reactors

Reactors in

power system

M.G.Morshad / ACM ( Elect.)Transformer Mtce. Division / TS II

References 1. REPORT - SYSTEM PLANNING & PROJECT APPRAISAL DIVISION – CEA

2. LOAD GENERATION BALANCE REPORT 2010-11 – CEA

3. INSTALLED CAPACITY - CEA

4. JOURNAL - TECH NEWS – AREVA

5. HV SHUNT REACTOR SECRETS FOR PROTECTION ENGINEERS -

Zoran Gajić , Birger Hillström, Fahrudin Mekić ABB Sweden, Västerås, Sweden

6. IEEE Guide for the Protection of Shunt Reactors

7. APPLICATION OF NUMERICAL RELAYS FOR HV SHUNT REACTOR PROTECTION –

Z. Gajić, B. Hillstrőm, M. Kockott , ABB Automation Technologies ,Sweden

8. TRANSIENTS DUE TO SWITCHING OF 400 KV SHUNT REACTOR

Ivo Uglešić, Sandra Hutter University of Zagreb Miroslav Krepela Siemens Božidar Filipović- Grčić Croatian National Electricity Franc Jakl University of Maribor

9. IS 5553 ( Part 2) 1990

10. IS 2026 ( Part 1 & 3)

11. Installation & commissioning of shunt reactor - BHEL

Reactors in power system

REACTORS

Series REACTOR

Shunt REACTOR

PURPOSE To reduce short circuit current

PURPOSE To reduce over

voltage

Z

Z

Acts as inductive load and reduce high voltage

by absorbing MVAR.

Acts as inductance and opposes the flow of short

circuit current.

AREA OF APPLICATION

Tie Lines

AREA OF APPLICATION

Bus , Lines, Tertiary winging of

ICT

Classification of shunt reactors

Shunt Reactors

Dry Type (system voltage

Below 72.5 KV)

Oil immersed Type (system voltage 72.5

KV & above)

Core less Gapped Core Air Core

•Star connected with neutral grounding• Range 30 to 300 MVAR •Connected at the terminals of transmission line

• Delta connected • Range below 30 MVAR •Connected at the tertiary winding of transformer

Switch on / off type Permanently connected type with thyristor

controlled

Back ground for installing shunt reactor

Reason for high grid voltage in Southern grid during off peak period – As per CEA

report

Availability 28450

MW

Peak Load demand 34224

MW

Off Peak Load demand 13000

MW

Installed Capacity

44220 MW

Grid voltage higher than rated (Max

1.05PU or 441KV)

Grid voltage lower than rated (Max

0.95PU or 399 KV)

11Hrs 15Hrs 22 Hrs

03 Hrs

Effect of high grid voltage

High over voltage cause – • Difficulty in regulating load flow through

HVDC line • Difficulty in synchronization inter grid

transmission line• instability in generator due to operation of

generator in under excitation zone near the pole slip region.

• Increase in line loss

Present practice to overcome overvoltage situation

• Keeping all 64 Nos Reactors (56 Nos Line &8 Nos Bus) are in service during off peak period

• Switching off all lightly loaded lines

Extract of PGCIL report (2006)

• With existing reactors and opening of the lines as per existing practice , the study reports indicate high voltage profile throughout the grid (the voltage ranges between 416 kV and 445 kV).

• Addition of 15 numbers of reactors of 63 MVAR each is not adequate to control the voltages under acceptable limits even with some of the transmission lines switched off.

• Even large generating stations like Ramagundam, Neyveli, Vijayawada, Raichur TPS are not able to hold their voltages as these are crossing reactive power absorption limit. As such these stations may also be considered for installation of bus reactors.

• With 10 more reactors (making it 25 ), the results indicate that when lightly loaded transmission lines are out of service, the voltages at various buses are generally controlled and are less than 420 kV and with the above lines in, the voltages are higher going up to 431 kV.

• Therefore, it is concluded that provision of large number of reactors are required to control the high voltages situation in the grid.

Calculation of grid reactive power

About 27 numbers of 63 MVAR shunt reactor are required in southern grid to absorb reactive power for bringing down grid voltage from 441 KV to 416 KV during off peak period

Formula Data

System Voltage Skv 420 KV

Fault Current level FkA 40 KA

Short Ckt MVA Scc= 1.732 X Skv X FkA 29097 MVA

Max Bus voltage V1 = 441 KV or (441/420) PU 1.05 PU

Acceptable Bus voltage

V2 = 416 KV or (416/420)PU 0.99 PU

Total reactive power

Sr = Scc ( V2 – V1)/V1 1763 MVAR

Standard capacity Sst 63 MVAR

Nos of reactor required

Sr / Sst 27 Nos

Recommended location for additional 25 Nos reactors

Advantages of installing reactor

Technical

1. Limited voltage rise on transmission lines at the time of light loads or after load shedding

2. Prevention of self excitation on generator on leading PF load.

3. Reduction of over voltage on sound phases during a line to ground fault.

4. Reduction of switching over voltage due to initial charging of lines.

Commercial

1. Being a capital item, investment on this equipment will be adjusted in the fixed cost portion of tariff so that entire amount ( capital + interest ) will return within the operating period of the reactors.

Basic operating principle

Basic operating principle

System Voltage, (V)

1. Initial charging current (i) produces pulsating flux in the coil

2. Induced voltage (e) = L (di / dt) is produced in opposite direction of the coil due to pulsating flux

3. As a result of two opposite voltages current equal to (V-e) / R passes through the coil

Shunt Reactor acts as inductive load when it is connected to Bus/Line and draws current for active & reactive load.

The reactive portion of current creates pulsating flux in the core and the power required for this purpose is known as reactive power (KVAR). KVAR = Current X system voltage X SinΦ

The active portion of current causes I2R loss and the power loss due to heating is known as active power (KW). KW = Current X system voltage X Cos Φ

Induced Voltage, (e)

Initial charging current.

Operating current (V-e) / R.

