Nicholas Rich, PE, LEED AP, Interface Engineering, Seattle
Although devices using power electronics can produce distortion in electrical distribution systems, it’s up to the engineer to apply effective solutions to mitigate them.
- Understand current and voltage harmonics in electrical systems, and their negative effects on the facility electrical system.
- Know how electronic power equipment such as VFDs creates harmonics.
- Understand characteristic and noncharacteristic harmonics.
- Understand IEEE 519 guidelines for the reduction of electrical harmonics.
- Learn design techniques for mitigating harmonics with recommended applications.
Harmonics and detrimental effects
In North America, alternating current (ac) electrical power is generated and distributed in the form of a sinusoidal voltage waveform with a fundamental frequency of 60 cycles/sec, or 60 Hz. In the context of electrical power distribution, harmonics are voltage and current waveforms superimposed on the fundamental, with frequencies that are multiples of the fundamental. These higher frequencies distort the intended ideal sinusoid into a periodic, but very different shaped waveform.
Many modern power electronic devices have harmonic correction integrated into the equipment, such as 12- and 18-pulse VFDs and active front-end VFDs. However, many nonlinear electronic loads, such as 6-pulse VFDs, are still in operation. These nonlinear loads generate significant magnitudes of fifth-order and seventh-order harmonics in the input current, resulting in a distorted current waveform (see Figure 1).
The characteristics of the harmonic currents produced by a rectifier depend on the number of pulses, and are determined by the following equation:
h = kp ±1
- h is the harmonic number, an integral multiple of the fundamental
- k is any positive integer
- p is the pulse number of the rectifier
Figure 1: Transformers are available in a variety of sizes and distribution voltages, and can be installed indoors or outdoors. All images courtesy: TLC Engineering for ArchitectureThus, the waveform of a typical 6-pulse VFD rectifier includes harmonics of the 5th, 7th, 11th, 13th, etc., orders, with amplitude decreasing in inverse proportion to the order number, as a rule of thumb. In a 3-phase circuit, harmonics divisible by 3 are canceled in each phase. And because the conversion equipment’s current pulses are symmetrical in each half wave, the even order harmonics are canceled. While of concern, harmonic currents drawn by nonlinear loads result in true systemic problems when the voltage drop they cause over electrical sources and conductors results in harmonics in the voltage delivered to potentially all of the building electrical system loads—even those not related to the nonlinear loads. These resulting harmonics in the building voltage can have several detrimental effects on connected electrical equipment, such as conductors, transformers, motors, and other VFDs.
Conductors: Conductors can overheat and experience energy losses due to the skin effect, where higher frequency currents are forced to travel through a smaller cross-sectional area of the conductor, bunched toward the surface of the conductor.
Transformers: Transformers can experience increased eddy current and hysteresis losses due to higher frequency currents circulating in the transformer core.
Motors: Motors can experience higher iron and eddy current losses. Mechanical oscillations induced by current harmonics into the motor shaft can cause premature failure and increased audible noise during operation.
Other VFDs and electronic power supplies: Distortion to the increasing voltage waveform in other VFDs and electronic (switch mode) power supplies can cause failure of commutation circuits in dc drives and ac drives with silicon controlled rectifiers (SCRs).
Establishing mitigation criteria
The critical question is: When do harmonics in electrical systems become a significant enough problem that they must be mitigated? Operational problems from electrical harmonics tend to manifest themselves when two conditions are met:
- Generally, facilities with the fraction of nonlinear loads to total electrical capacity that exceeds 15%.
- A finite power source at the service or within the facility power distribution system with relatively high source impedance, resulting in greater voltage distortion resulting from the harmonic current flow.
IEEE 519-1992, Recommended Practices and Requirements for Harmonic Control in Power Systems, was written in part by the IEEE Power Engineering Society to help define the limits on what harmonics will appear in the voltage the utility supplies to its customers, and the limits on current harmonics that facility loads inject into the utility. Following this standard for power systems of 69 kV and below, the harmonic voltage distortion at the facility’s electrical service connection point, or point of common coupling (PCC), is limited to 5.0% total harmonic distortion with each individual harmonic limited to 3%.
In this standard, the highest constraint is for facilities with the ratio of maximum short-circuit current (ISC) to maximum demand load current (IL) of less than 20, with the following limits placed on the individual harmonic order: (Ref. Table 10.3, IEEE Std. 519)
- For odd harmonics below the 11th order: 4.0%
- For odd harmonics of the 11th to the 17th order: 2.0%
- For odd harmonics of the 17th to the 23rd order: 1.5%
- For odd harmonics of the 23rd to the 35th order: 0.6%
- For odd harmonics of higher order: 0.3%
- For even harmonics, the limit is 25% of the next higher odd harmonic.
- The total demand distortion (TDD) is 5.0%.
There are various harmonic mitigation methods available to address harmonics in the distribution system. They are all valid solutions depending on circumstances, each with their own benefits and detriments. The primary solutions are harmonic mitigating transformers; active harmonic filters; and line reactors, dc bus chokes, and passive filters.
Learn more about Harmonics in this month's PQU Harmonics seminars: http://www.p3-inc.com/power-quality-university/seminar-info/harmonics-seminar
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