For certain applications, the most cost-effective solution for poor power factor, excessive voltage distortion, and IEEE Std 519-1992 violations is to install one or more larger harmonic filters at a distribution bus or busses.
Generally, an automatic harmonic filter bank will be installed on the secondary of each main transformer in the plant requiring power factor and harmonic compensation. Placement of multiple banks on a common low-voltage system can create problems by changing network harmonic flows and thereby increase the potential for overloading some of the filter banks.
Therefore, this practice is not generally recommended. Where power factor correction is most important, systems tuned to the 2nd harmonic or below can generally be safely applied in this manner. Parallel resonance at the 3rd harmonic must be carefully evaluated.
Caution must be exercised when a harmonic filter is electrically close to the main and is tuned to the 7th harmonic or higher. In this case, the potential exists to absorb large amounts of harmonic current from the utility distribution system.
Harmonic filters should be designed assuming the distribution system will have up to 3% voltage distortion at the harmonic nearest the tuning frequency per IEEE Std 519-1992. If the harmonic filter reactors are not equipped with taps, as is common with low-voltage filters, a good practice is to over specify the thermal rating so that additional capacitance may be added to detune the filter in the event a harmonic overload occurs.
If altering the tuning results in unacceptable filtering of in-plant harmonics, the utility can generally help to identify methods for reducing the available harmonic current. Possible utility-side solutions include the following:
Changing the size or status of capacitor banks to alter the impedance characteristics of the system
Enforcing IEEE Std 519-1992 limits on customers with excessive harmonic injection
Circuit reconfiguration to isolate harmonic injectors
Medium-voltage harmonic filters
During lightly loaded conditions, harmonic filters that are fixed on the bus can produce an overvoltage condition. The maximum per-unit voltage rise caused by the harmonic filter can be estimated as approximately equal to the harmonic filter power system frequency current (i.e., fundamental current) divided by the system three-phase short-circuit current at the harmonic filter location (see IEEE Std 1036-1992).
If overvoltage is a concern, an automatically switched harmonic filter should be considered. Switched harmonic filter(s) comprise a number of steps, each of which is an individually tuned harmonic filter.
Reactive current controllers (sometimes referred to as var controllers) that can switch steps in and out automatically as system reactive current (i.e., power factor) changes are readily available. Other switching alternatives include the use of current relays, time-of-day controllers, voltage controllers, or other sensing devices. Switching times become more important as the harmonic filter is tuned closer to its rated frequency.