Basic knowledge of relay protection
1、 Tasks and requirements of relay protection
1. Tasks of relay protection
In order to ensure the safe operation of the power supply and distribution system and avoid the influence of over-current caused by overload and short-circuit on the system, different types of over-current protection devices should be installed in the power supply and distribution system. The common over-current protection devices include fuse protection, low-voltage circuit breaker protection and relay protection. Among them, relay protection is widely used in high-voltage power supply and distribution system, which has many protection functions and is the basis of realizing power supply and distribution automation.
Relay protection device is a kind of automatic device which can reflect the fault or abnormal working state of electrical equipment in power supply and distribution system, and can act as an automatic device to send out prediction signal for circuit breaker tripping or starting signal device.
The main tasks of relay protection are as follows:
(1) Automatically, quickly and selectively remove the fault components from the power supply and distribution system, so that other non fault parts can quickly restore normal power supply;
(2) It can correctly reflect the abnormal operation state of electrical equipment and send out forecast signal, so that operators can take measures to restore the normal operation of electrical equipment;
(3) Cooperate with automatic devices of power supply and distribution system (such as automatic reclosing device, automatic switching device of standby power supply, etc.) to improve the operation reliability of power supply system.
The characteristics of relay protection of building power supply and distribution system are simple, effective, reliable, and have strong anti-interference ability.
2. Basic requirements for relay protection
The design of relay protection should be based on reasonable operation mode and possible fault types, and should meet the four basic requirements of selectivity, rapidity, reliability and sensitivity.
Selectivity means that the fault is first removed by the protection of the faulty equipment or the line itself. When the short-circuit fault occurs in the power supply system, the relay protection device acts to cut off the fault components only and minimize the scope of power failure, so as to reduce the impact of power failure. The ability of protective devices to select faulty components is called selectivity of protection.
In order to reduce the loss caused by the fault, reduce the user's working time under the low voltage when the fault occurs, and improve the stability of the power system operation, the relay protection device is required to act as soon as possible and remove the fault when the fault occurs. Fast removal of the fault part can prevent the expansion of the fault, reduce the damage of the fault current to the electrical equipment, accelerate the recovery of the voltage of the power supply system, and improve the reliability of the power supply system.
Reliability means that the protection device acts when it should act, and does not act when it should not. In order to ensure the reliability, it is better to select the simplest protection mode, adopt reliable components and as simple circuit as possible to form a device with good performance, and there should be necessary detection, locking and double measures. The protection device shall be convenient for setting, debugging, operation and maintenance.
Sensitivity refers to the ability of relay protection to respond to faults or abnormal working conditions within its protection scope. The sensitivity SP of over-current protection is expressed by the ratio of the minimum short-circuit current IK, min when the power system in the protection zone is the minimum operation mode, and the primary operation current of the protection device, that is, the action current of the protection device is converted to the value of the primary circuit, Op, 1, that is, SP = IK, min / IOP, 1
The sensitivity required for different protective devices and protected equipment is different, which is stipulated in "technical code for design of relay protection and automatic device of power equipment" (GB 50062; 1992).
In addition, the four basic requirements mentioned above are not all equally important for a specific protection device, but should be emphasized. For example, the power transformer is the most important equipment in the power supply and distribution system, so the sensitivity of its protection device is required to be higher; for the protection device of general power line, it is required to have higher selectivity.
2、 Basic principle of relay protection
Power system fault will cause the increase of current, the decrease of voltage and the change of phase between current and voltage. Therefore, most of the relay protection used in power system is constituted by the difference between physical quantity in fault and physical quantity in normal operation. For example, over-current protection reflecting current increase and low-voltage (or over-voltage) reflecting voltage decrease (or increase) Voltage) protection, etc.
The block diagram of relay protection principle structure is shown in Figure 1. It consists of three parts: (1) measurement part;; used to measure the input signals (current, voltage) of the protected equipment, and compare with the given setting value to determine whether it should be started; (2) logic part;; According to the size, nature and combination or output sequence of each output of the measurement part, the protection device can work according to certain logic program and transmit the signal to the executive part; (3) the executive part; according to the signal transmitted by the logic part, the protection device can finish the task borne by the protection device and give the trip or signal pulse.
As shown in Figure 2 is the schematic diagram of the basic principle of line over-current protection to illustrate the composition and basic principle of relay protection. In Figure 2, Current relay The current of the circuit is monitored by the current transformer of the line. Under normal operation, when the line passes through the maximum load current, the relay will not act; when the protected line k point is short circuited, the current on the line will suddenly increase, and the current at the secondary side of the current transformer TA will also increase correspondingly according to the transformation ratio. When the current passing through the current relay KA is greater than its setting value, the relay will act immediately, the contact will be closed, and the logic circuit will be connected Time relay In KT coil circuit, time relay starts and makes logic judgment of protection action according to the duration of short circuit fault. Time relay KT acts and its delay contact is closed to connect the tripping coil circuit of signal relay KS and circuit breaker QF in the execution circuit to trip the circuit breaker and cut off the short circuit fault.
3、 Composition and classification of breaker
1. Function of relay
Relay is a kind of automatic electrical equipment whose electrical output circuit is connected or disconnected when its input physical quantity (electrical quantity or non electrical quantity) reaches the specified value.
Relay is generally composed of three main parts: sensing element, comparison element and executive element.
(1) The sensing element synthesizes the changes of physical quantities (such as current and voltage) and sends them to the comparison element.
