Project Report

on
STATCOM
Operation & Maintenance

Ashish Rajpal
60004265
Executive Trainee

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Section- 1 : Introduction to FACTS

1.1 Introduction

The increasing industrialization, urbanization of life style has lead to increasing dependency on
the electrical energy. This has resulted into rapid growth of power systems. This rapid growth has
resulted into few uncertainties. Power disruptions and individual power outages are one of the
major problems and affect the economy of any country.
In contrast to the rapid changes in technologies and the power required by these technologies,
transmission systems are being pushed to operate closer to their stability limits and at the same
time reaching their thermal limits due to the fact that the delivery of power have been increasing.
The major problems faced by power industries in establishing the match between supply and
demand are:

1) Transmission & Distribution; supply the electric demand without exceeding the thermal limit.
2) In large power system, stability problems causing power disruptions and blackouts leading to
huge losses.

These constraints affect the quality of power delivered. However, these constraints can be
suppressed by enhancing the power system control. One of the best method for reducing these
constraints are FACTS devices. With the rapid development of power electronics, Flexible AC
Transmission Systems (FACTS) devices have been proposed and implemented in power systems.
FACTS devices can be utilized to control power flow and enhance system stability.
Particularly with the deregulation of the electricity market, there is an increasing interest in using
FACTS devices in the operation and control of power systems.
A better utilization of the existing power systems to increase their capacities and controllability by
installing FACTS devices becomes imperative. FACTS devices are cost effective alternatives to
new transmission line construction. Reactive power compensation is provided to minimize power
transmission losses, to maintain power transmission capability and to maintain the supply voltage.

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Series compensation is control of line impedance of a transmission line; with the change of
impedance of a line either inductive or capacitive compensation can be obtained thus facilitating
active power transfer or control.

In recent years, voltage stability and control are increasingly becoming a limiting factor in the
planning and operation of some power systems, mainly in longitudinal ones. However, a variety
of considerations constrains the construction of new transmission lines. This has been reflected in
the necessity to maximize the use of existing transmission facilities.
On steady state, bus voltages must be controlled on a specified range. A suitable voltage and
reactive power control allows to obtain important benefits in the power systems operation such as
the reduction of voltage gradients, the efficient transmission capacities utilization and the increase
of stability margins.
By different control means and operating techniques, the voltage control task in transmission
levels can be got; some solution technologies can involve a series voltage injection, or a shunt
reactive current injection in strategic sites of the power system. When a disturbance occurs,
changes in the voltage system are presented and the restoration to the reference values depends
on the dynamic response of the excitation systems and the control devices employed.
In the last decade commercial availability of Gate Turn-Off thyristor (GTO) devices with high
power handling capability, and the advancement of other types of power-semiconductor devices
such as IGBT’s have led to the development of controllable reactive power sources utilising
electronic switching converter technology. These technologies additionally offer considerable
advantages over the existing ones in terms of space reductions and performance. The GTO
thyristors enable the design of solid-state shunt reactive compensation equipment based upon
switching converter technology. This concept was used to create a flexible shunt reactive
compensation device named Static Synchronous Compensator (STATCOM) due to similar
operating characteristics to that of a synchronous compensator but without the mechanical inertia.
The advent of Flexible AC Transmission Systems (FACTS) is giving rise to a new family of
power electronic equipment emerging for controlling and optimizing the performance of power
system, e.g. STATCOM, SSSC and UPFC. The use of voltage-source inverter (VSI) has been
widely accepted as the next generation of reactive power controllers of power system to replace

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the conventional VAR compensation, such as the thyristor-switched capacitor (TSC) and thyristor
controlled reactors (TCR).

1.2 Flexible AC Transmission Systems

The concept of Flexible AC Transmission Systems (FACTS) was first defined by N.G. Hingorani,
in 1988. A Flexible Alternating Current Transmission System (FACTS) is a system comprised of
static equipment used for the AC transmission of the electrical energy. It is meant to enhance
controllability and increase power transfer capability of the network. It is generally a power
electronic-based device. FACTS is defined by the IEEE as “a power electronic based system and
other static equipment that provide control of one or more AC transmission system parameters to
enhance controllability and increase power transfer capability”. The primary advantage of FACTS
devices, over its conventional counterpart is the rapid control of current, voltage and/or
impedance. The conventional solutions such as capacitor, reactor and phase shifting transformers
are normally less expensive than FACTS devices but limited in their dynamic behaviour and are
less optimal.
STATCOM is defined by IEEE as “a self-commutated switching power converter supplied from
an appropriate electric energy source and operated to produce a set of adjustable multiphase
voltage, which may be coupled to an AC power system for the purpose of exchanging
independently controllable real and reactive power”.

