VOLTAGE REGULATORS

A voltage regulator consists essentially of a voltage-sensitive element and associated mechanical or electrical means to produce the changes necessary to maintain a predetermined, constant generator voltage.

An introduction to or explanation of the many different types and styles of voltage regulators manufactured today surpasses the scope of this chapter. All regulators accomplish the same job, but different solid-state devices and circuits may be used to arrive at the end result-voltage regulation. Two typical voltage regulators used in NCF generating equipment are the SCR voltage regulator and the transistor voltage regulator.

Silicon Controlled Rectifier Regulator

The SCR regulator precisely controls the output voltage of an ac electrical generating system by controlling the amount of current supplied to the exciter (or generator) field. This regulator may be used with brushless rotary exciters, brush type of rotary exciters, or direct excitation machines.

The SCR regulator (fig. 4-9) senses the generator voltage, compares a rectified sample of that voltage with a reference diode (zener) voltage,

Figure 4-9.\SCR voltage regulator block diagram. �

 

 

and supplies the field current required to maintain the predetermined ratio between the generator voltage and the reference voltage. The SCR regulator consists of five basic circuits. These are a sensing circuit, an error detector, an error amplifier, a power controller, and a stability network. The regulator also has an automatic voltage buildup circuit.

SENSING CIRCUIT.\This circuit consists of sensing transformer(s) T1 (T2), diodes CR1 through CR6, capacitors C1 and� C2, and filter choke L1 (fig. 4-10). The sensing circuit will sense the generator voltage, rectify and filter this voltage, and apply the resultant dc signal to the error detector and error amplifier. Transformer T1 (terminals E1 and E2) is used on

single-phase application, and transformer T2 (terminals E1, E2, and E3) is added to supply three-phase sensing.

ERROR DETECTOR.\The detector consists of reference (zener) diode VR1 and a voltage �

Figure 4-10.\SCR voltage regulator schematic diagram. �

 

 

divider network, consisting of resistors R1, R2, R3, and R5. This network provides a dc signal that is proportional to the generator voltage. The voltage at the junction of R3 and R5 is compared to the voltage of VR1 to develop the error signal, which is applied to the error amplifier.

ERROR AMPLIFIER.\The amplifier consists of two-stage transistor amplifier Q1 and Q2, unijunction transistor 43, emitter� follower 44, and their associated components. The error signal drives Q1, which, in turn, controls 42. Transistor Q2 controls the charging time of capacitor C4 in the emitter circuit of 43, thus providing phase angle control of the firing signal applied to the SCRs in the power, controller. Transistor 44 provides the correct voltage to base 2 of 43 to maintain uniform

SCR firing.

POWER CONTROLLER.\The power controller consists of SCRs CR11 and CR12 and diodes CR13 and CR14 in a bridge� rectifier circuit. The amount of output current depends upon the conduction time of the SCRs and the exciter field resistance. This circuit can be compared to a variable rectifier placed between the power souce (terminals 3 and 4) and the exciter field (terminals F+ and F-).

INPUT POWER.\The voltage applied to the regulator input power stage (terminals 3 and 4) must be 120 volts for single-�phase regulators and either 208 or 240 volts for three-phase sensing regulators. The input power may be taken from any generator phase that provides the correct voltage (line to line or line to neutral). The phase relationship of this input voltage in relation to other circuits is not important. When the generator output voltage is greater than the preceding values, a power transformer must. be used to match the generator voltage to the regulator input.

STABILITY NETWORK.\This circuit provides stable operation under all operating conditions. It consists of capacitors C6� and C7; resistors R27, R28, R29; and variable resistor R4. This resistance-capacitance (RC) network injects a stabilizing signal from the power stage to the error amplifier to prevent oscillations (hunting). R4 determines the amount of stability applied to the error amplifier.

AUTOMATIC VOLTAGE BUILDUP.\ Relay K1 provides automatic voltage buildup from generator residual voltage.� Normally closed contacts (relay de-energized) provide a current path around the control rectifiers (CR11 and CR12) to allow the generator residual voltage to be converted to dc by diodes CR13, CR14, CR15, and CR16 and applied to the exciter field. When the generator voltage reaches approximately 75 percent of rated voltage, the relay pulls in, removing the rectifier diodes, thereby allowing the SCRs to take control. A minimum of 3 percent generator residual is required for automatic voltage buildup. If less than 3 percent exists, external field flashing may be required.  

