A brief introduction to the wiring method and wiring coefficient of current protection
The wiring method of current protection refers to the connection mode between the current relay and the secondary winding of the current transformer in current protection.
There are three wiring methods for current protection: complete star wiring, incomplete star wiring, and two-phase current differential wiring.
In the power system, in order to facilitate the analysis and setting calculation of the protection, a wiring coefficient k w needs to be introduced, which is the ratio of the current i k a flowing into the relay to the current current i2 of the secondary winding of the current transformer. That is, k w=i k a i 2.
The purpose of introducing the wiring coefficient k w is to solve the error problem that may occur when the relay protection device is connected to the current transformer.
At the same time, it is also to improve the accuracy and reliability of relay protection devices, and provide more valuable reference information for the design, operation and maintenance of power systems.
In order to ensure the safe and stable operation of the power system.
The first. Three-phase three-relay wiring method.
The three-phase three-relay wiring method is also known as complete star wiring. This wiring method can well reflect the changes in three-phase current and voltage, and can also achieve a good protection effect. Its wiring is shown in the figure.
Three current transformers and three relays are connected to the three-phase power supply, and any of the contacts can be operated on the trip or start time relay after the contact is closed. It can respond to various short-circuit faults, in this wiring method, the current flowing into the relay is equal to the current transformer secondary winding current, and its wiring coefficient is equal to 1 in any short-circuit situation, that is, k w = 1.
This wiring method is mainly used in high-voltage and high-grounding current systems, such as zero-kilovolt and above neutral direct grounding systems. As protection against phase-to-phase and single-phase short circuits.
In conclusion, the full star wiring method of three-phase three-relay is a practical, reliable and economical wiring method, which is suitable for all kinds of power equipment and systems that need to be protected.
The second. Two-phase two-relay wiring method. The two-phase two-relay wiring method is also known as incomplete star wiring. It is a common way to wire relays. In this type of wiring, two relays are connected to a two-phase power supply and are connected together via a common terminal. Its content is shown in the image.
In the figure, the B phase is not equipped with current mutual inductance.
device and current relay, only two relays are used, so the cost is relatively low.
However, it cannot react to single-phase short circuits, only interphase short circuits, and is often used in three-phase circuits, and cannot be used in single-phase circuits.
In addition to this, due to the use of common terminals, if one of the relays fails, it can cause the entire circuit to not work properly. its wiring factor.
1 in various phase-to-phase short circuits.
This wiring method is mainly used for small grounding current systems for phase-to-phase short-circuit protection. It is used in neutral ungrounded systems or through arc suppression coil grounding systems. In practical applications, the appropriate wiring method should be selected according to the specific needs.
The third. Two-phase and one-relay wiring method. The current flowing into the relay is the secondary of the two-current transformer.
The difference in winding current, so it is also called two-phase current difference wiring.
As shown in Fig. In this wiring method, two current transformers are connected to a two-phase power supply and their secondary windings are connected together to form a differential circuit.
When the difference between the secondary winding currents of the two-phase current transformer reaches a certain value, the relay triggers the contact.
In the case of a two-phase short circuit in a and c diagrams, the coefficient k w = 2.
This coefficient is the factor when there is a short circuit in A, B, or B, C phases.
k w=1。
The wiring mode of two-phase relay can reflect all kinds of phase-to-phase short-circuit faults, but cannot fully reflect single-phase short-circuit and two-phase grounding short-circuit.
However, the wiring coefficient varies with the type of short circuit, and the protection sensitivity also varies. It is not as good as two-phase two-relay wiring, but it is simpler and more economical to use one less relay, which can reduce the impact of unbalanced current on the relay. It is mainly used for the protection of high-voltage motors.
There are also drawbacks to this type of wiring. When one of the current transformers fails, it can cause the entire circuit to fail to function properly. Due to the setting of the differential circuit, a certain phase deviation may occur, which will affect the operation of the relay. Therefore, in practical application, its disadvantages need to be noted.