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.