Application of common DC bus of the hottest genera

Application of common DC bus of general frequency converter in centrifuge

Abstract: This paper introduces the application of common DC bus of frequency converter in the centrifuge of vital pharmaceutical industry of Shijiazhuang Pharmaceutical Group

key words: frequency converter common DC bus centrifuge energy feedback energy saving

1 introduction

in the electrical drive of chemical enterprises, the frequency conversion drive of centrifuges is widely used. Due to various reasons of process and driving equipment, the phenomenon of renewable energy often occurs. In general frequency converters, There are two common ways to deal with regenerative energy: (1) dissipate it into the artificially set "braking resistance" in parallel with the capacitor in the DC circuit, which is called dynamic braking state; (2) If it is fed back to electricity, it is called feedback braking state (also known as regenerative braking state). The principle of DC common bus is based on the fact that all general frequency conversion devices adopt AC-DC-AC frequency conversion mode. When the motor is in braking state, its braking energy is fed back to the DC side. In order to better deal with the feedback braking energy, people adopt the way of connecting the DC side of each frequency conversion device. For example, when one frequency converter is in braking and the other frequency converter is in accelerating state, the energy can be complementary. This paper presents a scheme of common DC bus of general frequency converter in centrifuge of chemical enterprises, and expounds its further application in the feedback unit of centrifuge after purchasing a zigzag experimental machine

at present, there are many ways of DC common bus:

(1) share an independent rectifier

the rectifier unit can be either invertible or invertable. The former consumes energy through external braking resistance, while the latter can fully feed back the excess energy on the DC bus directly to the electricity, which has better significance of energy conservation and environmental protection. The disadvantage is that the price is higher than the former

(2) the large frequency conversion unit is connected to electricity

the small frequency converter shares the DC bus of the large frequency converter. The small frequency converter does not need to be connected to electricity, so it does not need rectification module, and the large frequency converter is externally connected with braking resistance

(3) each frequency conversion unit is connected to electricity respectively

each frequency conversion unit is equipped with rectifier and inverter circuits and externally connected. According to the fiber strength, the dynamic resistance is divided, and the DC bus is connected with each other. This situation is often used when the power of each frequency conversion unit is close. After disassembly, it can also be used independently without mutual influence

the DC common bus introduced in this paper is the third way, which has great advantages over the first two:

a. The common DC bus can greatly reduce the repeated configuration of the braking unit, with simple and reasonable structure, economy and reliability

b. The middle DC voltage of the shared DC bus is constant, and the capacitance and energy reserve capacity are large, which can reduce the fluctuation of electricity

c, each motor works in different states, and the energy feedback is complementary, which optimizes the dynamic characteristics of the system

d. The different harmonic interference generated by each frequency converter in the electricity can offset each other and reduce the harmonic distortion rate of the electricity

2 scheme of variable frequency speed regulation system before transformation

2.1 introduction to centrifuge control system

there are 12 centrifuges in total, and each control system is the same. The frequency converter is Emerson EV2000 series 22KW, constant torque type. The feedback units are energized IPC-PF-1s feedback braking units, and all control systems are centralized in a power distribution room. Two centrifuges share a GGD control cabinet, and only four of them are shown in limited space, and the other eight are similar. The system diagram is shown in Figure 1

Figure 1 Schematic diagram of frequency converter and braking unit system before transformation

it can be seen from Figure 1 that each frequency converter needs a feedback braking unit, and their respective control systems are completely independent.

2.2 braking work analysis during braking

when the centrifuge brakes, the motor will be in the regenerative braking state, and the mechanical energy stored in the system will be converted into electrical energy by the motor, The six freewheeling diodes of the inverter are sent back to the DC circuit of the frequency converter. At this time, the inverter is in rectification state. At this time, if no energy consumption measures are taken in the frequency converter, this part of energy will cause the voltage of the energy storage capacitor in the intermediate circuit to rise. At this time, the DC bus voltage of the capacitor will rise. When it rises to 680v, the braking unit will start to work, that is, feed back the excess electric energy to the power side. At this time, the DC bus voltage of a single frequency converter will remain below 680v (some 690V), and the frequency converter will not report overvoltage fault. The current curve of braking unit of single time converter is shown in Figure 2. The braking time is 3 minutes. The test instrument is fluke 43b single-phase power quality analyzer, and the analysis software is FlukeView power quality analyzer version 3.10.1

