Design of 24V DC uninterruptible power supply for

Design of a special 24V DC uninterruptible power supply for power distribution

1 introduction

in substation, DC power supply is very important for the secondary equipment of substation and the real-time communication of substation. If the DC power supply is unreliable, it will lead to relay protection failure, resulting in the expansion of power outage area, and also make communication errors, resulting in greater accidents. Therefore, it is required to have high reliability. Usually, the energy storage capacitor or battery is used as the backup power supply and directly as the DC output, but the output power quality is not high, and the battery management is complex. Therefore, this paper uses the switching power supply with battery to improve the reliability of the power supply, while the output power quality is high, which meets the requirements of communication. Generally, battery management is a very complex work. The system uses PIC16C73 single chip microcomputer to realize battery management, and the management scheme is simple and reliable

2 24V DC uninterruptible power supply system

24v power distribution dedicated uninterruptible power supply system schematic diagram is shown in Figure 1

Figure 1 Schematic diagram of 24V DC power supply system for substation distribution

2.1 performance index of power supply system

two AC inputs 90 ~ 130v

can be used for starting small DC motors in substations

output DC 240.03v

real time detection of battery working parameters and remote monitoring

the standby time is 10h when the load current is 3A

the battery management operation can be realized remotely and on site

working temperature range - 40 ℃ ~ ＋ 80 ℃

2.2 each mode speed of the power supply system

1) the first stage is a half bridge dc/dc circuit, which is used as a charger. In order to improve the reliability, the input is two ways of AC 110V, which can work at AC 90 ~ 130v, and the output is DC 28V (the battery is in the charged state). It is not only used to charge the battery, but also can start a small DC motor through the battery. The switch adopts transistor 2SC2625, and the control chip adopts TL494. Self excitation starting process of the circuit: the voltage dividing resistance on the DC bus makes the VBE of 2SC2625 0.6V, the transistor is turned on, the circuit starts self excitation, the instantaneous voltage is established on the auxiliary group winding, making TL494 work, and the circuit enters the normal working state

2) the latter dc/dc circuit adopts push-pull circuit structure, and the transformer is magnetized in both directions, effectively preventing magnetic saturation [1]. Since the battery terminal voltage can vary between 21V and 27V, the circuit can realize voltage rise and fall regulation, so that the output voltage is stable at 24V, meeting the load requirements

3) the battery management module adopts PIC16C73, and its block diagram is shown in Figure 2. PIC16C73 is a pic8 bit mid-range MCU launched by microchip company. It has only 35 single byte instructions and comes with 5 a/d conversion modules. It has good stability and can work in a harsh environment [2]

Figure 2 battery management block diagram

through the sampling circuit, the terminal voltage, discharge current, charging current, battery temperature, AC power failure, charger fault signal, etc. of the battery are sent to PIC16C73 for processing in real time. The processed data can be sent to the on-site LCD display for on-site inspection; The data is sent to the upper computer to realize remote monitoring

4) switching power supply integrated controller TL494 can output two complementary pulse control signals, and can also realize single ended output. The minimum dead band is 3% and adjustable, with voltage stabilization and overcurrent protection operational amplifier inside

3 battery management scheme and function realization

the hardware diagram of battery management is shown in Figure 3

Figure 3 hardware diagram of battery management

3.1 the selection of battery capacity

shall meet the requirements of continuous discharge under AC power failure. Under the condition that the manufacturer provides the storage battery capacity conversion coefficient KC, use formula (1) to calculate the capacity c[3]

c=ig/(KC t k) (1)

where: I is designed according to the maximum load current of distribution

g refers to the independent power supply time of the battery, which is determined by the distribution reliability level

t is the temperature coefficient of battery discharge capacity. Under the ambient temperature of 15 ~ 25 ℃, when the temperature increases or decreases by 1 ℃, the capacity increases or decreases with the temperature by 0.006 to 0.007 of the rated capacity, t=1 + 0.006 (t-25 ℃)

k is the aging coefficient of the battery, generally taken as 0.8

according to the above principles and combined with practical applications, the system selects two 12a h/12v lead-acid batteries in series

3.2 hardware realization of battery management

after AC power on, on the one hand, charge the battery of the hardness tester that is also provided by the company with hardness testing through R4 and D3, and at the same time provide input for the next level. At this time, relay K1 is engaged, but due to D4 reverse bias, the battery does not discharge the load; In case of AC power failure or battery discharge, D1 reverse bias is cut off. At this time, the battery supplies power to the load through relay contact and D4. When the battery voltage of 21V is detected, the power supply is stopped and the system is in the state of complete power failure, which should be avoided. R7 and R8 are used to detect the charging and discharging current of the battery. In the charging state, the 28V DC output is constant. When it is in the discharging state, the output will change

1) in the case of AC power loss, you can directly press S1, and the battery will supply power to the load. At the same time, R5 end is high level, and relay K1 is pulled in. Even if S1 is disconnected, the power supply will not be interrupted, forming a self-locking for S1

