Reducing harmonics with IEEE 519 practices, procedures
Methods for Harmonic Mitigation
A majority of large power (typically three-phase) electrical nonlinear equipment often requires mitigation equipment to attenuate the harmonic currents and associated voltage distortion to within necessary limits. Depending on the type of solution desired, the mitigation may be supplied as an integral part of nonlinear equipment (such as an ac line reactor or a line harmonic filter for ac pulse width modulation (PWM) drive) or as a discrete item of mitigation equipment (such as an active or passive filter connected to a switchboard). There are many ways to reduce harmonics, ranging from variable frequency drive designs to the addition of auxiliary equipment. A few of the most prevailing methods used today to reduce harmonics are explained below.
Delta-delta and delta-Wye transformers
This configuration uses two separate utility feed transformers with equal nonlinear loads. This shifts the phase relationship to various 6-pulse converters through cancellation techniques. A similar technique is also used in the 12-pulse front end of the drive, which is explained below.
An isolation transformer provides a good solution in many cases to mitigate harmonics generated by nonlinear loads. The advantage is the potential to "voltage match" by stepping up or stepping down the system voltage, and by providing a neutral ground reference for nuisance ground faults. This is the best solution when using ac or dc drives that use silicon controlled rectifiers (SCRs) as bridge rectifiers.
Use of reactors is a simple and cost-effective method to reduce the harmonics produced by nonlinear loads and is a better solution for harmonic reduction than an isolation transformer. Reactors or inductors are usually applied to individual loads such as variable speed drives and available in standard impedance ranges, such as 2%, 3%, 5%, and 7.5%.
When the current through a reactor changes, a voltage is induced across its terminals in the opposite direction of the applied voltage, which consequently opposes the rate of change of current. This induced voltage across the reactor terminals is represented by the equation below.
e = Induced voltage across the reactor terminals
L = Inductance of the reactor, in Henrys
di/dt = Rate of change of current through reactor in Ampere/second
This characteristic of a reactor is useful in limiting the harmonic currents produced by electrical variable speed drives and other nonlinear loads. In addition, the ac line reactor reduces the total harmonic voltage distortion (THDv) on its line side as compared to that at the terminals of the drive or other nonlinear load.
In electrical variable speed drives, the reactors are frequently used in addition to the other harmonic mitigation methods. On ac drives, reactors can be used either on the ac line side (called ac line reactors) or in the dc link circuit (called dc link or dc bus reactor) or both, depending on the type of drive design and/or necessary performance of the supply.
The ac line reactor is used more commonly in the drive than the dc bus reactor, and, in addition to reducing harmonic currents, it provides surge suppression for the drive input rectifier. The disadvantage of using reactors is a voltage drop at the terminals of the drive, approximately in proportion to the percentage reactance at the terminals of the drive.
In large drives, both ac line and dc bus reactors may be used, especially when the short circuit capacity of a dedicated supply is relatively low compared to the drive kVA or if the supply is susceptible to disturbances. Typical values of individual frequency and total harmonic distortion of the current waveform of a 6-pulse front end without and with integral line reactors are given in Figure 5.
Passive harmonic filters (or line harmonic filters)
Passive or line harmonic filters (LHF) are also known as harmonic trap filters and are used to eliminate or control more dominant lower order harmonics, specifically 5th, 7th, 11th, and 13th. They can be used either as stand alone parts integral to a large nonlinear load (such as a 6-pulse drive) or for a multiple small single-phase nonlinear load by connecting it to a switchboard. A LHF is composed of a passive L-C circuit (and also frequently resistor R for damping) that is tuned to a specific harmonic frequency that needs to be mitigated (for example, 5th, 7th, 11th, 13th, etc.). Its operation relies on the "resonance phenomenon" which occurs due to variations in frequency in inductors and capacitors.
The resonant frequency for a series resonant circuit, and (in theory) for a parallel resonant circuit, can be given as:
fr = Resonant frequency, Hz
L = Filter inductance, Henrys
C = Filter capacitance, Farads
The passive filters are usually connected in parallel with nonlinear load(s) as shown in Figure 4, and are "tuned" to offer very low impedance to the harmonic frequency to be mitigated. In practical application, above the 13th harmonic, their performance is poor; therefore, they are rarely applied on higher-order harmonics.
Passive filters are susceptible to changes in source and load impedances. They attract harmonics from other sources (such as from downstream of the PCC); therefore, this must be taken into account in their design. Harmonic and power system studies are usually undertaken to calculate their effectiveness and to explore the possibility of resonance in a power system due to their proposed use. Typical values of individual frequency and total harmonic distortion of the current waveform of a 6-pulse front end with integral LHF are given in Figure 5.
See next page for more ways to control harmonics, diagrams, and references.