- Same number of phases.
- Same phase rotation.
- Same frequency.
- Same voltage.
- Same voltage droop.
As discussed in the section on governors, it is not necessary for the mechanical or hydraulic governors to have the same speed droop. However, it is necessary for all the governors of units being paralleled to have 3% speed droop except one. This one governor can have 0% speed droop and be used to control the frequency of the system. The paralleling operation is performed with the assistance of a synchroscope or synchronizing lights to show when the frequencies of the generators are the same. With the synchroscope, paralleling is performed with the pointer, in a vertical position. With synchronizing lights, paralleling is performed when the lights are dark. Refer to the Operation and Maintenance Instructions for the electric sets being paralleled for more details.
When generators have been paralleled at no load and no operations have been performed except closing the circuit breaker, the machines are "floating" on the line with no power being transferred either between them or to a load. When a load is connected to the system, power is supplied from the paralleled units. If they all have the same rating and governor speed droop characteristics, the load will be divided equally among them. Any differences in kilowatt loading should be equalized by adjusting the governor controls. To increase the kilowatt load on a unit, the governor control should be advanced farther in the direction to increase speed. For small movements of the governor control, the change in system frequency will be negligible or zero. However, changes in kilowatt load among the engines may be appreciable. To insure balanced kilowatt loading of several electric sets, a separate kilowatt meter for each one is extremely helpful.
There is a misconception that an generator operating in parallel can have its kilowatt load changed by changing its excitation or magnetizing power. It seems logical that if the voltage level control setting is increased there will be an increase in kilowatt loading of the generator in a manner similar to that experienced when an generator is operating by itself. This is not true.
If the magnetizing power of a generator is increased beyond the value required to maintain the voltage of the generator at the same value as the system voltage while the generator is delivering the amount of power determined by the engine governor setting, the excess magnetizing power goes out from the generator into the system as MAGNETIZING POWER. This fact shows up as an increase in amperes being furnished by the generator.
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| Illustration 1 | g01059119 |
Consider an over excited generator operating in parallel into a resistive load. The result would be an increase in line amperes. Sample meter reading for this generator might be Ammeter – 100, Voltmeter 220, and Kilowatt meter 30.5. Note that the product of the volts times amperes times 1.732 (constant for three-phase generation) is 38.1 kilowatt and exceeds the kilowatt output by a substantial amount. The amperes required for 30.5 kilowatts at unity power factor and 220 volts are 80. The excessive amperes indicated on the meter represent magnetizing power circulation in the system. These are called reactive amperes or circulating current. | |
Illustration 1. There is no increase in kilowatt output since the kilowatt output is determined by the governor setting. The generator power factor is lowered. (Power Factor is briefly described in the following paragraphs. For a detailed description, refer to Systems Operations, "Power Factor" of this manual).
Three-phase power in a purely resistive circuit, such as one having only incandescent lamps or heaters, is defined as the product of volts, amperes and a constant. As an equation, this would be stated as Watts = V x A x 1.732. Three-phase power in a circuit having resistance and inductance, such as one having lamps and motors, is defined as the product of volts, amperes, a constant, and power factor. As an equation, Watts = V x Ax 1.732 x (P.F.). The power factor of a system varies between 1.0 (unity) for a system with all resistance and no inductance and 0 (zero) for a theoretical system with all inductance and no resistance. If there were such a thing as a perfect inductance with no resistance, no power would be required to circulate amperes through the system. This is shown by the equation W = V x A x 1.732 x (0). If the major portion of a load is resistive and a small portion is inductive, the equation might look like this: W = V x A x 1.732 x (.8). Most induction motors operate at a power factor near .8 when loaded near their rated horsepower. If a load operates at a power factor less than unity, it indicates that there is some inductance in the load. This inductance requires magnetizing current. This magnetizing current is necessary to form a magnetic field, but it does not contribute directly to the performance of useful work. The best example of this is the squirrel cage induction motor. This motor has no separate excitation source to form a magnetic field. It also draws active amperes to transform into shaft horsepower from rotation of its rotor.
The ratio of active amperes to total amperes is the same as the power factor value. Active and reactive amperes are not simply added to get total scope of this presentation. The ammeter in the generator switch-gear shows the value of amperes resulting from active and reactive amperes flowing at the same time.
Returning to the paralleled generator, if the magnetizing power of an generator is increased beyond the value required for the generator to deliver power to the system at the power factor value of the load, the excess magnetizing power flows into any other generators supplying the system and, as a result, their exciters reduce their excitation or magnetizing power since THERE IS ONLY ONE TOTAL VALUE OF MAGNETIZING POWER REQUIRED FOR ANY ONE SYSTEM. This value is fixed and is determined by the power factor of the load. All generators in the system should share it proportionately.
Voltage Droop
The method used to control the magnetizing power of an generator is called "voltage droop". It has been explained previously that speed droops in governors is used to divide the kilowatt loading of engine-driven generators. An engine that tends to speed up and carry more than its proportionate share of the load will, with a governor having speed droop, tend to speed up and again pick up its proportionate share of the load. Also, remember that active power (KW) loading is determined by the governor setting and the regulator should increase the magnetizing power in excess of its proportionate share for the system, it is necessary to reduce the exciter output of this unit to bring its level of magnetizing power back to the proper value.
