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.

5.5  Instability of Thyristor Firing Controls

It is possible that the waveform distortion caused by a Thyristor Converter system may not be tolerable to  control unit of that system as well as other thyristor converters. This is not unusual for equipment that did not include considerations for a "limited bus" or high impedance source in its original design. It is, therefore, very important to advise the converter supplier of the existence of a standby generator set as a potential power source. 

The SCR must be triggered on to begin conduction. Output control of a AC-DC Converter is achieved by phase angle control, where delay angle before triggering  is timed from the start of each half cycle or zero crossover. When the terminal voltage waveform is distorted zero crossover may move or there may be several zero crossings, causing erratic firing from one cycle to the next. Timing circuits must be designed to respond only to the 50 Hz fundamental output of the DG Set to avoid this problem. Filtering of the trigger circuits is possible and should be included by the converter manufacturer in original design. 

5.6 Some UPS Units may conclude that DG Set frequency is wrong !

Some UPS systems are also equipped with over/under frequency relays which may respond to a distorted waveform (due to multiple zero crossing and/or erratic zero crossings) and cause the UPS systems to reject the input source even though fundamental frequency is within tolerance. Filtering the sensing signal to these relays may correct this problem.

5.7 Some UPS-DG Set Specific Interaction Issues

5.7.1  UPS units do not like rapid and large frequency variations ...

A Utility Power System can be thought of as a large Synchronous Generator with low reactance and huge inertia. A huge inerta machine does not change its speed easily. Therefore the Utility frequency does not vary mush and when it varies it does not vary fast.

A DG Set  , in comparison , is a small inertia machine and consequently its frequency changes over comparatively large range whenever some load change takes place. And DG Set frequency varies rapidly. Of course if the load is constant the Governor System will bring the frequency back to its normal value. But Governor systems are never fast enough (neither should they be ; otherwise there can be instability problems in the Governor loop) to suppress frequency variations due to rapid load changes.

Hence , obviously , intermittent loads (like Air Conditioners , Pulsed Heater Controls etc) are bad loads on a DG Set.

UPS units generally synchronise to the mains input (except in low kVA range ; even there synchronising is needed if a synchronised transfer to bypass is required). This synchronising is done by some form of Phase Locked Loop(PLL) system. And all PLL systems have two limitations - (i) they can remain in lock only if the input frequency remains within a narrow range around the nominal frequency value (ii) they can remain in lock while the input frequency is changing only if the input frequency is changing sufficiently slowly i.e the rate of change of frequency (so-called "frequency slew rate") is less than some critical value.When the PLL loses lock the UPS refuses to take the input supply and switches over to battery , reporting some under/over frequency condition.

Now , if the UPS is the last load that comes on on the DG Set it may succeed in continuing on the DG Set.But if UPS is put on first and then some other heavy load (for e.g some large motor starting) comes on the DG Set , the DG Set voltage and frequency dip down and UPS moves over to battery either due to low voltage condition or due to low frequency or due to PLL losing lock due to fast rate of change of frequency. DG Sets can have their frequency slew at 3 to 5 Hz/sec and that will be too much for the UPS PLL systems unless the UPS is specifically designed for that.Even if a UPS is designed with a wider frequency window and larger tolerance band for frequency slew rate , it is better not to risk it - it is better to avoid large switching loads like A/C units with On/OFF control , Heater load with thermostatic ON/OFF control etc on the DG Set which supplies a large UPS load. 

5.7.2  DG Sets dislike the Inrush Power from UPS

If a UPS Unit is used only to tide over the first few minutes of mains power loss i.e till the AMF system of DG Sets bring them up , it is quite likely that the Battery Units have low AH capability. In that case the battery would have run down considerably by the time the DG Set comes on.Thus the UPS will suddenly transfer all the output load power and the full battery charging power to the DG Set when DG Set comes on.If the UPS load is only a small percentage of the DG Set rating it may be OK. But if the UPS load is the major load , sudden inrush power of this major load will cause the voltage to take a swing and the frequency to go down rapidly.And the UPS goes back to battery due to low voltage or due to high rate of change of frequency.It may have to be reset or it may attempt transfer automatically after a delay. But in both cases the same thing will happen again. In short , the DG Set will appear to be refusing to take load.

The solution lies in arranging a "Power Walk-in Feature" in the UPS Design.This is a design technique where the UPS slowly increases its power demand from the input supply immediately after transfer. UPS slowly ramps up the input power (usually over a period of 20 to 40 seconds) from 0 to 100% thereby avoiding sudden voltage and frequency changes and fast slew rates of frequency.During the ramping-up period the output of UPS is met partly from the Battery and partly from the mains.

If the total UPS kVA in a system is more than 50% of the DG Set Rating it may be desirable to have this "Power Walk-in" feature in the UPS Units.

