Power quality analysis in medium voltage facilities
Power quality begins with electricity generation. In theory, the nice generated sine should travel the whole way to the consumer and remain the same on the way back. In practice, this doesn’t happen. Power quality needs to be monitored at every station in the network. Measurements help maintain high quality of service and makes locating the sources of problems possible. A number of functional issues on equipment can be diagnosed using power quality analysis.
Many MV locations have more than a single 3-phase input and 3-phase output. Power stations can support a number of lines, while switchyards may have a number of incoming and outgoing lines. It is desirable that each is monitored. This can be done with a number of separate instruments or with a single large data acquisition system.
Most power quality analysers are made for low voltage measurements with ability of up to 1000 VAC or 1500 V DC. For measurements in medium or high voltage environment the use of calibrated measuring transformers are needed to change the levels to tolerable for the instruments. Sometimes, appropriate transformers are already present at the location, possibly even installed for measurement purposes, and it is preferable to use them. However, their calibration status and accuracy have to be checked to suffice for the desired accuracy class of the measurement.
Voltage dividers are an option for locations where transformers are not present or possible. They have to be appropriately accurate and calibrated. Optical link to the measuring equipment for safe separation is strongly recommended.
For current channels the best and simplest option are high range clamps. Sometimes, the location may have built-in measuring shunts that can be used. Breaking the connections to measure current is not a sensible solution. To achieve best accuracy in a high-voltage environment, openings of the clamps have to be turned as far away from other MV wires. The measured wire has to be centred in the clamp, which can be achieves using an insulating support. Shielded clamps lower the electromagnetic disturbances. For best accuracy measurements of stray voltages in the clamps at different positions may be necessary.
Connecting the measurement system is only possible when location is disconnected from power for both voltage and current channels. Firm MV side of the connection is best established by bolted connections. Rules for work under voltage or in vicinity of voltage must be adhered to while connecting. Safety distance between connections have to be considered. Earth loops can be a serious danger and are best avoided by keeping all the ground connections at the same point.
Once the safety measures and transformers are in place, one still needs to plan the power quality analysis session. More information can be found in the Metrel General catalog.
In Metrel, we always recommend our top of the line MI 2893 for measurements in a demanding environments. However, if a facility need a number of units for logging more than sounding alarms in case of events on network, a simpler device like MI 2883 might be the better choice.