The increase in magnetizing power shows up as an increase in line amperes so the use of a circuit sensitive to line amperes should furnish the necessary control. This circuit should act to reduce the magnetizing level of the generator as the line amperes increase. Since, for an individually operating generator, this would cause a decrease in voltage, the circuit is called a "voltage droop" circuit. It generally operates as follows: An increase in line amperes causes a voltage to be generated in the secondary of a current transformer. This voltage is added to the generator voltage impressed on the regulator. The regulator senses the increase in voltage and acts as though the generator terminal voltage is too high. It reduces the magnetizing power of the exciter field by reducing the exciter voltage and correspondingly reducing the power to the generator field and the magnetizing power of this field. Since the voltage of a paralleled generator cannot change, but must remain the same as the bus voltage, the reduction in magnetizing power acts only to reduce the magnetizing amperes being furnished by the generator. Conversely, if the magnetizing power of the generator is too low, this circuit acts, within its capacity, to cause the voltage regulator to increase the magnetizing power of the paralleled generator so it will be furnishing its share of magnetizing power to the system. A voltage droop adjustment causing a 3% to 8% drop in voltage from no load to full load at rated power factor is usually required for satisfactory division of ampere loading. The largest amount of droop that can be tolerated by the load should be used to insure stable operation.
Voltage droop circuits can mistakenly be connected in reverse. In this case, the voltage from the current transformer secondary subtracts from the voltage of the generator terminals. The regulator acts as though the generator voltage were too low and increases the magnetizing power of the generator. This causes the ampere output of the generator to become excessive. The increase in amperes acts to increase the voltage subtracting from the generator voltage impressed on the regulator and the regulator continues to act to increase the magnetizing power of the generator. This power can increase to the limit of the capacity of the exciter. The current can easily exceed the rating of the generator. The circuit should always be checked against the regulator manufacturers instructions. Proper connection of the voltage droop circuit can also be checked when the generator is loaded individually in the process of adjusting the amount of voltage droop prior to paralleling the generator. If the voltage falls when a load having a lagging power factor is connected to the generator, the droop circuit is connected properly. If the voltage rises, the circuit is connected improperly. Reversing the wires at the current transformer primary or secondary will usually correct the situation. Returning to the paralleled generator, when the voltage droop is adjusted to control the ampere output of the generator so the magnetizing power delivered by the generator to the load will not exceed the proportionate value for the generator, then the power factor of the generator output will be the same as the power factor of the load.
If the generator should furnish more than its share of magnetizing power, its power factor would be lower than that of the load. If it furnished less than its share, its power factor would be higher than the power factor of the load. Notice that things seem to be backward in the above statement? This happens because power factor decreases as magnetizing power increases, and vice versa. It is possible to have a paralleled generator operate at such a low magnetizing level that magnetizing power must flow into it from the system in order to keep the generator voltage at the same value as the bus voltage. In this case, the generator is said to be operating at a leading power factor. By definition, a generator delivering magnetizing power is operating at lagging power factor whereas an generator receiving magnetizing power is operating at a leading power factor. Also, by definition, a motor receiving magnetizing power from its power source is operating at a lagging power factor whereas a motor delivering magnetizing power to its power source is operating at a leading power factor. This latter case is somewhat rare and is represented by a synchronous motor operating with its magnetizing level higher than that required to furnish only the magnetizing power required to keep the motor in synchronism and carry its connected load. Leading power factor motors require special design characteristics for which a premium price is usually charged.
In order to cause the paralleled generator to deliver power to the system, the engine governor must be advanced so the engine will tend to speed up. This operation has already been described in the section on governors. The engine generator combination will not actually speed up. The generator rotor will advance several electrical degrees from the no-load position. This advance is made with respect to the neutral or center of the north and south poles of the rotating magnetic field. The generator rotor continues to rotate in synchronism with the rotating field produced in stator by virtue of its being connected to the system. As the governor is advanced, the generator will start to deliver power. It will also deliver magnetizing power if the system requires it. When the governor control has been advanced to the high idle position, the unit is capable of delivering power over its full range from no-load to full-load depending on the requirements placed on it by the system. The governor speed droop characteristic acts to cause the engine to deliver its proportionate share of the load. If the engine tends to take more load, it will tend to slow down and drop this additional load. If the engine tends to drop some load, it will tend to speed up and take some load. It requires some imagination to visualize these operations and understand them. Also, the generator voltage droop characteristic acts to cause the generator to deliver its proportionate share of magnetizing power to the system and the load. If the generator magnetizing power tends to rise above its proportionate value, the voltage droop circuit acts to decrease the magnetizing level of the generator to its correct value. If the generator magnetizing power tends to fall below its proportionate value, the voltage droop circuit acts to increase the magnetizing level of the generator to its correct value.
Most electric power stations have limited capacity so consideration must be given to the limitations of the installation. These limitations include slight variations in frequency and voltage, and limited power. Under these conditions, changes in governors operate along their speed droop characteristics and small changes in voltage as regulators operate along their voltage droop characteristics. The sudden application of large blocks of load will cause dips in frequency and voltage. The size of these blocks of load must be checked against the capacity of the system since too large a load suddenly applied can cause the system voltage to dip low enough that some loads may be disconnected automatically by low voltage protective devices. Also, slight discrepancies in the adjustment of governors or voltage regulators can result in frequency being high or low, voltage being high or low, load unbalanced, or unequal division of amperes between generators. A full complement of switchboard instruments including voltmeter, ammeter, kilowatt meter, and frequency meter for checking each generator is extremely helpful in balancing the operation of paralleled generators. This is particularly true for a power plant with more than two generators operating at any one time.
Under the conditions described above, and engine generator unit will automatically follow the load variations and continually deliver its proportionate share of the load requirements. A small amount of practice by an operator coupled with a study of the various operating steps and their effects on each other, will build confidence and bring understanding of the equipment to the operator.