Plc graduation thesis

On the basis of analyzing the requirements of emergency generator self-starting, combining with the present situation of equipment and the design requirements of distribution system, the existing equipment is reformed by using the advantages of PLC, and detailed design ideas and schemes are put forward for reference.

Keywords: PLC emergency generator scheme power distribution system

On the basis of analyzing the self-starting requirements of emergency generator, combined with the present situation of equipment and the design requirements of distribution system, a detailed design idea and scheme of transforming existing equipment by using the advantages of PLC (programmable controller) are put forward for reference.

Traditional generator control usually adopts backward relay contactor control mode. There are too many intermediate relays and time relays, which are bulky, with few functions, short service life, complicated lines and many contacts, resulting in many faults, poor reliability and difficult maintenance. However, due to the variety, large volume, long design cycle, low cost, complex technology, poor anti-interference and poor reliability of integrated circuit (IC) system chips, microelectronics technology is adopted. Programmable controller (PLC) is a new general automatic control device based on microprocessor and integrating computer technology and communication technology. It has the advantages of simple structure, superior performance, high reliability, flexibility, easy programming and convenient use. In recent years, it has been widely used in industrial automatic control, electromechanical integration and the transformation of traditional industries.

Using PLC to control emergency generator set has many advantages. It is mainly controlled by software, which saves hardware development work and has few peripheral circuits, which greatly improves the reliability and anti-interference ability of the system. Because of its simple programmable function, the control requirements of the system can be changed without changing the external hardware wiring of the system, which greatly improves the "flexibility" of the system.

Main design functions

In the process of production, when power is suddenly cut off, the emergency generator immediately continues to supply power to the equipment. The prime mover of emergency power supply generally adopts diesel engine with independent cooling and oil supply system, and is equipped with self-starting device to ensure that it can be started within 0-50 seconds after the main station is cut off. The emergency power grid is usually a part of the main power grid. Under normal circumstances, these appliances are powered by the main distribution board, but only in an emergency are they powered by the emergency generator set. Therefore, there is an electrical interlock between the main switch of the emergency generator on the emergency switchboard and the switch that the main switch supplies power to the emergency switchboard to ensure safety.

As an emergency power supply, the emergency generator set shall meet the following basic requirements:

1, start automatically

When the normal power supply fails (power failure), the unit can automatically start, accelerate and shut down to supply power to the emergency load.

2, automatic stop

When the normal power supply is restored and judged to be normal, the control switch completes the automatic switching from emergency power supply to normal power supply, and then controls the unit to slow down to idle stop.

3, automatic protection

During the operation of the unit, if the oil pressure is too low (less than 0.3MP), the cooling water temperature is too high (more than 95 degrees) and the voltage is abnormal, the unit will stop urgently and send out an audible and visual alarm signal. Whether there are faults such as high water temperature (above 90 degrees) and high oil temperature. Send audible and visual alarm signals to remind maintenance personnel to intervene.

4, 3 start function

The device has three starting functions. If the first startup is unsuccessful, it will be restarted after a delay of 10 seconds. If the second startup is unsuccessful, it will be restarted after a delay. As long as one of the three starts is successful, it will run according to the preset program; If the startup is unsuccessful for three consecutive times, it will be regarded as a startup failure, and an audible and visual alarm signal will be issued (another unit can also be controlled to start at the same time).

5, automatically keep the quasi-start state.

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

6, with manual and automatic operation mode.

Hardware design of control system

Emergency power supply mostly adopts 135 series diesel generator sets. Here, taking this as an example, the self-starting control of diesel engine is realized by PLC.

Circuit analysis

Design Description: There are "Manual/Automatic" selection knobs and "Start", "Acceleration", "Deceleration", "Close" and "Release" buttons on the control panel. The proximity switch (rotary encoder) is installed on the diesel engine to measure the rotating speed, the throttle motor is installed to control the rotating speed of the diesel engine, and the electromagnet is installed to stop and extinguish the engine. Voltage detection, water temperature and oil pressure are all external switching signals.

One-time starting process: after normal power failure, the "starting motor" starts for 4 seconds after 5 seconds. If the diesel engine is on fire, the proximity switch (rotary encoder) detects that the diesel engine reaches the starting speed, and the PLC immediately stops the "starting motor". After the diesel engine is idling for 30S, it starts to accelerate according to the signal of the proximity switch until the speed is stable and the generator starts to generate electricity. After the voltage is normal, the main switch is closed to supply power to the load. In operation, PLC automatically stabilizes the speed.

Three-time starting process: If the first start is unsuccessful, the proximity switch (rotary encoder) detects that the diesel engine can't reach the starting speed, and after 5 seconds, the diesel engine can't reach the starting speed, which is controlled by the timer inside PLC, taking 10 second as a cycle, and the three-time starting time is about 30 seconds, and an alarm is output after 32 seconds. If the proximity switch (rotary encoder) fails to detect the starting speed of the diesel engine during starting, the engine will restart.