Section- 2 : Review on Flexible AC Transmission System

2.1 Introduction

The FACTS is a generic term representing the application of power electronics based solutions to
AC power system. These systems can provide compensation in series or shunt or a combination of
both series and shunt. The FACTS can attempt the compensation by modifying impedance, voltage
or phase angle.

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The capability of FACTS in providing series or shunt compensation is explained with the help of
transmission line. In the case of a no-loss transmission line, voltage magnitude at receiving end is
the same as voltage magnitude at sending end: V1 = V2= V. Transmission results in a phase lag
δ that depends on line reactance X. Fig. below shows equivalent circuit and phasor diagram of no
loss transmission line.

As it is a no-loss line, active power P is the same at any point of the line is given by:

Reactive power at sending end is the opposite of reactive power at receiving end:

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As δ is very small, active power mainly depends on δ whereas reactive power mainly depends on
voltage magnitude.

2.2 Shunt Compensation

Shunt compensation is used to influence the natural electrical characteristics of the transmission
line to increase the steady-state transmittable power and to control the voltage profile along the
line. The shunt compensator like STATCOM can be operated either to provide capacitive or
inductive compensation depending on the specific requirement. The impedance of the shunt
controller, which is connected to the line voltage, causes a variable current flow, and hence
represents an injection of current into the line. As long as the injected current is in phase quadrature
with the line voltage, the shunt controller only supplies or consumes variable reactive power. The
ultimate objective of applying reactive shunt compensation in a transmission system is to increase
the transmittable power capability from the generator to the load, which is required to improve the
steady-state transmission characteristic as well as the stability of the system.

The main purpose of shunt compensation is to provide the following:

• Steady state and dynamic voltage control.
• Reactive power control of dynamic loads.
• Damping of active power oscillations.
• Improvement of system stability.

2.3 Series Compensators

The series Compensator could be variable impedance, such as capacitor, reactor, etc., or power
electronics based variable source of main frequency to serve the desired need. Various Series
connected FACTS devices are:
1) Thyristor Controlled Series Capacitor (TCSC).

2) Thyristor Switched Series Capacitor (TSSC).

3) Thyristor Controlled Series Reactor (TCSR).

4) Thyristor Switched Series Reactor (TSSR).

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5) Static Synchronous Series Compensator (SSSC).

2.4 Shunt Compensators

Shunt Controllers may be variable impedance, variable source, or a combination of these. In

principle, all shunt Controllers inject current into the system at the point of connection. Various
shunt connected controllers are:

1) Static Synchronous Compensator (STATCOM).

2) Static VAR Compensator (SVC)

2.5 STATCOM

The STATCOM (or SSC) is a shunt-connected reactive-power compensation device that is

capable of generating and or absorbing reactive power and in which the output can be varied to
control the specific parameters of an electric power system.
It is in general a solid-state switching converter capable of generating or absorbing independently
controllable real and reactive power at its output terminals when it is fed from an energy source
or energy-storage device at its input terminals.
Specifically, the STATCOM considered in this chapter is a voltage-source converter that, from a
given input of dc voltage, produces a set of 3-phase ac-output voltages, each in phase with and
coupled to the corresponding ac system voltage through a relatively small reactance (which is
provided by either an interface reactor or the leakage inductance of a coupling transformer). The
dc voltage is provided by an energy-storage capacitor.
STATCOM is defined by IEEE as a self-commutated switching power converter supplied from
an appropriate electric energy source and operated to produce a set of adjustable multiphase
voltage, which may be coupled to an AC power system for the purpose of exchanging
independently controllable real and reactive power.

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2.6 Typical Applications of STATCOM

1) Effective voltage regulation and control.

2) Reduction of temporary over voltages.
3) Improvement of steady-state power transfer capacity.
4) Improvement of transient stability margin.
5) Damping of power system oscillations.

6) Damping of sub synchronous power system oscillations.

