresses. these voltage stresses are determined by the shape and permittivity of the insulation materials used within the transformer but is generally concentrated in the insulating oil. Converter transformers are, however, exposed to AC voltage stress and DC voltage stress. the distribution of the DC voltage stress is predominantly defined by the resistivity of the insulating materials and hence more stress is concentrated in the winding insulation than in the insulating oil. this resistivity varies due to several factors including the temperature of the materials and the length of time the voltage stress is applied. this is why the internationally recognised testing requirements demand that the DC voltage stress be applied for a period of time in order to ensure that a steady-state voltage stress distribution is achieved. the converter transformer is the largest plant item to be shipped to site for an HVDC project. Hence transport restrictions such as weight or height, if the transformer has to go over or under a bridge for example, can have a major impact on the selected converter transformer arrangement. Figure 26.2 illustrates the commonly recognised transformer arrangements in HVDC schemes.
Figure 26.1: Comparison of AC and DC Voltage Stress Distribution in a typical Converter transformer
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Lowest cost can normally be achieved by minimising the number of elements the converter transformer is broken down into, hence the lowest cost is typically a 3-phase, 3-winding transformer. However, due to shipping limits, such a transformer may not be practical so another arrangement should be considered. Where a spare converter transformer is deemed necessary, based on an availability analysis of the scheme, then it is more cost-effective to use a 1-phase, 3-winding transformer arrangement, as one spare unit can replace any of the in-service units, whilst 2-winding arrangements require two spare units to be supplied. An important consideration in the design of a converter transformer is the selection of the leakage reactance as this will constitute the major part of the converters commutating reactance. the leakage reactance must primarily ensure that the maximum fault current that the thyristor valve can withstand is not exceeded. However, beyond this limitation, the selection of leakage reactance must be a balance of conflicting design issues, the most important of which can be summarised as follows: Lower impedance gives:
Lower regulation drop Higher fault current taller core Lower weight
Higher impedance gives: Larger regulation drop Lower fault current Shorter core Higher weight
typically the optimum leakage reactance will be in the range 0.12 pu to 0.22 pu.
1-Phase, 3-Winding Converter transformer for the 500 MW Chandrapur Back-to-Back scheme
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27.1 Reliability
Reliability is a measure of the capability of the HVDC link to transmit power above some minimum defined value at any point in time under normal operating conditions. Reliability is normally expressed as the number of times in one year the scheme is incapable of transmitting power above a minimum defined value. this inability to transmit above a defined power level is termed Forced Outage Rate (F.O.R.).
27.2 Availability
Availability is not commercially significant, for example, if the scheme is unavailable during times of zero loading, the unavailability of the scheme will have no impact. For HVDC schemes, the term is, therefore, used to represent energy availability. Energy availability is the ability of a HVDC scheme to transmit, at any time, power up to the rated power. Hence, a converter scheme which can transmit 1.0 pu power for 100% of the time would have an energy availability of 100%. Any outage of the HVDC scheme or, for example, the outage of one pole in a bipole, will impact the energy availability, reducing the figure to less than 100%.
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25% 56%
35%
AC Harmonic Flters
50%
HF Filter Auxiliaries
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68
69
Reactive Power
to establish the necessary sub-bank rating and switching sequence to meet the reactive power control requirements of the scheme. Reactive power bank/AC harmonic filter bank MVAr rating, switching sequence of banks under different operating conditions and converter reactive power absorption capability utilisation. Reactive power bank/AC harmonic filters. the HVDC converter absorption under all extremes of operating condition tolerances, measurement errors and operating DC voltage are established as part of the Main Scheme Parameters reports. From this converter absorption, the total reactive power required, allowing for the appropriate tolerance conditions, is established. Using the reactive power exchange limits, established switch points will be calculated which keep the net reactive power interchange of the converter plus AC reactive power banks with the AC systems within the established limits.
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Harmonic Filter
to evaluate the AC side harmonic currents generated by the converters as a function of DC power and to establish an AC harmonic filter solution which meets the harmonic limits of the project. Filter design topology, component values and rating data. AC harmonic filters. the analysis performed by Alstom Grid establishes the combinations of the AC system and converter conditions, such as frequency, temperature, transformer impedance, etc; which would give rise to the maximum levels of harmonic distortion at the terminals of the HVDC station. the harmonic currents generated by the rectifier and inverter of the HVDC converter are evaluated using a digital computer program called JESSICA. the JESSICA program calculates the magnitude of the individual harmonic currents from a mathematical analysis of the frequency domain behaviour of the converter. the performance of the AC harmonic filters and their operational losses are calculated using the network harmonic penetration program HARP. the program models the filters and the injected currents from the converters. A standard mathematical maximisation technique is used to search each harmonic impedance area to find the impedance which produces the maximum value of voltage at a chosen node, or of current in a chosen branch. this system impedance is inserted into the impedance matrix of the circuit being analysed for the harmonic current penetration study. the program then solves Ohms law, using standard matrix mathematics techniques. this procedure is repeated for each harmonic of interest.
