Hello everyone,
The case is:
A 500 KVA transformer is feeding a 200 KW DC motor. This motor is operated using controlled rectifier. The motor is operating in industrial processes. The motor works in a cycle: running then braking using regenerative braking.
The question is:
1) What is the effect of harmonics resulting from controlled rectifier on the sizing of: transformer, cables, protection devices.
2) What are the precautions on the operation of such a system due to harmonics.
3) Is their any software package capable of analyzing this case.
Thanks in advance
We supply DC drives and have done so for many years. I'll try to answer your questions. At 200kW, I'm assuming that it is a six pulse system.
In the same order as you presented them:
For ratings of devices on the three phase supply, I use Iac = 0.816Idc. That takes account of the non-sinusoidal current. For drive unit transformers, I add 20% the the kVA calculated from the previous relationship. That is 1.2*sqrt(3)*Vline*Iac/1000.
The non-sinusoidal current drawn results in harmonic voltage distortion both on the output of the local drive transformer and on the supply to it. To calculate the magnitude of that distortion requires data on harmonic current at each harmonic, the transformer parameters, and the supply characteristics. There are various ways of doing this but I'll leave that for the moment.
Precautions would depend on the degree to which the supply voltage gets distorted. The supply company or other authority may set limits on that. In UK, the benchmark is the Electricity Association G5/4. I don't know if Egypt has adopted that or has something similar. If your installation falls outside those limits (unlikely for a single 200kW machine) you may have to take remedial action. Again, I'll leave at that for now.
Another point to consider, not related to harmonics, is power factor. You mentioned that it is a controlled rectifier so I assume phase controlled SCRs (thyristors). The power factor varies throughout the speed range and that can be a bit problematic. Firstly in correcting a varying power factor though not insurmountable. The other issue is the voltage distortion mentioned above will result in the capacitors taking more or even a lot more than they would on a "clean" supply. Our normal practice is to put detuning chokes in series with them.
I suppose there are some software packages for calculating harmonics. I know one Variable Frequency Inverter manufacturer has one which was given to me. It looks like a custom MicroSim PSpice application. VF inverters mostly have an uncontrolled input rectifier so it isn't particularly useful for DC drives. But I'd be a bit surprised if a DC version of something like it didn't exist.
Over the years, I've developed a few for my own use that have served well. It was out of necessity since commercial software wasn't available when I first got into this.