7) Flicker control.
8) Power quality improvement.
9) Distribution system applications.

Section- 3 : Static Synchronous Compensator (STATCOM)

3.1 Introduction

STATCOM is a controlled reactive-power source. It provides the desired reactive-power

generation and absorption entirely by means of electronic processing of the voltage and current
waveforms in a voltage-source converter (VSC).
A single-line STATCOM power circuit is shown in Fig. 3.1, where a VSC is connected- to a
utility bus through magnetic coupling. In Fig. 3.1, a STATCOM is seen as an adjustable voltage
source behind a reactance—meaning that capacitor banks and shunt reactors are not needed for
reactive-power generation and absorption, thereby giving a STATCOM a compact design, or
small footprint, as well as low noise and low magnetic impact. The exchange of reactive power
between the converter and the ac system can be controlled by varying the amplitude of the 3-
phase output voltage, Es of the converter.

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That is, if the amplitude of the output voltage is increased above that of the utility bus voltage, Et,
then current flows through the reactance from the converter to the ac system and the converter
generates capacitive-reactive power for the ac system. If the amplitude of the output voltage is
decreased below the utility bus voltage, then the current flows from the ac system to the converter
and the converter absorbs inductive-reactive power from the ac system.
If the output voltage equals the ac system voltage, the reactive-power exchange becomes zero, in
which case the STATCOM is said to be in a floating state. Adjusting the phase shift between the
converter-output voltage and the ac system voltage can similarly control real-power exchange
between the converter and the ac system.
In other words, the converter can supply real power to the ac system from its dc energy storage if
the converter-output voltage is made to lead the ac-system voltage. On the other hand, it can absorb
real power from the ac system for the dc system if its voltage lags behind the ac-system voltage.
A STATCOM provides the desired reactive power by exchanging the instantaneous reactive power
among the phases of the ac system. The mechanism by which the converter internally generates
and/ or absorbs the reactive power can be understood by considering the relationship between the
output and input powers of the converter. The converter switches connect the dc-input circuit
directly to the ac-output circuit. Thus the net instantaneous power at the ac output terminals must
always be equal to the net instantaneous power at the dc- input terminals (neglecting losses).

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Assume that the converter is operated to supply reactive-output power. In this case, the real power
provided by the dc source as input to the converter must be zero. Furthermore, because the reactive
power at zero frequency (dc) is by definition zero, the dc source supplies no reactive power as
input to the converter and thus clearly plays no part in the generation of reactive-output power by
the converter.
In other words, the converter simply interconnects the three output terminals so that the reactive-
output currents can flow freely among them. If the terminals of the ac system are regarded in this
context, the converter establishes a circulating reactive-power exchange among the phases.
However, the real power that the converter exchanges at its ac terminals with the ac system must,
of course, be supplied to or absorbed from its dc terminals by the dc capacitor.
Although reactive power is generated internally by the action of converter switches, a dc capacitor
must still be connected across the input terminals of the converter. The primary need for the
capacitor is to provide a circulating-current path as well as a voltage source. The magnitude of
the capacitor is chosen so that the dc voltage across its terminals remains fairly constant to prevent
it from contributing to the ripples in the dc current. The VSC-output voltage is in the form of a
staircase wave into which smooth sinusoidal current from the ac system is drawn, resulting in
slight fluctuations in the output power of the converter.
However, to not violate the instantaneous power-equality constraint at its input and output
terminals, the converter must draw a fluctuating current from its dc source. Depending on the
converter configuration employed, it is possible to calculate the minimum capacitance required to
meet the system requirements, such as ripple limits on the dc voltage and the rated-reactive power
support needed by the ac system.
The VSC has the same rated-current capability when it operates with the capacitive- or inductive-
reactive current. Therefore, a VSC having a certain MVA rating gives the STATCOM twice the
dynamic range in MVAR (this also contributes to a compact design).
A dc capacitor bank is used to support (stabilize) the controlled dc voltage needed for the operation
of the VSC. The reactive power of a STATCOM is produced by means of power- electronic
equipment of the voltage-source-converter type. The VSC may be a 2- level or 3-level type,
depending on the required output power and voltage.
A number of VSCs are combined in a multi-pulse connection to form the STATCOM. In the
steady state, the VSCs operate with fundamental-frequency switching to minimize converter

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losses. However, during transient conditions caused by line faults, a pulse width–modulated
(PWM) mode is used to prevent the fault current from entering the VSCs. In this way, the
STATCOM is able to withstand transients on the AC side without blocking.