2) discharge control because in most cases, the battery is in a charged state, which has a great impact on the service life of the battery. Therefore, the battery should be discharged within a certain period of time. In this system, the discharge is divided into dynamic discharge and automatic timing discharge. The length of the timing is determined according to the user's requirements, which is generally set as 60 days. After the internal timer of the single chip microcomputer arrives, the discharge signal is given. Through the hardware circuit, the discharge signal is connected with a resistor R1 at the R3 end, which increases the potential of TL494 pin 1, narrows the control pulse, and lowers the output voltage, so that D1 is in the reverse bias cut-off. At this time, the battery supplies power to the load alone, and the previous dc/dc is equivalent to the no-load state. According to the suggestions of the battery manufacturer, set the end of discharge voltage of the battery to 10.5V 2=21v. When the battery voltage of 21V is detected, cancel the discharge signal, and the charger charges the battery and provides energy for the load at the same time

3) motor starting because the current required for motor starting is large, this purpose is achieved through battery discharge in this system. Before starting, an artificial signal is sent to reduce the voltage of the charger, and the battery is put into operation at this time

4) the charging voltage can be controlled. Changing the charging voltage of the battery is to change the output of the charger, and connecting different resistors R1 and R2 at the R3 end can change the output voltage. Users can set the voltage according to their needs

3.3 parameter detection of battery

1) charging current IC and discharge current ID when the battery is in the charging state, due to the clamping effect of D4, the load current IO is completely provided by the charger. At this time, the terminal voltage of R7 is ur7=ior7, id=0, ir8=io + IC. If r7=r8, then

ic= (ur8-ur7)/r7

when the battery supplies power separately, the clamping effect of D4 disappears. At this time, ur8=0, ic=0, therefore, id=io=ur7/r7. Therefore, as long as ur7 and ur8 are sent to the two a/d conversion channels of PIC16C73 through the differential amplifier to obtain a current signal of 0 ~ 5V, IC and ID at any time can be detected through the processing of the microprocessor

2) battery voltage since two 12V batteries are connected in series, the terminal voltage of the battery should be detected respectively, and the voltage output obtains the voltage sampling value through the resistance partial voltage. When | (ub1-12) |/120 (0 is the equalization rate, here take 4%) of battery voltage ub1 (ub2) is detected, it indicates that the battery voltage is unbalanced, and corresponding measures should be taken

3) the AD590 temperature sensor is used for the battery temperature, and the temperature sampling value is sent to the single chip microcomputer. When the battery temperature is detected to exceed 80 ℃ for more than 10min, the discharge control signal is immediately cancelled, and the high level of R5 is changed to low level, so that the relay is disconnected

4) battery capacity battery capacity detection has always been a difficulty in battery management. The usual methods include: capacity prediction based on electromotive force, capacity prediction based on battery internal resistance, capacity prediction based on battery internal resistance and electromotive force at the same time, capacity prediction based on current discharge rate, capacity prediction based on current time integration, etc. In this system, because the change of load follows io=0.2n (n is the number of parallel loads), the capacity detection adopts the capacity prediction of current time integration, which will make the detection simple and feasible. Battery discharge capacity C =iddt. Due to the switching of load, the change of current follows a fixed law, so C =0.2n1t1 + 0.2n2t2 (n1t1, n2t2 are the time of different loads). If the initial capacity Co of the battery before discharge is known, the changed battery capacity cx=co － C. This method is relatively simple and easy to implement, and the current sampling circuit of the system itself can be used without adding special equipment

3.4 prediction of remaining time

the purpose of battery capacity prediction is to obtain relevant information about the working time that the battery system can provide. Therefore, in fact, we only need to know the working time that the battery system can provide under current conditions (voltage, current, temperature). At a certain time, the voltage, current and temperature values can be measured, so we can predict the sustainable time of constant current discharge of the battery under this current, that is, there is a table in the system, which divides the voltage into several grades and the current into several grades, as listed in Table 1

Table 1 battery capacity prediction table

t (n, m) in the table is the time left when the voltage reaches VN with IM discharge.

the work of the system is to fill the table, and calculate the time that the battery can still run at this current from the table according to the terminal voltage and current at a certain time. The accuracy of battery residual capacity prediction is determined by the magnitude of voltage and current grading range. The working process of the system is illustrated by taking a 12a h/12v lead-acid battery as an example

1) there are two ways to initialize the table. One is to obtain data through the battery discharge curve provided by the battery manufacturer; The second is to obtain data from operation. The initialization data does not need to fill the table, but the amount of initialization data determines the accuracy of the prediction of the initial residual capacity of the flame retardant and foaming agent enterprise lines in the system. We divide the current into four levels: 0.05c/0.1c/0.15c/0.2c, and the voltage is divided into one level with 0.1V

2) the internal resistance and polarization voltage of the battery are different at different discharge currents. Therefore, the correction voltage under different discharge currents must be obtained first. Taking 0.05c as the benchmark, the discharge experiment of the battery is carried out, and the corrected voltage at different voltage points is obtained

3) predict the remaining time. According to the initialization results, obtain part of the data in the prediction table. If t (V1, I2) is known from the prediction table, predict the remaining time when I1 discharges to V1, and use the conversion formula (2) to predict

t (V1, I1) =t why is the domestic large aircraft called C919? (V1 － VX2 ＋ VX1, I2) i2/i1 (2)

where: t (V1, I1) is the remaining time when the battery voltage reaches V1

vxn is the corresponding correction voltage of each current

i2 the principle of proximity shall be considered in the selection of Longmei coal called "diversion with land" to ensure the accuracy of prediction. Select the latest discharge result to predict, for example, if 0.1C is used for discharge last time, and 0.2C is to be predicted this time, then I2 is taken as 0.1C. This is because the physical and chemical state of the battery is changing at any time. The closer the time is, the more accurate the result should be