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We installed 3000 HP, 12 pulse digital controlled thyristor system. 2 bridges are in parallel. Common transformer with 2 secondaries are feeding input voltage of 1000 vac.

While motor is loading, we found there is a difference of @ 50 V between 2 secondary and heavy distortion in both ac inputs. Can somebody explain why this is happening and how to overcome it?

Responding to dalvi’s 15-May-08 (01:13) query... the 2.9% current variation does not seem to be extreme! Also, some harmonic generation is a necessary evil in this situation. But, the distortion exhibited could be related to the capacity of the motor being too large compared to the input supply capacity!

1) Can you provide some comparative information showing input-supply measurements (eg: Volts, Amps, kW, pf, and cable-size) vs the motor’s circuit parameters?

2) Is the distortion more apparent when operating the motor at its nominal-speed rating, or when operating at off-nominal speeds?

3) What is the comparable supply and motor reactances or inductances?

4) What is the manufacturer’s reply to your observations?

Regards, Phil Corso (cepsicon@aol.com)

1) There are two 6 pulse regenerative thyristor converters (each converter consists of 2 bridges in parallel for redundancey) and connected to motor via a centre tap DC reactor. Both converters are in master-slave mode.

2) Input supply fed to both thyristor converter via transformer (delta pri- 11KV and sec-1 delta 1050 VAC and sec-2 star 1050 VAC) transformer rating is 3600 KVA. (sec currents 990A) PF is around .7 lag. Transformer is rated for 100% cont.

3) Motor is very old, around 50 years (as per my knowledge) 3000 HP, 2400A, 1000 VDC, 5000 A peak

4) Normally motor operates at 3900A for 10 seconds (it is 150%)

5) Distortion appears while motor is loading. Found 50 Volts difference in both input supplies.

6) We have provided capacitor bank to improve PF. Now the capacitor bank is not in operation.

If capacitor bank put in operation, will distortion be minimized?

Responding to p-dalhvi’s 20-May-08 (01:02)... a few more questions:

7) What is primary voltage of transformer?

8) What is capacity, in kVA, or MVA, (or inductance) of supply?

9) What is percent impedance (or inductance) of transformer windings?

10) What are phase currents and phase-phase voltage measurements of primary?

11) Once again, what is meant by distortion… in magnitude, or that voltage and current wave shapes being non-sinusoidal?

12) Can you provide a sketch depicting one-cycle (or Period) of the voltage and current? Even if hand-made, that is taken off of scope display?

13) Is this the first time the converter and motor have been used together?

14) Can you confirm Star-Star voltage-ratio is N while Star-Delta voltage ratio is Sqrt(3)xN?

15) Was distortion present before adding the PF correction capacitors?

Regards, Phil Corso (cepsicon@aol.com)