Power system operation and control 15EE81

Module-4

REACTIVE POWER COMPENSATION AND

VOLTAGE CONTROL

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Dept of EEE, GMIT Bharathinagara

 Power system operation and control 15EE81

Introduction:

A power system engineer has been encountering in the distribution

& transmission of power, a variety of problems such as voltage variations
with load, poor power factor, large losses, electromagnetic and
electromechanical oscillations followed by disturbances, supply voltage
distortions due to harmonics generated by non-linear loads, interference
with communications and so on. Their intensities may differ but all these
problems exist in the main transmission, sub-transmission and
distribution networks. The undertakings strive to provide un-interruptible
supply with quality, minimize losses to conserve energy [31] and operate
the system with timely actions in an attempt to overcome the adverse
effects due to internal defects and external causes. In recent times the
complexities in operation and control have increased due to a large
variety of highly non-linear loads and electronic controllers. The primary

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concern in the thesis work undertaken is related to reactive power
compensation, voltage control and energy conservation in a distribution
system. In this chapter the conventional methods employed for reactive
power compensation, their relative merits and demerits, desirable features
of an advanced compensator in a distribution system are highlighted.

VAR Compensation:

Reactive power compensation by appropriate means has become

the most economically attractive and effective solution technically for
both traditional and new problems at different voltage levels in a power
system. VAR compensation near load centre has gained more importance
in recent times. It limits the flow of load reactive current in lines and

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 Power system operation and control 15EE81

feeders, boosts the voltage, reduces KVA demand and leads to both
energy conservation and cost savings. Fig 3.1(a) & (b) show a typical
distribution transformer feeding inductive loads and three a winding
transformer at a receiving station requiring shunt reactive power
COMPENSATION.

The desirable characteristic features of a shunt compensator are as

mentioned below.

 Reactive power compensation of the load for power factor

improvement.
 Stepless control of reactive power continuously matching with the
prevailing load requirements from time to time.
 Maintenance of rated voltage at the point of common coupling
within a narrow range irrespective of the load acting during the

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day.
 Reduction in the main line / feeder current, the losses and to
conserve energy, throughout the day.
 Capacity to absorb line charging KVAr in very high voltage
system under light load conditions.
 In case the loads introduce harmonics, the compensator should
provide bypass paths for dominant harmonics and reduce the
distortion levels.
 Under disturbed conditions the compensator is expected to act fast
enough and damp out the oscillations.

Dept of EEE, GMIT Bharathinagara

 Power system operation and control 15EE81

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Traditional Methods of VAR Compensation:

This section deals with the conventional methods employed for

reactive power compensation and voltage control.

Synchronous Phase Modifier:

This is an ideal source having the capacity either to absorb or inject

reactive power. However, it has got number of limitations as pointed out
in section 1.5 (performance requirement). There are proven alternative

Dept of EEE, GMIT Bharathinagara

 Power system operation and control 15EE81

methods of compensation available which are practically equivalent to

SPM at low cost, more reliable, fast in response and giving trouble free
service.

Shunt Capacitors:

The use of shunt capacitors in conventional way through

mechanical switches has the following advantages:

 Overall cost is very low.

 The installation is simple requiring no strong foundations.
 Incur negligible losses
 Less maintenance problems
 More reliable in service with long life.

VTUPulse.com However, notable short coming are:

 Not possible to vary reactive power matching with load

demand continuously (only step variation).
 There exist a possibility for harmonics, if present, to get
amplified.
 There also exists a scope for series / parallel resonance
phenomenon to occur, which requires to be investigated
prior hand.
Hence the choice of a suitable compensator scheme calls for
detailed study and careful design before implementation.

Dept of EEE, GMIT Bharathinagara

 Power system operation and control 15EE81

Series Capacitors:

A capacitor bank can be interposed is a line to partially neutralize

the line reactance. Such an arrangement has the following attractive
features.

 It automatically provides reduction in line voltage drop

with increased loads.
 It increases the power handling capacity of a line by
reducing the transfer reactance.
 It reduces voltage flicker and damp out transient
oscillations.
 Quite effective in maintaining the voltage profile.

