Understanding voltage regulators

04/01/2010

Electric power international - quality and reliability: ongoing challenge for distribution systems

 

During initial operation, most utility and industrial electrical distribution systems present few problems with the voltage supplied to their loads. Progressive expansion, load growth and economic considerations eventually impose voltage conditions on these systems which are less than ideal. This can cause electric equipment to operate at reduced efficiency and overall power costs to rise. Increased industrial use of electronic equipment with its demand for voltage within rather critical limits, compounds the problem. Step-type voltage regulators are increasingly being used to solve this problem.

 

Theory of Operation

 

A step-type voltage regulator provides essentially in-phase voltage regulation (Figure 1). Regarding its construction, it can be compared to a basic auto-transformer, with its exciting winding connected across the line. This exciting winding in turn excites the serires winding by inducing voltage. The series winding, with no further modification, is capable of raising or lowering the system voltage by 10%, depending on whether it is connected in additive or subtractive polarity.

To obtain regulation in finer steps, the series winding is tapped into eight 1.25% sections. The taps are connected to stationary contacts on a dial switch panel (Figure 2).

 

In order to tap this voltage without breaking the load current, a set of two moving fingers are mechanically ganged together and physically spaced to that they can form a bridge between stationary contacts or be on the same stationary contact.

 

The moving fingers are connected to the two ends of a center-tapped preventive autotransformer that allows tap changes to be made without breaking load current. The autotransformer also provides a step size of 0.625% by introducing a bridging position between each 1.25% tap; this gives 10% regulation in sixteen o.625% steps from the top to the bottom of the series winding. Finally, the reversing switch, which connects the series winding in either additive or subtractive polarity, provides + or - 10% regulation in thirty-two 0.625% steps.

 

Continuous control of the voltage-regulator output is obtained by an automatic control system. Output voltage of the regulator is first sensed by a potential transformer (PT) or a tertiary winding (in the case of ANSI Type B design units) which steps the voltage down to a standard utilization level of 120V That voltage is then applied to a control circuit through a tapped sensing transformer which permits operation at any voltage between 106 and 134V.

 

Using microprocessor-based controls, the output of the PT is converted to either a derived average or a true root-mean-square (rms) value by a precision full wave rectifier. These values vary directly with the ac output of the regulator and are compared to a user-defined set point (voltage level setting). If it is above the fixed reference, power is applied through a time-delay circuit to the lower winding of the regulator tap-changing motor, and the regulator lowers its output voltage. Conversely, if the output of the circuit is low, the regulator raises its output voltage.

Application

 

Because the voltage change caused by one tap change is 0.625% at 120V or 0.75V, a bandwidth setting is required for proper operation. Possible bandwidth settings on a voltage regulator are determined by the size of the regulator step, because a smaller one would cause excessive tap changes as the unit searches for the nominal voltage (referred to as "hunting").

 

A one-step bandwidth is impractical because any small increase in the size of the step-- such as from increased excitation voltage -- also results in hunting. The smallest practical bandwidth is therefore somewhat larger than a one- or two-step voltage band. For regulators with 0.625% steps, the bandwidth would be 1.5% or +/- 0.75V.

 

Many system voltage fluctuations correct themselves. Example: When a motor starts, it draws a high current that causes the voltage to drop; however, as the motor comes up to speed, the current decreases and the voltage increases. To prevent the regulator from "chasing" this type of fluctuation, a time delay is introduced to the control system, which allows the regulator to "wait and see" before initiating a tap change. For flexibility, this time delay is adjustable in 10 second increments from 10 to 180 seconds.

 

As a general rule, regulators are located as close as possible to the load center. The farther away from the load, the greater the voltage drop that can be introduced between the regulator and the load. But practical considerations may require that the regulator be located some distance from the load. In this case, it is necessary to add a line-drop compensator to the control circuit to compensate for the voltage drop. With the line-drop compensator, a regulator minimizes the effects of resistance and reactance line drops and improves voltage at the load center. Also, average voltage at other points along the distribution line is obtained.

 

Note that the voltage drop is a function of load current; therefore, the regulator must be able to measure current and estimate the corresponding voltage drop. In addition to the current transformer, a resistor and reactor are added to the regulator control compensator circuit. These two circuit elements are variable and are adjusted to supply the necessary values to each individual application.

Figure 2

 

2.   To obtain regulation in smaller steps, the series winding is tapped into eight 1.25% sections; these taps are connected to stationary contacts on a dial switch panel.

 

Current from the current transformer flows through a portion of the resistance and reactance elements of the compensator and a voltage drop is introduced into the control circuit. This voltage adds or subtracts from the voltage in the voltage regulating relay and causes the regulator control to raise or lower the regulator output voltage.

 

By properly applying the resistance and reactance elements, the voltage rise or drop in the regulator control circuit will be proportional to the drop in the line caused by the load currents and the resistance and reactance in the lines. Thus, the regulator maintains a constant voltage at the selected load center and also reduces the voltage swing at the end of the line.

 

The calculation of correct compensator setting is based upon the size and spacing of conductors and the distance to the load center. When these values are known, they can be substituted into formulae supplied by the manufacturer to determine the proper compensator settings.

 

The industry standard for the range of regulation is + or - 10%. This range has proved to be adequate to correct the majority of voltage variations in electrical systems. Further changes can be made to the system if better regulation is required -- such as stringing heavier conductor to reduce load current, which reduces the voltage changes throughout the system.

 

Sizing

 

In selecting the correct regulator for an application, confusion can sometimes arise regarding the kVA rating required because regulators are rated in terms of kVA of regulation, not system kVA. Regulator kVA is a function of the current and voltage measured at the regulator and the percent regulation it delivers. Thus, single-phase kVA is equal to the product of the voltage applied across the exciting winding, rated line current, and percent regulation, divided by 1000:

where E = volts applied to the regulator and I = line amperes.

 

For example, a 100A primary bus is connected to a 13,800Y / 7960V circuit. For this system, the regulator is connected line-to-ground and three 7960 volt regulators would be required. According to the equation, the regulator kVA would be 79.6.

 

For a similar circuit connected to a 13 800V delta, the regulator is connected line-to-line and three 13 800V units could be used resulting in a regulator kVA of 138.

 

From the example, note that for the same transformer kVA, a different regulator kVA is required for a wye-connected system than for a delta-connected system. Two regulators rather than three, may be used for delta-connected systems. Connected in open-delta, they will deliver individual phase regulation on two phases while the voltage on the third phase will be the average of the others.

 

The kVA of a three phase regulator is equal to the square foot of three times the product of the line-to-line voltage, the line current, and the percent regulation, divided by 1000. Using the same circuit described previously:

This calculation of the three phase regulator kVA also provides a check for single-phase kVA calculations. The combined kVA of three single-phase regulators connected in  grounded wye will always be equal to the three phase regulator kVA for the system. Two regulators connected in open-delta to a 3-phase system, on the other hand, will always have a combined kVA greater than the comparable three phase regulator kVA.