Troubleshooting the voltage transients
Today’s factory floors usually include a proliferation of sensitive electronics, such as programmable logic controllers (PLCs), variable-frequency drives (VFDs), and network communications devices, all of which have led to tremendous gains in productivity. Ironically, however, many of these same electronic devices also contribute to productivity losses because of their sensitivity to voltage sags, swells, and transients. To further complicate matters, some of the electronic equipment is also a contributor to power quality problems — as in the case of VFDs. What does this mean to the average industrial facility? It means you need to monitor, troubleshoot, and preempt power quality problems now more than ever before.
Then and now. In the past, difficulties in pinpointing and troubleshooting electrical problems often led to disrupted or failed industrial equipment. This was especially true with transient voltage spikes, due to their short duration and often unpredictable pattern of occurrence. Identifying these transients involved the use of expensive, specialized, portable monitoring equipment. This meant renting or purchasing equipment or even bringing in a consultant specializing in power quality troubleshooting and analysis techniques.
Robotic arms and electronic equipment increase productivity, but they’re sensitive to PQ events.
Then, with the portable monitor in hand, personnel were faced with the difficult task of deciding where to look for the transient. Installing the portable device was equally difficult. Engineers were sometimes required to shut down a system, and they frequently had to relearn how to set up the device. Finally, they needed to find a safe location to place the unit while it waited for another occurrence. Unfortunately, all of this cost and complexity didn’t always lead to a satisfactory resolution of the problem. Today, specialized portable power quality monitors are no longer the only option for tracking down the source of voltage transients.
Some high-end, permanently installed circuit monitors can also do the job while serving as both a highly accurate digital meter for measuring and logging electrical quantities, such as current, voltage, power factor, and energy, and a monitor for watching for voltage transients and other power quality events, alarming on occurrence, and capturing waveforms and other useful data. Although circuit monitors have been able to detect and record steady-state power quality disturbances for some time, the ability to capture sub-cycle transients today requires much more sophistication when it comes to a piece of monitoring equipment. Only recently has the technology become available to design compact, affordable circuit monitors that perform high-speed transient detection without compromising accuracy, metering performance, data logging, or other functions. Detection requirements. If you need a circuit monitor capable of detecting and capturing voltage transients, you should look for a device that provides at least the following features.
Fast sample rate: The required sample rate depends on the frequency of the transients you monitor. Since most low-and medium-frequency oscillatory transients occur in the kilohertz frequency range, circuit monitors that sample voltage input channels at 256 or 512 samples-per-cycle can occasionally detect and capture oscillatory transients. On the other hand, impulsive transients are often shorter in duration and require much higher sample rates. ANSI and IEC standards require that electronic equipment designed for industrial environments, such as PLCs, VFDs, and circuit monitors, undergo a series of lab tests. These tests ensure the equipment can withstand high-voltage transients, such as a 1.2×50-µsec 2,000V impulsive transient. Technicians select test criteria that represent potential real-world scenarios.
A 1.2×50-µsec 2,000V impulsive transient rises from zero to its peak value of 2,000V in 1.2 µsec, then decays to half its peak value in 50 µsec. To reliably catch the peak of a 1.2-µsec rise-time transient, a monitoring device must sample data at a rate of at least twice per microsecond, which equates to a sample rate of 2 MHz per voltage channel or 33,333 samples-per-cycle. A monitoring device that samples data at the rate of 512 samples-per-cycle can sample data only once per 32.5 µsec. This is adequate for general disturbance monitoring and some oscillatory transients, but it’s far too slow to characterize a 1.2×50-µsec impulsive transient. Circuit monitors with the fastest transient detection capabilities sample voltage data at 5 MHz per voltage channel or 83,333 samples-per-cycle, which is fast enough to detect even very short impulsive transients.
