S TEP 3

Step 3: Current transformer check

Improper CT connection is the most frequent cause of problems during commissioning.

The following procedure will allow you to check the CT connection.

Warning: The secondary circuit of a loaded CT must never be opened otherwise extremely high voltages may appear which can lead to physical danger or destruction of the CT itself.

9.3.1. PQF connection diagram

Figure 9.1. shows the normal connection of the PQF. It must be noted that:

? ? L1, L2 and L3 rotation must be clockwise,

? ? The CTs must be on the supply (line) side of the PQF,

? ? One secondary terminal of the CT must be earthed.

L1 L2 L3

Load side Supply side

K L

k l

K L

k l

K L

k l

PQF X5.2

X5.3 X5.4 X5.5 X5.6

X5.1

L1 L2 L3

It is also seen that terminal X5.1 and X5.2 are related to the CT located in phase L1, terminal X5.3 and X5.4 are related to the CT located in phase L2 and terminal X5.5 and X5.6 are related to the CT located in phase L3.

9.3.2. Material needed & hypotheses for correct measurements

A two channel scopemeter with one voltage input and one current input is needed. Adequate sensors are also needed. A power analyser like the Fluke 41B can also be used.

Some minor knowledge of the load is also required. For instance, the method explained below is based on the fact that the load is inductive and not regenerative (i.e. the load current lags by less than 90° the phase voltage). If a capacitor bank is present, it is better to disconnect it before making measurements in order to ensure an inductive behaviour of the load. It is also assumed that the load is approximately balanced.

9.3.3. Checking the correct connection of the CTs with a two channel scopemeter.

The first channel of the scopemeter must be connected to the phase voltage referenced to the neutral or to the ground if the neutral is not accessible.

The second channel must measure the associated current flowing from the network to the load as seen by the CT input of the PQF.

99..33..33..11.. MMeeaassuurreemmeenntt ooff CCTT iinn pphhaassee LL11

For the voltage measurement (channel 1), the positive (red) clamp must be connected to the phase L1 and the negative clamp (black) must be connected to the neutral (ground).

For the current measurement (channel 2), the clamp should be inserted into the wire connected on terminal X5.1 and the arrow indicating positive direction of the current should point towards the PQF. Do not forget to remove the short on the CT secondary before making the measurement.

L1 L2 L3

Load side Supply side

K L

k l

K L

k l

K L

k l

PQF X5.2

X5.3 X5.4 X5.5 X5.6

X5.1

L1 L2 L3

Positive direction

Ch1 Ch2

Figure 9.2. Connection of the scopemeter for checking CT in phase L1.

On the scopemeter screen, two waveforms should appear. The voltage waveform should be approximately a sine wave¹ and the current waveform would normally be a well distorted wave because of harmonic distortion.

Usually, it is quite easy to extrapolate the fundamental component as it is the most important one (Figure 9.3).

I I1

Figure 9.3. Extrapolation of fundmental component from a distorted waveform.

From the fundamental component of both signals, the phase shift must then be evaluated (Figure 9.4). The time ?T between zero crossing of the rising (falling) edge of both traces must be measured and converted to a phase shift

? by the following formula:

? ?

? *360

T1

? T

where T1 is the fundamental period duration.

For an inductive and non regenerative load, the current signal should lag the voltage by a phase shift lower than 90°.

?T

T1

U

I1

Figure 9.4. Phase shift evaluation between two waveforms.

99..33..33..22.. MMeeaassuurreemmeenntt ooff CCTT iinn pphhaassee LL22 aanndd LL33

The same operations as those described in the previous paragraph must be repeated with the phase L2 (Figure 9.5) and phase L3 (Figure 9.6).

For a balanced load (which is usually the case in most of the three phase systems), the phase shift should be approximately the same for all the three phases.

L1 L2 L3

Load side Supply side

K L

k l

K L

k l

K L

k l

PQF X5.2

X5.3 X5.4 X5.5 X5.6

X5.1

L1 L2 L3

Positive direction

Ch1 Ch2

Figure 9.5. Connection of the scopemeter for checking CT in phase L2.

L1 L2 L3

Load side Supply side

K L

k l

K L

k l

K L

k l

PQF X5.2

X5.3 X5.4 X5.5 X5.6

X5.1

L1 L2 L3

Positive direction

Ch1 Ch2

Figure 9.6. Connection of the scopemeter for checking CT in phase L3.

9.3.4. Checking the correct connection of the CTs with two current probes.

If the main bus bar is available and all security rules are taken, it is possible to use the two channel scope meter in order to see if the current measured through the CT is matching the real current in the bus. Connecting the current probes as shown on Figure 9.7., the two traces must be in phase and of the same shape (the magnitude could be different as the gain are different) if the wiring is correct.

L1 L2 L3

Load side Supply side

K L

k l

K L

k l

K L

k l

PQF X5.2

X5.3 X5.4 X5.5 X5.6

X5.1

L1 L2 L3

Ch1 Ch2

Positive direction Positive direction

Figure 9.7. Connection of the scopemeter for checking CT in phase L1 by comparing the currents.

This operation has to be repeated for the remaining two phases for a complete check. The current probes have to be changed accordingly.

9.3.5. Checking the correct connection of the CTs with a Fluke 41B.

The Fluke 41B is a power analyser that allows measurements of one voltage and one current wave. Unfortunately, the device does not allow simultaneous display of both waveforms on the screen. But it is possible to synchronise the triggering on either the voltage or on the current. All phaseshift measurements are then referenced to the chosen origin. To read directly the phaseshift between the fundamental components, just select the spectrum window of the signal which is not chosen as the origin.

The instrument must be configured in single phase measurements.

The probes must be connected as shown on Figure 9.2, Figure 9.5 and Figure 9.6.