Most uninterruptible power supply (UPS) systems alone cannot handle inrushes and fault clearing. When these systems lack sufficient overload capacity, they must resort to an automatic bypass that directly connects the load to the utility or a standby engine generator that is synchronized with the UPS output. By choosing a UPS system that helps the standby generator gain and maintain synchronization, you will maximize bypass and system availability.

Matching Frequency

Modern power supplies designed for worldwide use have made frequency-sensitive loads a concern of the past. When necessary, data center computer synchronization is done on a digital signal level rather than on the power source's 60 Hz sine wave.

But frequency is still a concern when you attempt to seamlessly transfer critical loads to bypass or another distributed redundant UPS system. A seamless transfer requires matching of frequency, voltages, and phase.

One practice is to synchronize at the output of the various UPS buses (using the UPS as a frequency converter) and allow the engine generator to freewheel. New emissions requirements and improvements in electronic governor controls now make it possible to match frequency at the engine level.

“Engine generators can now be made to phase-lock synchronize with other sources as if they are geared together,” says Donald Becker, former president of the Electrical Generating Systems Association and senior area manager for Kohler Co., Sheboygan, Wis.

For example, you can synchronize the engine generator bus of one system to a utility reference, UPS output bus, or another engine generator output. This is also true for Cummins Power Generation, Minneapolis, MN, where senior market manager Steve Iverson says that their controls easily accommodate phase-lock synchronization at the engine generator level.

According to Steve Wetter, program manager for the Electric Power Group of Caterpillar Inc., Griffin, GA, all engine generators manufactured today are tested to ISO Standard 8528, Part 5, which dictates specific frequency recovery and transient response to loads.

Steady-State Loads

Kevin J. McCarthy, head of the Mission Critical Facilities Group of Einhorn Yaffee Prescott, Washington, D.C., observes that the demands on a load's power source are the least when the load is unchanging or in a steady state.

The capacity of the UPS's output inverter, which depends on the load and its characteristics, limits the UPS output. The higher output impedance (or lower output capacity) of the UPS places greater pressure on the system to make the bypass available and offset the lowered capacity.

A Stabilizing Topology

One UPS topology, used in conjunction with modern engine generators, is demonstrating that synchronization in critical power systems is the key to reliability and maximum availability. This topology, which is based on American Power Conversion's Delta Conversion technology, rapidly synchronizes the engine generator and incorporates a control loop to achieve engine-generator stability. With improved stability, generators can be synchronized with another utility source or distributed redundant UPS output bus using a simple master-bus synchronizer or in-phase monitor.

The Delta Conversion UPS (Fig. 1) uses a linear power walk-in to the engine generator, along with input unity power factor and input harmonic distortion active filtering. It requires no derating for load input current distortion and has no limits on crest factor. Its input power factor maintains at unity, even if the load power factor changes from 0.8 lagging to 0.9 leading, or the load current carries a high harmonic content.

In contrast, you must derate the Double Conversion UPS (Fig. 2) for harmonic-rich loads and set limits on crest factors. This UPS requires lossy input filters with capacitors and chokes. These modifications will raise the UPS's input power factor and attenuate heavy, self-generated harmonics to a marginal input .8 power factor lagging and typical 10% current total harmonic distortion (THD), respectively.

Most UPS systems handle steady-state load conditions but can't tolerate overload events. Overload events include step loads, starting inrushes, out-of-phase load transfers, and fault clearing. These out-of-phase periods prevent bypass availability. Without bypass, the UPS can only deliver a small percentage of the overload current needed. If the UPS fails or can't sustain adequate voltage quality during an overload event, the critical load becomes vulnerable to a hard crash. These events occur more often at facilities where numerous load changes and reconfigurations are made without shutting down the critical load.