PARALLEL COMPENSATION (REAC-TIVE DROOP).\When parallel operation is required,� additional components are required in the regulating system. These are resistor R25, transformer T3, and current transformer CT1. Two of the components are included in a parallelequipped voltage regulator. These are R25 and T3. Current transformer CT1 is a separate item and must be interconnected, as shown in figure 4-11.

These components allow the paralleled generators to share the reactive load and reduce circulating reactive currents between them. This is accomplished in the following manner.

A current transformer CT1 is installed in phase B of each generator. It develops a signal that is proportional in amplitude and phase to the line current. This current signal develops a voltage across resistor R25 (fig. 4-10). A slider on R25 supplies a part of this voltage to the primary of transformer T3. The secondaries of T3 are connected in series with the leads from the secondary of sensing transformer T1 and the sensing rectifiers located on the printed circuit board. The ac voltage applied to the sensing rectifier bridge is the vector sum of the steppeddown sensing voltage (terminals El and E3) and the parallel CT signal supplied through T3 (terminals 1 and 2). The regulator input sensing voltage (terminals E1 and E3) and the parallel compensation signal

(terminals 1 and 2) must be connected to the generator system so as to provide the correct phase and polarity relationship.

Regulators with single-phase sensing provide about 8 percent maximum droop while threephase-sensing regulators provide 6 percent droop. When generators are paralleled to the same bus and have different types of sensing, care must be taken to compensate for these differences using the slide wire adjustment on droop resistor R25.

When a resistive load (unity power factor) is applied to the generator, the voltage that appears'

 

Figure 4-11.\Parallel compensation interconnection diagram. �

across R25 (and T3 windings) leads the sensing voltage by 90 degrees, and the vector sum of the two voltages is nearly the same as the original sensing voltage; consequently, almost no change occurs in generator output voltage.

When an inductive load (lagging power factor) is applied to the generator, the voltage that appears across R25 becomes more in phase with the sensing voltage, and the combined vectors of the two voltages result in a larger voltage being applied to the sensing rectifiers. Since the action of the regulator is to maintain a constant voltage at the sensing rectifiers, the regulator reacts by decreasing the generator output voltage.

When a leading power factor (capacitive) load is applied to the generator, the voltage across R25 becomes out of phase with the sensing voltage. The combined vectors of the two voltages result

 

 

in a smaller voltage being applied to the sensing rectifiers. Then the regulator reacts by increasing the generator voltage.

When two generators are operating in parallel, if the field excitation on one generator should become excessive and cause a circulating current to flow between generators, this current will appear as a lagging power factor (inductive load) to the generator with excessive field current and a leading power factor (capacitive load) to the other. The parallel compensation circuit will cause the voltage regulator to decrease the field excitation on the generator with the lagging power factor and increase the field excitation on the generator with the leading power factor so as to minimize the circulating currents between the generators.

This action and circuitry is called reactive droop compensation (droop). It allows two or more paralleled generators to share inductive loads proportionally by causing a decrease or droop in the generator system voltage.

REACTIVE DIFFERENTIAL COMPENSA-TION (CROSSCURRENT).\Reactive differential� compensation allows two or more paralleled generators to share inductive reactive loads with no decrease or droop in the generator system output voltage. This is accomplished by the action and circuitry described previously for reactive droop compensation and by the addition of cross-connecting leads between the parallel CT secondaries, as shown in figure 4-12. When the finish of one parallel CT is connected to the start of another, a closed series loop is formed. This loop interconnects the CTs of all generators to be paralleled. The signals from the interconnected CTs cancel each other when the line currents are proportional and in phase. No system voltage decrease occurs. These regulators provide the necessary circuit isolation so that parallel reactive differential compensation can be used. THE REACTIVE DIFFERENTIAL CIRCUIT CAN BE USED ONLY WHEN ALL THE GEN-ERATORS CONNECTED IN PARALLEL HAVE IDENTICAL PARALLELING CIRCUITS INCLUDED IN THE LOOP. REACTIVE DIFFERENTIAL COMPEN-SATION CANNOT BE USED WHEN PARALLELED WITH THE UTILITY OR OTHER INFINITE (UTILITY) BUS.

 

Figure 4-12.\Crosscurrent compensation interconnection diagram. �