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Getting from utility power to standby power: Critical timing and power quality vulnerabilities

By Jack Smith

January 2012

The Fluke 435 Series II 3-Phase Power Quality Analyzer

The Fluke 435 Series II Three-Phase Power Quality Analyzer has a Power Wave measurement function fast enough to measure power system behaviors during switching operations.

In my two previous columns I wrote about problems associated with starting standby electrical generators - specifically genset battery health and battery-charging reliability. In this column I’ll touch on what happens when standby generators are called upon to perform.

Because the chain of events is a little more complicated than “utility power goes off; standby generator comes on,” I’ll focus on the critical timing during the transition from utility power to standby power, certain power quality vulnerabilities during power transfer, and how quantifying them can relate to other scenarios.

Transition timing is critical

In the simplest scenario, a standby power system consists of a normal power source - namely the utility; an alternate power source - namely a standby generator; and a way to transfer to standby power if utility power fails. For many types of facilities, this configuration works just fine.

However, if utility power fails, the facility’s entire electrical system is without power for around 10 seconds until the standby generator comes online. Typically, standby generators can start to accept loads when they reach about 90 % to 95 % of rated frequency. If the generator fails to start, emergency loads are lost.

Large facilities with significant power requirements and various load types may have multiple standby generators to accommodate different emergency scenarios. Some facilities may generate at least some of their power on-site to offset demand peaks. For those facilities, an alternate power source is immediately available in the event that utility power is lost. Depending on their switchgear and automatic transfer switch (ATS) configurations, these facilities may not experience any outages while waiting for additional generators to come online.

Health care facilities and data centers are among those applications considered to be “mission critical.” Although hospitals are required to restore power to life safety and critical branch loads in a maximum of 10 seconds, many mission-critical facilities can’t tolerate even a momentary power interruption. Facilities with this level of criticality should include an uninterruptible power supply (UPS) system in their emergency power strategy. Typical UPS systems can supply up to 25 seconds of ride-through power.

Some mission-critical facilities also require closed-transition transfers in which the ATS contacts operate in a “make-before-break” sequence. Closed-transition transfers allow critical loads to be transferred seamlessly from one power source to another by briefly paralleling the two sources, which is typically limited to less than 100 milliseconds (6 cycles).

Depending on the type of facility and the level of sophistication of its emergency power configuration, the generalized sequence of events of a typical standby power system includes:

  • Utility power source fails.
  • If the facility has a UPS, the ATS immediately transfers to UPS power and sends a start signal to the generator controls.
  • For facilities without a UPS, the ATS or utility switchgear relay (which senses loss of voltage and loss of frequency) sends a start signal to the generator controls.
  • Standby generator starts and reaches normal operating voltage and frequency, typically in around 10 seconds.
  • The ATS senses alternate power source availability or receives a permission signal from the standby generator control to transfer to standby power.
  • Emergency loads are switched to the emergency power bus sequentially according to load priority settings, if so equipped.
  • Normal utility power returns.
  • The ATS transfers loads back to the utility power source. This transfer can be done with a momentary power interruption or with no power interruption, depending on load criticality and automatic transfer switch functionality.
  • The generator or generators cool off and shut down.

Power transfers affect power quality

Not only is timing critical during transitions between utility and standby power - the way these transitions affect power quality are important as well. Large loads reduce engine speed, which causes a momentary reduction in generator frequency. The load also causes the generator voltage to dip until the standby power system exciter compensates for the increase in reactive power demand.

The opposite occurs when loads are removed from the standby power system. Engine speed increases temporarily, which causes the generator frequency to overshoot, and then return to steady-state operation.

The equipment and systems that standby generators must support during a utility outage determine the maximum allowable voltage dip. Most modern equipment restricts voltage and frequency to strict margins.

In addition to deviations in voltage and frequency during transfers between utility and standby power, harmonics-related problems can occur as well. Interactions between UPS systems and standby generators can occur if utility power outages are prolonged.

Until now, it has been difficult to quantify these parameters during the sequence of events associated with standby power switching operations. Fluke is introducing the 430 Series II 3-phase power quality analyzers. Within this series, the 435 model includes what Fluke calls the “Power Wave measurement function.” Power Wave is a fast capture system that samples and displays half-cycle RMS voltage and current, which is fast enough to measure power system behaviors during switching operations.

The 435 with Power Wave takes 512 samples every 60 ms cycle and records the data, making it the very first dedicated, portable analyzer to do so. Far more data, captured far more frequently, during the start-up sequence. Power Wave both displays that data and summarizes it as statistical values. So in addition to manually reviewing streams of data, operators can pull out the particular event information they need for load profiling, preventing motor/drive/load mismatches, and conducting motor and generator commissioning and start-up testing.

The new analyzer series also has a “Unified Power measurement function.” The impressive aspect of Unified Power is the ability to automatically quantify the energy wasted by harmonics and unbalance. If users input their utility rate structure, Unified Power can actually calculate the monetary cost of the wasted energy. To my knowledge, this is the first time a test tool has provided this type of functionality.

The new family of power quality analyzers provides users with a single tool for load profiling, measuring inverter or drive efficiency or degradation, preventing motor/drive/load mismatches, and for motor and/or generator commissioning and start-up testing. These features should provide plenty of material for me to write about for many months to come.

Until next time, keep standing on “Solid Ground.”

Read Jack’s two previous columns
“When the Lights Go Out, Will the Generator Come On ?” »
“Taking Charge of Genset Battery Health” »