5.7.3  DG Sets do not like those LC Filters at the input side of UPS Units

UPS Units in the 50kVA to 500kVA range generally use 6-pulse SCR converter front-end and handle the harmonic currents generated by this stage by adding some kind of harmonic filtering inside.This filtering maybe of two types. In the first design (which is the popular one) a passive LC Filter structure is connected in parallel to mains input.This series LC combination is tuned to the dominant harmonic (the fifth) thereby providing a shorted path for the fifth harmonic current taken by the 6-pulse converter.Thus the mains line current will have lesser harmonic content (< 10% usually). Now this LC Filter is a short at fifth harmonic and will attract all fifth harmonic currents -even those generated by other loads - into it ! Also , if the supply voltage has fifth harmonic in it for some reason , this over-enthusiastic filter is going to short it out ! Both are bad. So we need to isolate this filter from rest of the world.This is done by a 5% inductance in series with the mains input line before the line gets to the input side converter.

The SCR 6-pulse converter will also consume reactive power at fundamental frequency.It would be convenient to arrange the L and C value in the LC filter such taht the filter will behave like a Capacitance at 50Hz and will absorb the right value of leading Vars to make the entire unit behave like a UPF load.That is the way the LC filter is designed and the reactive power drawn by the LC Filter is matched to the Converter kVAr at full load. 

Now we have to distinguish between two cases.

Let us assume that the UPS is the major load on the DG Set. And precisely because of that this UPS will have "Power walk-in" feature. So when the mains supply from DG Set comes on UPS input gets connected to it , but it starts at low power - now we have the classic problem of an unneeded capacitor across the DG Set terminals. the capacitor comes from UPS input side filter.And since UPS is taking only small power to begin with, the DG Set is effectively on no load and it has a large capacitor across it - over voltage is the result. And , more seriously , the large leading VARs taken by the LC filter in the UPS may cause the AVR to reduce the excitation current to minimum or zero leading to instability in the DG Set.In any case the UPS will sense over voltage and trip its input breaker and promptly go to the battery.Again the DG Set will appear to be refusing to accept load. 

If the UPS is not the major load and/or it is brought in only after some other reactive power consuming load is put on the DG Set , the LC Filter may not create any problem.

The solution is to modify the UPS design such that the LC Filter is disconnected when the UPS is on the Battery and connect it again only after the "Power Walk-in" is completed successfully. UPS units of recent design from reputed manufacturers have this feature.

Passive LC filtering is not the only possible filtering. Some recent designs use "Active Harmonic Filtering" instead of passive filtering.The required components may be built into the UPS unit itself or it maybe available as an external add-on.In Active Harmonic Filtering , a PWM Inverter running from self-generated DC Bus works as a current source to inject a current that is equal in amplitude and phase to the harmonic current taken by the UPS front-end converter.Thus , this unit will supply the harmonic current required by the UPS front-end and UPS will draw more or less pure sinusoidal current from the mains.With this kind of filtering there are no heavy reactive power components and hence the problem of over-voltage due to the Capacitor in LC filter does not occur here.UPS units with integral active harmonic filter or external active harmonic filter are available from manufacturers like MGE.

5.7.4  Automatic Transfer Switch may create problems...

Most generator-UPS installations include automatic transfer switches that switch the UPS back to utility power once it becomes available again. There can be some problem if the transition is open transition i.e the DG Set is taken off the bus first and then the mains breaker is closed. Now the UPS will sense a mains failure , go to battery mode and will start a "power walk-in" when the bus is connected to utility power.Thus the UPS will behave like  a large capacitor (from LC filter) and near zero active power during the "open" period of 'open transition' and during the first few seconds of power walk-in. 

During the small interval when there was no connection to the bus (i.e DG set breaker was off and utility breaker was not yet on) the UPS LC filter capacitor will provide excitation currents to the motors running from the same bus.That turns the motors into Induction Generators and over-voltages may result. Moreover ,now when the Utility breaker is closed there is 'synchronising problem' with the induction generator voltages. If the UPS Input Filter Capacitor were not there , the rotor circuit flux decay in Induction Motor would have ensured that the back emf at the terminals of motor is near zero when the utility breaker closes.This problem is solved by by providing a fast means of detecting the transfer in the UPS and disconnecting the filter.Only a few UPS designs are available with this feature.

6.  An Illustrative Case Study

The subsequent sections of this article deal with the observed current and voltage waveforms at an Electronics Research Organization.This establishment received Electrical Power from K.S.E.B Grid (the Kerala State Electricity Board) through a 11 kV/415V, 250 kVA Transformer with a Contract Demand of 160kVA.The total Connected Load at the time of audit was 215 kW+19.5HP+58 kVA+50kVAr. Two 100 kVA DG Sets were installed as Standby Supply.The major energy consuming loads were: Flourescent Light fittings in the office area and Computer Rooms & Labs, Ceiling Fans, Pedestal Fans, Table Fans, Wall Fans, 32 Split A/C units of 2T each amounting to 64T of refrigeration, Water Coolers, two 5 HP pumps, 9 HP of workshop machinery Computers and terminals, Printers and 43 kVA of UPS load.Though the contract covered only Energy Audit a cursory Power Quality Audit was also performed as a part of Energy Audit since the system voltage showed considerable amount of distortion especially when the system was running on DG Set. This section presents the salient data and findings and contains extracts from the Audit Report.

