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Application of Industrial Boiler Control System


        Application of ACI Inverter in Boiler Control System


    This paper introduces inverters about the energy saving principle , application method of frequency inverter, the selection of frequency inverter, the protection device of frequency inverter and the selection of the conductor of the electric machine in application of control system in industrial boiler.

一、Introduction

    Frequency inverter is a power control device which uses the power semiconductor device to change the power frequency to another frequency. Its main circuit is AC - DC - AC circuit. 
     In theory, we can see that the speed of the motor F and power supply frequency N have the following relationship: 

      n = 60f(1 - s)/p (1)
     Among them: P-- motor poles S-- slip
     From formula (1) we can see that rotational speed n is proportional to the frequency f. If you do not change the motor's pole pairs, as long as the change in the frequency of F can change the speed of the motor, When the frequency of F in the range of 0 ~ 50Hz changes, the motor speed regulation is very wide. Frequency inverter achieves speed regulation by changing the frequency of motor power supply, which is an ideal means of high efficiency and high performance and widely used in the field of industry. With the frequency converter of the cost is reduced, the use of inverter driven induction motor speed control system constitutes a huge role.
二、
The Main Purpose of Frequency Inverter in Boiler Control System


    
The transducer is mainly used in the blower, the draught fan, the water supply system and the slag removal system in the automatic control system of industrial boiler. The main purpose is to save energy under the premise of industrial boiler to meet the production requirements. But the drum, the draught fan, the water pump is the square torque load, according to the principle of fluid mechanics, the relationship between flow, pressure, power and speed of the square torque load is as follows:

     Flow ∝speed, pressure∝torque ∝the square of the speed, power ∝flow* torque ∝ speed cube. Namely: the wind turbine or pump flow and speed of the primary side is proportional to, the pressure is proportional to the two power of the rotating speed, and the shaft power is proportional to the three power of the speed. Therefore, the ideal situation has the following relationship:

Flow (%)

Speed

Pressure (Lift) (%)

Power

100

100

100

100

90

90

81

72.9

80

80

64

51.2

70

70

49

34.3

60

60

36

21.6

50

50

25

12.5

By the above relationship, when the demand flow down, it can save a lot of energy by adjusting the speed. For example: when the traffic demand in half, such as through the frequency conversion, you theoretically can save 87.5% of the energy only by the amount of power of 12.5%. Such as the use of the traditional way of baffle regulating air flow, although it can be appropriate to reduce energy consumption, but the saving effect is compared with the frequency, which is a world of difference.   


三、Application of Frequency Inverter in Every Link of the Boiler Control System  

   1、Grate motor inverter and the drum, the lead converter type fan selection of different.

   Grate frequency choose general type that use ACI G11 series inverter; drum fan frequency, use energy-saving ACI P11 series inverter.

    2、The drum and the draught fan adopt frequency conversion control, which is the main part of the frequency conversion application in the boiler system, and is not only the most significant part of power saving, but also the largest part of the frequency conversion investment. Control process:

    ①.Induced air control: Through the negative pressure transmitter in the furnace, the furnace pressure standard electric signal is fed into the feedback channel of the PID controller, set the furnace negative pressure comparison after the treatment. Through the PID controller to control the induced air inverter and motor speed, which achieve automatic tracking of the blast furnace vacuum to maintain a constant purpose by the stable negative pressure. The speed of the wind turbine is changed with the change of the negative pressure value of the furnace, which guarantees the automatic operation of the boiler combustion part.

    ②. Blast control: Through the pressure transmitter on the steam pipe, the steam pressure signal is sent to the feedback channel of the PID controller of the blower frequency inverter. The PID controller controls the frequency converter to adjust the speed of the motor, so that the pressure of the steam pipe is basically stable at the set value,

    ③.Before the transformation of the frequency inverter, boiler control drum and draft fan air volume is to rely on manual according to the size of the steam pressure to operate air duct baffle opening control fan wind. When the steam pressure is lower than the required pressure, open the big wind shield. When the steam pressure is higher than the required pressure, close the small wind shield. As a result, the air flow generated by the motor running at full speed can not be fully used, and using baffle to control air flow can result in the loss of about 30% of the interception. The use of frequency converter can be adjusted according to the production requirements of the motor speed, so that the output air volume can be adjusted to improve the production efficiency and save energy and reduce consumption.

