Identifying the problem is just the first step
Discussions on power quality usually focus on either the fundamentals of power quality phenomena or interpreting what was recorded on a power quality monitor. Knowing what happened is often just half the process—fixing or eliminating the problem is the next step. Without some form of mitigation, your facility will keep experiencing the same process interruptions, and people will wonder why they bother to collect data unless things get better. As the Beatles sang, “I’m fixing a hole where the rain gets in, and stops my mind from wandering where it will go.”
Some companies are reluctant to invest in PQ mitigation, either because of lack of knowledge about available solutions or not wanting to spend the money because the return-on-investment isn’t clear. Perhaps a salesperson for a large UPS system came in and dropped a number on them that knocked them out of their seat, and they figure all solutions are too pricey. More than $6 billion is spent in the United States annually on a variety of solutions. But there is plenty that can be done with relatively low cost to improve the process. The first steps are knowing what the cost of downtime is, how often these losses are incurred, and what solutions are out there.
The most common type of power quality phenomena is the voltage sag, where the voltage reduces below 90 percent of nominal for up to several minutes. There are a number of different types of devices on the market that can provide the missing energy for short durations. On the high end, there are a series of devices that are designed to keep entire facilities operational through multi-cycle sags and interruptions. This include devices such as large-scale uninterruptible power supplies (UPS >100kVA), subcycle static transfer switches (to switch between separate utility feeds to a facility), low-speed flywheels, static VAR compensators, ultracapacitors and super-conducting magnetic energy storage (SMES) products. Generally, most of these solutions require some engineering efforts to properly design in the equipment to match the electrical demand and infrastructure of the facility.
Some things to investigate before installing a UPS are the load ratings and characteristic, both in kVA and type of loads that they can power. Some UPS devices don’t like loads with large power factors or high harmonic current content. Most low-end UPS devices specify that the laser printers and motors not be connected to the outputs, as they can draw high inrush currents when cycling. The ride-through duration is typically 5 to 15 minutes, which should be long enough for most processes to go through an orderly shutdown sequence. However, harmonic loading may derate the output, as well as when operating in colder temperatures. The duration also reduces as the batteries age. PF cap switching transients may trick some systems into thinking that an overcurrent condition is occurring, and force an abrupt shutdown of the UPS. And there are some units on the market that don’t put out a sine wave.
One of the most common devices that people buy for protection against power quality phenomena are TVSS or transient voltage surge suppressors. They can be used at an individual piece of equipment or for an entire facility. The primary function of these devices is to clamp the voltage at a fixed level. Some also include filters to suppress some of the higher frequency noise. These come in either voltage “strip” form or as separate components to be mounted at a distribution panel for voltage and/or communication lines
Some things to be aware of when purchasing and installing TVSS systems are that they cannot provide any energy, hence, they will not provide any benefit for sags or interruptions or voltage notches. You need to select ones with a clamping voltage below the value where your equipment would be damaged, yet above any normal high line conditions, otherwise, they will be conducting and clipping the voltage on every cycle. A good ground conductor and connection is very important, as the devices usually need to divert the energy into a low impedance ground source. Connecting them on the end of a long extension cord or into a duplex outlet without a ground conductor provides very little protection. They also have a maximum energy absorption rating (in joules) and cannot withstand more than their ratings without damaging themselves, which makes them ineffective for future transients (though you may not observe anything physically wrong).
Once you install any mitigation devices, it is good to go back and re-monitor to verify that you are getting the results that you expect. Next month, we’ll discuss some other solutions, such as improving the wiring and harmonic filters. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.
Fixing a Hole
Identifying the problem is just the first step