Regulator and Capacitor Voltage Control
Introduction
Fixed capacitors are often used to supply reactive power demand and to compensate for reactive power losses due to line impedance. These capacitors result in improvements in voltage profile but their use is limited by the increases in voltage at light loads. Switched capacitors can be used if greater improvements in line power factors are necessary as load increases.
The policies adopted for applying line regulators and switched capacitors for voltage control purposes varies considerably between utilities across the world. This article compares the advantages and disadvantages of regulators versus capacitors and quantifies the respective voltage control capabilities and the impact on operations and feeder utilization.
Voltage control is used primarily to maintain the voltage at each load point within defined statutory limits to comply with regulatory requirements and to avoid customer complaints because of poor supply quality. In particular, customers at the end of long feeders are often subject to wide variations in supply voltage if no voltage regulation compensating equipment is installed. For practical purposes, utilities specify operational limits for voltage that are within the statutory limits to allow some latitude for unusual operating conditions.
Line Regulators
Line regulators have been used extensively for voltage control purposes, sometimes even at step-down transformer substations to provide voltage control in the absence of load tap change capability. Since a regulator is capable of both bucking and boosting the feeder voltage, it offers a wide range of control to accommodate both high and low voltage problems within a single device. Regulators of modest kVA rating can be applied to feeders where the kVA flow is many times the regulator rating. The through kVA capacity of a regulator is defined by the equation:
Through kVA = Rated kVA x 1/pu max tap
where pu max tap is the maximum tap range from nominal (0%) tap in per unit quantities. For a regulator with +/-10% tapping range, the through kVA is ten times the regulator kVA rating. Generally, the regulator impedance is very low and is often ignored for analysis purposes.
Since a regulator functions like an autotransformer, the voltage difference between the input and output has a corresponding change in the current supplied to the regulator. For a boost setting, this results in a per unit current difference between input and output with a corresponding increase in the feeder current upstream of the regulator (Figure 1).
Figure 1 - Regulator effect on current
Applying a regulator for maximum boost or buck of the voltage may result in a step voltage change at the regulator (Figure 2) that approaches the operational limit in either direction. In practice, there is a limitation on how much boost/buck is practical and it may be necessary to apply regulators in series on order to limit the boost/buck at one location.
Figure 2 - Feeder voltage profile with regulator
Capacitors
Where fixed capacitors only are used for power factor compensation and voltage profile improvement, they an be optimally located to maximize the benefits from voltage regulation and/or line loss reduction. If only peak voltages are too low, switched capacitors are added. The criteria are that voltage limits must not be violated for either maximum or minimum load. A feeder system including both fixed and switched capacitors can be designed such that the location and size of capacitors and their locations are determined optimally to minimize feeder losses and comply with voltage constraints for all loading levels. The capacitor size applied at any one node is usually limited to a maximum for convenience of installation and to constrain the reactive power flow immediately upstream of the capacitor to avoid leading power factor currents at light load. Computer programs are available to perform the optimum capacitor allocation function.
As opposed to line regulator application, the allocation of switched capacitors (at B, C, D, E – Figure 3) along a feeder produces a much smoother voltage profile without voltage steps. Capacitor location and size is chosen to match the voltage and reactive power flows along each line section to minimize feeder power flows. This is particularly effective where load is unevenly distributed along a feeder resulting in large local voltage drops. Pole mounted capacitors are switched as an entity and are suitable for distributing along a feeder. The DESS optimization process allocates these capacitors where they have the greatest effect. Applying capacitors in this manner can be looked upon as a form of distributed control.
Figure 3 - Feeder voltage profile with capacitors
Control Techniques
Switched capacitors are controlled by one of several parameters, such as time of switching, kVAr flow through an adjacent branch, voltage at the capacitor (regulated) node, or power factor of the adjacent branch flow, each of which has advantages and disadvantages. The primary concern is that any load added to the feeder due to load growth or temporary feeder reconfiguration necessitates a resetting of control parameters. The value of the controlled variable determines when the capacitor is switched in or out. Time delay elements are incorporated to eliminate spurious switching for transient events. More sophisticated modes of control are now available with digital control technology replacing analog methods. These newer techniques address some of the concerns described.
Voltage controls for line regulators incorporate time delays to prevent unnecessary initiation of tap changing for temporary voltage variations. The control will have a deadband and the voltage can be controlled only within the deadband accuracy. Thus the voltage can vary above or below the control setting by the value of the deadband before control action is initiated. Typically, deadbands represent about 1-1.5% of the voltage setting. Line drop compensation uses a replica impedance that represents the portion of feeder between the transformer and a node downstream whose voltage is to be controlled. The replica impedance is supplied from a current transformer with a current representing the feeder load current. The resulting voltage across the replica impedance is subtracted from the voltage measured at the line regulator to simulate the voltage at the downstream node. These controls also are moving to digital technology.
More sophisticated controls for switched capacitors and line regulators have an economic benefit by reducing the number of operations per year with a consequent reduction in maintenance costs. A cost-benefit analysis would be required to determine the economic viability of using these controls
Application Considerations
Because regulators have the ability to both boost and buck voltage, they offer a flexible form of voltage control over a wide control range. The regulator is self-contained and does not require additional switches. Control is based on voltage conditions measured at the regulator although the use of line drop compensation does allow a location downstream of the regulator to be chosen as the point whose voltage is to be controlled.
If line drop compensation is used with regulators, care must be taken to ensure that the effective point of control is not within the control sphere of another downstream regulator otherwise hunting between regulators will occur and it will be impossible to achieve a stable control scheme. If more than one significant feeder spur exists downstream of a regulator, it becomes more difficult to determine line drop compensation settings. This is particularly true if a regulator is located close to a substation. A full evaluation of regulator tap parameters and compensation settings for the complete range of loading conditions is essential to ensure satisfactory settings.
Capacitors always function to reduce feeder reactive power flows; voltage control is achieved by switching in capacitors when voltage increase is needed and switching out when voltage reduction is needed. Capacitors can produce undesirable transients when switching or other disturbances occur and the application of surge control devices must be considered to ensure effective insulation coordination. Since it is not practical to perform transient analysis for every capacitor location, these surge devices are applied on the basis of good practice established from many years of experience.
The power flow on voltage-controlled feeders may reverse for certain operating conditions. In the event of generation connected downstream of the regulator, the real power flow may be positive but the reactive power flow negative. This, coupled with situations where the substation power transformer also has automatic control of the secondary voltage, requires careful evaluation of the controlled section of system to ensure proper coordination between control devices. This is accomplished by selecting suitable voltage settings and time delays to decouple, as far as possible, the action of individual devices. The alternative is the use of communications circuits to achieve the correlation. Unfortunately this can be expensive, although the gradual adoption of Internet technology is making this more of a practical proposition.
Economic Considerations
Voltage regulators function to raise or lower voltages by injecting a voltage in series with the feeder branches and have only a second order effect upon the losses incurred by feeder power flows. Capacitors generate reactive power and have a significant impact in reducing feeder reactive power flows as described. Hence total feeder flows and consequently line losses are reduced; at the same time conductor capacity is also released. Both consequences have economic benefits, the former in reducing the cost of losses and the latter in delaying the need for additional feeder capacity with load growth. The practice of lumping capacitors at distribution substations is of benefit in reducing transmission reactive capacity requirements but is of no value in reducing distribution feeder losses or improving voltage profiles, only inasmuch as these capacitors reduce losses and voltage drop in the substation power transformer.
©2009 Essex Energy Inc.