Consequences of Harmonic Currents and Voltages Generated by Nonlinear Loads

Voltage waveform distortion magnitude caused by the sinusoidal current demand of the nonlinear loads is a function of the source impedance. Source impedance is not an easily defined value in the case of a DG Set because generator reactance varies with time following a sudden load change. Generator subtransient reactance (X"d) and subtransient short circuit time constant (T"d) are primary parameters influencing distortion during the short SCR commutation periods.

A standby generator is characteristically of higher impedance than transformers. Significant differences in kVA ratings of the two sources often contributes to greater impedance differences. Utility transformers are frequently rated to carry the total plant load. DG Sets are often only sized to carry emergency or critical loads. Thus DG Sets may have 5 to 100 times greater subtransient reactance than normal source transformers. Consequently, nonlinear loads may work fine on utility, but may react entirely different when powered by a DG set. Using an oversized generator to reduce reactance may be of some benefit. However, to obtain a significant reduction in reactance is not economically feasible.

Section 4 of this article clarifies that there are two kinds of voltage harmonics created in the systems by non-linear loads - first we have the harmonic currents demanded by the non-linear loads flowing through various system impedances thereby distorting the bus voltages upstream and secondly we have the "voltage clamping" and "voltage notching and ringing" type of voltage-source type distortions at the converter input terminals.The closer a particular bus is to a coverter bus , the greater the effect of voltage clamping and notching.

In any case the net effect of non-linear loads is harmonic current flow everywhere and harmonic voltages everywhere.

5.1 Generator Heating

Harmonic currents produce high frequency flux change and cause heating in stator cores. Rotor losses also occur because harmonic currents in the stator will induce currents in the pole faces and amortisseur windings. Higher magnetic core temperatures result in a higher winding temperature. Generator stator heating is also a function of I 2 R loss. Winding heating is proportional to effective or RMS current squared. RMS current for a sinusoid wave is 1.11 times the average value. The RMS value of the distorted SCR circuit input current waveform is typically greater than 1.11 times average current. (See further discussion of instrument readings.) Derating or using a low temperature rise generator is a means of compensating for increased heat losses .

Of course harmonic currents cause increased resistive losses everywhere , not only in the Generator windings.

5.2 Effect on Capacitors and Motors

Increased harmonic voltage level during DG Set operation exposes the usual load i.e motors and capacitors to harmonic voltages.Motors can generally stand the resultant overheating. But capacitors will get overheated since their impedance goes down with frequency.It is true that the major power factor correction capacitors will be de-energised under DG Set operation.But the capacitors which are directly connected across the motors will not be de-energised and they may burn out or at least lose life fast. The inductive reactance level in the system is different when the system is on DG Set. Hence the parallel resonant freqencies of the power factor correction capacitor - generator reactance system will shif to lower levels and the risk of harmonic resonance should be studied.Best solution is to avoid having power factor correction capacitors on when the system is on DG Set.

5.3 Generator Voltage Regulation Problems

The Automatic Voltage Regulator (AVR) of a DG Set maintains the terminal voltage of the set at a constant value under steady state conditions.It senses the terminal voltage , compares it with the set value and corrects any error by suitably chaging the field excitation current. The sensing part is the most important part.

Many AVR designs sense the voltage across one phase ; some sense across two lines ; yet others sense all the three phases and average the measurements. Some AVRs calculate the rms value of the sensed voltage by using rms to dc converter electronics ; but they are few. Most of the designs use full-wave rectified average value and assume a form factor of 1.11 to convert that into rms.Obviously all these will result in DG Set voltage getting regulated at wrong level if there is considerable amount of harmonic distortion in the terminal voltage.Modern designs use proper filtering on the sensed voltages , sensing on all the three phases and true rms calculation (either using analog electronics or by using digital techniques in micro processors) to avoid voltage regulation problems in the case of DG Set serving non-linear loads.

5.4 Generator Speed Governor Problems

But there is a more serious problem that can emanate from the excitation control unit of a DG Set delivering power to SCR Converters. The Engine Governor system of the DG Set needs a speed feedback signal.And this signal is usually generated by measuring the frequency of the sensed output voltage inside the AVR unit. And this frequency calculation usually involves zero-crossing information from the waveform.Normally the zero crossing events are supposed to occur at about 10ms interval (assuming 50Hz operation).But in the presence of voltage notching and heavy ringing transients from Thyristor Converter units there will be multiple zer-crossings within one cycle of ac waveform.This leads to large magnitude random errors in the frequency signal prepared by the AVR unit for use by the governor unit. And Governor unit gets confused and there results instability of the speed governing system.The solution is good filtering on the sensed voltage or locking onto fundamental component of the sensed voltage by analog or digital phase locking techniques.

In short, the AVR unit of a DG Set has to be a specially designed one if that DG Set is going to handle predominantly non-linear load.