3.2 The V-I Characteristic

A typical V-I characteristic of a STATCOM is depicted in Fig. 3.2. As can be seen, the STATCOM
can supply both the capacitive and the inductive compensation and is able to independently
control its output current over the rated maximum capacitive or inductive range irrespective of
the amount of ac-system voltage.
That is, the STATCOM can provide full capacitive-reactive power at any system voltage—even
as low as 0.15 pu. The characteristic of a STATCOM reveals strength of this technology: that it is
capable of yielding the full output of capacitive generation almost independently of the system
voltage (constant-current output at lower voltages). This capability is particularly useful for
situations in which the STATCOM is needed to support the system voltage during and after faults
where voltage collapse would otherwise be a limiting factor.

Figure 3.2 also illustrates that the STATCOM has an increased transient rating in both the
capacitive- and the inductive-operating regions. The maximum attainable transient overcurrent in

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the capacitive region is determined by the maximum current turn-off capability of the converter
switches.
In the inductive region, the converter switches are naturally commutated; therefore, the transient-
current rating of the STATCOM is limited by the maximum allowable junction temperature of the
converter switches. In practice, the semiconductor switches of the converter are not lossless, so
the energy stored in the dc capacitor is eventually used to meet the internal losses of the converter,
and the dc capacitor voltage diminishes.
However, when the STATCOM is used for reactive-power generation, the converter itself can
keep the capacitor charged to the required voltage level. This task is accomplished by making the
output voltages of the converter lag behind the ac-system voltages by a small angle (usually in the
0.18–0.28 range).
In this way, the converter absorbs a small amount of real power from the ac system to meet its
internal losses and keep the capacitor voltage at the desired level. The same mechanism can be
used to increase or decrease the capacitor voltage and thus, the amplitude of the converter-output
voltage to control the VAR generation or absorption.
The reactive- and real-power exchange between the STATCOM and the ac system can be
controlled independently of each other. Any combination of real power generation or absorption
with VAR generation or absorption is achievable if the STATCOM is equipped with an energy-
storage device of suitable capacity, as depicted in Fig. 3.3.With this capability, extremely effective
control strategies for the modulation of reactive- and real-output power can be devised to improve
the transient- and dynamic-system-stability limits.

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 Section- 4 : STATCOM Lucknow

4.1 SLD of STATCOM system

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 1. Bay 601: It consists of STATCOM-1 of capacity ±150 MVAR.
2. Bay 602: It consists of STATCOM-2 of capacity ±150 MVAR.
Hence the overall capacity obtained using the STATCOM system is ±300 MVAR.
3. Bay 603: It consists of MSC-1 of capacity +125 MVAR.
4. Bay 604: It consists of MSR-1 of capacity -125 MVAR.
5. Bay 605: It consists of MSR-2 of capacity -125 MVAR.
6. Bay 606: It consists of Auxiliary Transformer that converts 38.5/0.433 kV.
7. Bay 607: It consists of Zig-Zag Transformer that is used for earthing purpose i.e. to
minimize the effect of circulating current.

4.2 STATCOM (Static Synchronous Compensator): Bay 601 & 602

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Reactor Capacity = 150 MVAR
Voltage level = 38.5 kV
CT Ratio (Phase side) - Primary = 3000
CT Ratio (Phase side) - Secondary = 1

STATCOM Reactor
Make – TRENCH
Equipment maximum voltage – 53.45 kV
There are 2 coils (C1 & C2) in this reactor.
Rated Inductance – 17 mH

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Protections provided against faults on STATCOM are
 Unbalance Protection
 Overcurrent Protection
 Earth Fault protection
 Thermal overload (Alarm) protection

1. Negative Phase Sequence protection: 46

Negative phase sequence (NPS) protection is provided against unbalance conditions
pertaining in the equipment’s or Loading. The protection should detect the unbalance
conditions and operate before the NPS withstand capacities are reached.

