CN105244859A - Method for preventing misoperation caused by voltage loss after starting distance protection of power grid - Google Patents

Abstract

The invention relates to a method for preventing power grid distance protection from starting first and then losing voltage and malfunctioning. The method is as follows: n consecutive sampling points satisfy the following criteria: ① The voltage mutation amount of any phase is greater than the set voltage mutation threshold value; ②The three-phase voltages calculated by the half-cycle algorithm are all less than 2V; ③The sudden changes in the three-phase currents within 20ms are all greater than the start-up value of the distance protection device; ④The zero-sequence current or negative-sequence current is less than the set current threshold; ⑤Line longitudinal connection The main protection is not open; where n is a positive integer; it is determined that the voltage transformer disconnection occurs within a period of time after the symmetrical fault outside the positive direction occurs; according to the determination result, the distance protection device is prevented from malfunctioning. Through this method, when the distance protection device is activated due to a symmetrical fault outside the positive direction and the main line where the distance protection device is located is a short line, it can be judged whether the voltage transformer disconnection occurs successively in a short period of time, thereby avoiding distance protection. Malfunction, improve the security of the system.

Description

Method to prevent power grid distance protection from starting first and then losing voltage and malfunctioning

technical field

The invention belongs to the field of power grid dispatching control, and in particular relates to a method for preventing a potential transformer disconnection from causing malfunction after a distance protection device of the power grid is started.

Background technique

The distance protection is an under-level protection in which the current is the action value and the voltage is the braking value. When the voltage braking capacity is lost after the voltage transformer is disconnected, the measured impedance tends to zero, and the distance protection will malfunction under the action of the fault current or load current outside the zone. At present, the widely used method of distance protection to prevent misoperation is to judge the disconnection and voltage loss of voltage transformers. Usually, the three-phase voltage vector sum is not zero to detect the disconnection of one-phase or two-phase voltage transformers; The method to detect the disconnection of the three-phase voltage transformer is generally delayed for 1-2s to block the protection and send out an alarm signal, and it is generally self-recovery. The above criterion presupposes that the protective device does not activate. At present, it is generally believed that once the system fails, the voltage transformer disconnection will not be judged after the starting element is started, because any kind of failure is characterized by the voltage transformer disconnection, regardless of the simultaneous occurrence of system failure and voltage transformer disconnection Case.

However, the actual operation situation shows that the possibility of system failure and voltage transformer disconnection occurring at the same time is very small, but the accidents of voltage transformer disconnection occurring in a short period of time (after 20 ms) due to the inducement of system failure have already happened. It occurs many times, and causes the distance protection of multiple circuits running on the same bus to malfunction, seriously threatening the safe operation of the system.

The analysis of the misoperation of the voltage transformer disconnection protection after the distance protection is started first is as follows:

1. As shown in Figure 1, the double-bus connection substation, lines 1, 2, 3, and 4 are system tie lines, and the double buses run in parallel. The secondary voltage circuits of the transformers are paralleled, so that the distance protection of lines 1, 2, 3, and 4 all use the secondary voltage of the II bus voltage transformer. But at this time, if the busbar II fails, the distance protection of lines 1, 2, 3, and 4 will all start. When the busbar protection action cuts off all the components of the busbar II, it will cause the distance protection of lines 1 and 2 on the busbar I to lose the measured voltage after starting. , because the start-up and widening time of the protection device is generally 5-7s, at this time, under the action of the load current, the distance protection of lines 1 and 2 may malfunction, and there is a risk of power failure in the whole station. Similarly, if the fault point is on the outgoing line of line 1 or line 2, but the outgoing circuit breaker refuses to move, the same consequences will also result.

2. Intermittent voltage transformer disconnection will also cause the line distance protection to malfunction when it starts first and then loses voltage. On-site often encounter voltage transformer disconnection caused by loose wiring screws of the voltage secondary circuit or poor contact of the secondary line. Sometimes such disconnection occurs intermittently and intermittently. If a fault occurs outside this time zone, the distance protection will start If the potential transformer disconnection occurs intermittently, the related line distance protection will malfunction under the action of the through-type short-circuit current. The disconnection of the voltage transformer still causes the risk of false operation of the distance protection.