Start failure and diesel engine stop: After start failure, the accelerator is pulled back to the "stop" position by electromagnetism. When the power returns to normal, the PLC sends out a brake opening signal, and the throttle motor slows down to 60S idle speed, and then the throttle is pulled back to the "stop" position by electromagnetism, and the diesel engine stops due to lack of oil.

Small man-machine interface can be added according to users' needs, and various values and states of diesel engines can be displayed in the form of words, indicator lights and patterns. And the value and state of the diesel engine can be changed by buttons on the panel. Time-related parameters can be modified, and the range of input data can be set, and data beyond this range can be rejected. Various faults of diesel engine can be displayed in text form, so as to find faults, such as three-time starting failure, high speed, high cylinder temperature, mains power supply, etc. With password protection function, it can prevent unauthorized users from changing important data and switch values.

Unit characteristics of automatic control

Unit (1) is composed of diesel generator set and central control cabinet, which can be controlled by single machine and single cabinet, double machine and single cabinet or networked automation (unattended).

(2) The core of the control cabinet is the programmable controller (PLC), and the CPU306 small programmable controller of Beijing Kaidian Company is usually selected, which is reliable in operation and stable in quality.

(3) Make full use of the instructions and functions of PLC for programming, and minimize peripheral control components and interfaces. The circuit is simple and easy to operate and maintain.

(4) Using the high-speed counter function of PLC, the rotating speed of the unit can be accurately measured, and the original tachogenerator and tachometer are not used, thus avoiding the installation difficulty and improving the reliability.

(5) The controller adopts 24V DC power supply and is equipped with advanced high-frequency switching DC charging equipment, which can float the battery and ensure the DC power supply of the control cabinet.

(6) 6) EPROM in PLC can solidify the program, so that the original program will not be lost for a long time.

(7) Using the communication function of PLC can realize centralized monitoring in short distance and long distance.

Technical requirements:

It is better to use a rotary encoder than a proximity switch.

Technical requirements of proximity switch:

Thread proximity switch detection distance 10 mm 10% working voltage DC type: 10-30VDC three-wire response frequency 400Hz.

Proximity switch, also called contactless proximity switch, is an ideal electronic switch sensor. When the metal detector is close to the induction area of the switch, the switch can send out electrical instructions quickly, without contact, pressure and sparks, and accurately reflect the position and stroke of the moving mechanism. Even if it is used for general travel control, its positioning accuracy, working frequency, service life, convenience of installation and adjustment and adaptability to harsh environment are incomparable to general mechanical travel switches.

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

According to the control requirements of the automation unit, the input point of the required PLC is 14 and the output point is 10. The control quantity of the system is basically a switch quantity, and only the voltage is an analog quantity. In order to reduce the cost, analog signals can be converted into switching signals through detection circuits. For example, voltage monitoring can be replaced by voltage protectors. In this way, you can choose a PLC without analog input. For small generators, there is no need to install throttle motor to control the speed of diesel engine. The system adopts CPU306 small programmable controller of Beijing Kaidian Company, which has high reliability and small volume. The number of input points is 14 and the number of output points is 10. The power supply, input and output voltages are all 24VDC.

Distribution PLC input and output

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

Table 1 input/output allocation table

I0.0

Power failure signal

Q0.0

Throttle acceleration

I0. 1

Proximity switch

(rotary encoder)

O0. 1

Throttle deceleration

I0.2

Proximity switch * *

(rotary encoder) * *

Question 0.2

Turn on the motor

I0.3

Voltage is normal

Question 0.3

open

I0.4

Low oil pressure

Question 0.4

Release brake

I0.5

High water temperature

Q0.5

Stop electromagnet

I0.6

Manual/automatic

Question 0.6

Breakdown signal

I0.7

Start button

Question 0.7

I 1.0

Acceleration button

Q 1.0

I 1. 1

Delay button

Q 1. 1

I 1.2

stop button

I 1.3

Close button

I 1.4

Brake button

I 1.5

Turn off the output signal

Note: I have DC 24V input, and the passive contact output of Q (24V3A) 1 means on and off.

See appendix1:for circuit design (opened in AutoCAD 2004).

The generator timing diagram is shown in Appendix 2: (Autocad2004 opens)

See the attachment for the generator PLC source program (download the latest version of EasyProg software from the website of Beijing Kaidean Automation Technology Co., Ltd. and open it). The source program is to install a proximity switch. The diesel engine sends out six pulse signals every revolution, and the diesel engine measures the speed at 1000 revolutions per minute with a period of 0.5 seconds. If a rotary encoder is used to measure the speed with a period of 0. 1 sec, the effect is better.

conclusion

The automatic diesel generator set controlled by PLC has the advantages of simple hardware structure, low cost, fast response, high cost performance and high reliability compared with single chip microcomputer system. The field test shows that the performance is stable and the operation is reliable. In addition, it can be easily expanded according to actual needs. As long as the program is slightly modified, it can meet the different control requirements of users. For modern intelligent buildings, the control system can also be incorporated into the monitoring system of the whole building through the communication module, which shows great flexibility and adaptability and has high practical promotion value.