Measuring Current Distortion
Measuring current distortion in any distribution system is much easier than many measuring equipment suppliers would want users to know. There is much talk about harmonics, negative sequence currents and other confusing terms. Although the measuring equipment supplier’s intentions are to assist the user in preventing and solving electrical problems, their continued use of terms which do little to aid the user of electrical equipment from truly understanding problem that may arise.
In the figure to the right, there are a number of different points in any distribution system where current distortion might exist. Each point identifies a point of common coupling or point which supplies power to electrical equipment connected to the system. As the demand for power moves closer to the utility source, the percent distortion will decrease. Since the recommended level of distortion is established by the capacity at each common point in the system, the effect on the system by the electrical system farthest from the common point will decrease. Essentially, as the demand for power moves away from the utility supply, the demand must be decreased. It is not normal practice to locate the greatest load on a system the farthest distance from the supply. If that is required, than the power feed to the load is designed with the lowest possible impedance.
Current distortion is determined by comparing an ideal sinewave of current against a non ideal current waveform. A simple explanation is that the rms value of current is compared against the peak value of current during one cycle. Two waveforms, having the same rms value, can have different values of current distortion if the amplitude of the peak current for each waveform is different. Electrical systems are generally designed to handle an ideal current waveform. The peak value, for an ideal current waveform is 1.414 times the rms value. A larger peak value of current for the same rms value can place a momentary stress on the electrical system that is different than what would be expected with an ideal sinewave. The question is how much extra stress can be expected.
If the extra stress is much greater than that caused by an ideal current sinewave, the user would generally increase the capability of the system. Larger transformers, larger wire, and larger fuses. If the stress is slightly greater than the normal level of stress, no extra capacity may be necessary unless the extra stress is continuous. Most electrical systems operate under a duty cycle demand. An electrical system rated to handle 100 amps continuously would have the capability of handling a greater demand as long as the value over time did not exceed the continuous rating. If the load demand during a 24 hour period varies from 90 amps to 110 amps so that the rms rating is no more than 100 amps, the user does not have to increase the size of supporting electrical equipment like transformers and wiring. In most installations, it is general practice to use less than 75 percent of the systems capacity on a continuous basis. Short term (less than one hour) may reach the full capacity of the system, however, the rms usage of most systems is generally less than 75 percent.
The operating or loading condition described above provide some insight for determining how to measure current distortion and how to determine safe operating values. Traditionally, electrical equipment was selected with more capability than would be used. No one would select a 75 kW transformer to supply voltage to a 100 HP motor. Typically, the 1.5 rule was used regarding transformer selection for motors. To operate a 100 HP motor, a 150kW transformer or supply was the minimum value selected. Traditionally. to start the motor across the line required the extra short term capacity to supply the locked rotor or 6 to 8 times full load amps. With the introduction of the energy efficient motor, a supply capable of 8 to 10 times full load amps became necessary. Line starting energy efficient motors on existing electrical systems has created some nuisance problems. Often the supply size had to be increased or staggering the starting of motor became necessary.
Current distortion like current will cause voltage drops as it flows through any impedance that exist in any circuit. Current distortion is determined in terms of percent distortion not amps. The distortion value will not provide the user with an indication of how much extra heating and where that heating will occur. It is the peak and rms values that will determine how much heating can be expected. If fewer rms amps are demanded, then less heating will result. If momentary peak values of current exist which do not significantly add to the rms value, then less heating will occur. That is the case when PWM, AFDs replace motor starters when operating a 3 phase ac motor. Less rms current is demanded from the supply. Although the waveform is no longer sinusoidal, the overall heating on the electrical distribution system is less.