Development and Application of New Constant Current Discharger for Heilongjiang Electric Power Cao Nan, Wang Shouwei, Ma Xiuyou, Du Zhaofeng, Ma Aihong, Zhao Xiaowei, Jin Donghai 2, Yan Hua 2, Yang Weidong 2, Gan Kai 2, Fan Wei 2, Li Yalin 2 (1 Qinhuangdao Guoda Electronic Control Equipment Co., Ltd., Qinhuangdao 066004, Hebei; 2. North China Petroleum Hydropower Plant, Renqiu 062507, Hebei, China) Developed a portable new type of discharger, which solved the problems of large workload and inaccurate discharge power caused by traditional arresters.
As a long-term storage device, the battery is an indispensable DC power supply for the power system. The most important indicator for measuring battery performance is the capacity of the battery. Its calculation formula is as follows: battery capacity = battery discharge current x battery discharge time.
Among them, the battery discharge time depends on the battery discharge termination voltage.
Conventional discharge mostly uses electric furnace or water resistance as the discharge load of the battery. As the discharge process progresses, the battery capacity gradually decreases, the internal resistance increases, the terminal voltage decreases, and the battery discharge current also decreases.
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Discharge by this method is not only labor-intensive, but also difficult to select load impedance and capacity, and the discharge current cannot be controlled, which makes it difficult to calculate the battery capacity.
In view of the various drawbacks of the above discharge method, we applied the chopping principle, using the PMW (pulse width modulation) method and the single-chip microcomputer control technology to develop a smart discharger for real-time monitoring of battery capacity.
1 The design idea of ​​the new constant current discharger is called chopping, which means that the direct current is pulsed. By changing the size of the pulse during the entire period, the average value of the current can be changed to achieve the purpose of adjusting the current.
In order to stabilize the discharge current of the battery, a steady-current closed-loop feedback system consisting of a feedback circuit from the sampling of the discharge input circuit to the pulse width modulation is required.
2 circuit principle and its analysis 2.1 main loop circuit analysis The main circuit design of the arrester uses a series chopper circuit, the principle is as shown.
f one battery; /C71 current chess block; B inductor; G electrolysis I 屯抒: C2 - capacitor; - resistance; Zha - load resistance; 1 > freewheeling diode; 1GHT - isolation gate bipolar transistor 2 The discharger main circuit diagram can be seen from the above, the battery discharge current turns into a pulse after the IGBT is turned on, that is, chopping. The discharge current can be controlled by controlling the IGBT to be turned on and off.
Since the IGBT is turned on and off, a high ripple current will be generated at the input end of the arrester. In order to eliminate the influence of the ripple current on the battery, the inductor L and the capacitor C can be connected in the input end of the discharger to eliminate the ripple current.
When the IGBT is turned off, it will produce a very good back EMF at the source and drain of the IGBT. In order to prevent this back EMF from breaking down the IGBT, a resistor A and a capacitor C2 are connected in parallel between the source and the drain of the IGBT to form a RC absorption loop for absorbing the back EMF generated when the IG-BT is turned off.
The discharge resistor can be considered as a pure resistive load under ideal conditions. However, in practical applications, there is a partial inductive load in the resistor, so that when the IGBT is turned on and off, a high back EMF will be generated, which is easy to break down the IGBT. The solution is to connect the freewheeling diode D in parallel with the IGBT output to eliminate The back EMF generated by the inductive load.
2.2 IGBT drive circuit principle analysis IGBT drive circuit schematic diagram as shown.
It mainly consists of the following parts: PLD control circuit: consists of adjustable resistor Rw, operational amplifier A, and integral capacitor C3. The discharge current is given by the adjustable resistor Rw. When the sampling current changes, the operational amplifier eight, the output voltage also changes.
Multi-resonant circuit: It consists of operational amplifiers A3, A4, resistor 3, potentiometer Rf and integral capacitor. The operational amplifier A outputs a square wave voltage, and a charge and discharge circuit is formed by the potentiometer and the integral capacitor C3, so that the operational amplifier 8.3 outputs a triangular wave voltage whose oscillation frequency is composed of a resistor, a potentiometer, and an integrating capacitor PMW (pulse width modulation) circuit. : consists of comparator 2, resistor i6 and transistor VT. Comparator into the 2 output terminal connected to the triode VT, the role is to improve the EXB841 input optocoupler current, to ensure that the positive EXB841 action is reliable.
