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SELECTION OF VOLTAGE REGULATING EQUIPMENT


SELECTION OF VOLTAGE REGULATING EQUIPMENT
How do we select voltage regulators for installation in lines and substations?

Distributors must consider many factors when deciding what regulating equipment to install in substations. These factors include:
  1. Equipment Maintenance Intervals, as related to the number of guaranteed tap-changer operations without maintenance.
  2. Capability of the equipment tocarry expected through-fault currents. Separate feeder regulators may be inadequate for the fault currents in some locations, whereas transformer LTCs are rated for the same fault duty as the transformer.
  3. Overall equipment purchase and installation costs.
  4. Space consumption in the substation yard.
  5. Whether or not separate regulating action is needed for each feeder, to include separate resistance and reactance settings for line-drop compensators and,
  6. Energy loss characteristics.
Although energy loss is only one of these factors, a loss evaluation should be part of the engineering and economic study involved in the choice of equipment. Since they exhibit the same types of characteristics, regulator losses can be evaluated using the methods presented previously for substation transformers. The only addtitional step required is to estimate the time the regulator will spend in various tap positions since losses vary for different tap points. To simplify calculations, the distributor should select an average or representative tap position based on engineering judgment and knowledge of expected conditions and base loss evaluations on characteristics at the chosen representative position.

Individual feeder regulators will contribute more to the efficiency of the distribution system than will group regulators or substation transformer tap changers. This is because the individual characteristics of each feeder can be accomodated by regulator settings, while group regulator settings must be based on composite characteristics for all feeders served. Substation that supply both urban and rural feeders are the most difficult to regulate properly by using group regualtion.

Sizing Voltage Regulators
Once a distributor decides on the type of substation regulating equipments to be installed, it must size the equipment for the expected loadings. In general, the choice of the most economical size isbased on the same factors explained previously for transformers. Regulators loaded above nameplate capacity may have excessive winding losses, and the installation of excessive regulator capacity will unnecessarily increase the regulator core losses. Individual circumstanceswillresult invaried recommendations,but optimal loadings will often approximate regulator nameplate ratings. Regulators are rate based on the product of the maximum regulation percentage and the total kVA on the feeder being regulated is 5,000kVA and regulation up to 10% boost or 10% buck is required, the regulator installation should be rated 1t 0.10 x 5,000kVA = 500kVA.
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