The power system relay protection and safety automatic device is an important automation technology and equipment used to quickly remove the fault and eliminate the abnormal situation when the power system fails and abnormal operation. When the power system fails or endangers its safe operation, the relay protection and safety automatic device can issue an alarm signal in time, or directly issue a trip command to terminate the event.
The basic task of relay protection is to reflect the abnormal operating state of electrical components and automatically, quickly and selectively trip a specific circuit breaker. Therefore, relay protection and its automatic devices are an indispensable part of the power system to ensure the safe operation of the power grid, to ensure that electrical equipment is not damaged or to reduce the degree of damage when a fault occurs. “Electrical Safety Work Regulations” clearly requires that electrical equipment is strictly prohibited from running without any protection. The relay protection of the photovoltaic power station is equipped with different protection devices according to the voltage level and the voltage level of the step-up transformer, but the basic line protection, bus differential protection, and transformer protection are necessary.
In the operation of the power system, various external factors (such as lightning strikes, bird damage, etc.), internal factors (insulation aging, damage, etc.) and human operations may cause various faults and abnormal operation. Common fault types are: single-phase grounding, three-phase short circuit, two-phase short circuit, two-phase grounding short circuit and disconnection.
1. Principle of relay protection
The relay protection device must have the function of correctly distinguishing whether the protected element is in normal operation or has a fault, and whether it is a fault within the protection zone or a fault outside the zone. In order to realize this function, the protection device needs to be constructed on the basis of the characteristics of electrical and physical quantity changes before and after the failure of the power system. After the power system fails, the main characteristics of GONGShun Electric’s energy changes are:
(1) The current increases. When a short circuit occurs, the current on the electrical equipment and the transmission line between the fault point and the power source will increase from the load current to greatly exceed the load current.
(2) Voltage drop. When an interphase short circuit and a ground short circuit fault occur, the phase-to-phase voltage or phase-to-phase voltage value at each point of the system drops, and the closer to the short-circuit point, the lower the voltage.
(3) The phase angle between the current and the voltage changes. During normal operation, the phase angle between the current and the voltage is the power factor angle of the load, which is generally about 20°. In the case of a three-phase short circuit, the phase angle between the current and the voltage is determined by the impedance angle of the line, generally 60°~ 85°, and when protecting a three-phase short circuit in the opposite direction, the phase angle between the current and the voltage is 180°+ (60°~85°).
(4) The measured impedance changes. The measurement impedance is the ratio of the voltage to the current at the measurement point (protection installation). During normal operation, the measured impedance is the load impedance; when the metal is short-circuited, the measured impedance is transformed into the line impedance. After the fault, the measured impedance decreases significantly, while the impedance angle increases.
When asymmetrical short circuit occurs, phase sequence components appear. For example, when two-phase and single-phase grounding is short-circuited, negative-sequence current and negative-sequence voltage components appear; when single-phase grounding, negative-sequence and zero-sequence current and voltage components appear. These components are not present during normal operation.
Using the change of electrical quantity during short-circuit fault, relay protection of various principles can be formed.
2. Basic requirements of power system for relay protection
(1) Speed: The relay protection device should be able to remove the fault as soon as possible to reduce the time for the equipment and users to operate at high current and low voltage, reduce the degree of damage to the equipment, and improve the stability of parallel operation of the system.
(2) Selectivity: When the equipment or line in the power system is short-circuited, its relay protection only removes the faulty equipment or line from the power system. When the protection of the faulty equipment or line or the circuit breaker refuses to act, the The fault is removed by the protection of adjacent equipment or lines.
(3) Sensitivity: It refers to the response ability of the protection device when the electrical equipment or line has a short-circuit fault or abnormal operation within the protected range.
(4) Reliability: including safety and reliability, is the most fundamental requirement for relay protection. Safety means that the tie-in protection does not act reliably when it is not required to act, that is, no misoperation occurs. Reliability requires that the relay protection act reliably when a fault that should act within the specified protection range occurs, that is, it does not refuse to act.