Multiple Load Transfers

Today's complex distributed redundant UPS configurations (Fig. 3) depend on synchronized, seamless load transfers between multiple redundant UPS buses. Even if the two source voltages match, dissimilar frequencies between these sources mean they are in phase (synchronized) only occasionally, proportional to the difference of the frequencies. Applying high-speed static transfer switches (STSs) will transfer critical loads from one UPS source to another, but it requires the two sources be closely synchronized (i.e., generally not more than 5 degrees apart).

When the utility feeds two UPSs, they are the same frequency and well within the STS phasing. But if the utility feeds one UPS while a freewheeling engine generator feeds another, that UPS must compensate, or the engine generator must be brought into synchronization. If a UPS compensates continuously for the engine generator's frequency and phase, the UPS bypass is not available, and the system is vulnerable. Out-of-phase transfers are especially risky due to their potential for high-inrush currents.

Largest Single Load

The UPS usually is the largest single load on the engine generator. Since both the generator and the UPS are regulated systems, their respective regulators tend to interact and can result in instability.

The design of the Delta Conversion UPS optimizes the UPS as the main stabilizing component load of the engine generator. It offers an extended range of programmable parameters for system stability, including a linear ramp-in to soft load the engine generator bus (see Fig. 4, on page 14). The UPS input current is increased from 0% to 100%. Once the engine generator is loaded, the UPS can be programmed with a slew rate specifically set for the engine generator and the particular critical load. Slew rate is the frequency's rate of change (e.g., from 50 Hz to 52 Hz in 4 sec or 0.5 Hz per sec). Switch-mode power supplies can accept 4 Hz per sec, but high-inertia loads (such as motors) may need a lower slew rate of 0.25 Hz per sec.

A Double Conversion UPS uses a power walk-in with current limit in nonlinear steps and lacks adjustable slew rate (Fig. 5). Most manufacturers recommend gross oversizing of the engine generator to offset their UPS as a destabilizing load.

Engine Generators

When choosing the proper engine generator, you must first size the prime mover or engine for the critical load, then oversize for UPS losses, input power factor, and battery recharge. The engine should be equipped with an isochronous governor to reduce oscillations from load changes and generator torque angle reactions. Second, you must size the generator for the critical load, UPS losses, battery charging, UPS harmonics, and power factor.

The Delta Conversion UPS represents a unity power factor load that is ideal for maximizing the power transfer of the engine or prime mover as well as for the generator. Unity power factor means the UPS appears the same as a linear load free of harmonic distortion. The engine and generator can be optimized for sizing with the Delta Conversion UPS at around 130% of the critical load. Fig. 6, below, is an actual 500kW-rated Delta Conversion UPS at 100% load running from an engine generator. The waveforms are of input current and voltage showing minimum distortion from an ideal sine wave for the engine generator.

If the generator is undersized when powering a Double Conversion UPS, it may experience severe problems when load is applied — especially with Double Conversion UPS line-commutation notches. Line-commutation notches result from the commutation of the input SCR or Thyristor based rectifier/charger. These can increase in amplitude due to higher source reactance. Subtransient reactance of generators or alternators (typically 16%) is much higher than that of similar rated distribution transformers (typically 4%), which means that the total harmonic voltage distortion (THVD) figure at the generator bus will exceed 5%.

Often the only alternative is to oversize the generator by a factor of at least two (in comparison to UPS rating) and allow 10% distortion on the load bus. This 10% voltage difference compared to the utility source and UPS output can be a serious problem for downstream STS load transfers. Fig. 7, above, shows the input waveforms of a typical Double Conversion UPS at full load.

Conclusion

Synchronized critical power systems offer the highest degree of power protection by assuring all components are integrated and prepositioned for maximum performance.

As the largest single load in the critical power system, the UPS that most proactively maintains synchronization is the best choice for reliability and robust response to power aberrations. The UPS topology that contributes to engine-generator stability and synchronization further assures maximum availability and 9s. System level synchronization at the engine generator bus and at the UPS output bus increases system availability in direct response to the power uncertainties for critical loads in today's climate.