Mitigating harmonics in electrical systems
Harmonic mitigating transformers
In a standard delta-wye transformer, zero-sequence currents flow through the secondary wye winding and are coupled into the primary delta winding where they are trapped (see Figure 2). These zero-sequence currents can cause excessive heating and voltage distortion. Harmonic mitigating transformers can be implemented in pairs to mitigate 5th, 7th, and higher-order harmonic currents by taking advantage of the transformer phase shifts relative to each other, to cancel a significant amount of the harmonic current at these higher frequencies.
One type of harmonic mitigating transformer uses a zig-zag configuration. The zig-zag transformer is configured by winding half of the secondary turns of one phase of the transformer on one leg of the 3-phase transformer, with the other half of the secondary turns on an adjacent phase (see Figure 3).
Note that harmonic mitigating transformers are not a panacea for the elimination of harmonics in an electrical system. Mitigation of 5th, 7th, and higher order harmonic currents requires the installation of multiple transformers with a 30-deg relative phase shift between the two, connected to a common bus in an electrical distribution system. Also, when mitigating these higher level harmonic currents by this means, balance of loads between the transformers is required. As shown in Figure 4, one transformer is a delta-zigzag configuration harmonic mitigating transformer with a 0-deg phase shift, and the second transformer is a delta-wye with a 30-deg phase shift.
Voltage distortion is normally greatest at the point where the equipment is connected to the distribution system. Therefore, to attain maximum benefit, harmonic mitigating transformers should be installed as close as practical to the load that they feed.
Installation of a non-phase-shift harmonic mitigating transformer provides an effective treatment of triplen (3rd, 9th, 15th, and so on) harmonic currents that are generated by loads connected to the transformer. Triplen harmonic currents are treated in the secondary windings of the transformer due to the transformer’s low zero-sequence impedance.
When a standard or K-rated delta-wye transformer is installed in an electrical distribution system, the addition of a non-phase-shift harmonic mitigating transformer offers an economical solution for treating higher order harmonic currents. The 30-deg phase-shift created between the standard or K-rated delta-wye transformer and harmonic mitigating transformer provides treatment of 5th, 7th, 17th, and 19th order harmonic currents to the extent of the balance of the load between the two transformers. In this configuration, the harmonic currents are canceled in the common electrical bus that feeds the transformers. Close coordination between the construction and location of the two transformers must be executed, as the impedance values of the transformers should be identical to receive the maximum mitigation of these higher-order harmonic currents.
Active harmonic filter (AHF)
The concept of an active filter is to produce harmonic components of the fundamental current waveform that are out of phase with—and thus cancel the harmonic components generated from—the nonlinear loads. Figure 5 conceptually illustrates how the harmonic current generated by the AHF is injected into the system to cancel harmonics from a VFD load. The AHF is installed as a parallel device and is scalable, making it a highly effective device that cancels multiple order harmonics in the distribution system. This method addresses harmonics from a systemic point of view and can save significant cost/space in many applications, with performance levels that can meet a TDD 5% target.
The active harmonic filter uses a current transducer to actively monitor the load current in real time to react to changes in load. Some AHFs are designed to also inherently synchronize the line current with the voltage to approach unity displacement power factor. The system typically performs fast Fourier transforms to calculate the amount of harmonics present for each harmonic order in the load current to determine the amplitude of the first 30 to 50 orders. The system logic processor filters out the fundamental frequency, and then directs the power converter to inject the phase-inverse of only the harmonic currents back into the circuit for cancellation of the harmonic content.
The benefits of AHFs include:
- Dynamic adjustment for virtual real-time correction of the nonlinear current
- Synchronization of the current and voltage waveforms
- Adjustment using a feedback loop to prevent leading power factor.
AHF equipment is available for implementation at the PCC of the facility to the utility, for connection to a distribution bus within 3-phase power distribution systems inside facilities, and within distribution and control equipment, such as motor control centers (see Figure 6).
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