System Voltage

Operating current (I)

Active current (I x Cos Φ ) to active load (KW)

Reactive current (I x S

inΦ )

to reactive load (KV

AR

)

Φ

Operating function of shunt reactors

(System voltage – Induced voltage) 1. Current drawn by each phase = (Resistance per phase winding)

2. Induced voltage (e) depends upon the construction and magnetization characteristic of the core

3. Shunt reactor is a device with the fixed impedance value. Therefore the individual phase current is directly proportional to the applied phase voltage (i.e. I=U/Z).

4. For balance three phase current (Ir + Iy + Ib = 0), no current flows through the neutral.

5.Only during fault when phase current becomes un balance (Ir + Iy + Ib ≠ 0), current less than 1 PU passes through the neutral. It is due to the fact that positive sequence reactance(X1) is more or less equal to zero sequence reactance (x0) in five limbs core configuration (Normally X1/Xo = 0.9).

6. During operation heat is produced as a result of copper loss ( due top I2R) and core Loss ( due to Hysteresis and eddy current)

7. Typical total core & copper loss (KW)= rating of reactor (MVAR) x 0.2%

8. Typical core loss & copper loss = 75% & 25% of total loss respectively

Ir Iy Ib

Induced voltage and Magnetizing characteristic

of the core

Characteristics of the induced voltage The magnitude of the inductive voltage (e) developed in the

reactor coil due to pulsating flux depends upon -

1. Flux density in the core ( Concentration of flux in the core )

2. Magnetization characteristic of the core ( Ability of the core to produce flux)

1. Flux density in the core

• The coil provided with iron core always have higher flux concentration than the coil without core (Air core).

• As higher concentration of pulsating flux creates higher induced voltage (e) in the coil, iron core is used for higher voltage (> 72.5KV ) Reactor and air core is used for lower voltage (< 72.5KV ) Reactor

Reactor - 72.5 KV and above•Star connected with solid / reactor grounding

•Oil cooled , gapped core•Rating 30 to 300 MVAR •Directly connected to bus / transmission line

Reactor - Below 72.5 KV •Delta connected •Air cooled , Air core•Rating below 30 MVAR •Directly connected to tertiary winding of transformer

2. Magnetization characteristic

V – I linearity range

•The flux produced by the core remains proportional to the current passes through the coil till the core gets saturated.

•After saturation of core it can not produce flux further with the increase of current.

•Because of this magnetic saturation of the core, reactor coil can not develop inductive voltage (e) further.

•In such condition impedance of the coil becomes lower with respect to the applied voltage and higher current drawn by the winding causing high I2R loss ( heat generation) or earth fault.

Components of Reactor

WTI

OTI

1. 400KV,1250 Amps OIP HV bushings (3 Nos)

2. 145 KV ,1250 amps neutral bushing (1No)

3. Air cell type conservator tank with silica gel breather

4. Radiator banks (8 Nos) with ONAN type cooling

5. Buchholtz relay, Pressure relief valve and sudden pressure valve for protection

6. Floor mounted marshalling box for providing WTI,OTI and required control circuit for protection

7. Provision of line side and neutral side CT terminals in each phase

8. Two numbers treated earth pit for earthing neutral through steel flat

9. Two number earth grid terminals for earthing reactor tank and other metal structure.

Components of reactor

Five limbed core construction is adopted to achieve high zero sequence impedance. In addition to the three gapped core limbs with windings, there are two continuous outer return limbs. The two unwound side limbs help in achieving zero sequence impedance approximately equal to the positive sequence impedance

Gapped core construction is preferred for high system voltages over coreless construction due to the high energy density that can be achieved in gapped core construction

The core sections between consecutive air gaps are moulded in epoxy resin to prevent movement between individual laminations. The spacers forming the air gaps are blocks of ceramics with a high modules of elasticity and the whole stacking of core modules is cemented together during the assembly to form a solid column without possibility of rocking , or rubbing between individual parts.

The core segments are of radial laminated configuration. The radial laminations prevent fringing flux from entering flat surfaces of core steel which would result in eddy current overheating and hot spots.

Interleaved disc winding has been used for rated voltages 220 KV and above. This type of winding configuration provides better impulse voltagedistribution. For lower voltage classes a continuous disc winding or a multilayer helical winding are used.

M 6M 5M 4

Loss in Watt / Kg

CRGO Steel Strip Hi-B grade

Thickness 0.3- 0.5 mm , Flux Density 1.6 Tesla Frequency 50 Hz, Gray colour ,E Carlite insulating laminated core

23 M0H

ZDKH

0.89

1.11

0.74

0.66

0.57

0.97

27 M0H

Grading of core according to loss

Vibration in reactor

1. As a result of magnetization of core, a magnetic attraction force (F=107xB2)/8π N / m2) approximately equal to (107x1.62)/8π N/m2 or 104 Ton / m2 is produced in the air gap between the core sections.

2. This force pulsates at double frequency (2 X 50 Hz = 100 Hz) due to sinusoidal flux of frequency 50 Hz.

3. Because of this double frequency pulsating force in the core sections, high vibration and noise is observed in reactor during operation.

To reduce the vibration within the limit ( 200 micron) following measures are taken during construction – 1. Air gap are filled with ceramic materials 2.

Earthing of Reactor

Treated earth pit

To earthing gridTo earthing grid LA LA LA

1. Neutral earthing – To provide return path for the fault / unbalance current, the neutral of the reactor is grounded to two separately treated earth pit

2. Tank earthing – To avoid heating of tank due to circulation of eddy current as result of voltage build up on tank due to continuous passing of fractional portion of main flux through it, the potential of the tank is made zero by connecting with earth grid.

3. Steel structure earthing – To avoid damaging of steel structure from lighting strike, entire steel structure is grounded through earth

grid.

Cooling of reactor

Approximate heat generation during operation of reactor due to core and copper loss = 0.2% x MVAR rating = 120 KW

Core loss due to hysteresis & eddy current is about 75% of total loss i.e. 0.75X120 = 90 KW

Copper loss due to I2R loss is about 25% of total loss i.e. 0.25X120 = 30 KW

The loss of 120 KW is converted to heat

50000 Litre of oil absorb the heat and rise the top oil temperature to maximum 45 Deg C

8 nos radiator banks reduces oil temperature to 15 Deg C by ONAN cooling process.