3. The secondary voltage circuit of the bus voltage transformer generally needs to be equipped with a voltage resetting circuit to prevent the voltage transformer from recharging from the secondary to the primary. This circuit is generally realized by driving the resetting intermediate relay with the normally open auxiliary contact of the voltage transformer knife switch. When the voltage transformer is out of operation and the primary switch is opened, the relay is released, and the secondary voltage circuit is disconnected from the voltage transformer; when the voltage transformer is put into operation, the primary switch is closed, the reset relay is excited, and the secondary voltage circuit Automatic access to the secondary side of the voltage transformer. The working power supply of the reset relay uses the power supply of the DC system of the substation. The operation reliability of the power circuit is directly related to the operation reliability of the protection equipment using the bus voltage transformer voltage, and the distance protection bears the brunt. In actual operation, the DC working power of the voltage reset circuit has been lost after a system failure for many times. The main reasons are the DC circuit breaker leapfrog tripping caused by the DC signal circuit failure after a system failure, and some are external interference when the system fails. The resulting trip of the DC circuit breaker is caused by the loss of the floating charging source of the DC system battery after the system failure, and the loss of the DC power supply caused by the battery disconnection. A system fault becomes the direct cause of power loss in the secondary DC system. In this case, all distance protections operating with the busbar have the risk of malfunction. In 2007, a 220kV substation in a large urban area lost power. The reason was that After the failure of the 110kV system in the station, the short circuit of the DC signal circuit led to the tripping of the DC space, which caused the secondary reset power supply of the 220kV voltage transformer to be lost. caused by a trip. The accident load loss was serious, and the lesson was painful.

To sum up, although the probability that the distance protection starts first and then loses the measured voltage is not too high, once it occurs, it is often caused by successive misoperations of the distance protection of multiple lines running on the same bus, and there is a risk of power failure in the whole station , the consequences seriously threaten system security. Therefore, it is necessary to take precautions based on the protection principle. It is necessary to improve the disconnection criterion of the distance protection voltage transformer, so that the distance protection can continue to judge the disconnection of the voltage transformer after it is started, so as to prevent the distance from the fault when the fault is outside the zone. Protection against misoperation.

Contents of the invention

The purpose of the present invention is to provide a method for preventing the distance protection device in the power grid from being activated due to a symmetrical fault outside the positive direction and the main line where the distance protection device is located is a short line, so as to prevent it from occurring again due to simultaneous (short time) A method of malfunction caused by disconnection of voltage transformer.

In order to achieve the above object, the technical scheme adopted in the present invention is:

A method for preventing power grid distance protection from starting first and then voltage loss and misoperation, which is used for judging when the distance protection device in the power grid is started due to a symmetrical fault outside the positive direction and the main line where the distance protection device is located is a short line. Whether the disconnection of the voltage transformer occurs within a certain period of time after the occurrence of the symmetrical fault outside the positive direction, so as to prevent the misoperation of the distance protection device. The method is: consecutive n sampling points meet the following criteria :

①The voltage mutation of any phase is greater than the set voltage mutation threshold;

②The three-phase voltage calculated by the half cycle algorithm is less than 2V;

③ The sudden change of any phase current within 20ms is greater than the start-up value of the distance protection device;

④ Zero-sequence current or negative-sequence current is less than the set current threshold;

⑤ The main protection of the line longitudinal connection is not open;

where n is a positive integer;

Then it is determined that the disconnection of the voltage transformer occurs within a period of time after the occurrence of the symmetrical fault outside the positive direction;

According to the judgment result, the above-mentioned distance protection device is prevented from malfunctioning.

The voltage mutation threshold value is greater than the voltage mutation amount during the minimum oscillation period.

The minimum oscillation period mentioned is 200ms.

