Monitoring Electricity Produced with current transducers

Frequently Asked Question: Can I monitor the renewable electricity I produce, using a Current Transducer?

We often have customers tell us that they’d like to monitor the electricity produced using CTs (current transducers). The current transducers have gained a lot of popularity recently, because they are a very useful way of measuring alternating energy currents. They are useful in many situations, for instance if the current is too high to directly apply the measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit. The current transducers are advertised to have excellent accuracy; they are usually claimed to have the accuracy of around or below 1%, which is not incorrect, or a usual overestimation of the quality of the product. What is often forgotten to mention is that even if you get the most accurate current measurements, it is only half of the “equation”.

The value you need to have as a result of your measurement is the electric power in order to calculate energy, because it is the electric power that does the work for you. And we all remember, back from our school days, that electric power equals to the current being multiplied by the voltage (watts = amps x volts, as some of you might remember it).

Voltage on any electric grid fluctuates, and it is not unusual for it to fluctuate to +/- 20%. The National grid operators often require less than 10% fluctuation for exporting to the grid but when importing and depending on location, things can get a lot worse. If you were to measure only the electrical current, and determine the electric power based on the assumption that the voltage in your system is constant, you could be introducing a large error in your measurements.

Let’s illustrate this with an example:

The nominal voltage of your grid is 240V. Given the fluctuation of the voltage of 20% in both directions, you get that your minimal voltage could be 192V, and your maximal voltage could be 288V (notice that the span is actually 40%). Let’s assume that your current is 40 A, which is the only number you’d get from a current transducer. If the voltage is not measured at the same time as the current, you’d conclude that the power is 240V (nominal voltage) multiplied by 40A (current), which is 9,600 W. But, the real life situation is, that because of the voltage fluctuations that appear in the system, your power could be as low as 7,680W, or as high as 11,520W!

Electrical and approved billing meters (often called kWh or Energy meters) tap into both voltage and current at the same time, because, as we saw in the example given, your power calculation can be up to 40% wrong, depending on the voltage of your grid.

Additionally, kWh meters consider other factors that might influence the power in your grid, such as real power of your circuit, ie. the capacity of the circuit for performing work at any particular time, and power factor of your system (ratio of the real and apparent power).

In conclusion

So, a message that we’d like to get across is that, while current transducers are a very valuable tool for measuring currents, they are not designed nor should be used as a tool to measure electric energy. Electric power (or energy) should be measured using electrical (kWh) meters, which measure the voltage and the current, and thus give you the real value of instant power and real cost of energy over time.

A current transducer only [CT] will always give you an approximation, an approximation that depending on your energy needs could be quite expensive.

Electric power (or energy) should be measured using electrical (kWh) meters, which measure the voltage and the current, and thus give you the real value of instant power and real cost of energy over time.

At Logic energy we supply CTs, and recommend their use, for the right reasons, to make an accurate measurement of power you must use a KWh meter, preferably with the Pulse output connected directly to the LeNET mobile, or if the KWh meter does not have pulse output terminals, then we you can use the KWh meter Irda interface cable to connect to the LeNET mobile.

KWh meters come in two main types, Single phase which ours is rated up to 45A and the Three phase which ours is rated up to 100A. Higher rating are possible, in the region of several 1,000 Amps if needed, contact us for more details.