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Chapter 3

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Chapter 3

Power System Harmonic Mitigation

- Power system harmonic issues have existed since the early 1900’s. The earliest discovered issues were third harmonic currents produced by saturated iron in machines and transformers.
- Today, there are many harmonic producing loads, making it increasingly necessary to address their influence on power systems.

- Two concepts must be taken into consideration for harmonics studies:
- 1. the nature of non-linear loads.
- 2. the interaction of harmonic currents and voltages within the power system.
- Here we will take a study case for corporate center in Las Vegas that uses over 1000 computers. These are powered from four 150-kVA delta-wye zig-zag transformers. SO large amount of third harmonic current flows on the neutral and phase conductors. The result of this current is heating of the conductors as well as the transformer.

- Using blockade device to blockthe flow of the third harmonic current in the neutral conductorand measure the energy consumption at the primary of one of the above transformers over repeated 15 minute intervals, first with and then without the blockade in the circuit.

Figure 1: Single Line of Building Distribution System

- The instrument used is a Yokogawa WT2030, which is a lab-grade power analyzer connected on the primary side.
- The Power sight 3000 harmonic analyzer device was used to measure current and voltage on the secondary.
- A Fluke 87 true RMS digital multimeter was used to measure neutral to ground voltage at the furthest
- outlet from the transformer.
- A Voltech PM100 single phase power analyzer to measure Blockade device energy consumption.

- Blockade device reduces the Neutral-to-Ground Voltage 66.30%which is clear by the following table

- For all these cases, the average energy savings is 2.13%.
- It should be noted that these savings are dependent on loading and circuit configuration.

- The biggest effect will occur on the secondary of the transformer, in both the line and the neutral current.
- The plots below show the current waveform and spectrum for the transformer secondary with and without the blockade.
- The current (especially the third harmonic) is reduced by 10.2%.

- Reducing harmonic problems by limiting the nonlinear load to 30% of the supply transformer rating.
- However, with power factor correction capacitors installed, resonant conditions could potentially limit the percentage of non-linear loads.
- The following equation to determine if aresonant condition is likely to occur at an undesirable frequency:

- If hr is close to a characteristic harmonic, such as the 5th or 7th, there is a possibility that a harmful resonant condition could occur.
- Using a Multi-pulse converter -a 12-pulse configuration - uses twelve diodes instead of six to eliminate the 5th and 7th harmonics.
- The disadvantages are cost and complex construction and also affects the overall drive system efficiency rating because of the voltage
- drop associated with the transformer requirement .

Typical Twelve-Pulse AC Drive Components

- Pseudo 12-pulse systems: This configuration uses phase-shifting transformers to cancel 5th and 7th harmonics at the common bus. For cancellation to occur, the non-linear loads must be operated simultaneously and have similar characteristics. One cost-effective
- implementation of this concept uses delta-wye isolation transformers -on a few large VFDs, while using smaller and less costly line reactors on smaller VFDs.
- Filters can be designed for several nonlinear loads or for an individual load, as shown

Typical Harmonic Trap Filter Configuration

- Larger conductors size are used ,or taking these problems into considerationin the initial design stage.
- Larger transformer (derated):In order to derate a transformer after it is installed, limits must be placed on allowed future loading.
- K-rated transformer:it transformer can be used to handle the expected harmonic currents if the transformer is being sized before installation.

- Energy savings by using the neutral current filter.
- The neutral current filter eliminates most of the third harmonic current in the neutral and phase conductors.
- Other solutions for harmonics are effective only if they are placed very close to the load.
- The neutral current filter does not dissipate much energy (only about 10% of that actually saved)