Delayed Current Zero Crossing Phenomena During Switching ... · Delayed Current Zero Crossing Phenomena During Switching of ... Voltage zero switching and also due to voltage

Minnesota Power Systems Conference

St. Paul, MN

November 7-9, 2017

Delayed Current Zero Crossing

Phenomena During Switching of

Shunt-Compensated Lines

David K Olson Pratap G Mysore

Paul Nyombi Pratap Consulting Services

Xcel Energy

Acknowledgement

2

•American Transmission Company (ATC) study

group for bringing up DCZ issues during the line

design studies on one of the CAPX lines.

•This prompted three utilities associated with CAPX

to determine the impact of shunt reactors on their

lines and to determine mitigation methods.

Transmission Line Representation

3

• Long Transmission lines can be represented as several series connected

modules made up of series resistance, Series Inductance and Shunt

capacitance.

• Lines are also represented with Lumped parameters where the line

capacitances are split between two ends.

Line Parameters – Typical

4

345 KV System

• Line Resistance: 0.033 Ω/Mile

• Line Reactance: 0.5-0.6 Ω/Mile

• Line Inductance: 1.459 mH/Mile

•Line Charging MVAR: 0.8-0.9 MVAR/Mile

•Line Capacitance:0.0189 µF/mile

Resistance can be ignored for our discussions.

Transmission Line Behavior

5

•During Light load or no load conditions,

Transmission line has the same effect as

capacitance connected to the system.

•System voltage increases with the connection of

open ended transmission Lines.

•Capacitance of the line is distributed over the entire

length. Remote end voltage increases with increase

in length – Also Known as Ferranti Effect

Shunt Reactor Application

6

•To Keep System Voltage below allowable

maximum value

•System voltage needs to be within the allowable

range to prevent connected Equipment failures.

- IEEE 1312- 1993 (R2004) “IEEE Standard Preferred

Voltage Ratings for Alternating-Current Electrical Systems and

Equipment Operating at Voltages Above 230 kV Nominal”

• Installed on

–Tertiaries of transmission Transformers

– On Transmission Lines –Either at both Ends or at

only one end.

–Middle of the line

Shunt Compensated Lines

7

•Transmission Lines with shunt reactors connected.

• Total shunt reactor MVAR_R = Sum of all shunt

reactors connected on the line.

•Degree of Compensation, M = _

_

Where, MVAR_C is the Line Charging MVAR

Degree of Compensation is determined by planning

studies- Can exceed 100%

Effect of Shunt Reactors

8

• Decreases the voltage by compensating for the capacitive charging

currents.

• Reactor current, IL lags voltage by 900

• Capacitor (line charging) current, IC leads voltage by 900

• Decreases the current through the breaker current (|IC |- |IL |)

(f ile Paper1.pl4; x-v ar t) factors:

offsets:

1

0

v :I_L1 1.00E-03

0

c:I_C2 - 1

0

c:I_L1 -I_L2 1

0

c:XX0002-I_L1 1

0

0 10 20 30 40 50 60 70*10 -3

-300

-200

-100

0

100

200

300

Steady State current and Voltage phase relationship

Line Currents During Energization

9

• Voltage waveform: V Sin (ωt +φ) where φ is the

delay angle of switching on the voltage waveform

from voltage zero.

•Charging (Capacitive) Current: I Sin(ωt+φ+90)

•Reactor Current: MI [Sin (ωt+φ-90)+ Cosφ e-t/τ]

(f ile Paper1.pl4; x-v ar t) v :XX0006

0.00 0.02 0.04 0.06 0.08 0.10-300

-200

-100

0

100

200

300

*103

Line Breaker Current

10

• IC = I Cos(ωt+φ)

• IL = MI[-Cos(ωt+φ)+ Cosφ e-t/τ]

•Breaker Current, IBRφ= IL+IC

• IBRφ= IL+IC =(1-M)I Cos(ωt+φ) + MI Cosφ e-t/τ

•Maximum DC offset is seen when the line with

reactor is switched at Voltage zero Crossing.

• IBR0 = (1-M)I Cos(ωt) + MI e-t/τ

Line Breaker Current Components

11

•AC sinusoidal wave has a peak value of (1-M)I

•DC component is exponentially decaying with time

constant of τ sec.

• X/R of oil filled reactors are in the range of 600-

750; τ can be as high as 2 seconds (~=750/377).

•DC component will decay to less than 2% of the

initial value after 4τ time.

Line Breaker Delayed Current Zero

12

• If AC component Peak (1-M)I is greater than DC

component MI, AC waveform will always cross

current zero axis or else, Current zero occurs after

a delay.