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4.3 MSC (Mechanically Switched Capacitor): Bay 603

Mechanically Switched Capacitor (Including Switchgear). A shunt-connected circuit containing

a mechanical power-switching device in series with a capacitor bank and a current limiting
reactor.
MSC is a switched 3-phase capacitor bank connected in shunt to the MV bus of STATCOM
station and switched automatically by means of Circuit breaker (with control switching device)
based on the command from STATCOM Station control system. The rated capability of MSCs
shall be at 400kV (Referred to as “Point of Common Coupling” or PCC) and in the steady state
frequency range of 48.5Hz-50.5Hz. The MSC Components shall be designed with the aim to
achieve operation according to the overall performance requirements of the STATCOM Station.
The individual components of MSC shall be able to withstand the condition imposed by system
over voltages and harmonics. The MSC consists of 3-ph AC power capacitor bank, current
limiting air core reactor as required, 3-ph MV Circuit breaker (SF6/Vaccum type), associated
current transformer, 3-phDisconnectorandassociatedsafetygroundingswitch.The MSC area shall
be fenced and castle key interlock with safety grounding switch shall be provided for human
safety.

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Reactor Capacity = 125 MVAR
Voltage level = 38.5 kV
Phase side - CT:
CT Ratio (Phase side) = 3000
CT Ratio (Phase side) = 1
Capacitor unbalance - CT:
Unbalance CT - Primary in Amps = 5
Unbalance CT - Secondary in Amps = 1

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MSC Reactor
Make – TRENCH
Equipment maximum voltage – 46.23 kV
Rated Inductance – 3.11 mH

Protections provided against faults on MSC are

 Capacitor Unbalance Protection
 Over voltage (current based) Protection
 Negative phase sequence protection
 Overcurrent Protection
 Earth Fault Protection

MSC Capacitor Banks

In these capacitor banks, they are star connected so that any unbalance current flowing through
the star network is made to flow through the Neutral CT.

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Capacitor Unit Failure Detection: The stages of capacitor units or element failure detection
shall be provided. A three step unbalanced current protection shall be provided in each capacitor
bank to initially generate an alarm when the unbalance limit is reached and finally to trip the
bank in case of limit being exceeded.

4.4 MSR (Mechanically Switched Reactor) – Bay 604 & 605

Reactor Capacity = 125 MVAR

Voltage level = 38.5 kV
Phase side - Winding a (Bus side CT)
CT Ratio (Phase side) - Primary = 3000
CT Ratio (Phase side) - Secondary = 1
Neutral side - Winding b: (Filter Neutral side CT)
CT Ratio (Neutral side) - Primary = 3000
CT Ratio (Neutral side) - Secondary = 1

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MSR Reactor
Make – TRENCH
Equipment maximum voltage – 40.09 kV
Rated Inductance – 35.4 mH

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Protections provided against faults on MSR are
 Differential Protection
 Negative Phase sequence Protection
 Thermal Overload (Alarm) protection
 Overcurrent Protection
 Earth Fault Protection

4.5 Auxiliary Transformer: Bay 606

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Transformer Total Capacity = 0.630 MVA
HV Voltage rating = 38.5 kV
LV Voltage rating = 0.433 kV
HV Side CT - Primary in A = 100
HV Side CT - Secondary in A = 1
Rated Voltage = 38.5 KV
Transformer % Impedance = 5.20
Vector Group of Transformer = Dny1

Protections Provided against Internal faults on HV side of Auxiliary Transformer are:

 HV Overcurrent protection
 HV Earth Fault Protection

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4.6 Zig-Zag Transformer: Bay 607

The 38.5KV system is a Delta connected hence zero sequence currents cannot flow in the system. As
we know that Delta connection acts as open circuit for zero sequence currents, the Earthing
Transformer (Zig Zag configuration) is provided in the MV Bus in order to measure the zero sequence
currents and restrict the flow of it.
As per the input provided the Earth fault current in the 38.5KV system in Lucknow substation has
been restricted to 270 Amps. Hence the Earth fault settings has been proposed considering single
phase to earth current as 270 Amps with a Definite Time delay.

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CIRCUIT BREAKER TESTING:

1. SCRM
2. DCRM
3. TIMING

1. Static Contact Resistance Measurement:

Static Contact resistance is measured to ensure the healthiness of the main contacts of the Circuit
Breaker when it is in closed position. It is significant to have very low resistance during closing
of the breaker as most of the time the breaker is in closed position and carries the entire load
current continuously. Static Contact Resistance is measured using precision low resistance
measuring instrument as the value will be in terms of micro ohms. Micro Ohm Meter of 100A
and above with associated accessories are used for thismeasurement.