Further, the voltage mutation threshold value is the sum of the voltage mutation amount at the minimum oscillation period and the floating threshold.

Further, the voltage mutation threshold value is formed by adding a setting margin on the basis of the sum of the voltage mutation amount at the minimum oscillation period and the floating threshold.

The threshold value of the voltage sudden change is 0.4U _n , wherein U _n is a rated voltage value.

The current threshold is the smaller value of 0.04 times the rated current and 1/8 times the positive sequence current.

The range of the period of time is 20ms.

The value of n is 3.

Due to the application of the above-mentioned technical solution, the present invention has the following advantages compared with the prior art: by this method, when the distance protection device is started due to a symmetrical fault outside the positive direction area and the main line where the distance protection device is located is a short line, Judging whether the voltage transformer disconnection occurs successively in a short period of time, so as to avoid the distance protection malfunction and improve the safety of the system.

Description of drawings

Accompanying drawing 1 is the wiring schematic diagram of double-bus connection substation.

Accompanying drawing 2 is the criterion principle block diagram of the method of the present invention.

detailed description

The present invention will be further described below in conjunction with embodiment.

Embodiment 1: A method for preventing power grid distance protection from starting first and then voltage loss and misoperation. This method is used for the distance protection device in the power grid to start due to a symmetrical fault outside the positive direction and the distance protection device is located on the main line In the case of a short line, it is judged whether a potential transformer disconnection occurs within a period of time after the occurrence of the symmetrical fault outside the positive direction area, resulting in a voltage loss, so as to prevent the malfunction of the distance protection device. The aforementioned period of time range is usually 20ms.

Specifically, the method is as follows: n consecutive sampling points (n is a positive integer, usually 3) satisfy the following criterion, that is, it is determined that after the positive direction out-of-area symmetric fault occurs and the distance protection device When the main line is a short line, the potential transformer disconnection occurs in a short time, and then the distance protection device can be prevented from malfunctioning according to the judgment result.

The criteria include the following five conditions, which must be met simultaneously:

①The voltage mutation of any phase is greater than the set voltage mutation threshold;

②The three-phase voltage calculated by the half cycle algorithm is less than 2V;

③ The sudden change of any phase current within 20ms is greater than the start-up value of the distance protection device;

④ Zero-sequence current or negative-sequence current is less than the set current threshold;

⑤ The main protection of the line longitudinal connection is not open.

Criterion condition for item ①: When the three-phase voltage transformer is disconnected, the voltage will have a certain amount of sudden change, and it will be large. The sudden change threshold should be greater than the voltage sudden change during the minimum oscillation period. The system oscillation period is considered as a minimum of 200ms. When the protection installation is located at the oscillation center, the voltage mutation value |U(t)-U(t-T)| will have the maximum value. U(t) is a sinusoidal waveform with a period of 200ms, and T is 20ms. but

$\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; t</span>}}{\mathrm{<span\; class="notranslate">\; 200</span>}}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; )</span>$

$<span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span><span\; class="notranslate">\; =</span><span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; t</span>}}{\mathrm{<span\; class="notranslate">\; 200</span>}}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 20</span>}}{\mathrm{<span\; class="notranslate">\; 200</span>}}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>$

According to the sum-difference product formula $\sin a-\sin b=2\mathrm{the\; s}i\mathrm{no}\left(\frac{a-b}{2}\right)co\mathrm{the\; s}\left(\frac{a+b}{2}\right)$

$\begin{array}{c}<span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; t</span>}}{\mathrm{<span\; class="notranslate">\; 200</span>}}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 20</span>}}{\mathrm{<span\; class="notranslate">\; 200</span>}}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 20</span>}}{\mathrm{<span\; class="notranslate">\; 200</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>\\ <span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; 10</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; cos</span>}\frac{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 10</span>}}{\mathrm{<span\; class="notranslate">\; 200</span>}}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; |</span>\end{array}$

It can be seen from the above formula that when t=10, the voltage mutation has a maximum value.

