The hottest new passive two-phase overcurrent rela

A new passive two-phase overcurrent relay

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 ability and working reliability, and its operation and maintenance are more simple and convenient

key words: inverse time limit of saturated transformer

0 introduction

at present, current transformers used in overcurrent protection and quick break protection devices of 6kv~35kv transformer neutral ungrounded or arc suppression coil grounded system lines in China adopt two-phase incomplete star connection mode to improve the reliability of power supply. Because the two-phase incomplete star connection mode has one-third less equipment than the three-phase star connection mode, it saves investment and improves the reliability of power supply of neutral indirect grounding system, so it has been widely used in bamboo powder

in the phased current protection, the two-stage current protection takes the instantaneous or time limited current quick break protection as the first section, and the overcurrent protection as the second section. The simplest two-phase incomplete Star 2. The Chinese interface wiring of cement pressure testing machine needs to be composed of four current relays, two time relays, two signal relays and one intermediate relay to realize two-stage line protection. Its expansion diagram is as follows:

Figure 1 two-stage protection expansion diagram composite flexible packaging materials: flexible packaging, aluminized film, iron core wire, aluminum foil composite film, vacuum aluminized paper, composite film, composite paper, BOPP

to realize this set of protection, we not only need to use a large number of relays, complex wiring, but also need to provide DC power supply. Therefore, we have designed a jgl-3/10 series two-phase overcurrent relay. Only one relay can realize all the functions of the above protection, which is introduced here

1 principle and composition the principle block diagram of

relay is shown in Figure 2

Figure 2 Relay principle block diagram

1.1 working power supply

relays are composed of integrated circuits, and relays composed of integrated circuits must have working power supply to maintain the operation of operational amplifiers and logic circuits. In recent years, due to the shortcomings of the active relay, such as the heating of the power supply, poor anti-interference performance and increased the probability of DC system grounding fault, more and more users are more willing to choose the passive relay in the selection. In addition to these shortcomings, the lack of external auxiliary power source is also a major advantage for users, such as simple wiring, economy and practicality

therefore, the power supply part is designed to sample and take the working power supply at the same time. The ± 12V power supply is obtained from the one speed saturation transformer after rectification and filtering. The speed saturation transformer adopts R-type iron core, and its saturation characteristics and voltage stabilizing circuit are used to prevent the internal components from overheating when the current increases, and the power supply voltage can still be maintained at ± 12V. This circuit has been proved by experiments to be stable in performance and low in power consumption. It is a good method for passive relays to obtain internal power supply

the original design is to take power from any phase, and the relay can still work normally. Because the two-phase incomplete star connection should be able to respond to all kinds of faults except phase B grounding short circuit, but in the actual system application, this design cannot respond to phase to phase short circuit other than the power phase and power phase grounding short circuit. Because assuming that the power supply is taken from phase a, phase B, phase C short circuit and phase a grounding short circuit, the phase a current is zero, the transformer output is zero, the internal power supply cannot be established, and all integrated circuits cannot work normally, so the whole machine cannot work, reducing the reliability coefficient of the original two-phase incomplete wiring protection. Therefore, the design was later changed to take power from both phases, so as to solve the above problems and still complete all functions of traditional protection

1.2 overcurrent protection

after rectification and filtering, the input signal is set through the dial switch. The input signal is amplified and compared with the threshold voltage. When the threshold voltage is exceeded, the trigger flips and outputs a high level. In order to prevent the system transient override and improve the anti-interference ability, the logic circuit is designed to receive three power frequency pulse signals continuously before the relay can output. Therefore, A one-stage delay broadening circuit is added to the overcurrent starting circuit, which is composed of RC charging circuit and four XOR gate circuit to prevent false exit caused by high-frequency interference signal. RC delay circuit is delayed for about 40ms

the overcurrent of any phase is output through the OR gate, so that when the overcurrent value of any phase exceeds the setting value, the following comparison circuit can be triggered. Li Jian, academician of the Chinese Academy of engineering, Professor of Northeast Forestry University and honorary chairman of the wood plastic composite Committee, sent a congratulatory message: "China wood plastic started from wood science research. After recognizing the overcurrent, the trigger signal gives a signal to the door input. At the same time, it starts the delay circuit. (when the delay is set to 0, the inherent action time of the whole circuit is not more than 50ms.)

1.3 overcurrent delay

definite time limit: the delay circuit is composed of CMOS integrated circuit, which has high delay accuracy and stability. The standard time base signal is generated by the quartz crystal oscillator. During normal operation, the quartz crystal has been working in the starting state, but the counter does not work. Only when the real overcurrent occurs, the time delay is started by the overcurrent signal, that is, the counter begins to count by frequency. When the count value reaches the setting value of the dial switch, the time delay circuit outputs a signal to the door

since definite time current protection is mainly used in electric line protection, the disadvantage is that its protection range is greatly affected by the system operation mode. Inverse time limit current protection can effectively protect the whole range of protected components, and has been widely used in distribution terminals and large capacity motors. Therefore, the development of inverse time limit function of two-phase overcurrent relay also has great market prospects

inverse time limit: inverse time limit is realized by using the change relationship between analog quantity and digital quantity of ad integrated circuit chip. The circuit is shown in Figure 3

f=vin/10rc (1)

according to the connection method in the above figure, the relationship of formula (1) is established. From the formula, it can be seen that the larger the input voltage is, the higher the output frequency is. The action characteristic of inverse time limit is that the larger the current is, the shorter the action time is. Using the above formula, such an action characteristic can be basically realized. However, in order to be close to the action characteristics of very inverse time limit in IEC international standards, it is necessary to design a correction circuit. Through experiments, after the overcurrent signal passes through the subtracter "-1", the voltage of VIN is formula (2),

k1 (i/is-1) (2)

is takes the overcurrent setting value/1.2. For the whole circuit, Vin is a voltage variable with the current. In the above figure, r=10k, c=0.047 μ F. The counter adopts a 10 frequency division counter. The frequency output by the ad integrated chip is counted by the frequency division of the counter. When the count value reaches the setting value of the dial switch, the delay circuit outputs a signal to the gate

referring to the time limit setting method of the earliest inverse time overcurrent relay gl-10 in China, the setting value of the dial is also set as the delay time under 10 times of overcurrent. Therefore, when 10 times of overcurrent, the output frequency is designed to be 1000Hz. According to formula (1), vin=4.7v and k1=4.7/11 at 10 times of overcurrent can be obtained. After the following derivation, the inverse time delay formula (4) can be obtained

t=1/f=4.7/vin (3)

because vin=k1 × (i/is-1) set k=4.7/k1=11,

to get

where t: inverse time delay time

t: action time under 10 times of action current, that is, dial setting value

is:id/1.2, ID: action current

i: relay input current value

2 main technical indicators of the product

3 conclusion

this new passive two-phase overcurrent relay has been used in the electrolytic aluminum project of Qinghai Aluminum and Power Co., Ltd. and the filter protection of the high-power power power electronics laboratory of Beijing Academy of electrical Sciences undertaken by the Engineering Center of Beijing Academy of electrical sciences. The field operation shows that the relay has reliable performance and can fully meet the requirements of the power system for the overcurrent relay in the two-phase incomplete star connection. It has passed the expert appraisal. (end)