Specifying equipment that addresses the problems associated with generators and UPS equipment compatibility isn' enough
Facility designers have been aware of problems associated with operating UPS equipment and gen-sets for years. These subsystem failures can put entire facility power systems at risk, and can create challenges for designers trying to get reliable power at critical sites. Over the years UPS and generator equipment manufacturers have worked together to solve common problems, and designers have become increasingly aware of these issues (Sidebar).
However, this awareness doesn't necessarily mean that you can now ignore the issue of UPS/generator compatibility. As always, finding and fixing 90% of the problems won't earn you accolades if you're expected to produce systems that are 100% operational and reliable under all conditions.
So in addition to specifying equipment that covers the known issues between generators and UPS equipment, you should be aware of several new issues that have become more common in recent years.
Constant temperature controls and gen-set instability.
Pulsed heating systems used to control the ambient air temperature in computer rooms can introduce what may be diagnosed as instability in the gen-set due to the nature of their operation. These systems provide very stable ambient temperatures, but create many load pulses per second on the generator by turning heating coils on and off. This pulsating load on the gen-set alters frequency faster than the engine and governor can adjust. This can prevent the UPS from ramping on to a gen-set or set off alarms in the UPS when it fails to synchronize a bypass.
However, design engineers aren't without options for solving the problem. It's possible to oversize the generator to reduce the effect of the pulsed loads. They can also alter the sequence of loads transferred to the generator so that the heating load is added last, thereby diluting its effect. And it's also possible to detune the performance of the gen-set to slow down its response to changes in the load. This keeps the generator frequency from oscillating as quickly.
Keep in mind that adjustment of gen-set transient performance may affect its ability to supply other loads on the system, such as large motor loads.
Power factor correction and gen-set instability.
It's possible to use digital voltage regulation equipment, separate excitation systems, and PWM-type control architecture to make generator equipment robust in the face of voltage distortion. However, UPS suppliers are more apt to employ filters to minimize the effect of waveform distortion on power systems. While these filters provide positive effects on the overall power system, they can be very disruptive to generator operation.
The voltage regulator monitors the voltage on the output terminals of a gen-set and controls the field strength in the alternator to maintain constant output voltage. Obviously, it takes a relatively low level of voltage regulator output to maintain voltage at no load.
UPS manufacturers size filter equipment for operation at the expected maximum load on the UPS. At light loads, there may be excess capacitance, causing a leading power factor with respect to the gen-set. This will cause the voltage of the power system to rise when the rectifier of the UPS is energized. Since rectifiers are designed to ramp on from zero load to minimize load transients, they almost always represent a leading power factor to the power source.
A utility supply system simply absorbs the reactive power output because it's extremely large relative to the filter system and has many loads that can consume this energy. With a gen-set, however, the rising voltage from the leading power factor causes the voltage regulator to turn down and reduce alternator field strength. If the gen-set voltage regulator turns all the way off, it will lose control of system voltage, which can result in sudden large increases in system voltage.
The UPS is designed to recognize high voltage as an abnormal and undesirable condition, so it may immediately switch off its rectifier when it sees this condition. The high-voltage condition is thus immediately relieved and voltage then returns to normal. To an observer, the generator seems to be unable to pick up the system loads.
The solutions to this problem lie in the inherent characteristics of a generator. Generators are physically unable to absorb more than a very small amount of real (kW) or reactive (kVAR) power. Remember the old problems with elevator motors and generators that went into overspeed? This is the same phenomenon, but it affects the alternator excitation system rather than the engine.
An alternator's ability to absorb power is described by a reactive capability curve, or operating chart (see Figure). It shows the capability of a machine to produce and absorb power. In this curve, the kVAR produced or absorbed is on the X-axis (positive to the right). The Y-axis shows kW (positive going up). Both are shown as per-unit quantities based on the rating of the alternator, although not necessarily that of the gen-set, which may have a different rating due to differences between engine and alternator ratings.
The normal operating range of a gen-set is between 0% and 100% of the kW rating of the alternator (positive) and between 0.8 and 1.0 power factor, which is represented by the green area on the curve. The heavy black lines on the curves show the operating range of a specific alternator when operating outside of normal range. Notice that as power factor drops, the machine must be de-rated to prevent overheating. On the left quadrant, you can see that near-normal output, which is represented by the yellow area, can be achieved with some leading power factor load, in this case, down to about 0.97 power factor, leading. At that point, the ability to absorb additional kVAR quickly drops close to zero, which is represented by the red area, indicating that the voltage regulator is “turning off” and any level of reverse kVAR greater than the level shown will cause the machine to lose control of voltage.
In other words, if the machine is rated for operation at 1,000kVA and 0.8 power factor (600 kVAR rated), with a reverse kVAR level of 0.2 per unit (rated), you'll exceed the machine's capabilities. So more than 120 kVAR reverse reactive power, which for most people is a surprisingly low level, creates a problem. The following solutions involve avoiding excess reverse kVAR levels through proper system design and operation:
Modify the sequence of operation for the facility so loads that require reactive power are already present on the bus when the UPS ramps on to the generator. The system loads will then consume the reactive power produced by the filters, and you'll avoid the loss of voltage control. This requires a rethinking of operation sequences in some cases. For example, you may have to place mechanical loads rather than a UPS on the generator first. Or you may have to break down the loads into smaller blocks of UPS and mechanical loads, rather than larger isolated buses of each.
