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Working principle and setting calculation of three stage current protection

What is three-stage current protection

          Three-stage current protection refers to the protection of the current instantaneous protection (the first paragraph), the current limit protection (the second paragraph), the definite time over-current protection (the third paragraph) constitute a set of protection, the following we will come to Introduce the working principle and setting calculation method of current protection during the third stage.

First, the current instantaneous protection (paragraph I)

Current breaking protection schematic.jpg

Simple network wiring diagram

          For current protection that responds instantaneously to current increases, it is called current trip protection. To give priority to ensuring the selectivity of relay protection , it is necessary to set the starting parameters of the protection device to ensure that the next line exit at a short circuit does not start, which in relay protection technology, also known as escape by the next line exit Department of short-circuit conditions set. Take the network connection shown in Figure 1 as an example. Assuming that each line is equipped with current-breaking protection, for the protection 1 installed at the A-bus, the starting current 2.png must be set greater than the short-circuit at point d2. Of the maximum short-circuit current, that is, in the maximum operating mode B-bus current on three-phase short circuit Three-stage current protection company.png, that is:

Current instantaneous trip protection formula.jpg

          When the primary current of the line to be protected reaches the value of the starting current, the protection 1 installed at the A-bus can start up and finally acts on the protection circuit 2 of the circuit breaker 1. According to the same principle, the starting current must be set Is greater than the short circuit at point d4, the maximum possible short-circuit current, that is, the maximum operating mode C-phase current on the three-phase short circuit Three-phase short circuit current.png, that is:

Three-phase short-circuit current calculation formula.jpg

          When the primary current of the protected line reaches the value of the starting current, the protection 2 installed at the B-bus can start and finally act on the tripped circuit breaker 2.

          Behind the paragraphs of the current line speed breaking protection setting principle Ibid.

          The main advantages of current breaking protection are: simple and reliable, fast action, which has been widely used. However, due to the introduction of a reliable coefficient Reliable system.png, it is not difficult to see that the shortcomings of the current instantaneous protection is: can not protect the full length of the line, and the protection of the direct impact of system changes in mode of operation. Operation proved that the protection of the current instantaneous protection is probably the line of 85% to 90%.

Second, the current limit protection (paragraph II)

1, the working principle and set the basic principles of calculation 

          Since the selective current instantaneous protection can not protect the entire length of the line, so we consider adding a new protection to cut off beyond the scope of the fault out of line to protect the full length of the line , But also as a backup current protection protection. Because it is required to protect the entire length of the line, its scope of protection must extend to the next line so that the short circuit occurs at the exit of the next line (at point d2 for protection 1 in Figure 1) It must be started, in which case, in order to ensure the selectivity of the action, the protection action must be given a certain time limit, but in order to make the time limit as short as possible, we consider its scope of protection Does not exceed the protection range of the next line quick-break protection (such as protection 2 in Fig. 1), and the operation time limit is higher than the next line quick-break protection by a time period

Current instantaneous trip protection formula.jpg

          As shown in Figure 2 (a), we can call it a fast-limiting current-trip protection because it can quickly cut down the fault within the entire line range with a small time limit. 

          Now take the protection 1 in Fig. 2 as the example, to explain the setting method of the instantaneous current breaking protection. 

          Set the protection 2 is equipped with instantaneous current protection, the starting current calculated in accordance with (1-3) is: formula.pngAssuming the protection range of B bus to d3 between the part, then the short circuit occurs at d3, the short circuit current is, formula.pngProtection 2 quick-break protection just to move. Based on the above analysis, the protection of a limited-time current protection should not exceed the current protection protection 2 protection range, because in the case of single-side power supply, its starting current should be set as:

Limit instantaneous current breaking protection setting formula.jpg

          In the above formula can take two current equal? If chosen equal, it means that the protection of the limited current instantaneous protection range of 1 and the protection of the current instantaneous range of protection coincide, which in the ideal case is possible, but in practice is not acceptable. Because the protection 1 and 2 are installed in different locations, the characteristics of the current transformers and relays they use are hardly exactly the same. If the current instantaneous protection of the protection 2 encounters a negative error, its protection scope is smaller than the calculated value, However, the positive current limit protection of protection 1 has a positive error and its protection range is larger than the calculated value. In practice, when the calculated end of the protection range (for example, point d3 in FIG. 1 or even a Point) short circuit, there will be protection of the current instantaneous speed protection can not move the protection 2, while the protection of a limited-time current instantaneous speed protection will start the situation, because the fault is located between the lines BC, even if the protection of the current instantaneous speed protection can not be started, It should also be cut off by its limited current instantaneous trip protection fault removal, and if the protection of a limited-time current instantaneous trip protection is also started, the result is that the two protection of the limited instantaneous trip at the same time acting on the trip, so the protection of a loss of selectivity. In order to avoid this situation, you can not use two current equalizing method, but must be adopted, the introduction of reliable coefficient, namely:

Limit instantaneous current breaking protection setting formula.jpg

          For the reliability factor: Reliable system.pngConsidering the short-circuit current in the non-periodic components have been decayed, it can be selected than the current quick-break protection is Reliable system.pngsmaller, generally taken formula.png

          It can be seen from the above that after the current quick-break protection and the fast-break current protection are installed on the line, both of them can be started if the fault is within the instantaneous current protection range of the protection 1, but the fast- In the time to ensure the selectivity, so the instantaneous instantaneous current protection cut off fault; and if the current protection in the protection of the instantaneous protection beyond the scope of the line at the same time within the scope of AB fault, the protection of a limited-time current protection action Shorter period (between 0.35 ~ 0.6s, usually up to 0.5s) to remove the fault. 

          Therefore, the circuit is equipped with current instantaneous protection and current limit protection after the instantaneous, their joint work can ensure that the fault within the entire line range can be cut within 0.5s time, under normal circumstances can meet the fast-moving Claim. Therefore, the protection of this performance can be used as the line of "main protection."

Time-limiting current protection action time limit with the relationship between the map.jpg

2, the protection device sensitivity verification 

          In order to be able to protect the full length of the line, the current limit protection must be the minimum operating mode of the system, the end of the line short circuit occurs when the two-phase, with sufficient response capacity, this capacity is usually sensitive coefficient to measure. The response to the numerical increase in the action of the over-protection device, the sensitivity coefficient of meaning is:

The meaning of sensitivity coefficient.jpg

          Where the fault parameters (such as current, voltage, etc.) calculated value should be based on the actual situation reasonable use of the most unfavorable to protect the operation of the system operating mode and fault type to be selected. 

          For the protection of the limit current instantaneous protection in Figure 1, the short-circuit current at the end of the line AB at the end of the line AB in the minimum system operation mode is taken Related parameters.pngas the calculated value of the fault parameter. Substituting the above formula is the sensitivity coefficient

Limited current instantaneous protection related computing company.jpg

For the limit current quick-break protection should be required. 

          If the sensitivity coefficient can not meet the requirements for various reasons, it is usually considered to further extend the protection range of the instantaneous current quick-break protection to match the fast-break current protection of the next line, as shown in Fig. 2 (b) Action time limit should be selected than the next line of the limit of the current instantaneous protection time limit again higher Related calculation formula.png, generally taken as 1 ~ 1.2s, this time

The related formula of instantaneous current breaking protection.jpg

          Therefore, with the extension of the scope of protection, will inevitably lead to increased time limit. 

          Problem: As shown in Figure 2, it is easy to see that the limit of instantaneous current protection for protection 1 is the same as the limit for instantaneous current protection for protection 1. It is assumed that a short circuit occurs at some point of the BC line (for example, point d2), which is within the range of the instantaneous current quick-break protection of the protection 1 in addition to the instantaneous instantaneous current protection and the quick- , It is assumed that the protection of current 2 of protection 2 can not be operated for various reasons. According to the requirements of selectivity, the protection outlet of protection 2 should be tripped by the current limit of protection 2. However, the protection of protection 1 of the protection of limited-time current can also be started and the operation time limit And the protection of the current limit protection of the same time limit 2, there will be the phenomenon of both exports at the same time, how should we solve it?

Third, the fixed-time over-current protection (Paragraph III)

          Over-current protection is usually refers to the starting current to avoid the maximum load current to set a protection. It does not start up during normal operation, but when the grid fails, it can react to the current increase. It can not only protect the entire length of the line, but also can protect the length of the adjacent lines to serve as backup protection effect.

1, the working principle and setting the basic principles 

          of calculation To ensure that during normal operation overcurrent protection never action, the starting current protection device must be set greater than the maximum load current that may appear on the line The maximum negative current.png, and in the actual determination of the starting current , It must also consider the issue of whether the protection can be returned after the external fault is removed. As shown in Figure 3, when d1 is short-circuited, the short-circuit current will flow through protection 5, 4, 3. These protection must be started, but according to the selective requirements, protection 3 should be used to remove the fault and then protect 4 and 5 As the current has decreased, it should be returned immediately.