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PLC filters, valve hall RFI screen. the components of the converter station will be modelled in the appropriate frequency range to the necessary level of detail. Of most importance will be the converter transformers, the converters and the PLC filters. the PLC frequency noise will be calculated at the relevant busbars with a range of transmission line impedances over the range of PLC frequencies of interest. the radiated interference at 15 m from the substation fence will be calculated taking into account practical levels of RF screening applied to the valve halls.
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Insulation Coordination
to establish the appropriate protective levels of station surge arresters and hence BIL, clearance and creepage of station equipment. the study will yield protective levels of station surge arresters, equipment BIL and creepages and clearances on the DC side of the converter transformer. All insulation. From the maximum valve winding and DC voltage along with the specified maximum AC system operating voltage, the appropriate surge arrester protective levels are calculated based on historical data. From this data, the insulation levels of primary equipment are calculated as well as the insulation levels of insulators. the calculated insulation level for insulators is corrected to provide an insulator which will provide the necessary withstand flashover probability for the DC side equipment. Clearances between equipment on the DC side of the converter transformer are also calculated.
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74
Evaluation of DC control and protection functions. Confirmation of stable operation. Confirmation of static characteristics and control set points. Evaluation of the performance of the AC/DC system for different DC system control modes. Evaluation of DC system performance for DC-side disturbances such as converter blocking, pole blocking, and valve winding faults, including demonstration of protective shutdown when required. Demonstration of the DC system response in accordance with the specified response criteria, including control system step responses and recovery from AC system faults. Demonstration of the DC system transient response for reactive component switching Studying the interaction with local machines during disturbances. Evaluation of the performance of the DC system during severe AC faults and subsequent to fault clearing. this will include the evaluation of DC power run-backs, if necessary, to achieve stable system recovery.
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Audible Noise
to investigate the acoustic noise levels at the site boundaries. Confirmation that the equipment design meets the maximum acoustic noise limits at the boundary. Acoustically-active equipment including converter transformers, converter transformer coolers, filter capacitors, air conditioning and valve cooling heat exchangers. Individual equipment suppliers information giving the acoustic noise predicted for individual items will be modelled in a graphical representation of the site layout. the predictor software performs the acoustic noise calculations using the methodology set out in the following standards.
ISO 9613-1: Attenuation of sound during propagation outdoors, Part 1: Calculation of sound by the atmosphere (first edition 1993-06-01). ISO 9613-2: Attenuation of sound during propagation outdoors, Part 2: General method of calculation (first edition 1996-12-15). VDI 2571: Schallabstrahlung von industriebauten (Sound emission from industrial buildings).
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Losses
to calculate the as manufactured losses of the converter station. total operational loss from plant. None. the converter station losses for operation under nominal AC system voltage and frequency conditions and with nominal equipment parameters at an outdoor ambient temperature of typically 20 C will be presented. the study will compile results from equipment factory tests along with the proposed nominal operating conditions and present the total converter station losses in accordance with the formulae defined in IEC 61803 or, if prefered by the client, IEEE Std 1158.
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30 REFERENCES
[1] [2] [3] [4] [5] [6] [7] PL Sorensen, B Franzn, JD Wheeler, RE Bonchang, CD Barker, RM Preedy, MH Baker, Konti-Skan 1 HVDC pole replacement, CIGR session 2004, B4-207. JL Haddock, FG Goodrich, Se Il Kim, Design aspects of Korean mainland to Cheju island HVDC transmission, Power technology International, 1993, Sterling Publication Ltd p.125. Bt Barrett, NM MacLeod, S Sud, AI Al- Mohaisen, RS Al-Nasser, Planning and design of the AL Fadhili 1800MW HVDC Interconnector in Saudi Arabia, CIGR Session 2008, B4- 114. RP Burgess, JD Ainsworth, HL thanawala, M Jain, RS Burton, Voltage/var control at McNeill Back-to-Back HVDC convertor station, CIGR Sesson 1990, p.14-104. NM MacLeod, DR Critchley, RE Bonchang, Enhancing the control of large Integrated AC transmission Systems using HVDC technology, Powergrid Europe conference, Madrid, Spain, May 2007. DM Hodgson, Qualification of XLPE tube systems for cooling high-voltage high-power electrical equipment, Power Engineering Journal, November 1991. CD Barker, AM Sykes, Designing HVDC Schemes for Defined Availability, IEE Colloquium (Digest), n 202, (1998), p.4/14/11
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31 APPENDIX
Questionnaire Data Requirements for an HVDC Scheme
the importance of each question is defined by the following categories: A A list of the minimum information required to enable a formal quotation to be prepared B A list of minimum information required to enable a budgetary proposal to be made, e.g. for feasibility study C A list of the additional, minimum technical definitions required at the time of an order award Please note that it is in the interest of the User to specify in the enquiry as much of this information as possible - for which purpose we would be pleased to offer our advice if required - since, although tenderers can make their own assumptions when data is missing, this can lead to difficulties when tenders are adjudicated.
