UNDERSTANDING NONLINEAR LOADS AND GENERATOR …

1 UNDERSTANDING NONLINEAR LOADS AND GENERATOR SETINTERACTIONG regory M. Williams, Inc.(309) 578-4995 INTRODUCTIONMany electrical power applications require continuous andhigh quality power. Standby diesel-electric generatingsets are integral parts of the genset sizing requires an UNDERSTANDING of thegenset characteristics and the connected load . Nonlinearloads, including; uninterruptable power supply (UPS),variable frequency drives (VFD), adjustable speed drives(ASD), and switched mode power supplies, present a specialchallenge to successful delivery of high quality powerunder all operating conditions. UNDERSTANDING how thisequipment interacts is sets are inherently a high reactance sourcecompared to a LOADS , not generators , cause should be sized for NONLINEAR LOADS to reduceheating and voltage waveform distortion.

2 Oversizing toequate distortion with a utility source may not load design should include circuits to acceptonly timing signals from the fundamental frequency andprovide some filtering of SCR commutation system component suppliers of a limited GENERATOR set supplier of NONLINEAR load details(clarify input and output , number of rectifieroutput pulses, other connected LOADS , etc.). SR4 generators do not require additionalfilters between GENERATOR and regulator. Three phasesensing and well-filtered voltage regulators ensurevoltage control and may not be accurate with may have adverse effects upon the power sourceor other LOADS connected to the same , problems arising after the fact must beresolved within the system and equipment external togenerator assuring stable operation are set suppliers cannot guarantee harmonicdistortion values with NONLINEAR LOADS .

3 Where LOADS areknown to require a low distortion voltage waveform, aconsultant with access to total system constants shouldevaluate total harmonic distortion effects and recommendfiltering as of NONLINEAR loadsNONLINEAR load (CURRENT NOT PROPORTIONAL TO VOLTAGE)Voltage supplied to a non linear system, either by utilityor GENERATOR set, is sinusoidal. For resistive and mostinductive LOADS , current is also sinusoidal, but rectifierscharging a battery draw an almost square wave currentpulse. As shown in Figure 1, AC line current will flowonly when the rectified instantaneous voltage exceedsbattery voltage. Original sine wave voltage from thesource now becomes distorted due to voltage drop across thesource impedance during the cycle portion when current 14 SCR LOADSR ectifier with SCRs for output voltage and current controlare often used in LOADS such as static UPS systems.

4 Withan SCR, current is maintained at zero until the SCR isgated "on" as shown in Figure 2. Current to the SCR at"turn on" causes a higher than normal voltage drop acrossthe GENERATOR source PHENOMENONIn a three-phase rectifier circuit, current is switched onby SCRs consecutively. Once switched on, an SCR conductsonly during the time when its particular phase voltage ismore positive than the other two-phase voltages. Inpractice, SCR turn on is delayed to regulate output anddoes not occur until the oncoming phase voltage issignificantly higher than preceding conducting phasevoltage. Due to inductance in the SCR source circuit,current cannot build up instantly in the oncoming phase norcan it decay instantly in preceding conducting 25In Figure 3, when the more positive oncoming Phase 2 withSCR-2 is gated on, there is momentary line-to-line shortingaction with Phase 1, which has SCR-1 in the decayingconducting mode.

5 The resulting short is of very shortduration, but produces a notch in input voltage waves 1 and2. Notch width and depth during this commutation periodare dependent upon supply system impedances, SCR firingangle, and load current. With a three-phase bridge, thereare six line-to-line notches per cycle as shown in EFFECTSA secondary phenomenon caused by the rapid switching ofSCRs is ringing effect. Ringing is a high frequencyoscillation following sudden "turn on" of an SCR as shownFigure 3 Figure 46in Figure 5. It is the result of high frequency resonanceoccurring in the rectifier source circuit due to inherentinductance and capacitance in the circuit and ringing effects can result in severe voltagewaveform CURRENTThe rectifier, because it draws nonsinusoidal current fromits source, along with notching and ringing effects,introduces distortion to the voltage wave from the is called harmonic distortion.

6 According to theoriesof waveform analysis, cyclical waveform is made up ofcomponents consisting of fundamental sine wave plus othersine waves, called harmonics, which are multiples of thefundamental frequency. Figure 6 shows separation of adistorted waveform into its component parts. The nonlinearsource, therefore, does not see distorted current waveformas a single waveform, but as multiple, fundamental plusharmonic, waves. Harmonics may have adverse effects uponthe power source or other LOADS connected to the samesource. It is important to note LOADS drawing harmoniccurrents cause voltage distortion at the source, the sourcedoes not produce the harmonic 57 CONSEQUENCES OF NONLINEAR LOADSWAVEFORM DISTORTIONV oltage waveform distortion magnitude caused by thenonlinear current demand of the rectifer/charger is afunction of the source impedance.

7 Source impedance is notan easily defined value because GENERATOR reactance varieswith time following a sudden load change. Generatorsubtransient reactance (X"d) and subtransient short circuittime constant (T"d) are primary parameters influencingdistortion during the short SCR commutation standby GENERATOR is characteristically of higherimpedance than transformers. Significant differences inkVA ratings of the two sources often contributes togreater impedance differences. Facility sourcetransformers are frequently sized to carry the totalfacility load . Standby gensets are often only sized tocarry emergency or critical LOADS . generators may have 5to 100 times greater subtransient reactance than normalsource transformers.

8 Consequently, NONLINEAR LOADS maywork fine on utility, but may react entirely different whenpowered by a GENERATOR an oversize GENERATOR to reduce reactance may be ofsome benefit. However, to obtain a significant reductionin reactance is not economically feasible. Doublinggenerator rating will reduce reactance by 68 Damping with other LOADS reduces effective bus , in particular, act as absorbers of momentaryvoltage irregularities and reduce harmonic content on theline. Typically, computer room support functions such asHVAC systems, chilled water systems, fire protectionsystems, and room lighting must also be connected to thestandby GENERATOR set. These support systems are often aslarge as or larger than the NONLINEAR load .

9 It isfrequently very desirable for system operation anddistribution reasons to have small, three phase,continuously running air handler motors in the computer andUPS room sharing a common feeder or power transformer withthe LOADS are effective in minimizing waveformdistortion caused by ringing effect. Resistance acts as aoscillation damper in a resonant circuit. Adding aresistance load is one technique used to minimize waveformdistortion caused by system oscillation. This, however, isonly effective if high frequency oscillations are the causeof a problem. If a resistance element is added strictlyfor treatment purposes, adding a capacitor in series withthe resistor will reduce fundamental current with minimumeffect on high frequency is theoretically possible to add a low pass filter tothe GENERATOR output for attenuation of prevailingharmonics.

10 However, it should be a last resortconsideration. Practical tuned circuit filters generallyrepresent compromise and may introduce more problems thanthey solve. Component size and expense are also limitingfactors. A better approach is to specify or add filteringor other harmonic attenuating options such as isolationtransformers at the distortion source. Consultation withthe device suppler usually reveals such options FACTORG enerators are rated for power factor. Connected loadsmay have a lower power factor. Displacement of current withrespect to voltage occurs with rectifier phase power factor can vary depending upon SCR conductionangle. Compounding this are the high frequency harmoniccurrents which primarily result in added kVArs.