This paper analyses the scheme of zero-line break detection controller in order to strengthen the operation control of low-voltage distribution network and improve the operation level of power grid. From January to September of 2001, 39 incidents of household appliances burning caused by zero-line disconnection occurred in a power supply company. The total number of household appliances burned was 1,800, with an economic loss of more than 290,000 yuan. The research of zero-line break detection controller is to prevent the damage to customers’household appliances caused by zero-line break of low-voltage lines, reduce customer complaints and improve the quality of service of power supply companies. But most of the time, single-phase load can not achieve balanced distribution. If zero-line is not considered, then the vector diagram is shown in Figure 1-2 (assuming phase A current overload and voltage pull-down). It can be seen that the relatively small load tends to get higher voltage. If the voltage change is larger, this voltage will often lead to the damage of electrical appliances, which we do not want to encounter, but at this time, if the zero-line is reliable connection. Input, unbalanced current as far as possible through the zero line access to the earth, equivalent to the zero line at the user side also appeared voltage, as shown in 1-3, through the vector diagram can be analyzed, although the voltage at this time is different from the voltage at the transformer grounding end, but observing the user side, its voltage is basically balanced. Thus, in the case of three-phase unbalance, the access of zero line is particularly important. In summary, zero-line plays a very important role in three-phase unbalance. How to routinely monitor the reliable connection of zero-line is particularly important. If the current flowing through the zero-line circuit can be detected, the reliable connection of zero-line can be judged. Current transformer can accurately detect the current of zero-line. In this case, 1000:1 current transformer and 100 ohm resistance are selected to detect the current of zero-line. 。 There is no current, but because of the three-phase balance at this time, although the zero line breaks, it will not burn out because of bias voltage, so we only need to effectively detect the zero line current in the case of three-phase unbalance to accurately judge the situation of the zero line and make corresponding protective measures. Zero-line break detection controller needs four parts: signal acquisition unit, central processing unit and relay output unit and power supply. Isolation sensor collects zero-line current and phase-line voltage vector separately. Data are analyzed and calculated by central processing unit to determine whether the real-time signal conforms to the specific electrical characteristics of zero-line break, so as to decide whether to do this kind of event or not. Output accordingly. According to the electrical characteristics of AC three-phase four-wire transmission system, the sensor A/D conversion microprocessor (with A/D capture function) is selected to achieve this goal (Note: Since most MCUs are built-in A/D conversion module, the latter two can be combined). The schematic diagram of system hardware is shown in Figure 2-1. According to the hardware schematic diagram, the hardware is divided into analog sampling and micro processing part, signal acquisition, signal amplification, relay output, power supply, communication and so on. In order to facilitate the modular development of hardware, each module is designed as an independent hardware module, and each module is assembled to form the required hardware system. The controller is designed as two printed boards. The power supply, signal acquisition and relay output are designed on the same board, and the signal amplification and microprocessing parts are designed on the same board for strong and weak isolation. As shown in Figure 2-2, the hardware module of the system is composed. Microprocessor module is the core part of the control and processing of the whole controller.
It is mainly used to process the amplified signal of the input sensor, to collect and convert the current and voltage values of 0-100A or 0-220V, and then analyze and process them according to the specific data. In the selection of processor chips, based on the application of this controller in power frequency occasions, electromagnetic interference is large, so try to choose chips with strong anti-interference ability. Thirdly, the controller detects the voltage, phase and zero-line current of three-phase current at all times, and the A/D sampler works abnormally frequently. Therefore, the A/D operation speed of microprocessor is required to be fast and the precision is high. According to past experience, it is suggested that the A/D operation of microprocessor should be fast and accurate. The choice between STC series MCU and PIC series MCU of American microcrystal company is made. Figure 2-3 is a general workflow diagram. The controller is designed as two printed boards. The power supply, signal acquisition and relay output are designed on the same board, and the signal amplification and microprocessing parts are designed on the same board for strong and weak isolation. The hardware module of the system is shown in Figure 1-2.
The power source of the controller is derived from zero line and a phase line, and then is transformed by a linear transformer. In view of the fact that the phase voltage changes frequently in non-equilibrium state, the voltage source with constant voltage will be obtained. The LM2576 regulator chip produced by National Semiconductor Corporation Ns will be used in this case. The LM2576 series voltage regulator is a 3A current output regulated switching regulator circuit manufactured by Z.
it contains a fixed frequency oscillator (52KHZ) and a reference regulator (1.
23V). It has a complete protection circuit, including current limiting and turn off circuit. The chip can use only a few peripheral devices to form an efficient and stable voltage circuit. When detecting zero-line current, it is necessary to process the AC current signal on the current sensor and amplify the small AC 50HZ signal to form the DC RMS signal so that the microprocessor can detect accurately. According to the performance of operational amplifier, positive half-cycle gain = 1 (R2 R3) / R1 and negative half-cycle gain = R3 / R2 can be obtained. In this case, positive and negative half-cycle signals need the same gain, so equation 1 (R2 R3) / R1 = R3 / R2 is required.
This case also needs to accurately calculate the reasonable resistance values of R1, R2 and R3 according to different current sensor selection and different sampling resistances. In addition, the case also includes negative voltage generation circuit, relay drive circuit, etc.
, due to space limitations, thermostatic element not a single review. The implementation of zero-line break detection and control scheme can clarify the operation of distribution network lines, and find out the fault of line operation in a relatively short time, so as to ensure the smooth operation of distribution network.