Harmonics Limit Amount of PV on a Distribution Circuit

Harmonics is a very specialized and not widely understood topic in the electric power field which can become a major issue when inverter-based photovoltaic (“PV”) generators, (popularly referred to as solar power), are added to existing distribution circuits. This Blog provides a quick overview of the phenomena, potential negative impacts, causal conditions, and mitigating measures associated with harmonics. The bulk of the material presented here is based on an oral presentation at the SOLAR 2012 Conference of the World Renewable Energy Forum (WREF 2012) held last May 13-17, 2012, at the Colorado Convention Center in Denver.

The full paper can be found here.

Harmonics Overview

First off, harmonics are a phenomena of alternating currents (“AC”), the sinusoidally varying electric power that characterizes 99% of today’s commercial and public supply. The fundamental frequency of AC is typically 60 cycles per second (or Hertz, shorthand as Hz) in the US and many other countries, and 50 Hz in the the UK and other countries. The harmonics of a waveform are components whose frequencies are multiple integers of the fundamental 60 Hz or 50 Hz wave. For example, 120 Hz, 180 Hz, 240 Hz, and 300 Hz are the 2nd, 3rd, 4th and 5th harmonic components of a 60 Hz waveform. In this sense, 60 Hz is the fundamental, while the other components are higher harmonics.
The effect of higher harmonics is referred to as distortion since superimposing them on the fundamental distorts the shape of that waveform.

Image source: http://www.doctronics.co.uk/

Higher harmonics are usually caused by non-linear devices in electric power systems. Power electronic converters, which are widely used in modern power systems, are some of the major sources of harmonics. Solar photovoltaic generation depends extensively on power electronic converters to produce alternating current output for interconnection purposes. Therefore, the harmonic issue is an important aspect affecting the integration of PV generation.

Potential Negative Impacts

The presence of harmonic distortion can lead to a variety of power system issues including overheating of transformers, motors, lines, and cables (leading to failure or premature aging), interference with communication systems (generally for services on the same electric pole such as cable TV and phone) and with the operation of sensitive loads, and outages associated with blown fuses and failed equipment.
The amount of harmonic distortion is defined by industry and utility acceptance criteria. For industry practice, the applicable standards are: IEEE Standard 519-1992, which specifies the limits on the amount of harmonics allowed in the power system, and IEEE Standard 1547-2003, which focuses on the interconnection requirements of renewable resources, including inverter-based PV. It should be noted that both these standards pre-date the rise in number of PV installations in the last 2-3 years, hence, distribution operators may impose other criteria more specifically attune to PV penetration.

Causal Conditions

A typical distribution circuit will have some level of harmonic distortion at all times. Just imagine the number and variety of electronic devices that the typical household may have. A sample distribution system is shown below.
Sample Distribution Circuit
To the background harmonics, add the spectrum introduced by the PV generation. The profile of harmonic content can vary significantly among the many commercially available UL listed PV inverters.
The combination of harmonic injection from the circuit background and PV units can be of such magnitude as to stimulate resonant modes in the distribution circuit. Resonant modes occur when components of the capacitor (shown in the upper right portion of the figure above) are online and interact with the distribution circuit.
In the sample system discussed in the paper, the combination of capacitor operating modes and harmonic injection results in maximum penetration levels of as low as 800 kW.

Mitigation Options

To be able to increase the maximum PV penetration levels, certain mitigation options may be applied. Harmonic filters tuned for specific excitation frequencies are typically the most cost-effective solution. The filters may be applied in conjunction with the available power factor capacitors on the circuit, an approach termed as “repurposing” the existing capacitors. In some situations, the existing capacitors may not qualify to be converted to harmonic filters due to their voltage margin. Hence, stand-alone filters may be specified, typically comprising of a high-pass filter and a notch filter that are paralleled with the existing capacitors to reduce both current and voltage harmonic distortions. This solution results in a smaller harmonic filter that operates independently of the existing capacitors, and is relatively more cost-effective compared to repurposing the existing capacitors as harmonic filters.

Conclusions

The maximum PV penetration is defined by the acceptance criteria adopted by the interconnecting utility, in combination with operating modes for capacitor banks connected to the same circuit and harmonic distortion existing on the circuit and introduced by potential PV units. If the PV units have a harmonic injection with less than 3% ITHD (current total harmonic distortion), a penetration level larger than 100% of the peak load could be achieved. This assumes that other technical issues do not limit PV penetration.
If the harmonic distortion issue becomes the bottle neck for PV penetration, passive harmonic filters can be applied to increase the PV penetration capability. By adding a combination of a high-pass filter and a notch filter in parallel with the existing capacitor, both current and voltage harmonic distortions can be reduced. This solution generally offers a more cost effective means of using smaller harmonic filter design that operate independently of the existing capacitors compared to repurpose existing capacitors to harmonic filter.