Applicable limits for SCRM are as follows:

Contact resistance 765kV 400kV 220kV 132kV

micro ohm per 75 75 100 100

break

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 2. Dynamic Contact Resistance Measurement:

The static Contact resistance measurement as discussed earlier shall monitor thehealthiness
ofthemaincontactonly. But inCircuit Breaker,the arcing contact participate more in isolation
of fault current and there is possibility of wear and tear during the service life of the breaker.
Though the arcing contact is specifically made using hard material such as copper tungsten
alloy or graphite, there is possibility of erosion, pitting on the contact due to interruption
of fault current. While doing static contact resistance measurement, arcing contact comes
in parallel to the main contact and hence any amount of damage is not revealed when the main
contact is in healthy position. Dynamic Contact Resistance primarily monitors the health of
both main and arcing contacts without opening the interrupter. This helps in decision
regarding major/ final overhauling/ inspection of main/ arcing contacts of the Circuit
Breaker.

DCRM can detect the following details/ defects on a Circuit Breaker:

1. Erosion of Arcing Contact

2. Erosion of main contact
3. Contact misalignments
4. Contact wipe of main and arcing contact

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5. Healthiness of damping system
6. Contact travel & speed

Figure: DCRM kit connection schematic

1. DCRM kit with sampling frequency of 10kHz(atleast) to be used.

2. DCRM to be taken for CO operation.
3. CO delay time to be atleast 300ms hence requiring plot length of about 450ms.
4. CB flanges where Current and Voltage cables are to be connected, should be thoroughly
cleaned by CTC etc.
5. Connection should be done only on Interrupter flanges and not on PIR or grading
Capacitor flanges.
6. Current leads to be connected outside whereas Voltage leads to be connected inside.

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Figure: The typical signature of DCRM

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 3. TIMING:-

Figure: TIMING TESTKIT

1. Breaker timings are very critical for stability of the power system.
2. Quick and reliable operation of breaker is required for system healthiness.
3. Close, Open, CO, OCO operations are performed on the Breaker using HISAC Ultima
Kit.
4. The closing time for a circuit breaker should not be more than 150 ms.
5. Trip timing is maximum 25ms (for 400kV), 35ms (for 220kV), and 40ms (for
132kV).
6. Close/Trip time pole discrepancy at rated operating pressure:
7. Phase to Phase (Max) – Close Operation 5.0 ms
8. Phase to Phase (Max) – Open Operation 3.33 ms
9. Break to Break (Max) of same pole 2.5 ms

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 10. CO time (min) 35 ms
11. Trip delay time tco 300 ms

Tan delta & capacitance test:

Purpose: The tan delta test is done to check the healthiness of the insulator. A pure insulator
when connected across line and earth act as a pure capacitor i.e. if the insulating material is
100% pure, the current flowing through it is only capacitive in nature. There is no resistive
component of the current flowing through the insulator in ideal condition.

But in practical scenario 100% pure insulator is not possible. Due to aging and accumulation
of dirt and moisture provides a conductive path to the current. This leakage current flowing
from line to the earth constitutes the resistive component of the current.

For a healthy insulator, this resistive component of the current should be low. So we can say
that for a insulator to be healthy, the ratio of the resistive component to the capacitive
component should be quite low. This ratio is called tanδ.

EQUIPMENTS USED: Megger Delta 4010 and Delta 4100 kits are used to check the tanδ
and capacitance values.

Megger Delta 4010: This is the booster kit which can supply upto 10KV voltage. HV lead is
taken from this kit.

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 Megger Delta 4100: This is the control kit. LV lead is connected back to this kit at the input
point.

HV control points and HV power point of the booster kit and control kit are connected. Also,
both the megger kits are earthed.

