8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

1/31

Coupling Capacitor VoltageTransformers TransientsFundamentals

Roger Hedding ABB Dousman,WI

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

2/31

Not this Transient

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

3/31

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

4/31

Agenda

Introduction

Capacitor Voltage Transformer Design

Transient Response

Transient Response Effect on DistanceRelays

Selection of Capacitor Voltage Transformer

Conclusion

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

5/31

Why Capacitor Voltage Transformer and What isthe Problem ?

Voltage transformer needed to transform system

voltage to voltage usable to IED.

Wire wound voltage transformers not economical

above 138kV.Capacitor voltage transformers consist of

capacitors and inductors that can discharge and

cause problems

Not a problem for e/m relays.

Faster sol id state and microprocessor relays need

attention

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

6/31

Capacitor Voltage Transformer Cross Section

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

7/31

CVT Schematic

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

8/31

CVT Equivalent Circuit

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

9/31

Transient response

Ability to reproduce rapid changes in

the primary voltage.

The remaining secondary voltage after aspecific t ime due to a short circuit on

the primary voltage

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

10/31

Transient response

Transient oscil lations in the secondary

Two components

High frequency component

600 4000 Hz

Damped out in 10 milliseconds

Low frequency component

2 15 Hz

Lasts much longer

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

11/31

Capacitor Voltage Transformer Transient Response

Transient Response function of

Equivalent Capacitance

Voltage Tap Ferroresonant Suppression

Connected Burden

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

12/31

Transient Response vs Capacitance

Early work by Rusnak and Sweetana vintage 1970s

345kV PCA-7 CCVT

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

13/31

Transient Response vs Tap Voltage

Early work by Rusnak and Sweetana vintage 1970s

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

14/31

Transient Response vs Capacitance and Tap V

Early work by Rusnak and Sweetana vintage 1970s

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

15/31

Transient Response Vs. Ferroresonant Suppression

Early work by Rusnak and Sweetana vintage 1970s

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

16/31

Transient Response vs Burden

Early work by Rusnak and Sweetana vintage 1970s

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

17/31

What has changed in modern day CVT design ?

Higher tap voltage up to 13kV

Higher capacitance

Changes in Ferroresonant suppression

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

18/31

Transient Response for Current Design

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

19/31

CVT Design Choices

Under control of

Designer

Stack

Capacitance

Ferroresonant

Suppression

Tap voltage

Under control of

user

Burden

Stack capacitance

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

20/31

CVT Transient Response Effect on DistanceRelays

Impedance relays rely on

accurate information of CTs

and PTs to calculateimpedance to fault.

CVT transients cause

erroneous information to be

presented to the relay Transients last a relatively

short period of time

Zone 2 and Zone 3 timers

are set much longer the

transient time period. Zone 2 and Zone 3

impedance relays not

effected by CVT Transient

Zone 1 relay with nointentional delay will be

effected

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

21/31

Factors effecting impedance relay

Type of fault

Fault incidence angle

Source Impedance Ratio of

protected line

Load current magnitude and

direction

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

22/31

Franklin and Horton Paper

How much does zone 1 setting need to

be pulled back so it wont overrreach due

to CVT transient ?

Used CVT with active Ferroresonant

suppression

Worst case

Phase phase fault

Heavily loaded line flowing toward

relay

Fault occurs at voltage zero

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

23/31

Impedance trajectories for Ideal VT and CVT

Franklin and Horton Determining Distance Relay Zone 1 Reach Settings to Prevent CCVT Transient Overreach

Phase Phase fault

SIR = 5

Zline = 0.1+J1.0

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

24/31

Maximum Reach to compensate for CVT Transient

Franklin and Horton Determining Distance Relay Zone 1 Reach Settings to Prevent CCVT

Transient Overreach for a particular relay

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

25/31

Modern Microprocessor relays

Use low pass filters and counters to mitigate

effects of CVT transients

Security comes at price of speed

Relay operation may be delayed by up to 10msec

Some manufactures use CVT Transient

filtering as setting (yes/no) in case

conventional wire wound VTs are used.

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

26/31

Capacitance Selection for CVT

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

27/31

Recommend EMTP Studies

Several models of CVTs have been developed for use with

EMTP Program.

To understand how a particular CVT will respond

on a given power system recommend EMTP Study

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

28/31

Conclusion

Reviewed CVT Design

For lower CVT Transient

High Capacitance

High Tap Voltage

Passive Ferroresonant Circuit

Lower Burden

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

29/31

Conclusion

Due to long time delay settings Zone 2 and Zone

3 relays typically not effected by CVT Transient

Zone 1 relays with no intentional delay subject to

overreach as a result of CVT transient.

Factors influencing Zone 1 overreach

SIR

Line Loading

Type of Fault

Fault inception angle

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

30/31

Conclusion

Modern relays employ filtering techniques to

lessen impact of CVT transient

For further questions on your system do

EMTP study

8/12/2019 1 4-3 am coupling capacitor voltage transformer transient fundamentals [frp-1]

31/31

Questions ?