Distance protection in the principle of line protection

Proximity protection is the distance (or impedance) from the point of reaction failure to the place where the protection is installed. And according to the distance of the distance and determine the action time of a kind of protection device. The main element of the device is the distance (impedance) relay, which can measure the impedance value between the protection installation and the short circuit point according to the voltage and current applied to the terminals, and this impedance is called the measurement impedance of the relay.

When the short-circuit point is close to the protection installation, the measurement impedance is small and the action time is short; When the short-circuit point is far away from the protection installation, the measurement impedance increases and the operating time increases, so that the protection selectively excises the fault line.

The relay protection composed of the ratio of voltage to current (i.e., impedance), also known as impedance protection, the impedance value of the impedance element is the ratio of the voltage to the current connected to the element: u i=z, that is, the impedance value from the short circuit point to the protection installation. Because the impedance value of the line is proportional to the distance, it is called distance protection or impedance protection.

Distance protection is divided into grounding distance protection and phase-to-phase distance protection.

The action behavior of the distance protection point reflects the distance from the protection installation to the short circuit point. Compared to current and voltage protection, the performance of distance protection is less affected by how the system operates.

In general, a distance protection device consists of the following four elements. Starting element: Starts the complete protection at the moment of failure and can be used as a distance protection section.

Overcurrent relays or low-impedance relays are often used for the starting element. Directional element: to ensure the directionality of the protection action, to prevent the protection malfunction in the event of a reverse direction failure.

The direction element can be taken with a separate power direction relay, or the power direction relay can be combined with the distance element to form a direction impedance relay. Distance element: The core part of the distance protection device.

Its function is to measure the distance from the short-circuit point to the protection installation. Impedance relays are generally used. Time Limit Element:

The required time limit characteristics are obtained according to the distance of the short circuit point to ensure the selectivity of the protection action. Generally, time relays are used.

The general line is uniform. Then, the equivalent impedance of the line.

The size is related to the length of the line. Suppose the impedance per unit length of the line is z. Then the equivalent impedance z= is the length of the line.

Understanding the above concepts, it is not difficult to understand distance protection:

A ground fault occurs in a general line. It is assumed that the excessive resistance is 0. Then, by measuring the voltage u and the current i, we can find the main reactance of the line, z=u i=zl

In this way, the fault distance l can be found. Protection can be achieved by determining whether l is within the scope of protection.

In practice, our protection scope x is known. Then, there will be a maximum impedance zmax=zx. The principle of protection is as follows:

z=u izmax The fault point is outside the protection range and does not operate. (For the sake of simplicity, the reliability coefficient is not considered) Reference: Zhang Baohui's "Power System Relay Protection".

The distance protection principle reflects the distance between the fault point and the place where the protection is installed, and the distance relay is a protection device that determines the operation time according to the distance.

When the system is in normal operation, it is impossible to always work in the maximum operation mode, so when the operation mode becomes smaller, the protection range of current protection will be shortened and the sensitivity will be reduced; As the name suggests, the distance protection measures the distance from the short-circuit point to the installation of the Dongsongming protection, which is less affected by the operation mode of the system and the protection range is stable. Commonly used for line protection.

The specific implementation method of distance protection is achieved by measuring the impedance from the short-circuit point to the protection installation, because the impedance of the line is proportional to the line length.

As we have already seen in the previous analysis, the voltage at the protection installation is equal to the voltage at the fault point plus the line voltage drop, that is, ukm=uk+ u; Among them, the line voltage drop u is not simply the line impedance multiplied by the phase current, it is equal to the sum of the voltage drops of the positive, negative and zero sequence currents on each sequence impedance, that is, u=ik1*x1+ik2*x2+ik0*x0.

How to implement distance protection.

In order to realize the protection of primary equipment, whether it is conventional protection or microcomputer protection, we must first determine a fixed action characteristic according to our wishes (there should be an action equation correspondingly), and if the calculated measured impedance zj falls inside the action characteristic, the relay will act.

1. For 220kV lines: according to the dual configuration (including reclosing), that is: double sets of main protection, double sets of backup protection.

The main protection is generally set up as the longitudinal protection that can reflect the rapid movement of the whole line, and the backup protection is generally set up with distance protection (grounding distance, phase distance) and zero sequence seepage protection before pin protection as the backup protection of the main protection. 220kV generally uses near backup.

2. For the 110kV network, the cluster tour ring network is generally not used to operate, and it is a radial network distribution, so the line is not provided with longitudinal protection, and only the distance protection (interphase, grounding) zero sequence is installed as the main protection of the line and the backup protection of the next line. The 110kV line adopts the long-distance backup mode.

KV and 10KV load lines are generally equipped with overcurrent as the main protection of the line (35kV: three-stage over-current, 10kV: two-stage over-current).

The protection range of section I of the distance protection is usually selected as the full length of the road to be protected ().

80%-85% (positive friends scatter accurate answers).

Distance protection is a commonly used protection method in the power system, and its basic principle is to judge the fault location according to the relationship between voltage and current in the power system, and remove the fault to protect the normal operation of the power system. The following will elaborate on the basic principles of distance protection, the classification of distance protection, and the application of distance protection.

First, the basic principle of distance protection.

Distance protection is a kind of indirect protection, and its basic principle is to calculate the fault distance by measuring the phase difference and amplitude of voltage and current at the fault point, and judge whether the fault is in the protection car section area or outside the protection area, so as to realize the accurate positioning and removal of the power system fault. The judgment of distance protection is based on the phase difference and amplitude of current and voltage, and the distance between the fault point and the protection device has a certain relationship with the phase difference and amplitude of fault voltage and current. Distance protection is a power system protection method with strong versatility, high reliability, good sensitivity, fast action speed, wide application range and strong anti-interference ability.

2. Classification of distance protection.

Distance protection can be divided into voltage type distance protection and current type distance protection according to the different working principles. Voltage-type distance protection is to use the change of the ratio of the voltage at the fault point to the terminal voltage to judge the distance of the fault point; Current-type distance protection is to use the change of the ratio of the line rapid excitation current to the current at the fault point to judge the distance of the fault point. According to the scope of the protected area, distance protection can be divided into main protection and reserve protection.

The main protection is to protect the main devices and lines of the power system; Backup protection is to play a role when the main protection action fails, protect the backup channel of the power system, and avoid the expansion of power system accidents.

3. Application of distance protection.

Distance protection is widely used in power systems, especially in transmission lines and substations. In the transmission line, distance protection is one of the main protection methods, which can protect the safe operation of the line and prevent the accident of the power system due to failure; In substations, distance protection can protect the safe operation of substation equipment and avoid power system accidents caused by equipment failure. Distance protection can also be combined with other protection methods, such as overcurrent protection, differential protection, direction protection, etc., to form multiple protections and improve the safety performance of the power system.

In short, distance protection is an important protection method in the power system, and its basic principle is to judge the distance of the fault point according to the phase difference and amplitude of the current and voltage, and remove the fault to protect the normal operation of the power system. The classification of distance protection includes voltage and current distance protection, as well as primary protection and backup protection. Distance protection is widely used in power systems, especially in transmission lines and substations.

The main band source protection for distance protection is (stupid high state).

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