      

    The inverter uses DLT-P11 ACI series. The starting mode of the control system is external remote manual control.

    (1)Speed regulation mode:

   A. according to the production needs of the pressure, set the frequency converter PID of a given pressure, The blower speed is adjusted automatically by the pressure of the PID. Frequency inverter built-in PID control fan speed to ensure that the furnace negative pressure is set value.

    B.Frequency inverter through the terminal to switch to automatic PID operation mode.

    C.The system can also realize the open loop operation with manual control and speed regulation by controlling the start and stop buttons and the potentiometer on the control cabinet or on the spot operation box.

    (2)Main interlock of the system:

    Blower operating conditions: only when the fan is put into operation, the blower is allowed to start running.

    B.Frequency conversion and power frequency chain. Frequency conversion operation and power frequency operation interlocking.

    C.The frequency conversion system sends out the sound and the light alarm signal when the fault occurs in the operation process, and reminds the duty personnel to deal with it in time.

    (3)System structure: the control principle diagram as shown below



四、 Advantages of Frequency Inverter in the Application of Industrial Boiler:

    1、Realize automatic control, opened a new chapter in the automation of boiler operation. The combustion process which makes it difficult to control realizes the automation and reduces the labor intensity. In the growing popularity of the network today, the frequency converter system which is connected by high-speed communication can reduce the system maintenance time, increase production efficiency and reduce operating cost.

    2、Control the starting current of the motor. When the motor is started directly, it will produce 7 to 9 times the rated current of the motor. This current will greatly increase the internal stress of the motor windings and generate heat, thereby reducing the electrical life of the motor. And variable frequency speed regulation can realize soft start. Once the frequency and voltage of the relationship between the establishment of the inverter can be in accordance with the V/F or vector control mode to drive the load to work. The use of variable frequency speed control can fully reduce the starting current, improve the winding endurance, the most direct benefit is that the maintenance cost of the motor will be further reduced, the life of the motor will be extended accordingly.

    3、Reducing voltage fluctuation of power line. At power frequency of the motor, the current surge in the same time the voltage of the power grid will be greatly reduced, the magnitude of the voltage drop will depend on the power of the starting motor and the power distribution network capacity. Voltage drop will result in failure of voltage sensitive equipment in the same power supply network or abnormal operation, such as sensor, proximity switch, contactor and so on. And the use of frequency control, due to the soft start, to the maximum extent to eliminate the voltage drop.

    4、Stepless speed regulation of the fan air volume can be smooth, so that the work of the fan in the best working condition, working curve is more consistent with the actual requirements, not only improves the efficiency of the fan, and avoid the "surge" phenomenon. Stable negative pressure in the furnace to meet the requirements of the working environment.

    5、Low speed operation can reduce wear and reduce noise, it is beneficial to prolong the service life of motor and fan.

    6、Significant energy saving effect. Because the final energy consumption than the establishment and the speed of the motor, so the frequency, the electric energy is saved, and faster return on investment, users are willing to accept.


五、Problems in Practical Application of Frequency Inverter

    1、The choice of frequency converter power

Selection of inverter, to fully consider the original motor system margin, otherwise the original motor system in normal operation, the use of variable frequency control, variable frequency motor capacity is not found, then the replacement of a variable frequency, will inevitably lead to unnecessary loss and trouble.

    2、Reliability of remote frequency conversion control

The control terminals of the converter are weak current DC signal or contact signal. When the remote control is used, the anti-interference and the loss of the line should be considered. Especially when the switch is used to control the acceleration and deceleration, it is possible to make the output of the node as far as possible with the frequency conversion, to prevent the passive node of the line impedance and interference caused by the control is not sensitive or failure.

    3、Other problems in design selection

  (1)Selection of air circuit breakers

    Since the inverter has characteristics of soft start and no impact, so the air switch can be selected according to the inverter capacity, do not have to consider the current impact of motor start.