2.3 Discharger principle analysis The current transmitter KT sends the current sample into the PLD control circuit. After adjustment, it outputs a certain amplitude voltage. Compared with the multi-resonance Heilongjiang power swash circuit, the triangular wave voltage is compared with the PMW circuit to output a certain width. The wave voltage (the frequency is the same as the triangular wave voltage generated by the multi-resonant circuit) is sent to the drive signal input terminal of the IGBT drive chip EXB841, so that the EXB841 also outputs a square wave voltage of the same frequency and width to control the on/off of the IGBT, thereby changing The average value of the discharge current is fed back to the drive circuit through the current transducer to form a steady-current closed-loop negative feedback circuit.
When the battery is discharged, the discharge current changes due to external factors or the discharge characteristics of the battery, the current sampling 丨(丨) PLD control circuit output voltage 丨(丨)——the PMW circuit output square wave voltage is narrowed (widened)-driven The output voltage of the chip EXB841 is narrowed (widened) by a switching transistor IGBT on-time t(i) a battery discharge current Ht), so that the discharge current is stabilized.
3 circuit parameters to design the maximum discharge current.
When calculating, the value should be selected appropriately. If the selection is too large, the IGBT loss is too large, causing the overall efficiency of the arrester to decrease. If the selection is too small, when the battery discharge voltage has not reached the discharge termination voltage, the IGBT is in full saturation conduction. The state makes the discharge current not adjustable, which affects the constant current accuracy.
The magnetic core material adopts a ferrite core structure; the b core shape adopts a ring structure and is wound by a wire; c. The diameter of the winding can be determined according to the rated discharge current.
Capacitor C, using electrolytic capacitors, can be connected in parallel to increase the capacity, reducing the impact of the pulsating current generated by the IGBT on and off the battery.
The current module KT uses a Hall element to improve the sampling accuracy of the discharge current.
When selecting the freewheeling diode D and IGBT, a certain margin adjustment should be considered, and the oscillation frequency can be changed, but it cannot exceed the maximum operating frequency of the driving chip EXB841 by 40 kHz. If the frequency is too large, the chip will be damaged.
The process op amps A.A3 and A4 use the CA3140E, a standard integrated op amp with high input impedance, high speed and wideband.
The IGBT driver chip adopts EXB841 type, which is equipped with a high isolation voltage optocoupler and overcurrent protection circuit. Single-supply operation, high-speed drive, drive signal delay of up to 1.5LS, maximum operating frequency of 40kHz. EXB841 is packaged in high-density SIL, each pin is defined as follows: pin 1 and pin 9 are connected for reverse The biasing source of the filter capacitor 47lF; pin 2 - power supply 20V; pin 3 - drive output, requires the drive circuit wiring should be small lm, and is a twisted pair; pin 4 - used to connect external capacitors, Prevent overcurrent protection (generally not connected); pin 5 - overcurrent protection output, optical coupler to make overcurrent signal remote transmission; pin 6 - collector voltage monitoring; pins 7, 8, 20, 11 is not connected; pin 9 - common end; Table 1 resistance and capacitance empirical value rated current / (A) 14 feet negative.
The effect of the middle and the (: 2 is to turn off the overvoltage protection, the empirical value can be taken from Table 1.
4 characteristics of the new type of discharger and its application The new type of discharger has the following characteristics: the main circuit uses a series chopper circuit, which makes the discharger smaller in size and easy to carry.
The switch tube uses a high-power isolation gate bipolar transistor IGBT to increase the rated discharge current of the arrester.
The IGBT drive circuit uses a high-precision PLD control system to improve the stability and measurement accuracy of the discharge current of the discharger.
The development of the new type of discharger greatly reduces the working intensity of the battery discharge, makes the storage capacity easier to calculate, and can be used as a detection device for the activation test and the verification experiment of the battery.
(Editor Zhou Naiwen)
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