For HV-LV winding: UST mode is used to calculate tan delta. Primary and neutral are shorted.
Both LV bushing are shorted. HV lead from megger is connected to neutral bushing. LV from
megger is connected to secondary bushing. Low frequency 2KV, 5KV, 10KV is injected through
HV of megger. Test is run in UST mode. Tan delta and capacitance (𝐶𝐻𝐿)calculated by megger is
noted down. Test is also carried out at variable frequency.
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ACCEPTANCE LIMIT:
Permissible limit for tanδ = 0.005(max)
Deviation of capacitance value from factory value should be within factory values.
+
5%
− of the
HV-Tank: When calculated in GST mode. HV lead from megger is connected to neutral
bushing. Primary and neutral are shorted. Both LV bushing are shorted. Low frequency 2KV,
5KV, 10KV is injected through HV of megger. Test is run in GST mode. Tan delta and
capacitance (𝐶𝐻𝐿 + 𝐶𝐻)calculated by megger is noted down. Test is also carried out at variable
frequency.
ACCEPTANCE LIMIT:
Permissible limit for tanδ = 0.005(max)
Deviation of capacitance value from factory value should be within factory values. +5% of the
−

HV-Tank: When calculated in GSTg mode. HV lead from megger is connected to neutral
bushing. Primary and neutral are shorted. Both LV bushing are shorted. Low frequency 2KV, 5KV,
10KV is injected through HV of megger. LV is used as guard and connected to secondary bushing.
Test is run in GSTg mode. Tan delta and capacitance (𝐶𝐻) calculated by megger is noted down.
Test is also carried out at variable frequency.
ACCEPTANCE LIMIT:
Permissible limit for tanδ = 0.005(max)
Deviation of capacitance value from factory value should be within factory values. +5% of the
−

LV-HV: UST mode is used to calculate tan delta. Primary and neutral are shorted. Both LV
bushing are shorted. LV from megger is connected to neutral bushing. HV from megger is
connected to secondary bushing. Low frequency 2KV, 5KV, 10KV is injected through HV of
megger. Test is run in UST mode. Tan delta and capacitance (𝐶𝐿𝐻)calculated by megger is noted
down. Test is also carried out at variable frequency.
ACCEPTANCE LIMIT:
Permissible limit for tanδ = 0.005(max)

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Deviation of capacitance value from factory value should be within factory values. +5% of the
−

LV-Tank: When calculated in GSTg mode. HV lead from megger is connected to secondary
bushing. Primary and neutral are shorted. Both LV bushing are shorted. Low frequency 2KV,
5KV, 10KV is injected through HV of megger. LV is used as guard and connected to neutral
bushing. Test is run in GSTg mode. Tan delta and capacitance (𝐶𝐿) calculated by megger is noted
down. Test is also carried out at variable frequency.
ACCEPTANCE LIMIT:
Permissible limit for tanδ = 0.005(max)
Deviation of capacitance value from factory value should be within factory values. +5% of the
−

LV-Tank: When calculated in GST mode. HV lead from megger is connected to secondary
bushing. Primary and neutral are shorted. Both LV bushing are shorted. Low frequency 2KV,
5KV, 10KV is injected through HV of megger. Test is run in GST mode. Tan delta and
capacitance (𝐶𝐻𝐿 + 𝐶𝐿)calculated by megger is noted down. Test is also carried out at variable
frequency.
ACCEPTANCE LIMIT:
Permissible limit for tanδ = 0.005(max)
Deviation of capacitance value from factory value should be within factory values. +5% of the
−

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HV Bushing: To calculate capacitance C1 and tan delta UST mode is used. HV is connected at
the top of bushing and LV connected at tan delta point. Low frequency 2KV, 5KV, 10KV is
injected through HV of megger. Tan delta and capacitance (𝐶1) calculated by megger is noted
down. Test is also carried out at variable frequency. For 𝐶2 GSTg mode is used. HV connected at
tan delta point and LV used as guard connected at top of HV bushing. Low frequency 1KV is
applied, test is run and tan delta and capacitance value is noted down.
ACCEPTANCE LIMIT:
Permissible limit for tanδ = 0.005(max)
Deviation of capacitance value from factory value should be within factory values. +5% of the
−

LV Bushing: To calculate capacitance and tan delta UST mode is used. HV is connected at the
top of secondary bushing and LV connected at tan delta point. Low frequency 2KV, 5KV, 10KV
is injected through HV of megger. Tan delta and capacitance calculated by megger is noted down.
Test is also carried out at variable frequency. Primary and neutral are shorted.
ACCEPTANCE LIMIT:
Permissible limit for tanδ = 0.005(max)
Deviation of capacitance value from factory value should be within factory values. +
−5% of the

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