Category 1. 1.1 AC System for each terminal Voltage nominal maximum continuous minimum continuous maximum short time and duration minimum short time and duration Frequency nominal maximum continuous minimum continuous maximum short time and duration minimum short time and duration Short circuit levels, maximum and minimum, for each stage of the development Insulation levels Creepage and clearance distances Harmonic impedance
AB AB AB A A AB AB AB A A AB AB A A
1.2
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A A A
Distortion and/or tIF limits for AC system. Which harmonics are to be assessed? Is the AC system solidly or resistance earthed? What are the design constraints (number of feeders, security criteria, current user practice, etc) for the AC switching station? Are there other large transformers connected to the converter station busbar? If so, give transformer ratings, reactances, tap range, etc. Give details of undervoltages and durations for AC system faults,both during the faults and during the fault clearance period for both main and back-up protection, for single phase and three phase faults if they are different. Give results of AC system disturbance studies including transient voltage and frequency variations, and details of any limits on acceptable VAr generation/absorption and switching during and after the disturbance. a) What is maximum permitted step voltage change arising from filter switching? b) What is the maximum permitted ramped voltage change and over what time duration? What is maximum temporary (<1 sec) overvoltage that existing equipment can withstand? Are there any other significant limits on permissible temporary overvoltage? How much reactive compensation is required (i.e. what power factor is to be achieved) at the AC terminals at various transferred power levels up to full load? Give details of outgoing AC lines from each converter station. Give negative sequence voltage on each converter station AC busbar, and existing harmonic voltages. DC System Is power flow required in both directions?
1.10
AB
1.11
AB
1.12
AB
1.13
AB
1.14
A A
1.15 1.16 2.
AB
2.1
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AB
2.2
AB
2.3
What is the nominal power to be converted into DC? (It is usually convenient to define this at the rectifier DC output terminals). Note that if the power flow is unidirectional, then the rating of the inverter can be made less than that of the rectifier. Is an overload capability required? If so how much and for how long and under what conditions of ambient temperatures? What are the ratings and parameters of the DC line, or cable? Where possible, include: Voltage rating and location at which this is to be defined. Current rating Resistance of each line or cable Inductance of each line or cable Capacitance of each line or cable Harmonic impedance of each line or cable Length of each line or cable Equivalent circuit for each pole
AB
2.4
with tolerances
AB AB AB AB
What are the permitted limits for harmonic current injection into the DC line? Is monopolar operation required either in emergencies orcontinuously? Is ground current allowed either in emergencies or continuously? Are ground (return) electrodes to be provided? If so give: Details as in question 2.4 above for both electrode lines Electrode resistance (predicted) Electrode type (sea or land) and approximate location
2.9
Information relevant to any possible electromagnetic coupling to other adjacent circuits, and the nature of the possible disturbances liable to be produced by such coupling. Details of any requirements for switching DC lines What is the time scale for any staged development?
A A
2.10 2.11
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A A
2.12 2.13
What are the capitalised costs for fixed and variable losses and at what power loading do they apply? In order that the quantities of spares required may be determined,give details of the energy availability and station reliability targets, preferred maintenance intervals and design criteria to be adopted (information to include cost of loss of service for availability and reliability optimisation). Generators (if applicable) What are the ratings and parameters of the generators? Include: MVA Voltage (including harmonic content) Power factor Reactances, both transient and sub-transient direct axis.