$\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; 10</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; no</span>}\mathrm{<span\; class="notranslate">\; 18</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0.618</span>}\mathrm{<span\; class="notranslate">\; p</span>}\mathrm{<span\; class="notranslate">\; u</span>}$

After increasing the floating threshold, the maximum sudden change in the voltage within one cycle is:

$\begin{array}{c}<span\; class="notranslate">\; |</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>\\ <span\; class="notranslate">\; =</span><span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>\\ <span\; class="notranslate">\; =</span><span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; t</span>}}{\mathrm{<span\; class="notranslate">\; 200</span>}}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 40</span>}}{\mathrm{<span\; class="notranslate">\; 200</span>}}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 20</span>}}{\mathrm{<span\; class="notranslate">\; 200</span>}}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>\\ <span\; class="notranslate">\; =</span><span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 20</span>}}{\mathrm{<span\; class="notranslate">\; 200</span>}}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; 5</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 20</span>}}{\mathrm{<span\; class="notranslate">\; 200</span>}}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span><span\; class="notranslate">\; =</span><span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 20</span>}}{\mathrm{<span\; class="notranslate">\; 200</span>}}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; |</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; 5</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>\end{array}$

It can be seen from the above formula that when t=70, the voltage mutation has a maximum value

$\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; |</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; 5</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0.382</span>}\mathrm{<span\; class="notranslate">\; p</span>}\mathrm{<span\; class="notranslate">\; u</span>}$

Considering that there is a certain margin for the threshold, the threshold value is 0.4Un. At the same time, considering the possibility that one or two phase faults have occurred outside the area, when a three-phase voltage transformer disconnection occurs, only the healthy phase There is a sudden change in the voltage, so this criterion meets the condition when the sudden change in any one phase voltage of the three-phase voltage is detected.

Criterion condition for item ②: When the voltage transformer is disconnected, the sudden change in voltage can be felt within one cycle, but based on the Fourier window of the full-cycle Fourier algorithm, the voltage Fourier value cannot be reduced to 0. If the half-cycle algorithm is used, the calculated Fourier value of the voltage is 0 after 10ms after the voltage transformer is disconnected, so the Fourier value of the three-phase voltage calculated by the half-wave algorithm is used. The steady-state amplitude of the three-phase voltage is less than 2V as the condition for three-phase loss of voltage of the voltage transformer.

Criterion condition for item ③: Since this condition considers that the secondary voltage circuit of the voltage transformer is disconnected at the same time as the symmetrical fault occurs, the current will also change while the voltage has a sudden change, so any increase within 20ms The condition that the sudden change of phase current is greater than the startup setting value.

Criterion condition for item ④: Since the premise of this criterion is that a symmetrical fault occurs outside the area, the three-phase voltage is less than 2V at the same time, which is a three-phase short-circuit feature. At this time, there should be no zero-sequence current I ₀ or negative-sequence current I ₂ , so If there is zero-sequence current I ₀ or negative-sequence current I ₂ , it is considered that the voltage transformer is disconnected. The threshold value can take the smaller value of 0.04 times the rated current I _n and 1/8 of the positive sequence current I ₁ .

Criterion condition for item ⑤: Since it is impossible to reliably distinguish the three-phase short-circuit fault at the exit from the three-phase short-circuit fault outside the short-line area, the condition of not opening the main protection of the line longitudinal connection is added to block. It can be blocked by using the low-value differential element of the split-phase current differential, or by using the opposite-side permission signal of the permissive-type longitudinal protection, or even by using the action result of the blocking-type longitudinal protection. It should be noted that the addition of the opening and blocking conditions of the longitudinal protection will affect the inherent action time of the distance I section when the three-phase short-circuit fault occurs at the exit. Therefore, this condition is only used on short lines. As long as the three-phase short-circuit fault outside the area under the maximum operation mode, the line protection measurement voltage is not less than 2V, then this condition does not need to be used. However, if the device does not have a longitudinal main protection, the exit distance section I protection for short lines should be considered, and the short line exit distance section I protection also meets the requirements of the setting regulations.

