This chapter describes the operation of the Load Sharing Module and its internal circuits. Figure 1-4 is a block diagram of the circuits of the module.
The Load Sharing Module senses the power output of a generator and provides a pulse-width modulated signal to the Caterpillar digital speed control to adjust the power output of the engine-generator set to match the reference power level. The Load Sharing Module can also produce a droop condition (instead of isochronous load-sharing) for the engine with Caterpillar digital speed control, making it possible to connect the generator set in parallel with either a generator set which is running isochronously, or with an infinite bus.
The internal power supply generates a regulated dc voltage for the operation of the circuits in the Load Sharing Module. The power supply gets its power from the engine 24 Vdc power source. To prevent damage to the Load Sharing Module that uses an alternator or battery charging device, make sure the charging device is turned off before disconnecting or connecting the 24 Vdc supply to the unit.
Generator load is measured by the power-sensor circuit of the Load Sharing Module. The power-sensor circuit senses current amplitude, and produces a load signal proportional to the current amplitude times the power factor. The potential input comes from potential transformers (PTs) and the current input comes from the current transformers (CTs). The circuit uses these two inputs to generate a load signal which is then filtered and sent to the controller circuit. The load signal voltage of each generator set will be proportional to the percentage of rated load of the generator set.
Ammeters and Voltmeters may be driven with the same PT and CT wires.
Load Comparator Circuit
In the load comparator circuit, the load signal voltage is balanced with the other generator sets in the system via the load sharing lines. The comparator circuit of each Load Sharing Module includes a load gain potentiometer to adjust each generator set's load signal so that the load signal voltage of each is the same at full load. This compensates for varying CT ratios or different generator set sizes. The load sharing voltage will be one-half the measured voltage of the load signal test point.
Speed Trim Circuit
A 10 k ohm 10-turn speed-trim potentiometer can be added to the Load Sharing Module to permit remote adjustment of the load or frequency of the generator. The speed trim is normally used for manual synchronization of a generator set with an existing bus or to change the load demand when in droop mode. If no speed trim is needed, add a 10 k ohm resistor between terminals 26 and 28.
Isochronous Load Sharing
Each comparator circuit compares the load signal voltage for its generator set to two times the voltage on the load sharing lines and produces an error voltage proportional to the difference. This error voltage is used to generate a pulse width modulated signal which is output to the Caterpillar speed control. This output biases the speed loop of the Caterpillar speed control until the load signal voltage is equal to that of other generators on the load sharing lines.
In droop operation, a portion of the load signal voltage is fed to the controller circuit. This voltage is used by the comparator circuit to reduce the control output by a percentage determined by the DROOP potentiometer. The output is reduced, and the Caterpillar digital speed control reduces the engine power according to the desired droop percentage.
When a generator set using the Load Sharing Module is paralleled in droop with other generator sets, the common load signal on the paralleling lines is not used. The frequency of the generator set will therefore vary with load, so it must be determined by a different means. In an isolated system with two or more generator sets paralleled, if isochronous speed control is required, one of the generator sets must be running in the isochronous (constant speed) mode. This generator set maintains the frequency system. If a generator set is in droop and is paralleled with an infinite bus, the bus determines and maintains the frequency. The DROOP percentage and the speed setting on the Caterpillar digital speed control determine the amount of the load that is carried by the generator, when running in droop.
The Woodward SPM-A synchronizer functions by biasing the output of the Load Sharing Module. All other Woodward auxiliary generating control equipment functions on biasing the voltage load-sharing lines.
Pulse Width Modulation
The Load Sharing Module output to the Caterpillar digital speed control engine is a frequency of 500 ± 100 Hz. The duty cycle ranges from a low of 10 percent for maximum decrease in load or speed to a high of 90 percent for maximum increase in speed or load. The duty cycle varies according to the magnitude of difference between the reference load and the actual power generation.