Significance of parameters

1. Capacity (S) – As reactor is used as inductive load which only absorb reactive power, it capacity is given in MVAR .

Rated Voltage 1 PU ( 420 KV) 1.05 PU (441KV)

Rated Capacity 63 MVAR 69.45 MVAR

Current ( MVARx100)/(1.732xKV) 86.6 Amps 90.92 Amps

2. Impedance / Positive sequence Impedance (X1) – It is the per phase AC resistance of the winding which decides the magnitude of per phase current. X1 = U / I = 420000/(1.732X86.6) =2800 Ohms

3. Zero sequence Impedance (X0) – It is the AC resistance of the neutral path which decides the magnitude of earth fault current through neutral.

The value of X0 depends on the construction of core. For five limbs core, the value of X0 lays between 90% and 100% of positive sequence impedance (X1). Therefore X0 = 0.9 . X1 = 0.9 X 2800 =2520 Ohms.

Because of high zero sequence impedance earth fault current is restricted within 1 PU

4. Winding resistance (R) – It is the per phase DC resistance of the winding which decides copper loss (I2R). Measured winding resistance per phase = -2.570 Ohms at 38 Deg C

5. Total loss – It is the total active power consumed by the reactor and converted in to heat. Total loss (core and copper loss) = 110KW

6. Power Factor – CosΦ = 110 / 63000 =

0.17%

110KW

630000KVAR630000KVA

Φ

Operation

Position of Bus reactors in ts ii

Stage I Gen (3 X 210MW)

Stage II Gen (4 X 210MW)

400KV Bus

400KV Extension

Bus

Reactor II

Tie Line to TS II Expn

ICT

Power Grid Feeder

Power Grid Feeder

Tie Line

230KV Bus Mines Feeder

State Grid Feeder

Tie Line to TS I

Reactor I

Power Grid Feeder

TS II Expansion Gen (2 X 250MW)

400KV Bus

voltage compensation methods in power system

Switchyard Bus 395 – 415KV

REC

CAP

Delivers reactive power to boost up

voltage

Absorb reactive power to reduce

bus voltage

Increasing tap to deliver reactive power for

increasing bus voltage

Decreasing tap to absorb reactive power for decreasing terminal voltage

Over excitation to deliver reactive power for increasing terminal

voltage

Under excitation to absorb reactive power for decreasing terminal voltage

Capacitive loading

Inductive loading

Current Leading MVAR

Lagging MVAR Current

105% Grid Voltage (445KV)



1. Reduce excitation 2. Reduce GT tap 3. Switch on reactor 4. Disconnect lines 5. Reduce Gen load

1. Increasing excitation 2. Increase GT tap 3. Switch on Capacitor 4. Reduce gen load

Operation of bus reactor

Switching ON : Whenever bus voltage goes to 4% higher than rated voltage i.e. 400 x104/100 = 416 KV

Switching Off : Whenever bus voltage goes to 2% less than rated voltage i.e. 400x98/100 = 392 KV

However switching On & Off are to be carried out as per the direction of SRLDC

With the switching on of bus Reactor, Station MVAR will increase depending upon the grid voltage

Since Reactor is a fixed impedance equipment , when grid voltage is lower , reactor will absorb lower current and generate lower MVAR Load.

Similarly when grid voltage is higher , reactor will absorb higher current and generate higher MVAR Load.

GRID VOLTAGE REACTOR CURRENT REACTIVE LOAD

400 KV 85 Amps 60 MVAR



Observation of parameters during switching on reactor

Switchyard parameters TimeGrid

voltageStation MVAR

Reactor current ( As per Meter)

Reactor MVAR ( As per Meter)

Reactor II (SL No 6007011

Location 17th Bay)

Before charging

10:05 Hrs 413 KV 295 MVAR 0 0

After charging

10:15 Hrs 411KV 335 MVAR 85 Amps 63 MVAR

Reactor I (SL No 6007012

Location 16th Bay)

Before charging

12:10 Hrs 414 KV 254 MVAR 0 0

After charging

12:17 Hrs 411KV 331 MVAR 87 Amps 63 MVAR

UCB IV Parameters TimeGen

Terminal Voltage

MVARBus

Voltage PF

Reactor II (SL No 6007011 Location 17th Bay)

Before charging

10:15 Hrs

15.9 KV 50 MVAR 410 KV 0.98

After charging

10:06 Hrs

15.9 KV 55 MVAR 405KV 0.98

Reactor I (SL No 6007012 Location 16th Bay)

Before charging

12:10 Hrs

16 KV 40 MVAR 408 KV 0.99

After charging

12:17 Hrs

16 KV 60 MVAR 405 KV 0.99

Site ParametersRunning Hours

OTI WTIAmbient Temp

Noise & Vibration

Label

Reactor II (SL No 6007011 Location 17th Bay)

Switched On: 10:13 Hrs

Switched Off: 11:15 Hrs

1Hour 30 Deg C 34 Deg C 28 Deg C Normal

Reactor I (SL No 6007012 Location 16th Bay)

Switched On: 12:15 Hrs

Switched Off: 13:10 Hrs

1 Hour 33 Deg C 34 Deg C 33 Deg C Normal

Behavior of shunt reactor during operation

Switching on of shunt reactor

Typical inrush current 3 to 5.5 times of rated current.

Due to closing of breaker poles in three phases at different point of cycle, unsymmetrical current is developed in three phases which persist for approximately 1 sec

Closing point of BRK for Lowest inrush current .

Closing point of BRK for Highest inrush current .

As a result of unsymmetrical current, 3rd harmonic current passes through the neutral that may cause spurious tripping on earth fault.

Due to unsymmetrical current, DC off set current is produced which decrease slowly because of low loss in reactor and may cause saturation of CT

N R Y B

waveform of inrush current with dc off set current during switching in

DC Off set current

Wave form of 3rd harmonic current through neutral during Switching in

Typical problem during switching in

One of the principal difficulties with shunt reactor protection scheme is false tripping during reactor energizing.