De-energize the filters in the UPS when operating on the gen-set or reduce the magnitude of filtering provided. If the generator has modern digital excitation control, the filters won't be needed to maintain generator operation, but you may still need them to properly serve other loads.
Special problems with paralleling applications
Paralleled generators maintain their synchronism due to the magnetic field in place between the rotor and stator in the alternator. When the field of the generator collapses because the voltage regulator turns off, the rotor can “slip a pole” and expose the machine to the same mechanical impact as an out-of-phase paralleling action. Obviously, you need to avoid this condition.
Gen-set manufacturers commonly provide reverse VAR (loss of field) protection for alternators in paralleling applications. This protection is effective in preventing pole slipping when properly tuned for the specific generator in use. For paralleling applications, the alternator supplier should provide reactive capability curves for use in your selection of protective equipment settings. Settings that are too conservative will cause nuisance shutdowns of the generator — and possibly the system — while settings that are too aggressive won't provide enough protection.
Remaining design issues.
While advancing technology and cooperation among manufacturers and facility designers have significantly reduced UPS/gen-set compatibility issues, there is still a potential for problems to affect power reliability.
Use the following design guidelines to minimize or eliminate compatibility issues between UPS systems and standby generators:
Continue design practices that have been successful in the past. For example, select gen-sets with low-temperature-rise alternators. These larger alternators also provide better motor-starting capacity and greater resistance to voltage waveform distortion.
Specify low subtransient reactance for alternators, digital excitation control, and voltage regulator power supplies with PWM. UPS applications also generally require electronic isochronous governors.
Always use closed-transition or open-transition, delayed-neutral position transfer switches.
Specify state-of-the-art UPS systems wherever possible. Later designs tend to reduce total harmonic distortion and minimize rectifier sensitivity to frequency changes. These newer systems also tend to have greater adjustability in dealing with generator and UPS interaction issues.
Understand the alternator's ability to absorb reactive power, and determine how much corrective kVAR is in the load. Determine which specific operational sequences will produce enough reverse VAR to cause problems and then specify what the system needs.
Follow these guidelines and exercise reasonable care during system design, and you can avoid most UPS/generator compatibility issues. When problems do come up, cooperation between generator and UPS equipment suppliers is critical to finding a quick solution.
Olson is technical counsel and acting product manager for paralleling and gen-set control products for Cummins Power Generation in Minneapolis.
Sidebar: Old Problems Solved
When discussing the issues that currently plague the process of integrating UPS systems and generators, it's easy to forget how far the industry has come in eliminating the following problems that once gave designers headaches.
UPS harmonic currents and resulting alternator heating. A UPS is “seen” as a nonlinear load to a gen-set, and the harmonic currents drawn by the UPS cause incremental alternator heating. If designers recognize this, they can specify oversized alternators to serve this type of load. Suppliers' sizing recommendations for combating the problem vary, but most agree that alternators that are 15% to 20% larger than required for a linear load of similar magnitude work well. Designers also recognize they can oversize alternators without oversizing engines, within limits. This eliminates the light load operation problems usually associated with diesel engines.
Waveform distortion. UPS-generated harmonic currents also cause voltage waveform distortion that's typically more pronounced while operating on gen-sets than while operating on the utility service. This is because the impedance of the generator is higher. To make the generator equal in impedance to the utility feed, it would need to be much larger than is practical to provide. Consequently, a system designer must often judge the acceptable level of harmonic distortion without solid guidance. Specifying a maximum allowable subtransient reactance on the alternator will result in the best practical performance. Make sure you properly evaluate proposals from various gen-set manufacturers.
Gen-set voltage regulator misoperation. Voltage waveform distortion can also disrupt the operation of gen-set voltage regulators. You can eliminate this problem by using 3-phase sensing digital voltage regulators, pulse width modulation (PWM) in the voltage regulator, and separate excitation power systems like permanent magnet generators (PMG). These steps can make a gen-set almost immune to voltage regulator misoperation caused by distortion.
Unstable governors. Voltage distortion problems with gen-set voltage regulation equipment sometimes made generator governors appear to be unstable, causing rectifier misfiring in the UPS equipment. Again, these issues are relatively uncommon now, since the advent of digital excitation controls. Some manufacturers offer digital governing systems that make engines more stable by sensing engine temperature and re-adjusting governing gains as a function of engine temperature.
Transient voltage and frequency conditions. Designers know that gen-set frequency will never be as constant as the utility service, and load changes will cause transient voltage and frequency conditions that can be disruptive to UPS equipment. Setting UPS equipment for slew rates that are achievable with generator equipment, which are typically in the 3 Hz/sec to 5 Hz/sec range, will minimize alarm conditions.
Transfer switch operation and UPS disruption. Occasionally in the past, UPS equipment was disrupted by transfer switch operation, mainly when the transfer switch operated too quickly between energized sources, causing rectifier failures or misoperation. Simple and commonly available “programmed transition” (short time delays in the open position) or fast closed transition transfer has solved these issues.