current transformer modeling in emtp-rv for transient

Academic User Group Meeting Sarajevo, May 2012

Božidar Filipović-Grčić Prof. Ivo Uglešić University of Zagreb Faculty of Electrical Engineering and Computing Department of High Voltage and Power Systems

Date: May 17, 2012

CURRENT TRANSFORMER MODELING IN EMTP-RV FOR TRANSIENT STUDIES Božidar Filipović-Grčić Prof. Ivo Uglešić University of Zagreb Faculty of Electrical Engineering and Computing Department of High Voltage and Power Systems

Outline


Physical background




Saturation of current transformer (CT) due to high DC offset in primary current (example: current measurement in a shunt inductor connected to 120 kV network)




Saturation of CT due to high short-circuit current (example of AC saturation)




Overvoltages due to opening of CT secondary




Effect of load connected to secondary winding on CT saturation

Introduction

• The performance of a protection relays depends on the quality of the measured current signal.

• Saturation of the CT causes distortion of the current signal and can result in a failure to operate or cause unwanted operations of some relay protection functions.

• Consequently CT saturation can have an influence on both the reliability and the security of the protection.

Physical background • Very short circuit breaker tripping times are achieved by fast operating protective relay systems, which must perform measurements at a time when the transient DC offset is present in short-circuit current. • A magnetic core in a CT will saturate very rapidly due to high currents and remanent flux. • After saturation occurs, the CT output will be distorted and the performance of the relay protection system will be affected. • For protection relays intended to operate during a fault the CT magnetic core output under transient conditions is of great importance.

Physical background – CT model and magnetization curve I n1 I n2

CT model

The CT model includes primary and secondary winding resistance (R1 and R2) and leakage inductances (L1 and L2). The magnetizing characteristic of this transformer is modeled by a nonlinear inductor (Lsat), connected in parallel with a resistance (Rm) representing the corelosses. CT magnetizing curve

Physical background – protection and metering cores Typical magnetizing curves for protection and metering cores

• The output required from a CT depends on the application and the type of load connected to it.

• Metering equipment or instruments measure under normal load conditions. These metering cores require high accuracy, a low load (output) and a low saturation voltage. They operate in the range of 5-120% of rated current. To protect the instruments and meters from being damaged by high fault currents, a metering core must be saturated typically between 5 and 20 times the rated current. • For protection cores, the information about a primary disturbance must be transferred to the secondary side. Measurement at fault conditions in the overcurrent range requires lower accuracy, but a high capability to transform high fault currents to allow protection relays to measure and disconnect the fault.

Physical background – transient CT cores For special measurement of fault current (transient current) including both AC and DC components, IEC 60044-6 defines the accuracy classes TPX, TPY and TPZ. The cores must be designed according to the transient current: − TPX cores have no requirements for remanence flux and have no air gaps. High remanence CT. − TPY cores have requirements for remanence flux and are provided with small air gaps. Low remanence CT. − TPZ cores have specific requirements for phase displacement and the air gaps are large. Non remanence CT.

Physical background – CT saturation

The effect of DC offset on magnetic flux Maximum DC offset of fault current can be achieved only if the fault occurs at a voltage determines the requirements on the CT core. zero crossing. Distortion in secondary current due to saturation:

Example • CT (2000 A / 5 A, 5 VA) is measuring the current in a shunt inductor connected to a 120 kV network. • The primary winding which consists of a single turn passing through the CT toroidal core is connected in series with the shunt inductor rated 69.3 MVAr. • The secondary winding consisting of 1·2000/5=400 turns is short circuited through a 1 Ω load resistance. In steady state, the current flowing in the secondary is 1000·5/2000 = 2.5 A (2.5 Vrms). • A simple 2 segment saturation characteristic is used in this example.

+

Modeling in EMTP-RV

AC1

120kVRMSLL /_0

Switches -> Ideal switch

+

SW1 + 20ms| 10| 0 120kVRMSLL /_0

AC1

Ratio=N2/N1=2000 A/5 A=400

Transformers -> Ideal unit Tr0_1

+

+

SW1 + 20ms| 10| 0 120kVRMSLL /_0

AC1 400

Modeling in EMTP-RV

120kVRMSLL/_0

Tr0_1

+

1 ?i R2 +

+

SW1 + 20ms|10|0 AC1

RLC branches -> R R1=0.625 mΩ R2=R3=1 Ω

400

RLC branches -> Ground RL1 ?i

+

1m,127.32uH

+

+

6.25e-6m,7.9577e-4uH 0.625m 400 Lnonl1

RL3

+

R1

Nonlinear -> L nonlinear

+ 1 ?i R3 +

+

693m,220.6mH

RLC branches -> R-L

RL2 Tr0_2

RL1-> (R=6.25e-6 mΩ, L=7.9577e-4 µH) RL2-> (R=1 mΩ, L=127.32 mH) RL3-> (R=693 mΩ, L=220.6 mH)

Scopes and simulation settings

1 ?i R2 +

AC1

EMTP->Simulation options:

400

RL1 ?i

RL2 Tr0_2

+

+ +

+ 0.625m

RL3

+

R1

400 Lnonl1

+ 1m,127.32uH

6.25e-6m,7.9577e-4uH

693m,220.6mH

120kVRMSLL /_0

Tr0_1

+

1 ?i R3 +

+

SW1 + 20ms|10|0

Scopes: RL1, R2, R3 – current Lnonl1 - flux

Closing at peak of source voltage (t=20 ms)


This switching produces no current asymmetry. As expected the CT secondary current is sinusoidal and the measurement error due to CT resistance and leakage reactances is not significant.

Primary current

Secondary currents

Closing at peak of source voltage (t=20 ms) Peak of calculated flux: 54.5 µWb

-4

6

Nonlinear characteristic plot

x 10

The flux contains a DC component but it stays lower than the the knee value.

Knee value of flux: 562.7 µWb

Flux (Wb)

4

2

0

0

500

1000

1500 Current (A)

2000

2500

3000

CT saturation due to current asymmetry The breaker closes at t=15 ms at a voltage zero crossing. This switching instant now produces a full current asymmetry in the shunt reactor. For the first 3 cycles, the flux stays lower than the saturation knee point (562.7 µWb). The CT secondary current follows the primary current. After 3 cycles, the flux asymmetry produced by the primary current causes CT saturation, thus producing large distortion of CT secondary current.

Primary current

Secondary currents

563 µWb (knee value of flux)

Flux

AC saturation due to short-circuit current (Isc=36.65 kArms) The breaker closes at t=20 ms at a voltage peak. This switching produces no current asymmetry. Due to high value of short-circuit current, AC saturation of CT core occurs.

AC saturation due to short-circuit current (Isc=36.65 kArms) 45.1 kA

Primary current

Secondary currents

563 µWb

Flux

Overvoltages due to opening of CT secondary The flux has a square wave shape chopped at ±580 µWb. Large dФ/dt produced at flux inversion generates overvoltages on CT secondary (273.5 Vpeak).

Secondary current

Flux

Overvoltages on CT secondary

CT secondary opening

Operating points of the CT according to its load

N - Number of turns (primary or secondary) Un - Rated voltage (primary or secondary) f - Rated frequency in Hz Aj - Core area in m2 Bn - Flux density at rated voltage (Tesla)

Rr < Rn It is obvious that proper operation of a protection relay is linked to the behavior of the associated CT and to its real load and not to the behavior of the CT associated with a theoretical nominal load.

Effect of CT secondary load on saturation

RL=1 Ω

RL=0.3 Ω