This false tripping typically occurs within some hundreds of millisecond or even 1 to 2 seconds after closing of circuit breaker. It also happens randomly and not with every reactor switching attempt

Most of the time, it trips on Restricted Ground Fault protection / Differential Protection / Ground fault protection during switching in

It should be noted that HV shunt reactors are typically switched in and out at least once per day or even more often depending on the power system loading patterns.

During switching in of shunt reactor relatively high and long lasting dc current component appears in one or more phases. This current waveform moves the operating point of CT magnetic core on the hysteresis curve in one direction and when the dc component diminish it leaves the main CT with certain level of residual (i.e. remnant) flux.

During normal operation reactor current is always around 1pu and therefore of a relatively low magnitude, which is never big enough to move the operating point towards the origin.

Therefore when next switching attempt comes, depending on the moment of switching, residual flux in the CT core can increase or decrease. Thus this mechanism will sooner or later cause CT saturation during reactor switch in operation.

This CT saturation then causes problems for protective relays, which lose the correct information about the primary current and therefore cause false operation of protective relays.

Cause for tripping during switching in

Switching offDuring switching off operation of reactor high transient over voltage is developed due to breaking of inductive current.

This switching transients are inversely proportional to the shunt reactor rated power

Typical over voltage in 400 KV reactors* ( Sav – Steepness of voltage)

Switching transients overvoltage can be reduced considerably by installing surge reactor and control switching operation i.e switching off during zero crossing.

Frequent transients overvoltage due to switching off operation always have the impact on the dielectric life of reactor and breakers.

Electrical faults in shunt reactors

Phase to ground fault at line side

Faults in shunt reactor

Internal fault External fault

Phase to ground fault

Phase to ground fault at neutral side

Phase to phase short circuit fault

Inter turn short circuit fault

Over load due to over voltage and

harmonics

Phase to earth fault - out side the reactor

•

•Shunt reactor is a device with the fixed impedance value. Therefore the individual phase current is directly proportional to the applied phase voltage (i.e. I=U/Z). During external fault voltage of the faulty phase becomes lower than other phases and a result of that unbalance is created in the phase current. Because of unbalance phase current , zero seq. current less than 1 PU passes through the neutral.

N R Y B

Zero seq current (<1 PU)

Un balance in phase current

Unbalance in phase current due to external earth fault

Low zero sequence current through neutral due to unbalance in the phase current

Phase to earth fault - at the line side

1. Short circuit current flow through the line side faulty phase and causes unbalance in the phase current

2. Zero sequence current typically 1 PU flow through the neutral due to unbalance phase current

N R Y B

Zero seq current

Un balance in phase current

Phase to earth fault – at the neutral side

Rated current passes through the line side phase

High current passes through neutral side due to transformer action

N R Y BRated current in the line side

Turn to turn short

Shunt reactor winding impedance is approximately proportional to the square of the number of active turns.

Short circuit between some number of turns will cause the decrease of the winding impedance only in the faulty phase and corresponding small raise of the shunt reactor neutral point current.

Currents during turn-to-turn fault are of the small magnitude and they will not produce any sufficient unbalance voltage.

Sufficient unbalance voltage is produced only when number of turn-to-turn short is high. In such condition it is possible to detect turn to turn fault with the help of sensitive directional zero seq relay connected on the HV side of the reactor.

Summary – protection

CAUSES EFFECTS RESULTS IN PROTECTION ACTUATES

Switching ON Unsymmetrical inrush current

•Zero sequence 3rd harmonic current through the neutral •Saturation of CT due to slow decaying of DC offset current

•Restricted Earth fault with time delay•Diff protection

Switching OFF

Transient over voltage

High voltage stress on the dielectric of the reactor & circuit breaker.

No protection is recommended for this purpose.

External phase to ground fault

Lower than rated current through the faulty phase

•Unbalance in three phase line current •Low zero sequence current ( 1PU) passes through the neutral

•Line side residual current protection •Differential protection

Internal phase to ground fault at line side

High current at line side in the faulty phase

Unbalance in three phase line current •Low zero sequence current 1PU) passes through the neutral

•Line side residual current protection • line side over current protection •Differential protection

Internal phase to ground fault at neutral side

High current at line neutral side

•Rated current at line side phases•High current at neutral side

•Neutral side over current protection •Differential protection

Internal Phase to phase short circuit

As the chance of this fault is very remote due constructional feature of the reactor , protection for this fault is not recommended.

Inter turn fault

Low magnitude voltage unbalance

Small rise in neutral side current

No protection is recommended since the magnitude of fault is very low

Electrical Protection& relay scheme

Reactor protectionPURPOSE OF SHUNT REACTOR PROTECTIONThe purpose of the protection relaying is to disconnect the reactor and limit damage in case of internal short circuits, earth faults, inter turn faults and over voltage or over load. The reactor forms certain impedance for rated frequency, and as it is shunt connected, as over load may be caused by over voltage or harmonics in voltage and current.

PROTECTION DEVICES INBUILT OR MOUNTED ON REACTORa) Oil immersed reactor usually have a gas detector and oil surge detector (Buchholz

alarm & trip devices), which are excellent for detecting internal faults.b) Temperature monitors for oil & winding provide good over load protection.c) Pressure relief device is provided to safe guard the reactor from high pressures.

REACTOR DIFFERENTIAL PROTECTIONIt is widely used as instantaneous protection for short circuit faults with in the differential zone. this is treated as main-1 protection for reactor. It can be of high impedance type or of a sensitive current stabilized type. High impedance differential protection relays require an equal CT turns ratio on the phase and neutral side. Sensitivity is 5% of nominal reactor CT current.

BACK-UP PROTECTIONA variety of relays are available a) Over current & earth fault protection. ( 50, 50N, 51, 51N, 67, 67N – any combination of

these)b. Under impedance / distance ( z<)(21r).c. Neutral displacement protection (un>)

RESTRICTED EARTH FAULT PROTECTIONIf, for some reason, a sensitive differential protection not chosen, a restricted earth fault protection can be utilized.