However, it poses serious problems during faults, prone for

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resonance phenomenon, complexity in control and likely to give rise to
sub-synchronous oscillations. Hence the series capacitors can be installed
after careful study only. They are employed widely in HV lines and
somewhat uneconomical for distribution networks, as the requirements in
both cases differ widely.

Static VAR Compensator:

This essentially consists of capacitor bank in suitable steps

(operated through mechanical switches / thryristors) and thyristor
controlled reactor across it of the size of minimum step. This combination
yields step less variation of reactive power over the entire range. When
SVC is applied at a receiving station it is possible to absorb line charging
KVAr produced under light load conditions. This will enable to avoid

Dept of EEE, GMIT Bharathinagara

 Power system operation and control 15EE81

over voltage phenomenon under light loads. The main theme of this thesis
work is application of multilevel advanced static VAR compensator with
a closed loop controller on a distribution transformer. The notable
features of SVC are[32, 33]

 Close matching of load reactive power

 Maintenance of power factor near unity
 Voltage control and reduction in losses
However, SVC has the following limitations.

 Switching of capacitor bank steps require appropriate

coordination.
 Complexity in the control of TCR.
 Generation of harmonics through TCR control

VTUPulse.com Harmonic Filters:

Most loads consume reactive power, highly non-linear

generate harmonics. The twin problems, reactive power compensation
and

and harmonics reduction are carried out using shunt passive filters. These
are tuned LC circuits to provide low impendence paths for dominant
harmonics. They are quite effective in reducing the total harmonic
distortion levels. An appropriately designed filter scheme can provide low
impendence paths for harmonics and inject reactive power at fundamental
frequency. The tuning reactor in every filter also serves the purpose of
limiting inrush / outflow currents during switching operations. A filter
scheme consisting of 2/3 selectively tuned filters for lower order
dominant harmonics and a high pass filter can meet the most commonly
encountered requirements in LT and HT applications. It is possible to

Dept of EEE, GMIT Bharathinagara

 Power system operation and control 15EE81

choose the appropriate filter scheme at the point of common coupling

depending on the load, its pattern of variation, harmonics present,
reactive power compensation at fundamental frequency so as to improve
the power factor, relieve the system from adverse effects due to
harmonics and improve the quality of power supply.

The advantages of shunt passive filters are:

 These are of relatively low cost, less complex, easy to operate and
reliable.
 Reduction in total harmonic distortion levels and improvement in
the quality of power supply.
 These have long life compared to active filters.
 Reactive power compensation and associated benefits similar to

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the use of shunt capacitors.
 Reduction in metering errors, communication interference, and
heating of electrical apparatus.

The limitations in their application are:

 Capacitors and Reactors are to be specially designed.

 Every filter in the scheme has to be provided with protection and
control arrangement.
 The scope for possible series / parallel resonance exists and should
be avoided by careful study before implementation.
 These do not offer 100% solution for harmonic suppression
similar to active filters.

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 Their performance is subject to parameter variations, ageing etc.

and precise tuning not possible.

Advanced Compensators:

The conventional techniques of reactive power compensation have

been dealt in the above sections. As seen, each method has its own merits
and limitations. Number of improvement have been brought out over the
years with the increased usage of high power rated thyristors and
advanced control techniques. There has been a growing tendency to
increase the number of functions to be carried out by a compensator,
either series, shunt or hybrid type. This section deals with the
requirements of a compensator and reviews the advances that have taken
place in the recent past.

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: Role of series / shunt Compensator:

Consider a transmission line with sources at either end, provided

with shunt and series compensator separately.

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A shunt compensator provided say at the middle of a line (Fig. 3.2

(a)) if effectively controlled can maintain the voltages Vs and Vr equal
irrespective of the directions of P & Q flows. This type of ideal
compensator doubles the power handling capability, improves the power
factor and maintains good voltage profiles. However, it is difficult to
practically realize fully such a condition of operation. It is quite effective
in providing reactive power compensation, improves steady state
performance and damps out the transient oscillations during disturbances.
It is usually a fast acting static VAR compensator.