Figure 2 current curve of braking single space when using more reasonable elements

it can be seen that each time braking, the braking unit must work, and the maximum current is 27a. The rated current of the braking unit is 45A. Obviously, the brake unit is in half load state

scheme of frequency conversion and speed regulation system after transformation

3.1 disposal method of common DC bus

the important point of using common DC bus is that the control of frequency converter, transmission failure, load characteristics and input main circuit maintenance must be fully considered when powering on. The scheme includes 3-phase incoming line (adhering to the same phase), DC bus, general frequency converter group, common braking unit or energy feedback device and some auxiliary components. For general-purpose frequency converters, figure 3 shows one of the widely used schemes. The main circuit system diagram after the third transformation scheme is shown in Figure 3. The air switches Q1 to Q4 in Figure 3 are the incoming line protection devices of each frequency converter, and KM1 to km4 are the power on contactors of each frequency converter. Kmz1 to kmz3 are parallel contactors of DC bus. 1 #, 2 # centrifuges share a braking unit to form a group, and 3 #, 4 # centrifuges share a braking unit to form a group. When both groups are normal, they can be connected together. At the same time, it is also based on the working sequence of on-site operators. 1 #, 2 # centrifuges are not braked at the same time, and 3 #, 4 # centrifuges are not braked at the same time. In normal operation, two centrifuges 1 #, 3 # are a group, and 2 #, 4 # are a group. Generally, four centrifuges will not brake at the same time. Due to the complex environment of the actual work site, it often leads to the shaking of electricity and the occurrence of high-order harmonics. It can also be used to increase the power impedance and help absorb the surge voltage generated when nearby equipment is put into operation and the voltage spike of the main power supply, so as to finally maintain the rectifier unit of the frequency converter. Each frequency converter can also use incoming reactor to effectively prevent the influence of these factors on the frequency converter. In the reconstruction of this project, since the original equipment is not equipped with incoming reactor, the incoming reactor and other harmonic treatment devices are not drawn

Figure 3 Schematic diagram of frequency converter and braking unit system after transformation

3.2 scheme of control system

control circuit is shown in Figure 4. After four frequency converters are powered on and each frequency converter is ready for operation, set the output option of output terminal of frequency converter fault relay as "frequency converter is ready for operation". Only after the frequency converters are powered on and normal, they can be connected together in parallel. If any one of them is faulty, The DC bus contactor will not be closed. The output terminals TA and TC of the frequency converter fault relay are normally open contacts. After power on, the frequency converter "is ready for operation", the TA and TC of each frequency converter are pulled in, and the DC bus parallel contactor is pulled in sequence. Otherwise, the contactor will be disconnected

Figure 4 Schematic diagram of parallel control of modified braking unit

3.3 features of this scheme

(1) a complete frequency converter is used instead of a simple rectifier bridge plus multiple inverters scheme

(2) there is no need for separate rectifier bridge, charging unit, capacitor bank and inverter

(3) each frequency converter can be separated from the DC bus independently without affecting other systems

(4) control the connection of the DC common bus of the frequency converter through the interlocking contactor

(5) interlock control to protect the capacitor unit of the frequency converter hung on the DC bus

(6) all frequency converters hanging on the bus must use the same three-phase power supply

(7) quickly disconnect the frequency converter from the DC bus after fault to further reduce the fault range of the frequency converter

3.4 main parameter settings of frequency converter

operation command channel selection f0.03=1

maximum operation frequency setting f0.05=50

acceleration time 1 setting f0.10=300

deceleration time 1 setting f0.11=300

fault relay output selection f7.12=15

ao1 output function f7.26=2

3.5 test data after transformation

incoming line voltage during shutdown: 3Ph 380vac

bus voltage: 530vdc

DC bus voltage: 650V

when one speed increases, the bus voltage decreases, while the other speed decreases. The DC bus voltage fluctuates between 540 and 670v, and the braking unit does not start at this time. The DC voltage of the braking unit is generally 680v, as shown in Figure 5

Figure 5 working current monitoring diagram of the modified braking unit

4 energy saving analysis

compared with resistance energy consumption braking, the feedback braking unit itself is an energy-saving application, but it is required that each frequency converter should be equipped with a braking unit when it needs braking. It is inevitable that several inverters must be equipped with several braking units. Why is it difficult to process units? The price of braking units is not much different from that of inverters, but the work continuity rate is not very high. The wide application of common DC bus frequency converter drive in centrifuges has better solved the problem of "one can't eat enough and one can't eat enough" when one frequency converter accelerates and the other frequency converter brakes. This scheme reduces the repeated setting of braking unit, reduces the number of work, reduces the number of interference with electricity, and improves the power quality of electricity. It is of great significance in reducing equipment investment, increasing equipment utilization, saving equipment and energy

5 conclusion

the wide application of common DC bus of general frequency converter has better solved the problem that the time period of electric energy consumption and electric energy feedback is not synchronized, which is of special significance to reduce equipment investment, reduce electric interference and improve equipment utilization

references

[1] Emerson EV2000 frequency converter Chinese technical manual. Emerson Network Energy Co., Ltd.

[2] IPC electric energy feedback and resistance braking unit user manual. Shenzhen power company.

[3] Du Jincheng. Electrical frequency conversion and speed regulation design technology [m]. Beijing: China Power Press, 2002.

[4] Zhong Mingzhen, Zhao Xiangbin. Low voltage frequency converter application manual [m]. Beijing: Machinery Industry Press, 2009.

Introduction to the author

Bai Xiangang (1979-), male, majoring in electrical engineering and automation, is currently working in the electrical management of Weisheng Pharmaceutical (Shijiazhuang) Co., Ltd. of Shijiazhuang Pharmaceutical Group. (end)