Introduction
The need for total ride-through of a power outage is becoming more evident. As productivity increases in the manufacturing sector, homeland security assumes increasing importance, and health care practices become more stringent and complex. It is paramount that the proper standby power solution be one that combines power quality (UPS, filters, etc.) on the front end, and power reliability (generator) on the back end to provide a seamless alternate power source.
Power quality is effectively maintained by correctly matching the power reliability source to the power quality system. By doing some up-front research, and applying some fundamental rules, successful integration of these modules can be accomplished.
Consideration of the efficiency and harmonic content of the UPS, its adjustability, and filtering properties when matched to the appropriate generator set are fundamental in implementation. Sizing the system to run in the “sweet spot” is also critical, as most UPS systems and generators operate most efficiently, and cleanly, in the 60 to 90 percent load range.
This brief article explores the fundamentals of selecting and integrating this kind of an emergency power system. Key points to consider are UPS to generator sizing ratios, harmonics (what they are and what to watch for), false triggering of the UPS and its root causes, UPS ride through, and genset transient and steady state response
Power Quality vs. Power Reliability 
It is important to differentiate between power quality, which is the conditioning of the normal source power to specific tolerances, and power reliability, which in the truest sense of the term is the supply of power to the load, regardless of quality. Naturally, power reliability has more stringent specifications, but the concept of reliability centers on supplying power continuously.
A seamless emergency power supply solution (EPSS) is one that combines power reliability and power quality to provide full ride through in the event of a normal source (utility) outage or variance outside of maximum allowable voltage and frequency tolerances. This solution is comprised of an uninterruptible power supply (UPS) that is a stored energy device used for instantaneous supply of power in the event of a power outage, or out of tolerance condition, and the generator, which satisfies longer duration (more than one minute) power outage events.
Power Reliability: The Generator Set
Fundamentals
In its most basic essence, the generator set (genset) is a prime mover (internal combustion engine) coupled to an alternator in order to generate electricity. When a demand is placed on the circuit, electrical power is produced. Selecting the proper size and type of generator set to supply the application is the most critical factor in successful implementation of the power reliability component of the EPSS. Key factors influencing selection are:
- Load Type
- Linear load (base incandescent lighting, fans, blowers, etc.)
- Non-linear load (high frequency welders, UPS systems, battery chargers, etc.)
- Heavy motor loads (high inrush current, etc.)
- Soft load via variable frequency drives, etc.
- Ambient Conditions
- Temperature
- Altitude
- Air quality (sand, dust, etc.)
- Electro-magnetic interference (noise) conditions
- Environmental Constraints
- Sound emissions
- Physical size
- Weight
- Fuel storage
- Exhaust emissions
- System Constraints
- Level of redundancy
- Expandability
- Reserve capacity
- Economics
- Code compliance (national and local)
- NEC: Generally, NEC applies to the EPSS
- UL 2200
UL 2200 is the governing standard for low voltage (600 volts or less) gensets. Its purpose is to a ssure a standard for the safe operation of stationary engine powered standby and emergency generators.
UL 2200’s scope applies to engine generators rated 600 volts or less installed in
stationary – ordinary (i.e., not hazardous) locations and in accordance with:
- NFPA 37 Stationary Combustion Engines and Gas Turbines
- NFPA 70 National Electrical Code
- NFPA 99 Health Care Facilities
- NFPA 110 Emergency and Standby Power Systems
These fundamentals must be considered to assure successful implementation of the EPSS generator.
Guidelines
There are some basic genset sizing guidelines that can be used to assist in the selection process.
Motor loads are calculated as follows:
- skVA load @ HP motor x 5.3 (assumes Code F)
- rkVA load @ HP motor (i.e., a 50 hp motor is approximately 50 kVA)
- skW engine @ skVA motor x .27 ( starting power factor)
- rkW engine @ HP motor x .85
Exceptions: specialty motors such as submersible pumps where locked rotor kVA is higher; therefore, skVA @ HP motor x 8
Where:
kW = kilowatt
kVA = kilovolt-ampere
sKVA = Starting kilovolt-amperes
rkVA = Running kilovolt-amperes
skW = Starting kW required by the electric motor
rkW = Running kW
Note: kW is the engine power component, and kVA is the alternator (electrical) component.
For air conditioning, use 1 ton : 3 kW as a general guideline.
Genset and UPS Compatibility
Much debate has occurred over the compatibility of gensets and UPS’s. Technology improvements to both platforms over the past fifteen years have made application of these systems a virtual success, as long as the systems are properly matched in the planning phases.
Advances in genset voltage regulation and engine speed governing have drastically reduced the potential for such anomalies as false triggering of a UPS event, or SCR (silicon control rectifier) tracking. Advances in the topology of the UPS have also resulted in less harmonic distortion throughout the power-band, and greater efficiencies have resulted as well.
Harmonic distortion is a byproduct of rectification, or changing alterna
ting current to direct current. Voltage waveform distortion occurs at points of commutation, where source voltages are shorted .