On the other hand a series compensator interposed in the

transmission line as shown in fig. 3.2 (b) either at sending end or
somewhere in the line is quite effective to provide partial neutralization
of line impedance and to reduce the voltage drop in the line. This
improves the power handling capability of the line and damps out

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electromagnetic oscillations. However, as compared to shunt
compensator, series compensator is complex to control and protect, costly
and must be carefully designed to avoid sub synchronous oscillations.
Both the methods have their own attractive features and limitations. It has
been established that a combination of shunt and series compensators
called hybrid scheme works out well. To have a understanding of the
advanced compensator in the modern power systems, consider the
following case as shown in fig. 3.3

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Under simplify conditions of operation (neglecting shunt paths). It

is well known that, the relative magnitude difference between Vs and Vr
determines the direction and magnitude of reacting power flow in the
line. On the other hand the relative phase angle displacement between Vs
and Vr will determine the direction and magnitude of real power flow. For
example, if Vs > Vr and Vs leads Vr then both P & Q flow from source-
1 to source-2. If Vs < Vr and still leads Vr then P flows from soucr-1 to

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source-2 and Q flows from source-2 to source-1. This clearly indicates
that the magnitude of P & Q and their directions of flow depend on the
voltage magnitudes and their phase angles. To have an understanding of
the influence of voltage control in its magnitude and direction, consider a
situation with nominal values of Vs, Vr and P0, Q0 in the line subject to
incremental changes in voltage deviation and phase angle difference. This
obviously gives rise to four quadrant operation with coordinate axis
around ‘O’ point corresponding to the nominal values. Fig 3.4 and table
3.1 gives the four quadrant operation for incremental values in V, ,
P, Q.

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Quadrant V1  P Q

1 + +  

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2 - +  
3 - -  
4 + -  

Table 3.1. Four quadrant operation for incremental changes

in V,  and the corresponding changes in P and Q.

A variety of compensating devices both in series and shunt forms

have been developed over the years to achieve complete control on a
voltage profile, the magnitude and directions of both P and Q flows. The
schemes in vogue are STATCOM, power conditioners, energy sourced
inverters, in phase and quadrature boosters and so on. The detailed

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treatment of this advanced compensators is outside the scope of present

work.

Requirements of Advanced Compensator for distribution Systems:

In recent years lot of developments have taken place in FACT

devices (flexible AC transmission) for their applications in interconnected
power systems[34, 35]. However, that much attention was not paid to the
compensators in the distribution system. It is in this perspective attempt is
made to develop an advanced shunt compensator as could be made
applicable on mass scale for the distribution transformers. The work
proposed aims at developing a static VAR compensator with the
following technical features:

 To design a static VAR compensator with capacitor bank in

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 To design a thyristor controledl reactor of KVAr capacity
equal to the lowest size of capacitor bank step.
 To design an electronic feedback controller with high gain
and low time constant for fast response.
 To sense the reactive power requirement as per the
prevailing load, at periodical intervals.
 To coordinate the switching ON and OFF operations of the
capacitor bank steps with permissible time delays from OFF
state to ON state.
 To initiate operation through feedback controller for
obtaining reactive power from the compensator.

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 To continuously monitor the reactive power injection,

voltage condition and to maintain the power factor near
unity.

The advantages contemplated with the use of above mentioned

static VAR compensator on a distribution transformer are as mentioned
below:

 Control of voltage and maintenance of power factor

 Reduction in feeder losses and conservation of energy
 Relief in tariff and reduction in maximum demand.
 Flexibility in control and reliability in operation.
 Limited generation of harmonics from TCR and reduction in
phase unbalance.

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 Minimization of neutral currents / potentials.
 Improvement in the quality of power supply.

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Transformer Tap Changer effect on Reactive Power

6.1 In line with IEGC clause 6.6.5 & 6.6.4, the transformer tap positions on different 765kV,
400kV & 220kV class ICTs & GTs shall be changed as per requirements in order to improve the
grid voltage. RLDCs shall coordinate and advise the settings of different tap position of ICTs in
their region. And any change in their positions shall be carried out after consultation with RLDC
only. Normally tap position of all the ICTs shall be reviewed/changed at every three month
interval.

