Abstract: This paper introduces a two-phase overcurrent relay suitable for two-phase incomplete star connection. Compared with the traditional protection system composed of multiple relays, it has higher anti-interference capability and operational reliability, and its operation and maintenance are simpler and more convenient. Keywords: saturated transformer, inverse time limit, time limit 0 Introduction Currently, the current transformers used in the overcurrent protection and instantaneous overcurrent protection devices of 6kV~35kV transformer systems with ungrounded neutral points or grounded through arc suppression coils in China all adopt the two-phase incomplete star connection method to improve the reliability of power supply. Since the two-phase incomplete star connection method requires one-third less equipment than the three-phase star connection method, it saves investment and can improve the reliability of power supply in systems with non-directly grounded neutral points, thus it has been widely used. In staged current protection, the two-stage current protection consists of instantaneous or time-limited instantaneous overcurrent protection as the first stage and overcurrent protection as the second stage. The simplest two-phase incomplete star connection requires four current relays, two time relays, two signal relays, and one intermediate relay to achieve two-stage line protection. The unfolded diagram is shown below: To implement this protection system, not only is a large number of relays required and the wiring complex, but a DC power supply is also needed. Therefore, we designed a JGL-3/10 series two-phase overcurrent relay, which can achieve all the functions of the above protection with just one relay. This is introduced here. 1. Principle and Structure The principle block diagram of the relay is shown in Figure 2. Figure 2 Relay Principle Block Diagram 1.1 Operating Power Relays are composed of integrated circuits, and relays composed of integrated circuits must have an operating power supply to maintain the operation of the operational amplifier and logic circuit. In recent years, due to the disadvantages of active relays, such as heat generation in the power supply section, poor anti-interference performance, and increased probability of DC system grounding faults, more and more users prefer passive relays. Besides these disadvantages, the fact that no external auxiliary power supply is needed is also a major advantage for users, as it simplifies wiring and is economical and practical. Therefore, the power supply section is designed to simultaneously sample and draw power from the operating source. A single-speed saturation transformer, after rectification and filtering, provides ±12V. The transformer uses an R-type iron core, utilizing its saturation characteristics and voltage regulation circuitry to prevent overheating of internal components when current increases, maintaining a power supply voltage of ±12V. This circuit has been proven through testing to be stable, have low power consumption, and is an excellent method for passive relays to obtain internal power. The original design allowed the relay to operate normally even when power was drawn from either phase. While a two-phase incomplete star connection should be able to detect various faults except for a short circuit to ground in phase B, in practical applications, this design cannot detect phase-to-phase short circuits other than those in the power supply phase, or short circuits to ground in the power supply phase. This is because, assuming power is drawn from phase A, short circuits in phases B and C, or a short circuit to ground in phase A, result in zero current in phase A, zero transformer output, no internal power supply, and malfunction of all integrated circuits, thus rendering the entire system inoperable and reducing the reliability of the original two-phase incomplete connection protection. Therefore, the design was later changed to allow power to be drawn from both phases, thus solving the above problems while still fulfilling all the functions of traditional protection. 1.2 The overcurrent protection input signal, after rectification and filtering, is set via a dial switch. The input signal is amplified and compared with a threshold voltage. When the threshold voltage is exceeded, the trigger flips, outputting a high level. To prevent system transient overshoot and improve anti-interference capability, the logic circuit is designed so that the relay can only output after receiving three consecutive power frequency pulse signals. Therefore, a delay-widening circuit is added to the overcurrent start-up circuit, consisting of an RC charging circuit and a four-XOR gate circuit, to prevent false outputs caused by high-frequency interference signals. The RC delay circuit delays for approximately 40ms. After any phase experiences overcurrent, the output is sent through an OR gate, so that when the overcurrent value of any phase exceeds the set value, the subsequent comparison circuit can be triggered. After confirming the overcurrent, a signal is sent to the input of the AND gate. Simultaneously, the delay circuit is started. (When the delay setting is 0, the inherent operating time of the entire circuit is no more than 50ms.) 1.3 Overcurrent delay time limit: The delay circuit is constructed using CMOS integrated circuits, which have high delay accuracy and stability. A standard time base signal is generated by a quartz crystal oscillator. During normal operation, the quartz crystal is always in the oscillation state, while the counter does not work. Only when there is a real overcurrent, the overcurrent signal starts the delay, that is, the counter starts to divide the frequency and count. When the count value reaches the setting value of the dial switch, the delay circuit outputs a signal to the AND gate. Since time-limited current protection is mainly used in the line protection of the power grid, its disadvantage is that its protection range is greatly affected by the system operation mode. Inverse-time current protection can effectively protect the entire range of the protected element and has been widely used in the end of the distribution network and large-capacity motors. Therefore, the development of the inverse-time function of two-phase overcurrent relays is also very promising. Inverse-time: Inverse-time is realized by using the relationship between the analog and digital quantities of the AD integrated circuit chip. The circuit is shown in Figure 3. f=Vin/10RC (1) According to the connection method in the figure above, the relationship of equation (1) holds. It can be seen from the equation that the larger the input voltage, the higher the output frequency. The action characteristic of inverse-time is that the larger the current, the shorter the action time. Using the above equation, such an action characteristic can be basically realized. However, in order to approximate the very inverse time-delay operation characteristics in the IEC international standard, a correction circuit needs to be designed. Through experiments, after the overcurrent signal passes through the subtractor "-1", the voltage of Vin is given by equation (2), K1(I/Is-1) (2) Is is taken as the overcurrent setting value/1.2. For the entire circuit, Vin is a voltage quantity that varies with the current. In the figure above, R=10K, C=0.047μF, and the counter is a 10-division counter. The frequency output by the AD integrated chip is divided and counted by the counter. When the count value reaches the setting value of the dial switch, the delay circuit outputs a signal to the AND gate. Referring to the time setting method of the earliest inverse time overcurrent relay GL-10 in China, the dial setting value is also set to the delay time under 10 times overcurrent. Therefore, under 10 times overcurrent, the output frequency is designed to be 1000Hz. According to formula (1), Vin=4.7V and K1=4.7/11 when it is 10 times. After the following derivation, the inverse time delay formula (4) can be obtained. T=1/f=4.7/Vin (3) Since Vin=K1×(I/Is-1), let K=4.7/K1=11, where T: inverse time delay time t: operating time under 10 times the operating current, i.e., dial setting value IS: Id/1.2, Id: operating current I: relay input current value 2 Main technical indicators of the product 3 Conclusion This new type of passive two-phase overcurrent relay has been used in the electrolytic aluminum project of Qinghai Aluminum & Electricity Co., Ltd. and the filter protection of the high-power power electronics laboratory of Beijing Electric Power Research Institute, which were undertaken by the Engineering Center of Beijing Electric Power Research Institute. Field operation shows that the relay is reliable and can fully meet the requirements of the power system for overcurrent relays in two-phase incomplete star connection. It has now passed expert appraisal.