LINE PROTECTION – I, LINE PROTECTION – II

CBIP Guidelines on shunt reactor Protection

Reactor Differential protection I) Shall be Triple Pole Type. ii) Have an operating current sensitivity at least 10% of nominal current.iii) Shall be tuned with system frequency.iv) Have an operating time not grater than 30 m sec at 5 times of setting.v) Have a suitable non-linear resistor to limit the peak voltage during in-zone faults in

case of high impedance type.vi) Shall be high or low impedance Principle type.

Reactor REF Protection.I) shall be single Pole.ii) Have an operating current sensitivity at least 10% of nominal current.iii) Shall be tuned with system frequency.iv) Have a suitable non-linear resistor to limit the peak voltage during in-zone faults in

case of high impedance type.v) Shall be high or low impedance Principle type.vi) Connection of restricted earth fault protection on the neutral side shall be from

residually connected Bushing CTs or from the ground side CT.

Reactor Backup Protection (Impedance type) I) Shall be Triple pole type.ii) Shall be single step Polarized ‘MHO’ or Impedance Distance relay suitable for

Measuring Phase to Ground and Phase to Phase to faults.iii) Shall grounds a Characteristic angle between 60-80 deg.iv) Shall have adjustable definite time delay with setting range of 0.2 to 2.0 sec.v) Shall have a suitable range for covering 60% of Reactor impedance. vi) Typical setting : Reach - 60% of Reactor Impedance, Time setting - 1 sec

ORReactor Backup Protection (Definite Time O/L & E/F).i) Shall be single stage Definite Time 3 Pole, Over Current relay with adjustable

current and Time.ii) Shall be connected for 2 O/C and 1 E/F connection and shall be non-directional

with reset ratio and low Transient Overreach. iii) Typical settings of o/c relays are: Current Setting- 1.3 x Rated current , Time

setting - 1 sec

Protection of bus reactor for double bus & transfer bus scheme

51N

21

87U

Bus I

Bus II

Transfer bus

87BB1

REACTORII

51N

21

87U

87BB1

REACTOR I

87BB287BB2

To Bus Bar protection

To Bus Bar protection

1. Differential protection

U1 V1 W1

U2 V2 W2

N

DP87

1s1

1s2

1s1

1s2

1s1

1s2

1s1

1s2

1s1

1s2

1s1

1s2

CT Specification1. Ratio: 200/1A2. Class: PS3. Knee point voltage: 200V4. Magnetizing current: 40 mA5. Secondary resistance: 1 Ohms

Purpose :Internal / external phase to ground fault.

Line side CT 1

Neutral side CT 1

Typical Relay connection for Differential protection

Type of relay : High impedance differential relay Setting : operating current sensitivity at least 10% of nominal current. operating time not grater than 30 m sec at 5 times of setting.

2. Residual earth fault protection

U1 V1 W1

U2 V2 W2

N

Instantaneous Residual over

Current Relay (50N) Or

AC Time residual Over Current relay

(51N)

1s1

1s2

1s1

1s2

1s1

1s2


Assigned Protection 1. External phase to ground fault.( Unbalance phase current) 2. Internal phase to ground fault at line side .( Unbalance phase current)

3. Circuit breaker pole discrepancy.( Unbalance phase current)

Typical relay setting •Set low set to 20% with time delay in between0.6s and 1s or even longer. •Use 2nd harmonic blocking. •Set high set to 175% with time delay of 0.1s.

4. Back up impedance protection Assigned Protection

Internal phase to ground fault at line side

Typical relay setting

•Set low set to 130% with time delay in between 0.6s and 1s. •Set high set to 250% with time delay of 0.1s.

U1 V1 W1

U2 V2 W2

N

21R – 3Ph, REACTOR BACKUP IMPEDANCE RELAY OF SUITABLY SHAPED CHARECRESTICS EITHER SINGLE / DOUBLE ZONE TYPE

1s1

1s2

1s1

1s2

1s1

1s2


Typical Relay connection for back up impedance protection

5. Line protection main & backup

U1 V1 W1 Line Side

U2 V2 W2Neutral Side

N

Bus Bar protection I & II

1s1

1s2

1s4

1s3

500/1A

1000/1A

2000/1A

1s1

1s2

1s4

1s3

500/1A

1000/1A

2000/1A

1s1

1s2

1s4

1s3

500/1A

1000/1A

2000/1A

Assigned Protection

Bus bar protection

Electrical Test on reactor

Prepared byM. G. Morshad / Additional Chief Manager

( Elect.) Transformer Maintenance Division

Thermal Power Station II Neyveli Lignite Corporation Ltd

Electrical test as per IS 5553

Routine Test ( To confirm the operating criteria)

1. Measurement of WR

2. Measurement of IR & PI

3. Measurement of impedance by bridge methods

4. Measurement of loss and current at rated voltage and ambient temperature

5. Isolation test

Dielectric Test ( To confirm the dielectric strength of the insulation)

1. Separate source voltage withstand test at 230 KV for one minute

2. Induced over voltage withstand test with PD indication at 364 KV AC (1.5/√3 PU ) for 30 minutes during which the PD level shall not exceed 500pc

3. Full wave lighting impulse voltage withstand test at 1300 KVp on line terminal

4. Switching impulse voltage withstand test at 1050 KVp on line terminal

Type Test ( To confirm the design criteria)

1. Temperature rise test along with DGA before and after test

2. Full wave lighting impulse voltage withstanding test at 550 KVp on neutral terminal

3. Measurement of zero sequence reactance

4. Measurement of acoustic noise level

5. Magnetizing curve test / knee voltage measurement

6. Measurement of capacitance and tan delta between winding and tank

Special Test ( To confirm design and operating criteria )

1. FRA test

2. DGA test before and after electrical test

3. Jacking test on reactor tank

4. Vacuum test on reactor tank

5. Oil leak test

6. Snap back test on HV bushing

Measurement of WR Purpose - To measure DC resistance per phase of coil for calculating I2R loss in the coil, which in turns decides the temperature rise. The measurement also shows whether the winding joints are in order and the windings are correctly connected.

Measuring methods -

1. Only at the stable value of current (I), corresponding voltage (V) value is taken for measuring the value of resistance ( R = V/I).