Delayed

Current

Zero

-0.5

0

0.5

1

1.5

2

2.5

3

3.5 Delayed Current Zero (DCZ)

DC Component

Breaker Current

Criterion to Prevent DCZ during

Normal Switching

13

•Boundary Condition: (1-M)I = MI;

•M = 0.5;

•Under normal switching, the degree of

compensation, M cannot exceed 50% to prevent

Delayed Current Zero on Breaker Current.

Time to first Current Zero

14

• If the Degree of compensation is greater than 50%,

the AC waveform will cross current zero line when

the peak equals or exceeds the decayed DC

component

• (1-M) = M * e-t/τ

•Solving for t , tSeconds τln

;

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10

Deg

ree

of

Co

mp

ensa

tio

n

Time for first current zero

Degree of Compensation VS Time to

first Current Zero (τ =2 sec)

Effect of Source Impedance

15

•The time constant of DC Component is the overall

time constant of the system: (Xtotal/RTotal))

• If the source is another transmission line behind the

line, X/R is typically around 17.

•Overall X/R reduces. DC component decays faster.

•ZSource = 2.491 +j47.5 Ohms for 2500 MVA, 345

kV source (X/R=17)

•ZReactor = 3.174 +j2380.49 (X/R =750)

•Ztotal = 5.665 +j2428 (X/R=428.6)

Energizing Faulted Line

16

•Fault Location: at the shunt Reactor

•Voltage Shift on un-faulted phase is the maximum

at the fault location and is maximum for double

line to Ground Fault

•Voltage of B-Phase (PU) for A-G fault

• | VB| = |e-j240 -

| ; K=

•B-C-G fault, VA (PU) =

•For K=2.8, A-Phase voltage increases to 1.273 PU

for B-C-G fault

•B-phase voltage increases to 1.23

M Limit - Effect on DCZ due to Faults

17

•Voltage rise on un-faulted phase of the shunt

reactor results in higher DC component due to

Voltage zero switching and also due to voltage rise.

•Capacitive current doesn’t increase in the same

proportion due to distributed Capacitance of the

line.

• Is affected by inter-phase capacitance

•Transient studies are needed to determine the

reduction in M from 50% to prevent DCZ

System Simulation

18

Energization of Shunt Compensated

Line

19

Simulated case 1a: Radial line energization from South

with A- and B- phases poles closing at voltage zero

a. With both shunt reactors

connected (approx. 86%

compensation)

b. With both shunt reactors

connected and utilization of

425ohm pre-insertion resistors

inserted for 13ms

(f ile North_South_345kV.pl4; x-v ar t) c:STH_BA-STH_LA c:STH_BB-STH_LB c:STH_BC-STH_LC

0.10 0.25 0.40 0.55 0.70 0.85 1.00-700

-525

-350

-175

0

175

350

525

700


0.190 0.252 0.314 0.376 0.438 0.500-200

-150

-100

-50

0

50

100

150


Line

20

Simulated case 1b: Radial line energization from South

with A- and B- poles closing at voltage zero

a. With only North line-end shunt

reactor connected (approx. 43%

compensation)

b. Energizing the transmission

line into a close-in BC-Ground

fault with only one reactor

connected. A-Phase current is

shown below


0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50-600

-400

-200

0

200

400

600

(f ile North_South_345kV.pl4; x-v ar t) c:STH_BA-STH_LA

0.15 0.24 0.33 0.42 0.51 0.60-600

-450

-300

-150

0

150

300


Line

21

Observations:

Energization of the line with only one reactor connected

eliminates DCZ – 43% shunt compensation

DCZ may also be minimized by using pre-insertion

resistors and/or increasing pre-insertion time

Several faults may need to be studied as their impact on

DCZ depends on other factors like system parameters and

line configuration. 43% compensation was found to be

adequate in this case for minimizing DCZ for all switching

scenarios.

Switching of Shunt Reactor on an

energized line

22

a. Switching South line-end shunt

reactor during limited/no

loading on the line. South line

end breaker currents are shown

below:

b. Switching of the shunt reactor

with at least 46MW flowing

from South to North

substation. DCZ on South

line-end currents is

eliminated.


0.3 0.4 0.5 0.6 0.7 0.8-120

-80

-40

0

40

80

120


0.38 0.40 0.42 0.44 0.46 0.48 0.50 0.52-200

-150

-100

-50

0

50

100

150

200

Switching of Shunt Reactor on an

energized line

23

Observations:

Switching of shunt reactor at voltage zero crossings creates

DC offsets at line-end breakers, with the strongest source

experiencing the greatest offset.

Line breakers experience DCZ if the offset is greater than

the capacitance from the reactor up to the breaker location.

