Harmonic distortion is caused by nonlinear devices in the power system. A nonlinear device is one in which the current is not proportional to the applied voltage. Harmonic distortion is present to some degree on all power systems.
Fundamentally, one needs to control harmonics only when they become a problem. Harmonic distortion is not a new phenomenon on power systems.
Concern over distortion has ebbed and flowed a number of times during the history of ac electric power systems.
There are three common causes of harmonic problems:
- The source of harmonic currents is too great.
- The path in which the currents flow is too long (electrically), resulting in either high voltage distortion or telephone interference.
- The response of the system magnifies one or more harmonics to a greater degree than can be tolerated.
When a problem occurs, the basic options for controlling harmonics are:
- Reduce the harmonic currents produced by the load.
- Add filters to either siphon the harmonic currents off the system, block the currents from entering the system, or supply the harmonic currents locally.
- Modify the frequency response of the system by filters, inductors, or capacitors.
Reducing harmonic currents in loads
There is often little that can be done with existing load equipment to significantly reduce the amount of harmonic current it is producing unless it is being misoperated. While an overexcited transformer can be brought back into normal operation by lowering the applied voltage to the correct range, arcing devices and most electronic power converters are locked into their designed characteristics.
PWM drives that charge the dc bus capacitor directly from the line without any intentional impedance are one exception to this. Adding a line reactor or transformer in series will significantly reduce harmonics, as well as provide transient protection benefits.
Transformer connections can be employed to reduce harmonic currents in three-phase systems. Phase-shifting half of the 6-pulse power converters in a plant load by 30° can approximate the benefits of 12- pulse loads by dramatically reducing the fifth and seventh harmonics. Delta-connected transformers can block the flow of zero-sequence harmonics (typically triplens) from the line. Zigzag and grounding transformers can shunt the triplens off the line.
Purchasing specifications can go a long way toward preventing harmonic problems by penalizing bids from vendors with high harmonic content. This is particularly important for such loads as high-efficiency lighting.
The shunt filter works by short-circuiting harmonic currents as close to the source of distortion as practical. This keeps the currents out of the supply system. This is the most common type of filtering applied because of economics and because it also tends to correct the load power factor as well as remove the harmonic current.
Another approach is to apply a series filter that blocks the harmonic currents. This is a parallel-tuned circuit that offers a high impedance to the harmonic current. It is not often used because it is difficult to insulate and the load voltage is very distorted. One common application is in the neutral of a grounded-wye capacitor to block the flow of triplen harmonics while still retaining a good ground at fundamental frequency.
Active filters work by electronically supplying the harmonic component of the current into a nonlinear load.
Modifying the system frequency response
There are a number of methods to modify adverse system responses to harmonics:
- Add a shunt filter. Not only does this shunt a troublesome harmonic current off the system, but it completely changes the system response, most often, but not always, for the better.
- Add a reactor to detune the system. Harmful resonances generally occur between the system inductance and shunt power factor correction capacitors. The reactor must be added between the capacitor and the supply system source. One method is to simply put a reactor in series with the capacitor to move the system resonance without actually tuning the capacitor to create a filter. Another is to add reactance in the line.
- Change the capacitor size. This is often one of the least expensive options for both utilities and industrial customers.
- Move a capacitor to a point on the system with a different short-circuit impedance or higher losses. This is also an option for utilities when a new bank causes telephone interference—moving the bank to another branch of the feeder may very well resolve the problem. This is frequently not an option for industrial users because the capacitor cannot be moved far enough to make a difference.
- Remove the capacitor and simply accept the higher losses, lower voltage, and power factor penalty. If technically feasible, this is occasionally the best economic choice.