2. Applied current must not be higher than 15% of the rated current

3. % error in measurement increases with increases of applied current due to increases in I2R loss at higher current.

4. Value to be measured between (R – N) , (Y-N), and (B-N), and all the measured value must be equal.

N R Y B

Resistance measuring Kit

Voltage lead

Current lead

R-N

Y-N

B-N

Room Temp ( 37 Deg C) 75 Deg C

2.570 Ω 2.570 Ω

2.570 Ω 2.570 Ω

2.570 Ω 2.570 Ω

Measurement of IR

Minimum IR value Below 6.6 KV 6.6 - 11KV 22 – 33 KV Above 66 KV

K = 1.00 30 Deg C 200 MΩ 400 MΩ 500 MΩ 600 MΩ

K = 1.65 40 Deg C 121 MΩ 242 MΩ 303 MΩ 363 MΩ

K = 2.60 50 Deg C 77 MΩ 153 MΩ 192 MΩ 230MΩ

K = 4.20 60 Deg C 47 MΩ 95MΩ 119 MΩ 142 MΩ

K = 6.6 70 Deg C 30 MΩ 60 MΩ 75 MΩ 90 MΩ

K = 10.5 80 Deg C 19 MΩ 38 MΩ 47 MΩ 57MΩ

(Polarization Index) PI Value = (15 minutes IR / 60 minutes IR )

Less than 1 Dangerous

Above 1 to 1.1 Poor

Above 1.1 to 1.25 Questionable

Above 1.25 to 2.0 Fair

Above 2 Good

Purpose - To ascertain minimum insulation strength (IR Value) and dryness level (PI Value) of the winding required to charge the reactor.

N R Y B

5 KV IR measuring Kit (Megger)

Procedure – Measure IR and PI value between (R+Y+B +N) – (Tank + E) with 5 KV Megger and confirm the minimum IR and PI value as per the table given below.

_ +

Isolation testPurpose - To ascertain that the reactor core is insulated from the tank and core frame.

GCL Core BoltCore clamp Core Tank

CL – connected to core lamination

CC – Connected to core clamp

G – Connected to tank ( Earth )

CC

Procedure : 1.Disconnect the closing link that connects the two terminals CL-G.

2.Connect the tank with earth

3. Use a Megger and measure IR value between CL and CC + G by applying 3.5 KV for 1 minute

4. The measured IR value shall be minimum 1000 kohms ( 1 M Ohms)

5. There is no general requirement on the insulation level CC-G .

Measurement of impedance by bridge methods

Purpose : To measure the per phase impedance ( AC resistance) of the winding which controls the flow of current through the windings.

Impedance (Z) = √[(Resistance) 2 + ( Reactance)2] = 420KV / 86Amps

Reactor windings

Temp (0C)

U =

100/5

C4 (μF)

M3 (mH)

CN (pF)

Lx (H) = (M3/U)X(C4/CN)x1000

X (Ω) =2∏fLx

U 38 20.00 8.98966 1.0032 50.915 8.856355604 2783.43V 38 20.00 8.96700 1.0032 50.915 8.834031621 2776.41W 38 20.00 8.96210 1.0032 50.915 8.829204282 2774.89

Where

U = CT Ratio ( 100/5)

M3 = Mutual inductance = 1.0032 mH

CN= standard capacitance = 50.915 pF

C4= measured bridge capacitance in pF

X= calculated impedance of the winding in Ohms

Measurement of loss and current at rated voltage and ambient temperature

Purpose: To measure the loss ( core & copper loss) in reactor at rated operating condition.

Reactors windings

Temp

(0C)

V = 420/√3

KV

R4 ( KΩ)

C4 (μF)

ω = 2∏f

Tan delta = 1000/(C4xR4xω)

X (Ω)

Loss ( KW) at rated voltage =

V2 *(Tan Delta/X*1000)

U 38 242.49 210.15 8.98966 314.29 0.0016842350 2783.43 35.58

V 38 242.49 201.15 8.96700 314.29 0.0017640389 2776.41 37.36

W 38 242.49 332.75 8.96210 314.29 0.0010669585 2774.89 22.61

Phase Voltage

(KV)

X

( Ohms) Phase current = KV*1000/X ( C )

Rated Amps ( R )

Calculated loss at rated voltage

(KW) Calculated loss at rated current = KW X ( R/C)2

U 242.49 2783.43 87.12 86.60 35.58 35.16

V 242.49 2776.41 87.34 86.60 37.36 36.73

W 242.49 2774.89 87.39 86.60 22.61 22.20

Total loss at 38Deg C at rated current = (35.16+36.73+22.20) KW = 94.09 KW

Purpose of dielectric test

Voltage level as per IS 2072

Normally reactors are operated at the rated operating voltage. During its operation it is exposed to various transient over voltages like power frequency over voltage (1.5 x BIL, due to system over voltage), Lighting impulse (due to lighting), switching impulse (due to switching off). To avoid abrupt failure of insulation due to these transients, insulation is designed considering all the aspect. Dielectric test confirms the capability of the insulation to withstand these transient overvoltage which is subjected to the reactor during its service life.

Operating voltage

KV rms

Highest system voltage (BIL)

KV rms

Power frequency voltage

(KV rms)

Switching Impulse

(KV Peak )

Lighting impulse

(KV Peak )

0.415 1.1 3 - -

3.3 3.6 10 - 20/40

6.6 7.2 20 - 40/60

11 12 28 - 60/75

15 17.5 38 - 75/95

24 50 - 95/125

33 36 70 - 145/170

52 95 - 250

66 72.6 140 - 325

123 185/230 - 450/550

145 230/275 - 550/650

170 230/275/325 - 550/650/750

230 245 325/370/395 - 750/850/950

300 395/460 750/850 950/1050

362 460/510 850/950 1050/1175

400 420 570/360 950/1050 1300/1425

Separate source voltage withstand test at 230 KV for one minute

PURPOSE : To verify the operating voltage withstanding capacity of the minor insulation ( paper ) used in line terminals and windings.

