Alternators Reactance For Nonlinear Loads - White Paper - 26jul13
Alternators Reactance For Nonlinear Loads - White Paper - 26jul13
Alternators Reactance For Nonlinear Loads - White Paper - 26jul13
Introduction
Widespread invocation of IEEE Std 519 on systems powered by generators, together with increased use
of equipment that draws harmonic currents, mean that the impedance of emergency or standby power
sources has become more important. Particularly for data centers and other facilities strongly
dependent on data processing equipment, electronic loads may be more than half of the total load. If
the harmonic contribution of these loads is not accounted for in the overall design, the emergency or
standby generators may need to be oversized so much that they are not practical or economical.
Because these rectifiers draw high harmonic currents, filters are often added to bypass these currents
and make the input current more sinusoidal. The large capacitors used in these filters cause the input
power factor to be leading, which can cause problems with generator stability.
Harmonic Loading Effects
Harmonic currents drawn by nonlinear loads interact with the system impedance to induce harmonic
voltages on the power line. These harmonic voltages may cause misoperation of electronic equipment,
overheating of transformers, motors, and switchgear, and noise on analog signals such as telephone
lines among other problems [1]. Because the generator is small by comparison with the generation
capacity of the utility, it has a much higher impedance, so any harmonic currents cause much larger
harmonic voltages than they would operating on utility power. Loads connected line‐to‐line do not
draw any current from the neutral. They only have line‐to‐line harmonics, and if they are balanced,
none that are multiples of three. The impedance seen by these harmonics at the generator is the
negative‐sequence reactance or X2, because they “rotate” at a different rate than the fundamental
current in the same way that negative‐sequence currents do. This impedance is sometimes not found
on the generator data sheet, but the subtransient reactance X”d is usually a fair approximation for
machines with a fully‐connected damper cage.
Single‐phase loads connected from line to neutral (possibly including large numbers of computers with
single‐phase power supplies) may also draw high third‐harmonic currents. Third‐harmonic currents and
multiples have the same phase angle on all three phases. These currents add together in the neutral
and may cause heating of the neutral if it is not sized correctly. The important impedances for third
harmonic are the stator leakage reactance and the zero‐sequence reactance. If a three‐phase UPS or
delta‐wye transformer is connected between the generator and load, the generator will be isolated from
these third‐harmonic currents, and they should not be considered for generator selection.
Apart from the problems caused by the negative‐sequence harmonic voltages to other connected
equipment, they can cause additional generator heating, mainly in the rotor. Third‐harmonic currents
do not couple to the rotor very much, so do not cause extra heating in the rotor. A generator with fully‐
connected copper damper windings should normally not need to be derated more than about 10% to
account for the extra heating.
Harmonic Mitigation
Equipment manufacturers and consultants have proposed various remedies for harmonic voltage
problems. Following is a table showing some typical suggestions:
The maximum amount of reactance that should be specified in the generator varies depending on many
factors such as:
1. How much of the load is UPS or other harmonic generating equipment.
2. How much harmonic current is produced by each UPS or other device.
3. How much voltage total harmonic distortion (THD) can be tolerated
4. How many generators can be online at any given time.
5. Presence of any other power sources.
6. Presence of harmonic filters or capacitors on the power system.
7. Transformer connections in the power system (by arranging to have some transformers
connected delta‐wye and some wye‐wye, a considerable amount of fifth‐ and seventh‐harmonic
current cancellation occurs).
A full system harmonic analysis is usually advisable to determine the actual reactance requirement.
Using a rule of thumb such as 12% maximum X”d risks either wasting money on an excessively oversize
generator, or failure of the system to meet harmonic requirements. A very rough calculation can be
performed to get an idea of the order of magnitude of impedance required. The formula is:
0.01379 P
where THD is the percent total harmonic voltage distortion, X2 is negative‐sequence impedance (%), and
PH is the percent of load that consists of rectifiers. This assumes typical six‐pulse load [2]. For twelve‐
pulse the distortion is one half of the amount calculated. No account of the generator harmonics is
taken, so they need to be added. No account is taken of transformer or cabling impedances, or the
effect of filters or other system loads. Figure 1 is a nomograph that may be used to estimate the
distortion according to the above formula.
Per IEC‐60034‐4, reactances of a synchronous machine are normally declared in the data sheet with a
tolerance of ±15%. Given six‐pulse load, it will be evident that exceeding 25% nonlinear load will require
maximum X2 less than 15% (nominal less than 12.75%). Especially for larger generators, 15% is a low
value for X2, and to achieve this as a guaranteed maximum will require special design along with a
significant increase in size and cost. Guaranteed values of X2 less than 10% may not be practical, since
the increased size or inertia may make it impossible to couple the generator to the engine. The system
designer needs to investigate other techniques for reducing harmonics instead of requiring the
generator to do the job by itself. These might include:
1. Specifying that electronic equipment loading the system have reduced harmonic current, by
using higher pulse number rectifiers or active rectification, or by adding filters to these loads.
2. Adding system‐wide passive or active filtering to reduce harmonic voltages.
3. Adding phase‐shifting transformers to some part of the nonlinear load to effect harmonic
cancellation with a fully‐loaded system, or using a six‐phase generator to achieve the same
effect.
Conclusions
The practice of requiring a fixed value of generator reactance as a panacea for any sort of nonlinear load
is misguided and will likely lead to failure of the system to meet requirements, unnecessary expense of a
greatly oversized generator, or both. A system‐wide harmonic study followed by careful consideration
of all available means of harmonic mitigation is the correct solution to this problem. For preliminary
quotation as much information as possible should be obtained from the user, then reasonable
assumptions should be made and clearly stated in the quotation.
Figure 1. Nomogram for harmonic load calculation.
References
1. “Adjustable‐Speed Drive and Power Rectifier Harmonics – Their Effect on Power System
Components,” David Rice, IEEE Transactions on Industry Applications, Volume IA‐22, Number 1,
Jan‐Feb 1986.
2. “A Detailed Analysis of Six‐Pulse Converter Harmonic Currents,” David Rice, IEEE Transactions on
Industry Applications, Volume 30, Number 2, Mar‐Apr 1994.
3. “Understanding Static UPS Systems and Generator Set Application Considerations,” Caterpillar
Engine Division, 1989.
4. “Understanding UPS and Motor Generator Set Compatibility,” William A. Barcus, Liebert
Corporation (undated)
5. “Generator Set and UPS Compatibility,” PT‐6014‐genset‐ups‐compatibility‐en.pdf, Gary Olson,
Cummins Power Generation (undated).
6. “Calculating Generator Reactances,” Timothy Loehlein, Cummins Power Generation, 2006.
7. “UPS Application Guide,” Part 19 “Emergency Generator”, GE Digital Energy, 2008.
8. “Frequently Asked Questions,” Association of Manufacturers and Suppliers of Power Systems
(AMPS), www.amps.org.uk (undated).
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10. “Specifying Generators for Mission‐Critical Environments,” Keith Lane, P.E., Pure Power, Spring
2011.
11. IEEE Std 519‐1992, “Recommended Practices And Requirements For Harmonic Control In
Electrical Power Systems,” IEEE, 1992.
12. “Tutorial on Harmonics Modeling and Simulation,” IEEE TP‐125‐0, IEEE Power Engineering
society Task Force on Harmonics Modeling and Simulation, 1998.
13. “The Impact of Non‐Linear Loads on Generator Sizing for Dedicated Loads,” Michael L. Sieberg,
Kato Engineering Report #2086.
14. “The Effects of Electrical Power Variations Upon Computers: an Overview,” U.S. Department of
Commerce, 1973