I. Principles of Relay Protection

Relay protection primarily operates on the principle of utilizing the variations in electrical quantities (such as current, voltage, power, frequency, etc.) during short circuits or abnormal conditions in the power system to trigger protective actions. Additionally, other physical quantities, such as the significant gas generation and oil flow rate increase accompanying faults in transformer oil tanks or the rise in oil pressure, are considered. In most cases, regardless of which physical quantity is being monitored, relay protection devices consist of measurement sections (including setting adjustment sections), logic sections, and execution sections.

1)Measurement Units

These units isolate and convert the physical quantities (parameters) of the power system being protected into signals acceptable by the comparison and identification units within the relay protection device. They typically consist of sensors such as current and voltage transformers.

2)Comparison and Identification Units

This includes setting units which compare the signals from the measurement units with predetermined signals to determine the type of signal to be sent to the next processing unit (normal, abnormal, or fault status). Comparison and identification units can comprise four current relays, two for instantaneous protection and two for overcurrent protection. The setting value of the current relay serves as the setting unit, while the current coil of the current relay receives the current signal from the measurement unit (current transformer). When the current signal reaches the set value, the current relay operates, sending a signal to the next processing unit to trip the circuit breaker; if the current signal is below the set value, the current relay remains inactive, and the signal to the next level remains inactive. The identification signals "instantaneous" and "overcurrent" are transmitted to the next unit for processing.

(1)Processing Units

These units receive signals from the comparison and identification units and process them according to the requirements of the comparison and identification units. The protection device's operation is determined based on the magnitude, nature, and sequence of the output from the comparison section. These units typically consist of time relays, intermediate relays, etc. For example, for current protection: instantaneous action triggers intermediate relay, while overcurrent triggers time relay.

(2)Execution Units

Fault handling is implemented through execution units. These units are generally divided into two categories: audible and visual signal relays (such as sirens, bells, flashing signal lights, etc.), and the tripping coils of circuit breakers to trip them.

(3)Control and Operating Power Supply

Relay protection devices require their independent AC or DC power supply, and the power rating varies depending on the number of devices being controlled. AC voltage is generally 220V or 110V as per "GB50053-2013 Design Code for Substations of 20kV and Below".

II. Quadrants of Relay Protection

For relay protection that operates by tripping, four basic requirements are generally considered: Selectivity, Speed, Sensitivity, and Reliability.

1.Reliability

Reliability refers to the protection device's ability to operate when necessary and to remain inactive when not required, ensuring that only faulty equipment or lines are disconnected.

2.Sensitivity

Sensitivity implies that the protection device should have a necessary sensitivity coefficient when metallic short circuits occur within the protected range of equipment or lines. It ensures that any fault within the specified protection range is detected promptly and accurately.

3.Selectivity

Selectivity means that the protection device should prioritize disconnecting the faulty equipment or lines themselves. Only if the protection or circuit breaker of the faulty equipment or lines fails to act should adjacent equipment or lines' protection or circuit breaker be allowed to operate to clear the fault, avoiding widespread power outages.

4.Speed

Speed refers to the protection device's ability to promptly clear short-circuit faults, aiming to enhance system stability, reduce damage to faulty equipment and lines, limit the extent of fault propagation, and improve the effectiveness of automatic reclosing and the automatic insertion of standby power sources or equipment.

The basic task of relay protection is to automatically disconnect faulty equipment from the system or signal operators to eliminate the source of abnormal conditions in the shortest possible time and within the smallest possible area, thereby reducing or avoiding damage to equipment and minimizing the impact on power supply to adjacent areas.

These four fundamental requirements serve as the basis for designing, configuring, and maintaining relay protection systems and are fundamental to analyzing and evaluating relay protection systems. While these requirements are interrelated, they often involve trade-offs. Therefore, in practical work, a dialectical approach should be adopted based on the structure of the power grid and the nature of the users.

III. Sequence of Quadrants

Reliability

Sensitivity

Selectivity

Speed

IV. Contradictions in Quadrants

Sensitivity and Reliability are mutually contradictory. Ensuring high sensitivity may compromise reliability since setting the protection too sensitively may result in false or unnecessary tripping, such as during overload conditions when disturbances occur.

Selectivity and Speed are mutually contradictory. For example, in three-stage protection, coordination is needed among the stages based on different settings and times. This necessitates time to avoid cross-protection zone issues. When the primary protection fails, backup protection must act, but the time delay of 500ms may not meet the requirement for speed.