January 15, 2008

Protecting plant equipment from voltage sags

Andreas Eberhard, Power Standard Labs

Common failure mechanisms

The most common failure mechanism is lack of energy. This can manifest itself in something as simple as insufficient voltage to keep a critical relay or contactor energized or something as complex as an electronic sensor with a failing power supply giving an incorrect reading, which would cause EUT software to react inappropriately (Figure 6).

6. Anatomy of voltage sag. To test a new device, a voltage sag is introduced in the power source (a). The waveform, which was about 40 amps peak before the sag in this example, then increases to 450 amps peak after the voltage sag (b). The same current, this time expressed as an RMS value, is shown. The next graph shows the same current, this time as an RMS value. Before the sag, it was about 23 amps RMS (this equipment was rated at 30 amps), but after the sag the current increased to 175 amps RMS. This behavior is not unusual (c). The final graph shows the output of a DC supply during this sag (d). Courtesy: Power Standard Labs

The second most common failure mechanism, surprisingly, occurs just after the sag has finished. In such cases, all of the bulk capacitors inside the EUT recharge at once, causing a large increase in AC mains current. This increase can trip circuit breakers, open fuses, and even destroy solid-state rectifiers. Most design engineers correctly protect against this inrush current during power cycling, but many do not consider the similar effects of voltage sags. Be careful when the test procedure is developed; if you use a sag generator that lacks sufficient current capability it will incorrectly pass the equipment if there is insufficient current available to blow a fuse or trip a circuit breaker in a half-cycle.

Another common EUT failure mechanism occurs when a sensor detects the voltage sag and decides to shut down the EUT. In a straightforward example, a three-phase EUT might have a phase-rotation relay that incorrectly interprets an unbalanced voltage sag as a phase reversal and therefore shuts down the EUT. A more atypical example would be if you had an airflow sensor mounted near a fan, it detected that the fan had slowed down momentarily, and the equipment software misinterpreted the message from this sensor as indicating that the EUT cooling system had failed. In this case, a software fan failure signal delay is the solution to improve sag immunity.

Another common EUT failure mechanism involves an uncommon sequence of events. For example, in one case, a voltage sag was applied to the EUT and its main contactor opened with a bang. But further investigation revealed that a small relay, wired in series with the main contactor coil, actually opened because it received an open relay contact from a stray water sensor. That sensor, in turn, opened because its small 24-VDC supply output dropped to 18 V during the sag. The solution was an inexpensive bulk capacitor across the 24-VDC supply.

Many other failure mechanisms can take place during voltage sags. The question to the test engineer will always be: How do we fix this problem? Usually, there is a simple, low-cost fix once the problem is identified.

Protect your equipment

There is no one best place to locate a protective device for all your plant equipment. An equipment protection program should begin with identifying specific equipment items that are sensitive to voltage sags, either through hard experience or with the support of the manufacturer. The ubiquitous UPS may not provide enough of the right protection.

However, there are areas where voltage sags have a history of interfering with plant operations by affecting programmable logic controllers as well as relays and contactors in sensitive equipment. The best approach to handling those problems is to specify new equipment according to a particular voltage ride-through specification, such as SEMI F47.

If you have recently upgraded to adjustable-speed drives (ASDs) in your plant, you are in luck. ASDs can ride through voltage sags because of the inertia of the motor and the connected load. Some ASD manufacturers offer an optional voltage sag ride-through feature.

Very short sags can be tolerated with ferroresonant transformers, magnetic synthesizers, or active series compensators. Others have employed static transfer switches and fast transfer switches that can operate within two cycles to protect overly sensitive loads.

At PSL, we believe that only in extreme cases should devices that eliminate voltage sags on the AC circuit be considered because this is the most expensive possible solution. However, the final selection of a solution requires weighing the cost of equipment and production losses against the cost of protection.

—Andreas Eberhard (aeberhard@powerstandards.com) is vice president of technical services for Power Standard Labs.