3. AB 3.1
AB
3.2
What are the ratings and parameters of the generator transformers? Include: MVA Voltage ratings Percentage reactance Connection
A A AB
Will the generators be designed to absorb harmonics from the converters? What are the in-service dates for the generators? Will control be provided to limit AC busbar voltage variation? If so what will the voltage limits be, and what maximum reactive power can the machines safely absorb? Is the generating station at the same site as the converter station? If not, give details of the AC lines between generating station and converter station including: Length of lines Number of lines, voltage ratings Impedance and characteristics of each line
3.6
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4. A 4.1
Auxiliary Supplies, for each terminal Give details of preferred voltages and frequencies to be used for the auxiliary power system and sources of auxiliary power supply, i.e. will supply be provided from a generating station or will contractor have to supply auxiliary power transformers connected to the AC busbars? Define whether start up of auxiliaries is to be manual or automatic. If auxiliary supply is being provided by the customer, give details of reliability of supply and disturbances. If the auxiliary supply is to be provided by the Contractor, define redundancy requirements. Controls and Telecoms Locations from which control instructions may be received Control philosophy to be explained. to what extent is automatic control preferred to giving detailed responsibility to the operator? Any operation requirements to be defined, i.e. power and/or current control, rate of change of power, etc. Define the performance required during and after disturbances in either AC system Define any restrictions on the recovery from DC disturbances arising from the requirements of the AC system Define the disturbances liable to occur in either AC system and the control objectives required of the DC system in such circumstances such as supplementary control signals in response to AC frequency or voltage at one or more terminals. Control desk or panel requirements Protection requirements Requirements for line fault protection (a) what are the particular requirements?
A A
4.2 4.3
5. A A 5.1 5.2
5.3
C C
5.4 5.5
5.6
C AB A
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A A A A A A
Requirements for line fault location Requirements for alarm annunciation Requirements for sequential event recording Requirements for disturbance recording Supervisory system requirements Is an HVDC power line carrier (or fibre optic cable communications) system to be provided by the contractor and, if so, what information will be carried on it? Details of any interface between power line carrier (or fibre optic cable communications) and any other telecommunication system(s). Arrangements for communication during the construction/commissioning stage, between sites and from each site to national and international circuits for telephone, printer, fax, etc. Requirements for permanent facilities to be provided by the contractor for telephone, telex, fax, etc., circuits. Map of route of DC line (if applicable) showing sites, and respective distances, for power line carrier repeater stations, indication of any suitable auxiliary power supplies that may be available at these sites.
(b) What are the system practices which should be followed for operational convenience?
5.16
5.17
5.18
5.19
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6. A 6.1
The following general information is required for each site: Extent of supply to be clearly defined especially any services or equipment to be provided outside the converter station. Interfaces with equipment or services outside the scope of supply to be clearly defined Site conditions: (a) Ambient temperatures (i) nominal (ii) maximum (iii) minimum (iv) maximum wet bulb and coincident dry bulb (b) Altitude (c) Maximum wind speed (d) Maximum depth of snow (e) Rainfall (f) Isokeraunic levels (g) Range of relative humidity (h) Incidence of air pollution (salt or industrial)
A AB
6.2 6.3
A A A AB
What are the RFI limits and where are they to apply? What are the low frequency electromagnetic field limits ? What are the audible noise limits and where are they to apply? Are the sites in an earthquake zone; if so what forces do structures and building have to be designed to withstand? Local structural/building codes, defining what factors have to be applied to forces due to wind and/or snow, when designing buildings and structures. What are the maximum loading gauges and weight restrictions at the ports and on the routes to each site?
6.8
AB
6.9
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6.10
Maps of the areas of the sites showing the areas available for the converter station and those available for ground or sea-shore electrodes. Site surveys including soil analysis especially in the areas of the ground electrodes if these are required. Maps showing the location of the outgoing AC lines and DC linesfrom the converter stations. Details of auxiliary supplies that will be available, during construction and installation. Details of water supplies, available at the sites, including flow rates and chemical analysis. What language should be used on drawings and instructions? Details of any preferred materials that will utilize local resources, e.g. copper or aluminum, brick or concrete. Standards and specifications to be used, stating the order of precedence. this list should also include specific standards relating to items of equipment including Busbars, transformers, Switchgear, Cabling and Wiring, Insulation Oil, Civil Works and Structures, Equipment Finishes, Painting, Drawings and Drawing Symbols. If the customer has any standard specification for control and protection circuits, e.g. control circuits for circuit breakers, then copies of these should be provided.
A A A A A A
6.17
A A
6.18 6.19
Are IEC test Standards applicable? Define what office and workshop facilities are to be provided, e.g.should workshops be capable of handling major items such as converter transformers? Define any restrictions applicable to indoor, oil-filled equipment.
6.20
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Systems-L4-HVDC Basics -2165-V2-EN - Alstom, the Alstom logo and any alternative version thereof are trademarks and service marks of Alstom. The other names mentioned, registered or not, are the property of their respective companies. The technical and other data contained in this document are provided for information only. Neither Alstom, its offi cers nor employees accept responsibility for or should be taken as making any representation or warranty (whether express or implied) as to the accuracy or completeness of such data or the achievements of any projected performance criteria where these are indicated. No liability is accepted for any reliance placed upon the information contained in this brochure. Alstom reserves the right to revise or change these data at any time without further notice. Printed on paper made with pure ECF (Elemental Chlorine Free) ecological cellulose produced from trees grown in production forests under responsible management, and selected recycled three-layer fibres.
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