Overcurrent protection starting current and operating time network diagram.jpg

          In fact, when the external fault is removed, the current through the protection 4 and 5 is still in the running load current, due to the voltage drop in the short circuit, substation B bus load on the motor is braked, so in When the voltage is recovered after the fault is removed, the motor must have a self-starting process. The self-starting current of the motor should be greater than the current of the normal working current, which is about 6 times of the normal working current. Therefore, we introduce a self-starting factor parameter.pngto indicate the ratio of the maximum current during startup to the Maximum current.pngmaximum load current during normal operation The maximum negative current.png, ie:

 Correlation coefficient and formula of overcurrent protection.jpg

          Since the protection device is started and returned through the current relay to achieve, therefore, the relay return current and the starting current relationship also represents the protection device return current and the starting current, for the introduction of the relay return coefficient parameter.png, the protection device The starting current is 

 Related calculation formula.jpg

2, set the action time limit 

          Overcurrent protection action time limit according to the selective requirements to set, as shown in Figure 4, assuming that each electrical component

The Choice of Time Limit of Overcurrent Protection in Unilateral Power Network.jpg

          Are equipped with overcurrent protection, the protection of the starting current are in accordance with the protection of the components are to avoid the maximum load current to set. In this way, when d1 is short-circuited, protection 2, 3, 4, and 5 may start under the action of short-circuit current, but to meet the selective requirements, only protection 1 action should be taken to cut off the fault and other protection should be immediately after the fault removal return. This requirement can only be met by having different protection devices with different deadlines. 

          Protection 1 is located at the very end of the power grid. As long as the internal fault of the motor, it can be momentary action to give excision, t1 is the inherent action time of the protection device itself, for protection 2, in order to ensure the selectivity of action when d1 is short-circuited, The action time t2> t1, the introduction of time phase △ t, then the protection of 2 action time limit

Calculation formula.png

          After the time limit of the protection 2 is determined, when the d2 point is short-circuited, it will cut off the fault by the time t2. In this case, in order to ensure the selectivity of the protection 3 action, t3> t2 must also be set and the time period Δt is introduced. Then, the protection 3 action The time limit is

Similarly, followed by analogy, the protection of 4,5 action time is

Overcurrent protection setting calculation.jpg

          T1 is the operation time of No. 1 (motor) protection; t2 is the operation time of No. 2 (transformer) protection; t3 is the operation time of No. 3 (line BC) protection. So t4 should actually take the largest of them. 

          This protection action time limit, after the set calculation is confirmed, that is, by a special time relay to be guaranteed, the starting time limit and the size of the short-circuit current has nothing to do, so called the fixed-time over-current protection. 

          The shortcomings of this protection: When the fault is closer to the power supply terminal, the greater the short-circuit current, but the longer the protection action to cut off the fault instead. Therefore, in the power grid, current quick-break protection and instantaneous current quick-break protection are widely used as the main protection of this line to quickly remove faults and use definite-time overcurrent protection as back-up protection for this line and adjacent lines. Line called near backup protection, for adjacent lines, called far backup protection. 

          It can also be seen from Fig. 4 that the protection devices (such as protection 1 and 2) located in the vicinity of the terminal of the power grid have a limited time limit for overcurrent protection, so in this case, it can serve as the main protection Backup protection, without the need to install the current instantaneous protection or limit the current instantaneous protection.

2, the fixed time over-current protection sensitivity coefficient of the calibration 

          Over-current protection sensitivity coefficient of the calibration is still using (1-7), when the overcurrent protection as the main line protection, the minimum run mode should be used at the end of the line When the short-circuit two-phase current verification, requirements Claim.png; when the adjacent line backup protection, the minimum operating mode should be used when the end of the adjacent line two-phase short circuit current to verify, this time requirements Claim.png

Fourth, the summary

          As the current fast-break protection can not protect the full length of the line, and the limit of the current fast-break protection can not be completely as the backup of adjacent lines of protection, therefore, in order to ensure rapid and selective cut-off, often the current instantaneous protection, Time-limiting current protection and timing overcurrent protection combined together to form a phase-type current protection. Specific application, you can only use the current quick-break protection plus a fixed time over-current protection, or limited-time current protection plus a fixed time over-current protection, you can also use the three at the same time.

This article comes from:http://www.relay-protector.com/technical/875.html.Please refer to the source!

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