A brief discussion on the application of PLC in automated diesel generator sets

1 Main functions

As a backup power source, the automated diesel generator set should have the following basic functions:

(1) Automatic start

When the mains fails (power outage, undervoltage, overvoltage, phase loss), the unit can automatically start, automatically increase speed, and automatically close the switch to supply power to the load.

(2) Automatic shutdown

When the city power is restored and judged to be normal, the switch is controlled to complete the automatic switch from power generation to city power, and then the unit is controlled to slow down and automatically shut down after idling for 3 minutes.

(3) Automatic protection

During the operation of the unit, if the oil pressure is too low, overspeed, or voltage is abnormal, the unit will be shut down immediately and sound and light alarm signals will be issued. If the water temperature is high or the oil temperature is high, sound and light alarm signals will be issued and the unit will shut down normally after a delay.

(4) Three-time start function

The unit has three start functions. If the first start fails, it will start again after a 10-second delay. If the second start fails, it will start for the third time after a delay. As long as one of the three starts is successful, it will run according to the pre-set program; if three consecutive starts are unsuccessful, it will be regarded as a start failure, and an audible and visual alarm signal will be issued. It can also control the start of another unit at the same time.

(5) Automatically maintain the quasi-start state

The unit can automatically maintain the quasi-starting state. At this time, the unit's automatic periodic pre-oil supply system, automatic oil and water heating system, and automatic battery charging device are put into operation.

(6) With maintenance startup function

When the unit has not been started for a long time, it can be started for maintenance to check the performance and status of the unit. The maintenance start does not affect the normal power supply of the mains. If the mains fails during the maintenance start, the system will automatically switch to the normal start state and be powered by the unit.

(7) It has two operation modes: manual and automatic.

2. Hardware Design of Control System

2.1 Select the PLC model according to the required input/output points

According to the control requirements of the automation unit, the required PLC input points are 11 and the output points are 12. The control quantity of the system is basically a switch quantity, and only the voltage and speed are analog quantities. In order to reduce costs, the analog quantity can be converted into a switch quantity through a detection circuit. In this way, a PLC without analog input can be selected. This system uses a C28P-CDR-D small programmable controller with high reliability and small size. The number of input points is 16 and the number of output points is 12. The power supply, input, and output voltages are all 24VDC.

2.2 Allocating PLC input and output

According to the control requirements and electrical schematic diagram of the automation unit, the PLC input and output signal distribution table is shown in Table 1.

Table 1 Input/output allocation table

When the switch of the automated unit is turned to the automatic position, the unit is in a quasi-starting state. The PLC performs uninterrupted cyclic detection of the mains, and the pre-supply oil pump periodically supplies oil to the diesel engine lubricating oil channel. The cooling water and oil temperature of the diesel engine can ensure that the unit can be started at any time in an emergency. In the quasi-starting state, once the mains fails, such as power failure, undervoltage, overvoltage, and phase failure, and is confirmed after 5 seconds, the PLC controls the unit to start automatically, automatically increase the speed to the rated speed, and automatically protect it. When the unit is working normally, the control switch is switched to the power generation state, and the unit is switched to the power supply state. When the mains returns to normal, after a delay of 5 seconds to confirm, the unit output switch is disconnected, and the control switch is switched to the mains state. The unit automatically slows down to the idle state, and automatically stops normally after running for 3 minutes. The unit is in a quasi-starting state.
According to the control process and input and output allocation table of the automated unit, a ladder diagram controlled by PLC can be compiled.

4 Conclusion

The automated diesel generator set controlled by FLC has simple hardware and good technical and economic indicators. It has been used on different units and has high reliability. With a slight modification of the program, it can meet the different control requirements of users.