CR1220 battery.Design of battery charging, discharging and detection system based on PWM technology

In order to solve the shortcomings of traditional battery charging and discharging devices such as low power factor and high harmonic pollution, a battery charging and discharging device and detection and monitoring system based on pWM rectifier and inverter technology were designed for 15kVA batteries used in electric locomotives. When the device is used as a charging power source, it adopts a current double closed-loop control system to achieve staged constant current mode or constant voltage mode charging; when used as a load for battery discharge tests, energy is fed back to the grid, and the discharge power can be flexibly regulated through SpWM modulation. . Test and inspection results show that the device has the characteristics of bidirectional energy flow, sinusoidal grid-side current, high power factor, and flexible power regulation.

0 Preface

The battery used in electric locomotives is responsible for powering the auxiliary system before the locomotive is lifted. The quality of the battery is crucial. At present, most battery charging and discharging devices for electric locomotives use traditional phase-controlled rectification charging technology. Although the technology is mature and low-priced, it has long adjustment cycles, slow dynamic response, low power factor, and relatively large harmonic pollution, which can easily cause damage to the power grid. pollute. In order to ensure quality, batteries for electric locomotives must undergo aging tests before leaving the factory. Most of the current factory test and aging tests use energy-consuming loads such as cement resistors as the load of the power supply product under test. Although energy-consuming loads are low in cost, their energy is consumed in vain, resulting in a large amount of waste of electrical energy.

This article studies a new battery charging and discharging device based on pWM inverter rectification. It has low energy consumption and high power factor. It can realize constant current or constant voltage charging and discharging and realize flexible adjustment of load size, and can feed back the energy during the test. Back to the grid, energy reuse is realized.

1 Principle of battery constant current/constant voltage charging and discharging device

The battery charging and discharging device in this article adopts a method with voltage pulse rectifier as the core. The pWM control method can easily realize the two-way flow of energy. Depending on the power grid, a single-phase or three-phase pWM pulse rectifier can be used. The system principle is shown in Figure 1.

The battery charging and discharging device is mainly composed of DC/DC converter, three-phase pulse rectifier (pWM rectifier), isolation transformer, control system and other auxiliary circuits. Since a voltage type pulse rectifier is used, the DC input side is connected to a voltage type DC power supply.

As can be seen from Figure 1, during the aging experiment, the output energy of the battery DC power supply, except for a small part of the energy required to maintain the system's own operation, is mostly fed back to the power grid, so it can significantly reduce the energy during the experiment. consumption to achieve the purpose of saving electrical energy.

When charging the motorcycle battery, in order to charge quickly and extend the service life of the battery, the charging and discharging processes are divided into different stages. You can choose constant current or constant voltage mode charging and discharging in stages depending on the specific situation. The constant voltage charge and discharge mode adopts voltage-oriented control mode, and the control block diagram is shown in Figure 2. The control strategy adopts a double closed-loop structure, with the outer loop controlling the DC voltage and the inner loop controlling the current. The purpose of the current inner loop is to improve the dynamic performance of the system and implement current limiting protection, and the purpose of the voltage outer loop is to ensure the stability of the DC side voltage. The error signal after comparing the DC output voltage given signal and the actual DC voltage Vdc is sent to the p1 regulator. The output of the pI regulator is the amplitude of the AC input reference current of the main circuit. After the current error is obtained by comparison, the current error is pI Adjustment is used to slow down the sudden change of current in the dynamic process. Then it is compared with the space vector of the input voltage for control. Finally, the corresponding 6-channel drive pulses can be generated through the SpWM modulation algorithm to control the on-off of the three-phase rectifier bridge IGBT, indirectly controlling the grid-side current and realizing the regulation of the grid-side power factor.

2 Design and analysis of power control link

2.1 Design of buck-boost chopper circuit

The DC/DC converter is the energy control link in the entire system. The circuit is shown in Figure 3. T1~T4 are IGBTs, and C is the intermediate support capacitor.

In order to increase the switching frequency of the charge and discharge device, the DC/DC part adopts a frequency dual design. If the switching frequency of a single IGBT is 20kHz, the equivalent switching frequency can be increased to 40kHz. The dual design can reduce the size of the device and reduce voltage and current ripples.