NR Y B

50 Hz, AC Generator

Method Test voltage from a 50 Hz, sinusoidal source is applied between (HV +N) and (E + Tank) through a step up transformer

Maximum test voltage ( 400 KV /1.732) or 230 KV rms

Duration of test 60 sec

Measurement of test voltage

Direct reading for RMS type voltmeter or (Reading / √2) for Peak type voltmeter (KV)

Confirmation The test is declared to be successful if the test voltage does not collapse during the test.

KV

Induced over voltage withstand test with PD indication at 364 KV AC (1.5/√3 PU ) for 30 minutes during which the

PD level shall not exceed 500pc ( Method 2) PURPOSE : To verify the power frequency voltage withstanding capacity of the minor insulation (Paper ) used in line terminals & windings and the maximum level of PD observed during the test.

Method Test voltage from a 160Hz ( to avoid saturation of core), sinusoidal source is applied at line terminals through a step up transformer keeping neutral terminal and tank grounded. PD is measured by PDD connected with impedance ( z) & capacitors as shown in the fig

Maximum test voltage ( 1.5 x 420KV /1.732) or 364 KV rms

Duration of test 30 minutes in steps at various voltages level as shown below

Measurement of test voltage

Direct reading for RMS type voltmeter or (Reading / √2) for Peak type voltmeter (V)

Confirmation The test is declared to be successful if the test voltage does not collapse during the test. and the PD level is observed within 500pC

364KV 420KV 364KV

5 Minutes

5 Sec

30 Minutes

N

RYB

50 Hz, AC Generator

KV

Z

PDD

Full wave lighting impulse voltage withstand test at 1300 KVp on line terminal

PURPOSE : To verify the impulse voltage withstanding capacity of the major insulation ( pressboard) used between the windings, line terminals caused by lighting strike.

N R Y B

Impulse Generator

0.1 Ώ

Recorder

-

+

Method Test voltage from an impulse generator is applied at line terminals keeping neutral grounded through 0.1 Ώ resister and other terminals directly grounded. The sequence for applying impulse is - one impulse of a voltage between 50% and 75 % of the full test voltage, and three subsequent impulses at full voltage

Test voltage 1300 KV peak with Front time

Duration of test T1 = 1,2μs ± 30% and Time to half-value T2 = 50 μs ± 20%

Confirmation The test is successful if the test voltage does not collapse during the test.

Switching impulse voltage withstand test at 1050 KVp on line terminal

N R Y B

Impulse Generator

500Ώ

Recorder

-

+

Method Test voltage from an impulse generator is applied at line terminals keeping neutral grounded through 500 Ώ resister and other terminals directly grounded. The sequence for applying impulse is - one impulse of a voltage between 50% and 75 % of the full test voltage, and three subsequent impulses at full voltage

Test voltage 1050 KV peak

Duration of test

Front time Tp> 100μs , Time above 90% Td> 200μs and Time to the first zero passage T0> 500μs ( preferably 1000 μs )


PURPOSE : To verify the impulse voltage withstanding capacity of the major insulation (Pressboard) between the winding ,line terminals caused by switching operation.

Type Test

1. Temperature rise test along with DGA before and after test

2. Full wave lighting impulse voltage withstanding test at 550 KVp on neutral terminal

3. Measurement of zero sequence reactance

4. Measurement of acoustic noise level

5. Magnetizing curve test / knee voltage measurement

6. Measurement of capacitance and tan delta between winding and tank

Temperature rise test

The purpose of the measurement is to check that the temperature rises of the oil and the windings do not exceed the limits agreed on or specified by the standards.

(T1)Hot / top oil temp

Cold winding resistance at (T) 38 deg C R1 2.569 Ohms

Hot winding resistance after switched off (to be derived from graph)

R2 2.743 Ohms

Ambient temperature after switched off Ta 34.23 Deg C

Calculated winding temp at Time of S/D Tw = R2/R1 (235+T)R1 – 235 (2.743/2.569) x ( 235+ 38) – 235 = 56.49 Deg C

Average oil temp at S/D T1 55.30 – ½(55.30 – 4425) = 49.22 Deg c

Winding temp gradient Tg = (Tw – T1 ) 56.49 – 49.22 = 7.27 Deg c

Average oil temp rise Td = (T1 - Ta) 49.22 – 34.23 = 14.99 Deg C

Corrected winding temperature Tc = (Tg + Td) 7.27+ 14.99 = 22.26 Deg C

Winding temperature rise (Tc – Ta)

Temp rise

Time

Steady state temp

T 55..30 / B 44.25

T 35.00 / B 33.00

8 Hours

Full wave lighting impulse voltage withstanding test at 550 KVp on neutral terminal

NR Y B

Impulse Generator

500Ώ

Recorder

-

+

Method Test voltage from an impulse generator is applied at neutral terminals keeping other terminals grounded through 500 Ώ resister and tank directly grounded. The sequence for applying impulse is - one impulse of a voltage between 50% and 75 % of the full test voltage, and three subsequent impulses at full voltage

Test voltage 550 KV peak

Duration of test

T1 = 1,2μs ± 30% and Time to half-value T2 = 50 μs ± 20%


PURPOSE : To verify the impulse voltage withstanding capacity of the major and minor insulation (Pressboard and paper) used in neutral terminals and side of the winding caused by switching operation.

Measurement of zero sequence reactance

N R Y B

50 Hz, AC Generator

V

A

A

Purpose : To measure the AC resistance (Impedance) of neutral path which controls the earth fault current (zero sequence current) through neutral during internal or external earth fault.

Method •Two phase AC supply is applied between (R+Y+B) and( N) through a step up transformer which is fed by a generator. •Applied voltage is increased till 70% of the rated current flow through the neutral .

Zero seq impedance

(3 x Applied voltage) / Neutral current x rated frequency / test frequency.

Current through neutral = 62.5 AmpsApplied Voltage = 56.40 KVTest Frequency = 49.80 Hz Zero Sequence Impedance = [(3 x 56400)/ 62.5] x [50.0/49.8] = 2718.07 Ohms

Measurement of acoustic noise level and vibration

1 Meter

2 MeterMicrophone for picking up noise

The purpose of the sound level measurement is to check that the sound level of the reactor meets the specification requirements given in relevant standards .

