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Application of PLC in diesel generators

Diesel generator set is a type of ac power supply equipment of self-provided power station. It is a small and independent power generation equipment. Diesel generating set is also called mobile power station, is a kind of standby power supply, when external power grid power supply tension, the suspension of power supply failure or overhaul, but by starting diesel generator power supply, in order to maintain the normal power supply, which in the case of temporary power supply, continuous production suddenly loses power and plays an important role in fire control, etc.

Programmable Logic Controller (PLC). PLC is the core part of the microprocessor, which not only has the logic control function, but also has the operation, data processing and data transmission, and other functions, is a device which has the function of computer special industrial control. The compact design of programmable controller, good expansibility, low price, strong instruction, high reliability and simple maintenance meet the requirements of automatic control almost perfectly.

1 composition of the system

Based on PLC, the system consists of diesel generator set and programmable logic controller. Through this control system can be realized: when the power grid is normal, the load is supplied by the power grid; When the power grid is not normal, the control system starts the diesel generator set immediately to realize the power supply from the diesel generator set to the load. When the power grid returns to normal, the system will restore power to the grid and shut down diesel units. This system ensures the normal output of the load.

1.1 start and stop the diesel engine
When the programmable controller passes the three-phase voltage protection relay and detects the power supply is abnormal, start the diesel generator set immediately. The starting of diesel generator sets is divided into the following two stages. 1) PLC output point Q0.1 output, make the diesel engine running into the state, and the output Q0.0 closed make KA1 power, start the motor M run, drive the diesel engine running, when after the success of the diesel engine start, PLC output point Q0.0 output point immediately disconnect, KA1 lost electricity, starter motor and diesel engine flywheel separation. (2) when after the success of the diesel engine start into the idle running after the 30 s, PLC output point Q0.2 closure, the diesel engine to run at full speed, electronic governor drive actuator, a diesel engine throttle more, into a state diesel engine running at full speed.

The shutdown of diesel generators can also be divided into the following two stages. (1) in determining the diesel generator external power supply switch is disconnected, namely stops to load power supply, PLC output point Q0.2 disconnected, the diesel engine to run the idle speed, electronic governor drive actuators, will decrease the diesel engine throttle diesel engine into the idle running condition. The output point of PLC Q0.1 is disconnected after ringing 30s, causing the diesel engine to stop.

1.2 switching power supply control between diesel units and municipal power
Overload and short-circuit protection circuit breakers QF1 and QF2 are disconnected in normal state. When the system is open, the circuit breaker is closed. The utility provides power to the load through the contactor KM1. The unit supplies power to the load through the contactor KM2. The two contactors are interlocked through a contact cabinet to prevent parallel mechanical interlocking. The three - phase voltage protection relay on the power supply is used to check whether the power supply is normal. When the mains is normal and stable phase protection relay signals a mains normal instructions to PLC, PLC detected after the mains is normal, by controlling the power supply output unit contactor KM2 disconnect, closed mains power contactor KM1. At this point, the power station supplies power to the load through the municipal power supply.

When the mains is not normal, three-phase protection relay will give P L C signal a grid abnormal instructions, PLC detected after the mains is not normal, through the PLC to start the engine, to make it run at full speed, controlling the output will be mains power contactor KM1 disconnect, closed unit power contactor KM2. At this point, the power station supplies power to the load through the unit.

1.3 software design
PLC consists of central processing unit CPU, memory, input, output unit, power supply and programmer, etc. PLC is a working mode of cyclic scanning, that is, each change of state requires a scanning cycle. The cycle scanning time of PLC is usually several milliseconds to several tens of milliseconds. The whole process is divided into internal processing, communication, input processing, execution program and output processing.

PLC program is run from the starting address of 0000 to the last address (that is, the END instruction), for repeated scanning circuit, in strict accordance with the ladder diagram, logic diagram lines logical sequence and logic logic elements are arranged from top to bottom, from left to right word for word processing program. In this way, the node competition that relay control system is difficult to solve and the phenomenon that delay relay is not accurate will not occur, thus ensuring the reliability of control system.

There are three programming methods for the s7-200 programmable controller, namely, ladder-shaped graph (LAD), statement table (STL) and functional graph block (FBD). Trapezoid diagram is more intuitive, programming and debugging are very convenient; Statement table programming speed is slow, debugging is more complicated; A functional block can view logic box instructions like a normal logic gate graph but is relatively complex and less used. The handheld programmer of phrase table programming has the highest cost performance, and users can choose the programming language according to their habits.

2. System notes

1) PLC installation location should avoid sun direct illuminate, ensure adequate cooling space and ventilation, avoid interference installed in severe high temperature, high humidity, dust, clean and no corrosion gas environment. In this case, the PLC is installed in the control panel with damping measures.

The power cable and control cable should be laid separately to avoid interference.

Press power connection: PLC usually USES a single-phase ac power supply. When wiring, it is necessary to distinguish the "N" end "zero line" and "ground" end on the terminal. The ac power line of PLC shall enter into the control cabinet from the top of the machine alone, and shall not be tied together with other dc signal lines and analog signal lines to reduce the interference to other control lines. To curb on power supply and input, the output interference, should give special PLC connected to ground, the ground should be ground separate diesel generator set, usually pay attention to check whether the PLC grounding is good.
(4) control PLC working environment (0 ℃ to 50 ℃ advisable), forced air cooling mode should be used when necessary, can effectively improve the work efficiency and life span [3].

4 conclusion

PLC control of automatic diesel generating sets, mainly through software control, eliminating the hardware development work, few peripheral circuit, greatly improving the system reliability and anti-interference ability, and its hardware structure is simple, low cost, the corresponding speed, stable performance and reliable operation. In addition, it can be extended according to the actual needs, and the program can meet users' different control requirements with a little modification, which shows great flexibility.