Sonel PQM analyzers case study #15: Phase synchronisation

Comprehensive diagnostic measurements of power quality very often require measurements at several points simultaneously. However, connecting several analysers at different points will be of no use if they all do not have exactly the same timing and their measuring terminals are not connected to exactly the same phases.

Description of the identified problem

Measurements were taken of the operation of a 20 kV/2x690 V MV resin transformer. The transformer windings were in a D/y11/d0 arrangement. Three class A analysers with external time synchronisation using built-in GPS modules were used for the measurements. Can the phase correspondence between the measurement points be precisely verified?

Measurement tools used

• 3 x Sonel PQM-702 analyser (see successor Sonel PQM-710)
• A set of F-3A clamps (In=3000 A) and a set of C-6A clamps (In=10 A)
• Sonel Analysis software

Figure 1 Oscillograms of instantaneous voltages and currents on the MV side in triangle arrangement (D)

 

Figure 2 Oscillogram of instantaneous voltages and currents on the LV side in star arrangement (y11)

 

Figure 3 Oscillogram of instantaneous voltages and currents on the MV side in triangle arrangement (d0)

PRELIMINARY CONCLUSIONS:

1. All the measurement points preserve the direction of rotation (sequence) of the phases.
2. On all three oscillograms, the phases L1, L2 and L3 are consistent with each other with accuracy to 2ms.

In order to analyse the data in more detail, a 50 Hz network is characterised by the following relationships:

• one complete grid period lasts 20ms, corresponding to a 360° rotation of the voltage vector,
• each 30° shift of the voltage vector corresponds to a time of 1.67ms,
• the phase shift angles for triangle and star arrangements are always given as multiples of 30°.

When analysing the passage of the waveforms through zero for the phase-to-phase voltage U L1-2 (marker no. 1 in Figs. 1, 2, 3), one can see that the times of the same measuring points are respectively:

• on the MV side in triangle arrangement (D) – 18:00:00.122 
• on the LV side in triangle arrangement (d0) – 18:00:00.122
• on the LV side in star arrangement (y11) – 18:00:00.120

FINAL CONCLUSIONS:

1. An active GPS is required for a precise synchronisation of the analysers.
2. The analysis of simultaneous steady-state oscillograms makes it possible to identified phase compatibility at multiple measurement points.
3. For the triangle-delta winding pair (D/d0), the phase shift is 0ms (0°), which is as expected, as such an arrangement of the connections should not introduce a phase shift. An hourly offset of “0” specifies that there is no shift in the voltage vectors between the primary and secondary sides of the transformer.
4. For the triangle-star winding pair (D/y11), the phase shift is approximately 2ms (approx. 36°). Taking measurement errors into account, this confirms that the voltage vectors in the “y” star arrangement are consistent with the voltages in the “D” triangle arrangement. The hourly offset “11” defines a shift in the voltage vectors between the primary and secondary sides of the transformer of 330° (the voltage on the secondary side precedes the voltage on the primary side by 30°).

 

Authors:
Krzysztof Lorek, Marcin Szkudniewski