When the above five conditions are all satisfied, and last for three sampling points, it can be judged that the voltage transformer disconnection occurred within a short time after the above-mentioned symmetrical fault outside the positive direction occurs, as shown in Figure 2 .

In the process of starting the distance protection through the above method, if it is judged that the disconnection of the voltage transformer occurs, the distance protection will be blocked, and the judgment of the disconnection of the voltage transformer will not be carried out during the entire starting process until the protection group is reset. Use the conventional voltage transformer disconnection criterion to continue to judge the voltage transformer disconnection. In order to prevent misjudgment that the voltage transformer is disconnected when the three-phase fault occurs on the back side bus, when the three-phase voltage returns to normal, the disconnection judgment returns immediately.

At present, the longitudinal protection dominated by optical fiber phase-separated current differential protection is the main protection of high-voltage transmission lines and an important guarantee for the safe and stable operation of the power grid. The staged distance and zero-sequence protection as the backup protection are necessary supplements to the protection of high-voltage transmission lines. Only by improving the reliability of the backup protection can the powerful advantages of the main protection be reflected, which is of great significance to ensure the safe operation of the power grid.

The above-mentioned embodiments are only to illustrate the technical concept and characteristics of the present invention, and the purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly, and not to limit the protection scope of the present invention. All equivalent changes or modifications made according to the spirit of the present invention shall fall within the protection scope of the present invention.

Claims (9)

1. A method for preventing power grid distance protection from starting first and then losing voltage and misoperation, which is used when the distance protection device in the power grid is started due to a symmetrical fault outside the forward direction and the main line where the distance protection device is located is a short line , judging whether a potential transformer disconnection occurs within a period of time after the symmetrical fault outside the positive direction area occurs, thereby preventing the misoperation of the distance protection device. It is characterized in that: the method is: consecutive n The sampling points meet the following criteria: ①The voltage mutation of any phase is greater than the set voltage mutation threshold; ②The three-phase voltage calculated by the half cycle algorithm is less than 2V; ③ The sudden change of any phase current within 20ms is greater than the start-up value of the distance protection device; ④ Zero-sequence current or negative-sequence current is less than the set current threshold; ⑤ The main protection of the line longitudinal connection is not open; where n is a positive integer; Then it is determined that the disconnection of the voltage transformer occurs within a period of time after the occurrence of the symmetrical fault outside the positive direction; According to the judgment result, the above-mentioned distance protection device is prevented from malfunctioning. 2. The method for preventing power grid distance protection from starting first and then voltage-loss malfunction according to claim 1, characterized in that: the voltage mutation threshold value is greater than the voltage mutation amount during the minimum oscillation period. 3. The method for preventing power grid distance protection from starting first and then losing voltage and malfunctioning according to claim 2, characterized in that: the minimum oscillation period is 200ms. 4. The method for preventing power grid distance protection from starting first and then losing voltage and misoperation according to claim 2, characterized in that: the voltage mutation threshold value is between the voltage mutation amount during the minimum oscillation cycle and the floating threshold and. 5. The method for preventing power grid distance protection from starting first and then losing voltage and misoperation according to claim 2, characterized in that: the voltage mutation threshold value is between the voltage mutation amount at the minimum oscillation period and the floating threshold The sum is formed on the basis of increasing the set margin. 6 . The method for preventing power grid distance protection from starting first and then losing voltage and malfunctioning according to claim 5 , characterized in that: the voltage mutation threshold value is 0.4U _n , where U _n is a rated voltage value. 7 . 7. The method for preventing power grid distance protection from starting first and then losing voltage and misoperation according to claim 1, characterized in that: the current threshold value is the smaller of 0.04 times the rated current and 1/8 times the positive sequence current value. 8. The method for preventing power grid distance protection from starting first and then losing voltage and malfunctioning according to claim 1, characterized in that: the range of the period of time is 20ms. 9 . The method for preventing power grid distance protection from starting first and then voltage loss and malfunctioning according to claim 1 , wherein the value of n is 3. 10 .