(2) The comparison element compares the physical quantity sent by the sensing element with the physical quantity (setting value) given in advance, and sends instructions to the executive element according to the comparison result.
(3) According to the instructions from the comparison element, the actuator automatically completes the tasks of the relay, such as sending trip pulses to the circuit breaker or performing other operations.
2. Classification of relays
There are many kinds of relays. At present, the general classification method is as follows:
(1) The relay can be divided into electromagnetic type, induction type, rectification type, polarization type, semi conducting type, thermal type and so on.
(2) According to the nature of physical quantity reflected by relay, it can be divided into current, voltage, time, signal, power, direction, impedance, frequency and so on.
There are two types of electrical quantities that can be added to the relay. The former is an over current relay, such as an over-current relay, while the latter is an under voltage relay.
In addition, there is also a kind of relay which reflects non electrical parameters, such as gas, gas, relay, temperature relay, etc.
Representation of relay
The compilation of relay model in China is represented by Chinese Pinyin alphabet, which is composed of action principle code, main function code, design serial number and main specification code. The representation form is as follows:
The design serial number and main specifications are represented by Arabic numerals. The main specification code of relay is often used to indicate the form and quantity of contact. For example, dl-11 / 10 represents an electromagnetic current relay, where the first digit "1" represents the design serial number (10 series), and the second "1" indicates that there is a pair of contact points,
"10" indicates that the maximum operating current is 10A.
4. Common relay
The relay protection devices (including power supply system) of power lines and electrical equipment in the power network of 35kV and below are still widely used in addition to the microcomputer protection. The following focuses on the structure, principle and characteristics of several electromagnetic relays reflecting a single electrical quantity.
(1) Electromagnetic current relay
Electromagnetic current relay is used as starting element in relay protection device. The internal structure and internal wiring diagram of DL series electromagnetic current relay are shown in Fig. 3.
The working principle of electromagnetic current relay is as follows: when coil 2 passes through current IKA, electromagnetic torque M1 tries to make movable tongue 3 rotate clockwise. In normal operation, due to the small IKA, the electromagnetic torque generated is not enough to overcome the resistance torque m2 of spring 4, so the tongue 3 will not rotate and will not drive the movable contact 5 to close with the static contact 6; in case of short circuit fault, IKA will increase greatly, M1 > M2, which will make the tongue 3 rotate and drive the movable contact 5 to contact with the static contact 6 to make it close.
The current IKA value that can make the over-current relay just act and make the contact close is called the action current of the relay, expressed by IOP. The IKA is gradually reduced after operation. When the relay just returns to the original position, the corresponding IKA value is called the return current, which is represented by ire. The above definition can also be said that the minimum current that makes the normally open contact of the relay close is called the action current IOP; the maximum current that causes the normally open contact of the relay to open is called the return current ire. The ratio of the return current ire of the relay to its action current IOP is called the return coefficient kre (its value is generally less than 1), that is, kre = ire / Io
(2) Electromagnetic voltage relay
The structure and working principle of electromagnetic voltage relay and electromagnetic current relay are basically the same. The difference is that the coil of voltage relay is voltage coil with more turns and smaller wire diameter, while the coil of current relay is current coil with less turns and thicker wire diameter.
There are two kinds of electromagnetic voltage relay: overvoltage and undervoltage, among which undervoltage relay is widely used in factory power supply system
Similar to over-current relay, the operating voltage UOP of under voltage relay is the maximum voltage to make it operate, and its return voltage ure is the minimum voltage to make it return, and the return coefficient kre = ure / UOP. Since the return voltage ure of the under voltage relay is greater than the action voltage UOP, its return coefficient kre is more than 1, generally 1 ~ 1.2.
(3) Electromagnetic time relay
The time relay plays a time-delay role in the protection device to ensure the selectivity of the protection device action.
The internal structure of DS series electromagnetic time relay is shown in Fig. 4, which is mainly composed of electromagnetic mechanism and clock delay mechanism. The electromagnetic mechanism is mainly used to lock and release the clock delay mechanism, and the clock delay mechanism plays an accurate delay role. The coil of time relay is designed for short time operation.
(4) Electromagnetic type Intermediate relay
The function of the intermediate relay is to expand the contact number and capacity of the relay at the outlet of the protection device. It can also make the contact close or open with little delay (0.4 ~ 0.8s), or adapt to the needs of the protection device through the self holding of the relay.
The working principle of intermediate relay is generally composed of electromagnetic principle, as shown in Figure 5 is the structure diagram of DZ series electromagnetic intermediate relay.
(5) Electromagnetic signal relay
The signal relay is used in the circuit of each protection device as the indicator of protection action. The signal relay is generally composed of electromagnetic principle. The electromagnetic starting mechanism of the relay adopts the type of attracting armature and is powered by DC power supply. The structure of DX series signal relay is shown in Figure 6. Under normal conditions, there is no current passing through the relay coil and the signal relay is in the normal position. When there is current flowing through the relay coil, the signal board falls down or protrudes, indicating that the signal relay has dropped. In order to analyze the cause of the fault, the signal indication should not disappear with the disappearance of the electrical quantity. Therefore, the signal relay must be designed as manual reset type.
Signal relay can be divided into series signal relay (current signal relay) and parallel signal relay (voltage signal relay). Its wiring mode is shown in Fig. 7. In practical use, current type signal relay is generally used.