6.2 Transformers with tap-changing facilities constitute an important means of controlling voltage
throughout the system at all voltage levels. Coordinated control of the tap changers of all the
transformers interconnecting the subsystems is required if the general level of voltage is to be
changed.

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6.3 As per CEA Manual on Transmission Planning Criteria, in planning studies all the
transformers may be kept at nominal taps and On Load Tap Changer (OLTC) may not be
considered. Hence the effect of the taps should be kept as operational margin for system operator.

6.4 The OLTC allows voltage regulation and/or phase shifting by varying the turns ratio under
load without interruption. Large power transformers are generally equipped with ―voltage tap
changers,‖ sometimes called ―taps‖ with tap settings to control the voltages either on the primary
or secondary sides of the transformer by changing the amount and direction of reactive power
flow through the transformers. Transformer taps can be controlled automatically based on local
system conditions or manually.

6.5 Generating Transformer: - Power generated at generating station (usually at the range of
11kV to 25kV) is stepped up by generating transformer to the voltage level of 220, 400, 765kV
for transmission. It is one of the important and most critical components of power system. They
are generally provided with off circuit tap changer with a small variation in voltage because the

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voltage can always be controlled by the field of generator. Generating Transformer with OLTC
also used for reactive power control.

6.6 Interconnecting Transformer: - Normally autotransformers are used to interconnect two

grid/systems operating at two different voltage levels ( i.e.400 and 220kV). They are normally
located between generating transformer and receiving end transformer. In autotransformer there is
no electrical isolation between primary and secondary. Some volt-amperes are conductively
transformed and some are inductively transformed.

REACTIVE POWER

Reactive power is defined for AC systems only. Reactive power is produced when the
current waveform is out of phase with the voltage waveform due to inductive or capacitive loads.
Current lags voltage with an inductive load and leads voltage with a capacitive load. Only the
component of current in phase with voltage produces real or active power that does real work like
running motors, heating etc. Current is in phase with voltage for a resistive load like an

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incandescent light bulb. Reactive power is necessary for producing the electric and magnetic
fields in capacitors and inductors.

Reactive power is present when the voltage and current are not in phase,
one waveform leads the other, Phase angle not equal to zero and power factor less than unity. It is
measured in volt-ampere reactive (VAR). It is produced when the current waveform leads voltage
waveform (Leading power factor). Vice versa, consumed when the current waveform lags voltage
(lagging power factor).

The additional current flow associated with reactive power can cause increased losses and
excessive voltage sags. Transmission system operators have to ensure that reactive reserves are
available to handle system contingencies such as the loss of a generator or transmission line
because increased current flow after the occurrence of contingencies can produce greatly
increased reactive power absorption in transmission lines.

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The transmission lines generate VARS under No load or less loaded conditions and
consume VARS under loaded conditions. At any given point of time the power system can
experience different voltage levels at various locations.

In general, under peak load conditions, voltages are high at reactive source points and are
low at load points and the direction of reactive power flow is from source to the load, whereas,
under the off peak conditions, the reactive power flow is from load points to source.

The transmission of VARS over transmission elements during peak load conditions further
burdens the transmission elements and as a result, the voltages at the load end become further
less. Hence it is desirable to meet the reactive power requirement locally and necessary planning
of reactive compensation to be carried out. Even at nominal frequency and satisfactory voltage
operating conditions, voltage collapse cannot be ruled out as voltage is a local phenomenon.

System voltage levels are directly related to the availability of reactive power. System
events, such as the loss of a transmission line, create an instantaneous change in the reactive

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power demand. Shunt capacitors are not able to switch fast enough to supply the increase in
demand and prevent further voltage decline.

VOLTAGE MANAGEMENT

3.3.1 Control of voltage levels is accomplished by controlling the production, absorption,

and flow of reactive power at all levels in the system. Unlike
system frequency, which is consistent throughout an interconnected system in the steady
state, voltages experienced at points across the system form a "voltage profile" which is
uniquely related to local generation and demand at that instant, and is also affected by the
prevailing network arrangements.
3.3.2 Controlling the voltage is a local problem. In other words,the voltage control
problems need to be solved separately by each control area. This can be achieved by
providing sufficient reactive power sources for controlling voltage level as specified in
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IEGC. The voltage controlling problems can be divided into two situations, which are
normal situation and emergency situation.