A sound spectrum analyses is used for measuring sound level. The sound spectrum indicates the magnitude of sound components as a function of frequency. The sound pressure level is the measured at various points around at a distance (D) of 30 cm for ONAN or 2 m for ONAF cooling system spaced at an interval (X) of 1 meter.

Limit

Sound level

Vibration level Within 200 microns

Within 81db

Magnetizing curve test / knee voltage measurement

Test procedure • Each phase of the reactor is charged one by one with direct current. • When the maximum test current reached, supply is switched off and the reactor

winding is short-circuited simultaneously by DC current breaker.• The decaying current in the circuit is registered by a computer assisted data

acquisition system.• The saturation curve Flux (Ø) / Flux (Ø) nom. versus I / Inom is then determined

using the formula.

Imax Maximum measured current Calculation

Imin Minimum measured current L(I) = (IxR) / (di/dt)

I nom Nominal AC current / √ 2 Ø nom = I nom x L mean

I mean Calculated inductance from I min to I nom Ø (I ) = ∫( Imin L(I)) +(Imean x L mean)

Ø nom Nominal flux at I nom

R Circuit resistance (RL + Rs+ Rc)

I / I nom

Ø / Ø nom

2

2

Measurement of capacitance and tan delta between winding and tank

Angle δCapacitance (pF)

[(I/2 f V π) x Cos δ ]Tan δ = Sin δ =Cos Φ(PF)

Condition of the insulation

0.0 Deg (I / 3140) x 1.000 0 Pure capacitor

0.5 Deg (I / 3140) x 0.999 0.002 Very good

0.5 Deg (I / 3140) x 0.999 0.004 Good

0.5 Deg (I / 3140) x 0.999 0.006 Fairly good

0.5 Deg (I / 3140) x 0.999 0.007 Acceptable

0.5 Deg (I / 3140) x 0.999 0.008 Not acceptable

90.0 Deg 0 1.0 Pure resistance

N R Y B 10 KV Tan

Delta Kit

Purpose : To ascertain the condition of the solid insulation of the windings.

+

+

+

+

-

-

-

-

I

IcI

Ir

Ir

V

δ

Φ

I = total current drawn by the capacitor formed between winding and tank and the value of capacitance is [(I/2 f V π) x Cos δ ] (pF)

Ic = Capacitor charging current

Ir = Current flow through the capacitor due to impurities / disintegration of the insulation between winding and tank. As this current is in phase with applied voltage, it is dissipated in heat.

Measured Capacitance = 9887 pF, Tan δ = 0.0031 at temp 39 Deg C

Special Test

1. FRA test

2. DGA test before and after electrical test

3. Vacuum test on reactor tank

4. Oil leak test

5. Snap back test on HV bushing

FRA test Purpose- Frequency Response Analysis (FRA) is carried out to detect displacement (or movement) of the windings. Usually the first measurement in the factory is used as a fingerprint. Results of later measurements are compared with the first one in the factory.

The software controlled sine wave generator produces output voltage of max. 4 Vrms with frequency range of 50 Hz to 1 MHz. It has 75 Ω output impedance. Input impedance is 75 Ω.

Voltage from the generator is applied to the one transformer terminal (one winding end) and response voltage is measured on another terminal (the other winding end).

FRA test report Impedance value Z in kΩ versus frequency is plotted on the diagram with indication of terminals with applied and response voltage.

Or attenuation A (or damping) in dB (20 log (Uoutput / Uinput ) versus frequency is plotted on the diagram with indication of terminals with applied and response voltage.

The reactor is said to be healthy if no deviation is observed between the results taken in factory and field

DGA test

5 gms of silver nitrate (AgNO3) dissolved in 100 ml distilled water

A week solution of ammonia in water is slowly added to 100 ml of solution 1, until a white curdled precipitate which forms first disappears in the mixture.

Chemical analysis of gas : The gas analyser loaded with these solution is connected to the top pet cock. Small quantities of gas collected in the gas relay (Bucholtz relay) is allowed to pass through the two solutions.

Vacuum test on reactor tank

Oil leak test

Snap back test on HV bushing Purpose : Snap back test was carried out on the above bushing to determine the natural frequency and damping factor. Following equipment were used to conduct this test.1: Piezoelectric accelerometer B & K 4371.2: PL 202 Real Time FFT Analyzer

Methods : Two nos. Piezoelectric accelerometer were mounted 90 degree apart at the bottom of the bushing, one in the direction of applied force (X) and other 90 degree to the applied force (Y). A force of 250 Kg was applied at the top and then it was cut-off. The resulting vibrations were recorded on the FFT. The recorded signals were analyzed on FFT Analyzer to determine the natural frequency and damping factor of the bushing.

X

Y

FFT

X direction

Y direction

Naturalfrequency

From FFT reading 3.25 3.25

Damping factor

[(100 / 2 π n) * log (Y 1 / Yn+1)] Where: n = No. of cycle Y n+1 = Amplitude of (n+1) cycle peak Y1 = Amplitude of1st cycle peak

1.421% 2.03%

The test is successful if no evidence of physical damage is observed on the bushing after the test

Final Observations

Though oil immersed, shunt reactor and power transformer are viewed alike, there are distinct differences between construction and operating characteristics of these two devices.

As NLC is going to install two numbers 63 MVAR bus reactor for the first time in TS II and the operating and maintenance staffs are not properly exposed to its operating data, following information need to be collected from any southern grid thermal power plant ( not from substation) presently operating with similar capacity bus reactors for successful and trouble free operation of reactor in TS II –

Average number of switching operation of the reactor per day Numbers of operating hours achieved since commissioningNumbers of forced / planned shutdown taken after

commissioningProtection co-ordinations and its settings Number of false/actual tripping, if any, since commissioning and

its reasonsMaximum & minimum bus voltage for switching in and out of

reactorsReduction in leading MVAR and bus voltage after switching in of

the reactorAny abnormalities observed in generator excitation during

switching in/out Average reactor current, winding and oil temperature Any failure of parts like bushings, LA, gaskets etc since

commissioning Remarks of the operating staff on the performance of the

reactors

Thank you

reactors

Technology