When it comes to power quality events, it always helps to know what you’re looking for—before you begin your search.
You’ve heard the term a million times, but what exactly is a transient in the electrical world? We typically define this power quality pest as a change in the steady-state condition of voltage, current, or both. What triggers a transient? Lightning strikes and faults are common causes of transients; however, they can also occur when someone opens or closes a switch. To complicate matters, transients can readily transfer from one conductor to another by means of electrostatic or electromagnetic coupling. Now that you know what a transient is, where do we go from here?
The key to understanding power quality events is knowing the duration and magnitude of the event. For example, let’s look at a common event known as the power surge. A surge is a transient condition consisting of a series of peak sine wave voltages that exceed the set standard voltages for a substantial period of time. This can be as short as ½ of a cycle or as long as several hundred cycles. The key here is time: This event could last from 8 ms up to a couple of seconds.
An impulse is also a transient condition, but the duration is of a completely different level. Most impulses last less then a millisecond and in many cases are repetitive or oscillatory in nature (see figure, above). A common misconception among many of us is impulses are always positive in nature or add to the voltage (often referred to as a spike). However, impulses can also be negative in nature and cause notches, which is a subtractive impulse.
What about the magnitude? Surges are high-energy transients typically associated with current induced by lightning strikes. Voltage transients are lower energy events and typically result from switching loads. You can find the most common source of these spikes in the building itself.
Now that we know what we’re looking for, where can you find these internal voltage spikes? You can start with your motor control center. Did you know the coil on a motor starter can generate a 2000V to 3000V spike? The coil in the starter uses current to generate a strong magnetic field, which pulls in the contactor. Of course, the larger the contactor, the bigger the coil. This is fine until you have to de-energize the coil to open the contactor. The magnetic field collapses across the same coil windings. Since this process takes less time than it does to produce the magnetic field, you get a larger kickback spike. This counter electromotive force (CEMF) can be 10 to 20 times greater than the voltage supplied to the coil. Repetitive cycling of the starter, as in fast starts-stops, aggravates the problem.
Sometimes we find out we’re the source of the problem. If you’re not careful, the type of meter you use in electrical troubleshooting can cause problems. The Wiggy (a solenoid voltage tester) can actually create a voltage spike in the circuit when you remove it after testing. This situation is similar to what happens with the coil in the starter. This voltage can travel back to the source and destroy solid-state control components, such as SCRs and TRIACs (commonly found in PLC-based control systems).
This problem isn’t limited to industrial applications. I recall troubleshooting an office lighting system, where the lights would turn off by themselves. A fast spike (originating from a laser copier) turned off the solid-state controller for the lights. The best way to address voltage spikes of this nature is get right to the source of the problem. You should install a clamping diode or metal-oxide varistor (MOV) at the load. In the case of the motor starter, the MOV connects in parallel with the connections to the coil. When you remove voltage from the coil and generate the CEMF, the MOV clamps the excessive voltage before any harm is done. In fact, many manufactures sell an add-on MOV, which mounts on the starter itself.
Turkel is a senior instructor, ATMS Technical Training Co., Owings Mills, Md.
Sidebar: The Case of the Bad UPS
A company experienced some power quality problems, so they installed a central UPS. Several months later, the UPS blew and the load took a hit. The company then accused the UPS manufacturer of making a bad product. But, was it really bad? Investigation revealed why the manufacturer was not at fault. The building had no lightning protection, no TVSS ahead of the UPS, and a grounding system grossly out of compliance with the NEC Art. 250.
The lesson here is you need a complete solution to protect your system from surges. Divide a facility up into zones—moving from the outside to the point of use. Lightning protection and bonding of all metal objects on (or adjacent to) the building (per NFPA 780) addresses the outer zone. Then, you need TVSS at the service entrance, and additional protection (e.g., MOVs) at the point of use. Bear in mind that most surge protection equipment works to divert the surge to ground. And ground doesn’t necessarily mean earth. A solid review of your facility against Art. 250 (and NFPA 780) is paramount for surge protection. Only when your facility complies with these documents can surge protection devices do the job they’re meant to do.
By Mark Lamendola, Technical Editor