In order to enable energy to flow in both directions, the power control link uses a Boost/Buck bidirectional converter topology. When charging, it is equivalent to a Buck step-down chopper converter. T1 and T3 are turned on, and energy is transferred from Ud to Uo; T2 and T4 are always turned off during the charging process, but their internal anti-parallel diodes VD2 and VD4 are turned on. Its output voltage and charging current are:

In the formula: ton is the time when T1 and T3 are in the on state; tooff is the time when T1 and T3 are in the off state; T is the switching period; a1 is the duty cycle; R is the internal resistance of the battery. It can be seen from equation (1) that adjusting the duty cycle a1 of T1 and T3 can adjust the charging power.

When discharging, it is equivalent to a Boost chopper converter. T1 and T3 are cut off, T2 and T4 are working, and form a boost chopper circuit with VD1 and VD3. The energy is transferred from Uo to Ud. The relationship between the two is:

It can be seen from equations (3) and (4) that the discharge power can be adjusted by adjusting the duty cycle a2 of T2 and T4.

The DC/DC converter shown in Figure 3 can not only convert the DC input voltage, but also adjust and control the DC input current to achieve energy regulation in the DC/DC stage.

2.2 Analysis of power control circuit and charge and discharge switching

The charging and discharging power control principle is shown in Figure 4.

When charging, S1 is closed, UL1 output by the logic control circuit is high level, UL2 is low level, AND gate D1 outputs a driving pulse, and D2 has no driving pulse. It can be seen from Figure 5 that by changing the size of the pWM rectifier carrier signal uc, the duty cycle of the pWM circuit will change accordingly, thereby achieving the purpose of changing the power. When uc is added, the duty cycle a1 is added and Uo is added. It can be seen from equation (1) that the charging current Io is added and the charging power is added.

When discharging, S2 is closed, the logic control circuit output UL1 is low level, UL2 is high level, and the AND gate D2 outputs the driving pulse. When uc is added, the duty cycle a2 is added, and Uo is added, according to equation (4) It can be seen that as the discharge current Io increases, the discharge power increases, thereby achieving the purpose of controlling the discharge power by the duty cycle.

During the switching from charging to discharging, when S1 is disconnected and S2 is closed, in order to prevent both T1 and T4 from being turned on, causing the power supply E to be short-circuited through T2 and T4, a certain value must be set between the output pulses of D1 and D2. In the dead time, D1 is blocked and delayed by one click, and then the output pulse of D2 is enabled.

Design and Analysis of 3pWM Rectifier

3.1 Mathematical model of pWM rectifier and inverter

The DC/AC part of the battery charging and discharging device uses a three-phase pWM rectifier, as shown in Figure 2. The purpose of the three-phase pWM rectifier is to invert the stable DC voltage output by the DC/DC converter into a three-phase AC voltage. By adjusting the size of the three-phase output voltage of the pWM rectifier and the phase difference between the control and the grid voltage, the pWM rectifier not only The energy sent from the DC/DC converter can be fed into the three-phase AC power grid, and the power factor on the AC side of the battery charging and discharging device can also be effectively controlled.

This article uses SpWM modulation method. In Figure 2, the three-phase modulation signals uru, urv and urw are sine waves with phases that differ by 120° in sequence. The control methods of phase a, b, and c self-turnoff switching devices are the same. Now take phase a as an example: in each interval of uru>uc, the upper arm power transistor V1 is given a conduction drive signal, and the lower arm V4 is given a conduction drive signal. to turn off the signal. In each interval of uru<uc, V1 is given a turn-off signal and V4 is given a turn-on signal. Figure 5 is the phase voltage waveform of the three-phase output of the three-phase bridge pWM inverter circuit with respect to the load neutral point N.

Assume that the switching device is an ideal switch, with no transition process, and its on-off state is described by the switching function. The meaning of the switch function expression is:

The essence of the circuit is to optimize the switching functions Sa, Sb, Sc so that the AC input terminal voltages ua, ub, uc of the three-phase bridge AC input terminal are equivalent to a three-phase AC voltage source to achieve rectification and inverter operation.

3.2 Equivalent circuit and vector analysis of pWM rectifier and inverter

Figure 6 is the phasor diagram of phase a during rectification operation and inverter operation. In the SpWM modulation mode, the fundamental wave component uao appearing in the grid voltage ua and ua is a sine wave, and the current flowing through the inductor La is also a sine wave. As shown in Figure 6, it can be easily seen from the phasor diagram of the a-phase circuit that the three-phase voltage source type pWM bidirectional converter can achieve unit power factor operation.

Figure 6(a) shows unit power factor rectification operation, and Figure 6(b) shows unit power factor inverter operation. Since the phase current ia can operate 180° out of phase with the grid voltage ua, energy can be fed back to the grid, thereby realizing a two-way flow of energy. From the above analysis, it can be seen that by setting the modulation wave uc of the three-phase voltage source pWM bidirectional converter, the switching state of the three-phase voltage source pWM bidirectional converter can be controlled, so that the input current changes according to a given rule.