There is about 43kVA of Single Phase UPS Load in the form of 1kVA, 2kVA, 3kVA and 5kVA Single Phase UPSs used in various laboratories and Computer Centres in the System. All UPS units in the system are of Single-Phase input type and the front end of all the units are of phase controlled thyristor converter type.These UPS units collectively consume about 22kW of power from the mains and contribute to prominent harmonic distortion of line currents in the System. Heavy distortion of currents drawn result in harmonic distortion of Voltages too, and prominently so when the System is on the 100kVA DG Set.

6.1  Harmonics When the System is on Grid Supply

Single Phase UPS units draw a current which is rich in odd harmonics-the predominant ones being third, fifth and seventh. The third and all other triple-n harmonic currents add in the neutral conductor irrespective of distribution of UPS loads among the three phases.

If the current drawn from a transformer has more than 15% T.H.D , the transformer losses increase in a significant manner. However, the resultant harmonic distortion in voltages will be marginal due to small short circuit impedance of the transformer. The waveforms shown below in Figure 6.1 illustrate these points. These waveforms show the phase voltage and line currents at the transformer secondary at the Establishment recorded on 10-3-1999 at around 2:00PM. These waveforms were recorded one at a time using Tektronix THS 710A DSO and hence do not reflect the correct phase relationships between various quantities. 

The line currents are heavily distorted due to the single-phase UPS loads. The THD in R Phase current is 14.5%, that of Y Phase is 16.2% and that of B Phase is 28%. The phase voltages are more or less clean and have THDs less than 5%. The R phase current of the 50kVAr capacitor shows a THD of 15.8% and shows a predominant component at 11^th harmonic (See Figure 6.2 below).

A detailed analysis of current waveforms coming from various feeders were carried out and it was found that there is no load that contributes this much of 11^th harmonic in the system. Hence this predominant 11^th harmonic in capacitor current and transformer current was seen to be the result of partial harmonic resonance between the 50kVAr capacitor at the main bus and the leakage reactance of the 250kVA transformer. Calculations using 4.52% leakage reactance value (found from the nameplate of transformer) reveal that a 50kVAr capacitor can resonate partially at 11^th harmonic with this transformer. This partial resonance stresses the capacitor and transformer in addition to the extra stress brought in by the harmonic currents demanded by the non-linear loads.

6.2  The Neutral Current - Grid Supply

The neutral current at the transformer secondary is shown in the waveform below in Figure 6.3. It shows a fundamental component and a third harmonic component that is more than even the fundamental component. This third harmonic is exclusively the result of single-phase UPS loads. The r.m.s value of this neutral current is 68 Amps and is almost as much as the r.m.s value of phase currents indicating heavy neutral loading.

6.3 Harmonic Content When the System is on DG Set

However, the harmonic distortion picture changes considerably when the system is run on one of the 100kVA DG Sets. Even small degrees of harmonic distortion in generator currents will result in large amount of harmonic distortion in generator voltages through the large sub-transient impedance of the generator. Also harmonic currents cause increased losses in stator and field windings of generator.

The harmonic distortion in phase currents and phase voltages in the DG Set were measured during the audit on 10-3-1999 and 11-3-1999 and were found to be excessive (See Figure 6.4 below). Waveform observation confirmed sizeable distortion in currents and voltages. It is well known that a distorted voltage results in loss of capacity of induction motors and overheating of motors. Also distorted voltage has been identified as a major reason for frequent failures of power factor correction capacitors connected across the motors.

[Note-These waveforms were recorded one at a time and hence do not reflect proper phase relationships. They can be used only for harmonic analysis and not for power analysis or power factor analysis.]

The generator output voltage shows prominent distortion and ringing due to large harmonic current flow through its winding impedance. The Generator neutral current has a large third harmonic component that is even more than the fundamental amplitude (See Figure 6.5).

Switching on the 50kVAr capacitor at the main bus was found to result in a relatively clean output voltage waveform from the generator. This happens due to the filtering action of the capacitor. All the harmonic demand of the load goes through the capacitor instead of through the large synchronous reactance of the generator. This will result in a clean voltage waveform at the expense of overheating and possible burnout of the capacitor.

Unequal third harmonic currents in the three phases of the generator brought about by unbalance in UPS load among the three phases will result in third harmonic appearing  in line to line voltages. Third harmonic content in line voltage will result in circulating currents in delta connected motors which will go undetected by the overload relay. Also, third harmonic in line voltages interferes with AVR of generator and voltage sensing circuits of servo stabilisers etc.