Power Factor, Capacitors, Harmonic Filters, and Resonance — Part 1

Feb 1, 2009 12:00 PM, By Tom Shaughnessy, PowerCET Corp.

How adverse effects of unintended resonance impact the use of capacitors and harmonic filters

According to the “IEEE Standard Dictionary of Electrical and Electronic Terms,” power factor (PF) is defined as “the ratio of the circuit power (watts) to the circuit volt-amperes.” This is described by the equation PF = W ÷ VA. Put another way, PF is the ratio of useful power to perform real work (commonly termed “active power”) to the power supplied by an electric utility (commonly termed “apparent power”). Usually, the goal is to get the PF of a facility to about 0.90 or higher.

Fig. 1. Plot of a 45° lagging sinusoidal waveform (i.e., green trace).

Low PF (below 0.90) occurs when the volt-ampere drawn by a load exceeds the wattage used by the load. From an electric utility perspective, low PF is primarily caused by inductive loads and overhead, pole-mounted electrical distribution systems. Improving PF by adding shunt capacitance to power distribution networks has been and remains a common electric utility practice traceable back to the earliest uses of alternating current. Large industrial and commercial firms have also used capacitors to add offsetting reactive currents to balance out reactive currents from inductive loads within the facility and thereby improve PF.

Fig. 2. Representation of a nonlinear load and source inductance (L).

While electric utilities use capacitors for voltage support and to maintain distribution capacity, the use of capacitor networks by end-users is usually driven by electricity rates or the need to free up distribution capacity. The practice of adding offsetting reactive loads (capacitive or inductive) falls into a general category that we can conveniently call displacement PF.

There is a time difference or phase angle between voltage and current waveforms, and cos U reflects the PF caused by the phase angle. For instance, a 45° displacement equates to a 0.707 displacement PF. Figure 1 shows PF correction of sinusoidal, displaced waveforms. The green trace is a 45° lagging waveform from an inductive load; the blue dashed line is a 90° offsetting waveform from shunt capacitance; and the red trace is the corrected in-phase waveform. The amplitude of the corrected waveform (red) is smaller than the original waveform (green) because the inductive components are balanced out. You can calculate the offsetting current by using the following equation:

I_correction = sin U × I_displacement

Fig. 3. Plot of service bus voltage with no nonlinear loads operating.

In this case, sin U = 0.707, so the offsetting current is 7.07 (10 × 0.707).

Armed with all of this PF knowledge, nothing can go wrong, right? Wrong! All sorts of mischief can arise, leaving you confused and searching for answers. At best, the problems can be embarrassing; at worst, they can be catastrophic. At the root of these problems are the unforeseen effects of resonance, and both simple capacitor banks and complex harmonic filters can be adversely affected. Source inductance (normally transformer leakage inductance) can interact with harmonic currents and cause unwanted voltage distortion. Worse yet, source inductance and shunt capacitance can interact, forming resonant tank circuits (Fig. 2).

The short form equation for resonance is as follows:

h_r = 1 ÷ (2ϖ√LC) (Equation 1)

Fig. 4. Plot of service bus voltage with nonlinear loads operating.

This equation clearly shows that the two major players are inductance (L) and capacitance (C). If the resonant circuits occur at integer multiples of the fundamental frequency, then all sorts of mayhem can occur, such as fuses blowing, breakers tripping, capacitors blowing up, and loads tripping off-line.

Let's see how the effects of resonance can affect a single stage capacitor bank.

Resonance case study

Our case history involves a facility where voltage distortion increased markedly, following nonlinear load concentrations, at various times throughout the day. Figure 3 shows service bus voltage without nonlinear loads operating while Fig. 4 shows service bus voltage with nonlinear loads operating. As you can see, voltage distortion increased from below 2%_THD (without nonlinear loads operating) to 6.9%_THD (with nonlinear loads operating). Dominant frequencies and distortion levels (%_fundamental) for Fig. 4 were the 11^th order harmonic at 6.1% and the 13^th order harmonic at 2.3%.

Fig. 5. Plot of capacitor bank currents with and without nonlinear loads operating.

When the voltage distortion rose sufficiently inside the facility, variable-speed drives would turn off and disrupt operations. Also, fuses would blow causing total disruption, and capacitors occasionally failed catastrophically.

Fig. 6. Plot of capacitor bank currents squared.

Looking at the current flowing through the capacitor bank, the effects of the harmonic currents are even more evident. Figure 5 shows current flowing through the bank with and without nonlinear loads operating. With nonlinear loads operating, the 13^th harmonic currents were dominant (44.6% of the 269A fundamental or 120A). The harmonic currents added to the fundamental root-mean-squared (rms) level of 60 Hz current. Without harmonic loads operating, the rms current was 236A; with harmonic loads operating, the rms current increased to 311A. At some point, fuses gave way but this was only part of the problem. Figure 6 shows the squared values of the currents shown in Fig. 5.

Remember, I²R losses involve resistance and squared current. The highest squared value with harmonic currents present is 2.48 times larger than the highest squared value without harmonic currents. This amounts to substantial instantaneous heating differences, resulting in blown fuses and capacitors — not exactly what was intended when the capacitors were installed.

Harmonic and reactive currents

Fig. 7. Representation of a harmonic filter applied to a VFD.

With the advent of harmonic currents from nonlinear loads, users have learned that low PF is also caused by harmonic currents. However, they may not have totally embraced the negative consequences caused by mistaking the content. The equation for true PF (PF = W 4 VA) is not limited solely to displacement effects. It also addresses both the effects of harmonic currents and displacement at the fundamentals. However, users must recognize that measured values for PF contain both fundamental displacement and harmonic components, along with harmonic content. Ignore the harmonic content and problems will occur.

The following IEEE standards provide recommendation guides you can use to either offset reactive currents or to address harmonic currents:

IEEE Std. 18-2002, “IEEE Standard for Shunt Power Capacitors”
IEEE Std. 1036-1992, “IEEE Guide for Application of Shunt Power Capacitors”
IEEE Std. 519-1992, “IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems.”

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