Current Transformer (CT) and Potential Transformer (PT) Sensors - How to Measure Electrical Power Usage?

Question

I am not from Electrical Engineering background and so, does not have much idea about how energy power meters work.

I want to measure electrical power usage of a three-phase induction motor. So, I need three CT sensors and three PT sensors to collect the current and voltage readings from the power transmission line from which the induction motor is taking power. And then, using that current and voltage readings, I have to measure how much power the Induction Motor is using? Is that correct? Actually, I am confused because, when I asked the manufacturer to provide me all the instruments needed to measure electricity usage, they gave me only three CT and one PT sensors. Can any body please explain to me how the CT & PT sensors are used to measure electricity usage data (may be with a simple diagram). I just want to understand how current and voltage data are collected. The electrical power usage calculation I can do by myself. Thanks in advance.

Answer 1

You need to measure the instantaneous voltage and current at lots of little time slices. Each (voltage x current) is the power over that time slice. To get the average power, you average all the values from the short time slices over a larger time range.

Usually you want to be able to correctly handle up to about the 100th harmonic. For 60 Hz power, that means up to 6 kHz, which means at least 12 kHz sample rate. Fortunately, significantly faster is still easy to do and compute in even a modest microcontroller.

I'd probably sample at 100 kHz because that should still be easily doable. That means you get 10 µs of computation per sample, which should be more than enough. For example, that's 700 instruction cycles on a dsPIC33EP series.

The reason they suggest only a single voltage measurement is probably because the voltage isn't going to change much regardless of what the motor is doing. You can measure one of the phases and just assume that applies to the other phases, with the appropriate phase shifts of course.

Answer 2

Figure 1. A three-phase power meter showing instantaneous voltage and current readings on the left and a phasor diagram on the right. Source: London Electronics.

Given your lack of experience I would recommend that you purchase an industrial grade meter. These are available in fairly simple LED 7-segment display type up to full colour models as shown in Figure 2. The meter will give connection diagrams for the various CT and voltage probe wiring configurations. The meters typically give readings of

• Phase current and 3-phase average.
• Phase to phase voltage and 3-phase average voltage. (This requires three voltage probes.)
• Phase to neutral voltage.
• Instantaneous energy (kW).
• Instantaneous reactive power (kVA).
• Consumed kWh and kVArh.

A point worth considering is that the meter has to be installed near the measuring point due to limits on the length of the CT wiring. If there is a possibility of connecting it to the building Ethernet you would have the option of remote monitoring and logging.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 2. A likely wiring scheme for a 3-phase meter with one voltage connection.

Answer 3

The usual method of measuring power to a three-phase load is to use two wattmeters. That would require two current transformers and two potential transformers. The measurement is done as described in the answer of Andy aka to wattmeter average power. His answer to What's the most economical way to digitally measure 240V mains voltage, current and power factor? describes digital measurement. Note that three-phase motors are three-wire loads. There is never a neutral connection and the three phases and normally very well balanced. However the two-wattmeter method is also valid for four-wire, and unbalanced loads. A lot of additional information can be found by searching "two wattmeter method."