Common power quality culprits and how to minimise downtime
GANESH GANESHKUMAR, product manager industrial group at Fluke Australia, shares which common culprits of power quality issues you should be mindful of and which steps you should take to ensure facilities run efficiently and downtime is minimised.
Facilities have enjoyed enormous benefits from the evolution of solid-state technology; however, the microelectronics at the heart of that technology require clean power. Faster speeds and lower voltages mean that there is less tolerance for poor quality power. Power quality (PQ) covers a wide range of issues, from voltage disturbances – such as sags, swells, outages and transients – to current harmonics, performance, wiring and grounding.
The symptoms of poor PQ include intermittent lock-ups and resets, corrupted data, premature equipment failure and overheating of components for no apparent reason. The ultimate cost is downtime, decreased productivity and frustrated personnel. Following these steps and being mindful of the common culprits of PQ issues will ensure facilities run efficiently and downtime is minimised.
POWER QUALITY CULPRITS – FROM UTILITY TO RECEPTACLE
- Lightning can be extremely destructive if proper surge protection is not installed. It also causes sags and undervoltages on the utility line if far away. If close by, it causes swells and overvoltages.
- Utility automatic breaker reclosure causes short duration sags/outages.
- Utility capacitor switching causes a high-energy voltage disturbance, which looks like an oscillating transient riding on the wave. If the cap bank is near the facility, this transient can propagate all the way through the building.
- Generator sets not sized for harmonic loads and excessive voltage distortion affects electronic control circuits. If silicon-controlled rectifier (SCR) converter loads are present, notching can affect frequency control circuits.
- Applying power factor (PF) correction capacitors without considering the effects of harmonics causes issues. Harmonics and caps don’t mix.
- Inrush currents from high torque motor loads started across the line causes voltage sags if the load is too large or the source impedance too great. Staggered motor starts can help.
- Undersized neutrals at panel board are a problem. In the era of the third harmonic, neutrals can easily carry as much current or more current than the phase conductor. Keeping them undersized leads to overheated lugs, potential fire hazards and high N-G voltage.
- Running power and signal cables together causes issues. Think of the signal cable as a single-wire transformer secondary and the power cable as the primary. The opportunities for coupling are endless.
- Loose conduit connections and lack of green wire grounding conductor cause an open or high impedance ground circuit.
- Shared neutrals on branch circuits cause load interaction and overloaded neutrals.
- Laser printers and copiers sharing branch circuits with sensitive loads result in periodic voltage sags and switching transients.
- Miswired receptacles (N-G swapped) are guaranteed to put return currents on the ground conductor and create a noisy ground.
- Data cables connected to different ground references at each end shows up as voltage between equipment case and the data cable connector.
- Concerning hi-frequency noise, the most effective high frequency grounding technique is the installation of a signal reference grid (SRG).
- Isolated ground rods are a safety hazard because the earth is a high impedance path and they will prevent enough current from flowing to trip the breaker.
- Illegal N-G bonds are guaranteed to put return currents on ground. Not only is it a PQ problem, it’s a plumbing problem. Circulating ground currents cause corrosion of water pipes.
TROUBLESHOOTING PQ PROBLEMS: START AT THE SCENE OF THE CRIME
To troubleshoot PQ problems, one approach is to start as close to the ‘victim load’ as possible. The ‘victim load’ is the sensitive load, typically electronic, that is somehow malfunctioning. If poor PQ is suspected, part of a technician’s job is to isolate PQ as a cause. This bottom-up approach relies on having a sharp eye and taking some basic measurements.
An alternative is to start at the service entrance (connection to the utility) using a three-phase monitor and work back to the ‘victim load’. This is most useful if the problems originate in the facility. As a general rule, PQ is best at the service entrance and deteriorates as technicians move downstream through the distribution system. This is because the facility’s own loads are causing the problems. For this reason, a logical troubleshooting flow is usually the best way to diagnose the electrical infrastructure of the building.
1. Make a map: obtain or create a current one-line diagram
It’s tough to diagnose PQ problems without having a working knowledge of the site. Start by locating or reconstructing a one-line diagram of the site. The one line will identify the AC power sources and the loads they serve. The As Built one line, which is the one with red lines, is what technicians are looking for.
If a technician works on site, the map may already exist in their head, but it is always better on paper. If coming to a worksite for the first time, getting an up-to-date one line means identifying new loads or other recent changes in the system. Systems are dynamic; they change over time, often in unplanned and haphazard ways.
Furthermore, while some problems are local in origin and effect, there are many problems that result from interactions between one part of the system and another part. A technician’s job is to understand these system interactions. The more complete the documentation, the better. The sites that need the most help are the ones least likely to have good records. At this point in the investigation it is essential to get the best documentation possible.
2. Walk around the site
A visual inspection can offer immediate clues, including:
- a transformer that is too hot
- wiring or connections discoloured from heat
- receptacles with extension strips daisy-chained to extension strips
- signal wiring running in the same trays as power cables
- extra neutral-ground bonds in sub-panels, and
- grounding conductors connected to pipes that end in mid-air.
At a minimum, a technician will get a sense of how the facility is wired and what the typical loads are.
3. Interview affected personnel and keep an incident log
Interview the people operating the affected equipment to get a description of the problem. It’s also good practice to keep a record of when problems happen and what the symptoms are. This is most important for problems that are intermittent.
The goal is to find a pattern that helps correlate the occurrence of the problem in the ‘victim load’ to a simultaneous event elsewhere. Logically, this trouble logging is the responsibility of the operator closest to the affected equipment.