  (2)Selection of motor thermal protection

    Because the inverter is controlled by a single motor, it is not required to choose the thermal protection relay. However, if it controls multiple motors at the same time, the thermal protection of each motor to be calculated and selected separately. It is selected by the current situation of the motor at low speed, rather than selection by the motor rated current. This current is much larger than the rated current of the motor.

  (3)Because the current and voltage of inverter output changes with frequency, so the current and voltage detection of frequency conversion equipment should be carried out at the end of the converter. The current transformer and voltage meter should be designed at the end of the line after the air circuit breaker and in front of the frequency inverter.

  (4)Wire problem

    When frequency inverter put line, it can be appropriate to reduce the margin. Because the inverter power saving feature is to reduce the line current. But the inverter outlet to be appropriate to increase the margin, especially  the long-term low speed operation of the inverter device, whose output current is quite large.


六、Application of ACI Inverter in Power Plant Fan:

    Guangdong Guanghui Energy Co. Ltd. (garbage power plant) is a energy saving and environmental protection enterprises, which uses garbage and industrial waste to generate electricity.Design scale is the daily processing of 600 tons of garbage, generator installed capacity of 2X6WM. Waste incineration power generation project in December 2007 formally and grid connected power generation. Furnace blower, electric control system all use SIEMENS electric motor, frequency converter, contactor (frequency conversion). The use of a few years later, a number of SIEMENS inverters have damaged. Because the repair cycle is long, high cost of repair, Guangdong Guanghui Energy Limited company leadership decided to adopt domestic inverter instead of SIEMENS inverter. After a comparative analysis, the company has selected several domestic inverter, but in the use of a few months after the inverter has a phenomenon of the inverter, which seriously affect the safety of production.


    After investigation, Guangdong Guanghui Energy Limited company decided to use ACI G11 general inverter. At the installation site, the technical personnel of our company found that the main reason is that the power grid voltage is very high when the power plant is very low at night.It is sometimes over 450V. At night, the amount of waste is less, and the amount of air required is less. Fan is a large inertial load, in the reduction process will be converted to mechanical energy into electrical energy feedback to the inverter, which makes the converter bus voltage rise to near the overvoltage protection value. If the transducer is internally integrated with conductive dust or humidity is too large and the frequency converter insulation level is reduced, the inverter may be due to internal insulation breakdown and damage.

    After analyzing the cause of the frequency converter, we have adopted the following measures:

    ①To enhance the electrical insulation, improve the voltage from the hardware level;

    ②Adjust the display value of DC bus voltage, the converter bus voltage display value is greater than the value of about 5% (such as the actual value is 540V, the display value is about 567V), the purpose is to reduce the stall point overvoltage (software restriction minimum 120% stall overvoltage point play a role in 115%);

    ③Adjust relevant parameters, reduce the over-voltage stall point from the default value (i.e., 140% stall voltage is 750V, close to the overvoltage protection value of 780V, if the bus voltage in the vicinity of 750V if the long-term frequency may be reduced insulation insulation breakdown and the damage to the drive) reduced to 120% (645V), the bus voltage inverter in the display value of 645V wind cannot slow down, to prevent the fan motor in the power generation state of the inverter bus voltage is increased further;

    ④Set inverter shutdown mode for free parking, to prevent the inverter from over voltage damage. After adopting the above measures, I produced the blower, 1 P11-220KW 1 P11-315KW fan from March 2010 to now has been running, no downtime phenomenon, stable production, while saving a lot of energy.

7.Concluding Remarks

    

  Industrial boiler control system adopts frequency converter to realize boiler control, which has high stability, high reliability and good control performance. Since the inverter can be very smooth and stable regulation, which makes the operation staff can flexibly control the combustion system and water supply system, which improves the efficiency of the boiler and reduces the working intensity. The frequency conversion speed regulation makes the motor significantly improved, and the maintenance quantity is obviously reduced, and the speed changing mechanism and the control mechanism of the mechanical system are greatly reduced. The system is more convenient to operate, and the efficiency of the equipment is improved obviously, The system adopts overcurrent and overvoltage protection andinstantaneous power, short circuit, under voltage, lack of equal protection, so this avoids the impact caused by the production of direct and indirect economic losses of motor burning, more important is its energy saving effect is very obvious, the economic benefit is very considerable.







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