A minimum of 46MW, power flow from South to North, is

found to be adequate in mitigating the delayed current zeros

when energizing the second shunt reactor

Switching of line/shunt reactor for

transmission line fault events

24

Simulated case 3a: South line-end A-Phase reactor current

following a close-in A-G fault on transmission line

Under this study:

A-Phase to ground fault is

considered to occur at A-Phase

voltage zero crossing

Both reactors are connected

with the line closed through

Shunt reactor current takes

several cycles to decay to zero;

X/R ~ 1.68s

(f ile North_South_345kV.pl4; x-v ar t) c:STH_LA-STH_RA

0 2 4 6 8 10-120

-80

-40

0

40

80

120

Transmission line de-energization

under no fault conditions

25

Simulated case 3b: South line-end B-Phase line voltage

following line de-energization with both reactors connected

Trapped energy in the shunt

capacitance and shunt reactors

creates high frequency transients

that may take several seconds to

decay

A- and C- Phases also do

experience voltage transients.

Only B-Phase is shown for

clarity.

(f ile North_South_345kV.pl4; x-v ar t) v :STH_LB

0 2 4 6 8 10-300

-200

-100

0

100

200

300

400

*103

Switching of line/shunt reactor for

transmission line fault events

26

Observations:

Shunt reactors should not be tripped for faults on

transmission line.

Disabling high-speed reclosing is recommended especially

for lines that have more than 50% compensation. Adequate

time may be required before safely reducing the shunt

compensation for re-energization.

Delayed Current Zero Mitigation

Methods

27

1. Use of pre-insertion resistors on line breakers

Limitations:

Breaker manufacturers do typically guarantee only 8-12ms

of pre-insertion time. Pre-insertion time of at least 13ms

was required in our case study during normal energization.

The time duration was not enough to eliminate DCZ during

faulted line energization.

Depending on the breaker closing mechanism, increasing

pre-insertion resistor size created new transients when the

main contact by-passes the pre-insertion resistor


Methods

28

2. Limit degree of line compensation during line

energization

Limitations:

Reduced line compensation increases remote end line

voltages. Remote end line connected equipment must be

rated adequately – Surge Arrestors, CCVTs, etc


Methods

29

3. Utilization of controlled closing on shunt reactor

breakers

Limitations:

Implementation of this may be challenging on existing

shunt reactor breakers as this may require breaker upgrade


Methods

30

4. Consider moving reactors (or some of them) from

transmission line to substation buses

Limitations:

Implementation of this may be challenging for existing

substations

Tripping and reclosing considerations for

shunt compensated lines

31

Radial energization of a transmission line with less 50%

shunt compensation is recommended.

Disabling high-speed reclosing is recommended on lines

with more than 50% shunt compensation.

Tripping and reclosing considerations for

shunt compensated lines

32

• Delaying line breaker tripping for transmission line faults is

suggested – allows for appearance of current zero crossing

for faults (during light load periods) on lines with more than

50% compensation. South line-end currents are shown

below for close-in A-G fault

(f ile North_South_345kV.pl4; x-v ar t) c:STH_BB-STH_LB c:STH_BC-STH_LC

0.35 0.39 0.43 0.47 0.51 0.55-300

-200

-100

0

100

200

300

400

500

Conclusions

33

a. On line energization to prevent DCZ:

If shunt reactor(s) on the line is not required to limit

open end voltage to acceptable level, energize the line

without shunt Reactors.

Keep Shunt Compensation below 50% during line

energization.

Energize the line through breakers equipped with pre-

insertion resistors that provide enough damping to

produce current zeros within breaker interrupting time.

This is dependent on the resistor value and duration of

insertion. It may not work under all the cases if the

degree of compensation is close to 100%.

Conclusions

34

b. On Shunt reactor energization onto energized lines,

to prevent DCZ:

Switch shunt reactors less than 50% of the total

charging current.

Switch shunt reactor at voltage maximum point on the

wave.

Switch shunt reactors if their inductive MVAR is not

greater than the minimum load on the line, in MW.

Conclusions

35

c. Tripping for line faults:

Trip only the line breakers and not shunt reactors.

Shunt reactors need to be tripped only after several

seconds delay to allow full decay of reactor current on

the faulted phase.

Total interrupting time of faults on lines with shunt

compensation greater than 50% may need to be at least

few cycles to allow presence of current zeros on healthy

phase(s) during faults. This is dependent on the zero

sequence currents flowing on the healthy phases or the

minimum load on the healthy phases.

Conclusions

36

d. Tripping for shunt reactor faults:

Trip the shunt reactors only if they are equipped with

breakers.

Conclusions

37

e. Reclosing on Lines:

Instantaneous reclose is disabled on lines where shunt

reactor switching is required to reduce the degree of

compensation.

Time delay reclose is enabled after the reactor is

switched out.

Synch-check reclosing at the other end after energizing

the line may generate offsets on the currents. DC offset

is dependent on the load picked up after restoration.

Questions

38

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Delayed Current Zero Crossing Phenomena During Switching ... · Delayed Current Zero Crossing Phenomena During Switching of ... Voltage zero switching and also due to voltage