4 Design of battery charging and discharging device detection and monitoring system

In order to verify the function of the battery charging and discharging device designed for the project, a detection and monitoring system was designed. Part of the interface of the software is shown in Figure 7.

Before charging and discharging, according to different battery charging and discharging requirements, first set the voltage range and current range to protect the charging and discharging system to avoid missetting the output parameters beyond the controllable range.

The battery charging and discharging device has two operating modes:

(1)Constant current mode. The constant current mode uses the output current as the feedback quantity, and the control system keeps the output current of the battery charging and discharging device constant;

(2)Constant voltage mode. The constant voltage mode uses the output voltage as the feedback quantity, and the control system keeps the output voltage of the battery charging and discharging device constant.

When charging the battery, in order to charge quickly and extend the service life of the battery, the entire charging process is divided into different stages in a complete charging process. Different stages use different operating modes and operating parameters. Between different stages Set the stage conversion conditions. When the operating status of the battery charging and discharging device meets the stage conversion conditions, the battery charging and discharging device can change from the current operating stage to the next stage. The battery charging and discharging device can divide a charging process into 1 to 4 operating stages, and the discharging process into 1 to 2 operating stages. The operating parameters of each stage include:

(1) Operation mode. Constant current or constant voltage.

(2) Given parameters. If the operating mode is constant current mode, the given parameter is to output a given current; if the operating mode is constant voltage mode, the given parameter is to output a given voltage.

(3) Limit parameters. Regarding the battery load, under constant current conditions, the control system may cause the output voltage to exceed the maximum voltage limit of the battery pack in order to meet the set output current value. Under constant voltage conditions, in order to meet the set output voltage value, the control system may cause the output current to exceed the maximum current limit of the battery. In order to solve this problem, there is a limiting output part in the control system of the battery charging and discharging device. In the constant current state, the output limit part compares the output voltage with the set maximum limit voltage. If the output voltage is less than the maximum limit voltage, the control system keeps the output current equal to the given current. If the output voltage is greater than the maximum limit voltage, the control system The system will no longer keep the output current equal to the given current, but ensure that the output voltage is less than the maximum limit voltage; the limit parameters are similar to it in the constant voltage state. The purpose of adopting the above measures is to protect the battery and prevent it from being damaged during the charging process. Therefore, a limit output value is included in the operating parameters of each stage. If the operating mode is constant current, the output value is limited to the maximum output voltage. If the operating mode is constant voltage, the output value is limited to the maximum output current.

(4) Stop parameters. The meaning of the stop parameters is that when the actual operating status of the battery charge and discharge device meets the set stop parameters, it will automatically transfer to the next stage of operation. If the current operation is the last stage, the control system will shut down the battery charge and discharge device. The following explains the meaning of the stop parameters in constant current mode; when the operating mode is constant current, the user can select two stop conditions: output voltage or operating time. If the user selects the stop condition to be output voltage, when the battery voltage reaches the set stop output voltage value during constant current charging, the system ends the operation of this stage and moves to the next stage of operation; if the user selects the stop condition of operation time. If the running time of this stage is equal to the set stop time, the system ends the operation of this stage and moves to the next stage.

Combining operating modes and stop conditions, the battery charging and discharging device can have four operating modes: constant current and voltage limiting, constant current on time, constant voltage hate current, and constant voltage on time. At the same time, in addition to the settings of the above parameters, the system can also realize cycle charging and discharging. You can select the starting point and ending point of the cycle, as well as the number of charging and discharging.

5 Conclusion

The technical parameters of the battery charging and discharging device designed in this article are:

Battery side: rated power 15kVA, rated voltage DC110V. Among them, the voltage variation range during charging is 60~160V, and the voltage variation range during discharging is 75~160V; the rated current during charging is 65A, and the rated current during discharging is 130A.

AC side: three-phase AC (380±76) V, grid-side current meets relevant standard requirements, power factor is higher than 0.95, and current distortion is less than 5%;

The bidirectional battery charging and discharging device based on pWM rectification solves the power waste problem of traditional devices and feeds 90% of the test energy consumption back to the power grid, realizing a two-way flow of energy. The SpWM modulation method can make the grid-side current sinusoidal and have a high power factor. Able to achieve flexible adjustment of charging and discharging power. The controllability of discharge power simplifies the work of operators, while also improving the reliability of data and the safety of equipment.