3.3.3 Voltage changes continuously according to the varying electrical demand,

transmission lines utilization etc. Reactive power (VAR) is required to maintain the
voltage to deliver active power (watts) through transmission lines. When there is not
enough reactive power, the voltage sags down and it is not possible to push the power
demanded by loads through the lines.

VOLTAGE STABILITY

a) Voltage stability‖ is the ability of the power system to maintain steady acceptable
voltages at all buses in the system under normal operating conditions and after being
subjected to disturbance. A system enters steady voltage instability when a disturbance
(An increase in load demand, or change in system conditions) causes a progressive and

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uncontrolled drop in voltage.

b) A system is voltage unstable, if for at least one bus in the system, the bus voltage
magnitude decreases as the reactive power injection in the same bus is increased.

c) ‗Voltage Instability‖ is basically caused by non-availability of reactive power support in

some nodes of the network, where the voltage uncontrollably falls. Lack of reactive power
may essentially have two origins,
i. Gradual increase of power demand where the reactive requirement at some buses cannot
be met.
ii. Sudden change of network topology redirecting the power flows
in such a way that the reactive power cannot be delivered at some buses.

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d) The increased load is always accompanied by a decrease of voltage except in the case
of a capacitive load. When the loading is further increased, the maximum loadability point
is reached, from which no additional power can be transmitted under those conditions.

In case of constant power loads, the voltage in the nodes become uncontrollable and
rapidly decreases.

VOLTAGE COLLAPSE:

a) When voltages in an area are significantly low or blackout occurs due to the cascading
events accompanying voltage instability, the problem is considered to be a voltage
collapse phenomenon. Voltage collapse normally takes place when a power system is
heavily loaded and/or has limited reactive power to support the load.

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PROCEDURES FOR CONTROLLING VOLTAGE AND REACTIVE POWER:-

a) The control of voltage level is accomplished by controlling the production, absorption

and flow of reactive power at all levels in the system. (Refer Table –1 for sources and
sinks of reactive power.)

b) Primary Voltage Control: RLDCs shall control primary voltage by providing specific
voltage levels to generators according to the requirement. The generators shall adjust the
AVR which will vary the excitation of generating units in order to achieve the specified
voltage levels. For other voltage control equipment such as SVCs or automatic tap
changing transformers, they are considered to be a part of primary voltage control.
The maximum and minimum values in the above table are the outer limits and all the
regions shall endeavour to maintain the voltage level within the above limits. The steady
state voltage is maintained within the limits given in above table. However, the step
change in voltage may exceed the above limits where simultaneous double circuit outage

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of 400 kV lines are considered. In such cases, it may be necessary to supplement dynamic
VAR resources at sensitive nodes.
c) SLDC/RLDC may direct a wind farm to curtail its VAr drawal/Injection on considering
system security or safety of personnel/equipments.

d) The control centers shall apply the following mechanism for voltage control in general.

i) Generating units of all the region shall keep their Automatic Voltage Regulators (AVRs)
in operation and power system stabilizers (PSS) in AVRs shall be tuned in line with clause
5.2(k) of IEGC.

ii) The transformer tap positions on different 765kV, 400kV & 220kV class ICTs & GTs
shall be changed as per requirements in order to improve the grid voltage.

Switching off of the lines in case of high voltage:-

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i) In the event of persistent high voltage conditions when all other reactive control measures as
mentioned earlier including opening of redundant HT lines within the state system by the
concerned SLDCs have been exhausted, selected 400 / 230 / 220 / 132 / 110 KV lines shall be
opened for voltage control measures.
ii) The opening of lines and reviving them back in such an event would be carried out as per the
instructions issued by RLDC/NLDC in real time and as per the standing instructions issued from
time to time. While taking such action, RLDC/NLDC would duly consider that to the extent
possible the same does not result in affecting ISGS generation as well as the system security &
reliability is not affected.

