What Makes a PCC a PCC?
The concept of a point of common coupling may not be as simple as you think — Editorial Director John DeDad offers some insight. And Mark McGranaghan, vice president of Electrotek Concepts, compares sine-wave and square-wave inverters.
I've heard the term “point of common coupling” used in discussions on harmonic distortion and seen it in technical papers. To what does the term refer and why is it important?
DeDad's answer: The term “point of common coupling” (PCC) gained popularity and importance after the release of IEEE 519, “Standard Practices and Requirements for Harmonic Control in Electrical Power Systems,” which defined it as “the interface between sources and loads on an electrical system.” The late Warren Lewis, a true power quality pioneer, did the best job of defining the PCC and detailing its importance.
In his view, the purpose of IEEE 519 was to help define conditions existing at only this point on the wiring system. Most power quality professionals originally thought the PCC to be at the service transformer. This definition/location benefits the electric utility, which doesn't want harmonic currents above a certain level to be impressed upon the neighborhood electrical distribution wiring system from a connected customer. Typically, a utility uses a transformer to connect the whole supply to the whole load via primary and secondary windings that are magnetically coupled together in the transformer. As you can see, the PCC, as described here, correctly points out the physics of what actually happens.
As Lewis pointed out, the main PCC is at the service transformer on most premises; there's typically one PCC since there's usually just one service transformer.
However, if the service equipment has a back-up engine gen-set or similar power source, the interface to this power source becomes the PCC when the transfer switch connects to that source and when the normal electrical supply disconnects.
Lewis also noted that if an electrical system's service equipment were subdivided into smaller sections each served by a transformer installed as a separately-derived AC system, you would have a subset of PCCs across each transformer.
His main point was this: By using the IEEE 519 definition of the PCC, you can call the interface to any power source serving a load a PCC. This includes any motor-alternator set, engine-alternator set, or UPS where the output comes from a DC-fed inverter driving a transformer.
The PCC is important because it's the point where the interaction of the served nonlinear load's distorted current waveform creates the voltage waveform's distortion. In other words, it's where the resulting source's internal IZ drop — at each harmonic current's frequency — creates a corresponding distorted voltage that's algebraically additive to the fundamental voltage waveform produced at the output of the source. This resultant voltage waveform is then provided to the bus or feeder served by the power source. From there, it can propagate to all served downstream loads as a power quality problem related to the distorted voltage waveform.
Feeders also have an effect on the type of power quality problem referred to above because they have impedance that increases as a function of their length. So a panelboard that serves nonlinear loads will exhibit greater voltage waveform distortion at its buses, as compared with the upstream end of the feeder where the PCC exists. Branch circuits have the same general impedance considerations.
Does a square-wave inverter respond differently than a sine-wave inverter to the effects of power factor? It seems to me there would be less energy loss with the square wave than with a sine wave, both loads being equal.
McGranaghan's answer: This is a good question warranting some thought.
When we say “the effects of power factor” in this case, we're talking about the effects of waveshape. The waveshape affects the “true” power factor. Power factor is real power divided by the total apparent power, or P÷S. The real power includes components at the fundamental frequency and harmonics. For a square-wave supply, real power (P) is expressed as P=P1+P3+P5+…
Also, there has to be both voltage and current at each component in order for there to be real power. The in-phase components of the voltage and current determine the real power. This depends more on the load than on the supply.
For instance, if the load is a resistor, it doesn't matter whether you supply it with a square wave or a sine wave. The power factor is unity. The real power equals the total apparent power. If you supply it with a square-wave voltage, it draws a square-wave current with all harmonic components in-phase with the voltage. If you supply it with a sine wave, it draws a sine-wave current in-phase with the voltage. Either way, you have unity power factor.
It gets interesting when you consider loads like a diode bridge that's part of a switch-mode power supply. This draws a current that's different from the voltage in either case, resulting in a power factor less than unity. It's very possible that the power factor of this type of load with a square wave might be higher than the power factor with a sine-wave supply. That could also translate to lower losses because power factor loads result in losses associated with the additional current components flowing through the resistance of the supply system. This may require a simulation or test to prove the point, but chances are you're right.
A motor is another story. When powered by a square-wave supply, it will draw a current waveform very different from the voltage. The harmonic current components in this case cause a much lower true power factor for the square-wave supply and they cause additional losses in both the motor and supply system.