Harmonics affect a generator since it is a high impedance source, and not able to displace the harmonic energy like the normal source (utility) is able to. Harmonically induced currents within the genset alternator windings cause heat build-up beyond what a normal load at or near a unity (1) power factor would induce. Severe harmonics may cause heat build-up within the alternator to the point where damage to the insulation may occur, resulting in premature alternator failure.
The old rule of thumb that a genset alternator needed to be 3 times the size of the UPS output has been replaced by a more reasonable ratio of 2 : 1 or 1.5 : 1 or less, depending on the efficiency of the UPS, and other factors such as rectification type (full or half pulse) and pulse frequency. Generally, 6 or 12 pulse are the industry standards today, where pulse refers to the number of semiconductors in the device (usually silicon control rectifiers or SCR’s). Additionally, the use of isolated gate bipolar transistors (IGBT’s) has further reduced the amount of harmonic content in UPS’s. Within the matrix, TDH refers to total harmonic distortion, RSC refers to the short circuit ratio as it relates to autotransformers used in the circuit, and RMS indicates root mean squared.
Assuming the UPS is tunable (voltage and frequency trigger points), and it operates in the 92+ % efficiency range, most EPSS applications can utilize a genset that has an alternator that is 1.5 times the rated kVA of the UPS. Additionally, voltage dip of both the alternator and prime mover must be taken into consideration, as these can affect UPS stability as it relates to operation with the genset during load swings. It is important to size the genset to operate below the maximum allowable voltage and frequency dip of the UPS, unless the UPS is of the type that can condition voltage to its load during out of tolerance events in concert with the generator, or utility.
The type of alternator excitation system is important when matching the genset to the UPS. Typically, a permanent magnet excitation system is utilized in applications using a UPS. A permanent magnet excitation system is able to react to transient conditions brought on by heavily induced loads in a more stable and quicker manner than other types of excitation systems. Also, any commutation spikes created by the voltage regulator are not transferred into the alternator windings and thus the power output.
The alternator itself is a source of impedance. Unlike the utility supply or normal source, which is considered an infinite bus for practical purposes, the generator set alternator is a finite bus, and is much more sensitive to harmonic distortion from the attached load.
Some basic rules on alternator size in relationship to the type of UPS rectification and amount of UPS load are:
- 6 Pulse UPS
- Upsize the genset alternator 2 to 2.5 times the amount of non-linear load, or select an 80 oC temperature rise continuous duty alternator.
- Limit the subtransient reactance (x”d) to 6% (based on non-linear load).
Consult alternator manufacturer data for x”d.
- 12 Pulse UPS
- If UPS load is less than 25% of genset rating
- Standard alternator with automatic voltage regulation
- If UPS load is greater than 25% of genset rating
- Upsize alternator to 105 oC insulation (typically Class F)
- Variable Frequency Drives (VFD’s)
- Size generator alternator to achieve x”d less than or equal to 15%
Leading power factor loads also must be dealt with accordingly, as they result in a voltage regulation output above the nominal set point voltage of the regulator. In some cases, voltage runaway due to unstable voltage regulator output may result, which can damage both the alternator and connected equipment. Any leading power factor less than 0.95 is suspect.
Harmonics can be created by the supplied load or by the UPS itself. Most of the newer UPS systems are filtered on the input side. Harmonic distortions of less than 7% are common and a typical generator will operate well within those limits. Trouble can occur when adding a genset to an existing UPS system that has been in service for several years. These older systems are not well filtered and can cause instability of the generator because of the harmonics induced backward toward the generator. These harmonics distort the sine wave to the extent that the voltage regulator cannot control the output voltage. A 6 pulse rectifier, unfiltered, will create 35% distortion on the incoming sine wave.
Filters are typically employed to minimize harmonic distortion in non-linear load devices such as UPS’s. Filters can be either active or passive. An active filter is essentially a switching device such as an IGBT. A passive filter is typically a device such as a transformer that acts as a choke.
If the UPS is sized correctly, it will be operating at about 80-95% of its rating. However, some systems are installed with growth in mind and operate at much less than the full load rating. The problem occurs when the generator is sized to the lower rating. When an outage occurs, the UPS takes over and supplies the power. When the generator comes on line, the batteries are now slightly discharged and the UPS will turn on to its full rating to bring the batteries back to full charge and this is regardless of the load on the UPS.
Larger UPS systems step into the load to prevent the genset from overloading, but the initial load that the generator will see will be the full output of the UPS to the batteries, not the connected load.
In summary, when sizing a UPS system to a generator, the type of UPS, the amount of filtering that the UPS is equipped with, and the load that the UPS is carrying are important considerations to take into account.
When operating a generator set with a high amount of non-linear load, be sure to consider:
- The alternator size in relation to the amount of non-linear load, as well as the total load;
- Permanent magnet excitation;
- Make sure the prime mover (engine) has adequate power to recover from large single step (block) loads;
- For three phase loads, consider implementing a voltage regulator with three phase sensing; and
- Isochronus (constant speed) governing.
The more information that is available, the closer the genset can be sized to the UPS. Above all, consult with both the UPS provider and the generator set provider to assure successful implementation of the EPSS. These professionals can assist in sizing the system appropriately.