h) VAR Exchange by regional constituents for Voltage and Reactive Control:

i. Each constituent shall provide for the supply of its reactive requirements including appropriate
reactive reserves, and its share of the reactive requirements to support safe and secure power
transfer on interconnecting transmission circuits.
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ii. The RLDC and constituent states shall take action in regard to VAR exchange with the grid
looking at the topology and voltage profile of the exchange point. In general, the beneficiaries
shall endeavour to minimize the VAR drawl at interchange point when the voltage at that point is
below the nominal value and shall not inject VARs when the voltage is above the nominal value.
In fact, the beneficiaries are expected to provide local VAR compensation so that they do not
draw any VARs from the grid during low voltage conditions and do not inject any VARs to the
grid during high voltage conditions.

i) VAR generation / absorption by generating units: - In order to improve the overall

voltage profile, the generators shall run in a manner so as to have counter balancing
action corresponding to low/high backbone grid voltage and to bring it towards the
nominal value. In order to achieve the same, all generators shall generate reactive
power during low voltage conditions and absorb reactive power during high voltage
conditions as per the capability limit of the respecting generating units.

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Load Management for controlling the Voltage:- All the regions shall identify the radial feeders
in their areas in consultation with SLDCs which have significant reactive drawls and which can be
disconnected in order to improve the voltage conditions in the event of voltage dropping to low
levels. The details of all such feeders shall be kept ready in the respective control rooms of
RLDC/SLDC and standing instruction would be given to the operating personnel to ensure the
relief in the hour of crisis by disconnecting such feeders.

k) Following corrective measures shall be taken in the event of voltage going

high / low:-
1. In the event of high voltage (when the bus voltage going above 410 kV), following specific
steps would be taken by the respective grid substation/generating station at their own, unless
specifically mentioned by NLDC/RLDC/SLDCs.

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i) The bus reactor be switched in
ii) The manually switchable capacitor banks be taken out
iii) The switchable line/tertiary reactors are taken in.
iv) Optimize the filter banks at HVDC terminal
v) All the generating units on bar shall absorb reactive power within the capability curve.
vi) Operate synchronous condensers wherever available for VAR absorption.
vii) Operate hydro generator / gas turbine as synchronous condenser for VAR absorption
wherever such facilities are available.
viii) Bring down power flow on HVDC terminals so that loading on parallel EHV network goes
up resulting in drop in voltage.
ix) Open lightly loaded lines in consultation with RLDC/SLDC for ensuring security of the
balanced network.

2. In the event of low voltage (when the bus voltage going down below
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390kV), following specific steps would be taken by the respective grid substation/generating
station at their own, unless specifically mentioned by NLDC/RLDC/SLDCs.
i) Close the lines which were opened to control high voltage in consultation with RLDC/SLDC.
ii) The bus reactor be switched out
iii) The manually switchable capacitor banks are switched in.
iv) The switchable line/tertiary reactor are taken out
v) Optimize the filter banks at HVDC terminal.
vi) All the generating units on bar shall generate reactive power within capability curve.

SYSTEM FREQUENCY & VOLTAGE CONTROL:-

i) This option is rarely used say for example when two islands has to be synchronized and voltage
has to be controlled at the end where line has to be synchronized.

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ii) Voltage of the large interconnected grid can also be controlled by controlling the system
frequency. As per Modern Power Station Practice, System Operation Volume-I (2), the general
synchronous machine equations shows that voltage levels are directly proportional to frequency
and for good voltage control extremes of system frequency must be avoided.
E=4.44øf N. Where: E is the EMF Generated; f is the Frequency, ø the flux.
iii) Times of low frequency are usually associated with plant shortage. The reactive capability is
low as the units are running at rated MW capacity; any increase in reactive power would only be
at the cost of reduction in MW output, something that is not usually allowed as per the Indian
Electricity Grid Code section 6.6 Para 6.

ADVERSE WEATHER CONDITIONS AND VOLTAGE CONTROL

As per Modern Power Station Practice, System Operation Volume-L [2], Fog or other conditions
of high humidity give an increased risk of insulation flashover which can be minimised by
reducing voltage levels. However under critically loaded conditions, it is judged that the risk of
running with reduced voltage levels outweighs.

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