GE EPM 6000 Multi-function Power Metering System Inductive and Capacitive Impedance

CHAPTER 2: ELECTRICAL BACKGROUND

frequency waveforms. These higher frequency waveforms are referred to as harmonics. The following figure shows the content of the harmonic frequencies that comprise one cycle of the distorted portion of the above waveform.

	250
	200
	150
(amps)	100
	50
Current	0	t
		a
	-50
	-100
	-150
	-200
	-250

FIGURE 2–12: Harmonics for Distorted Current Waveform

The waveforms above provide an indication of the impact of combining multiple harmonic frequencies together. The broken lines represent the 3rd, 5th, and 7th current harmonics. The solid line represents the sum of the three harmonics.

When harmonics are present, it is important to remember that they are operating at higher frequencies. As such, they do not always respond in the same manner as 60 Hz values.

2.5.2Inductive and Capacitive Impedance

Inductive and capacitive impedance are present in all power systems. We are accustomed to thinking about these impedances as they perform at 60 Hz. However, these impedances are subject to frequency variation.

XL = jω L and XC = 1 ⁄ jω C (EQ 2.3)

At 60 Hz, ω = 377; but at 300 Hz (5th harmonic) ω = 1885. As frequency changes, the impedance changes and system impedance characteristics that are normal at 60 Hz may be entirely different in the presence of higher order harmonic waves.

Traditionally, the most common harmonics have been the low order odd frequencies, such as the 3rd, 5th, 7th, and 9th. However newer, new-linear loads are introducing significant quantities of higher order harmonics.

2.5.3Voltage and Current Monitoring

Since much voltage monitoring and almost all current monitoring is performed using instrument transformers, the higher order harmonics are often not visible. Instrument transformers are designed to pass 60 Hz quantities with high accuracy. These devices, when designed for accuracy at low frequency, do not pass high frequencies with high

EPM 6000 MULTI-FUNCTION POWER METERING SYSTEM – USER GUIDE 2–15