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Current Transformer Saturation

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This example shows the measurement distortion due to saturation of a current transformer (CT).

G. Sybille (Hydro-Quebec)

Description

A current transformer (CT) is used to measure current in a shunt inductor connected on a 120 kV network. The CT is rated 2000 A / 5 A, 5 VA. 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, 69.3 kV (120kV/sqrt(3)), 1 kA rms. The secondary winding consisting of 1*2000/5 = 400 turns is short circuited through a 1 ohm load resistance. A voltage sensor connected at the secondary reads a voltage which should be proportional to the primary current. In steady state, the current flowing in the secondary is 1000*5/2000 = 2.5 A (2.5 Vrms or 3.54 Vpeak read by the voltage measurement block V2).

Open the CT dialog box and observe how the CT parameters are specified. The CT is assumed to saturate at 10 pu and a simple 2 segment saturation characteristic is used.

The primary current reflected on the secondary and the voltage developed across the 1 ohm resistance are sent to trace 1 of the Scope block. The CT flux , measured by the Multimeter block is converted in pu and sent to trace 2. (1 pu flux = 0.0125 V *sqrt(2)/ (2*pi*50) = 5.63e-5 V.s)

The switch connected in series with the CT secondary is normally closed. This switch will be used later to illustrate overvoltages produced when CT secondary is left open.

Simulation

1. Normal operation

In this test , the breaker is closed at a peak of source voltage (t = 1.25 cycle). This switching produces no current asymmetry. Start the simulation and observe the CT primary current and secondary voltage (first trace of Scope block). As expected the CT current and voltage are sinusoidal and the measurement error due to CT resistance and leakage reactances is not significant. The flux contains a DC component but it stays lower than the 10 pu saturation value.

2. CT saturation due to current asymmetry

Now, change the breaker closing time in order to close at a voltage zero crossing. Use t = 1/50 s. This switching instant will now produce full current asymmetry in the shunt reactor. Restart the simulation. Observe that for the first 3 cycles, the flux stays lower than the saturation knee point (10 pu). The CT voltage output V2 then follows the primary current. However, after 3 cycles, the flux asymmetry produced by the primary current causes CT saturation, thus producing large distortion of CT secondary voltage.

3. Overvoltage due to CT secondary opening

Reprogram the primary breaker closing time at t = 1.25/50 s (no flux asymmetry) and change the secondary switch opening time to t = 0.1 s. Restart the simulation and observe the large overvoltage produced when the CT secondary is opened. The flux has a square waveshape chopped at +10 and -10 pu. Large dphi/dt produced at flux inversion generates high voltage spikes (250 V).