1. Primary circuit – The main electrical wiring from the generator through the transformer and transmission and distribution lines to the electrical equipment is usually called the primary circuit.
2. Secondary equipment – Secondary equipment refers to auxiliary equipment used for monitoring, measuring, operating, controlling, and protecting the primary equipment. Examples include instruments, relays, control cables, and control and signaling devices.
3. Secondary circuit – A circuit formed by connecting secondary equipment in a certain order is called a secondary circuit or secondary loop.
4. Low-voltage switch – a switching device used to connect or disconnect AC and DC circuits below 1000 volts. This differs from the low-voltage definition (voltage to ground below 250 volts) in the safety regulations.
5. Contactor – A low-voltage switch used to remotely connect or disconnect the load current in a circuit, widely used in circuits for frequent starting and controlling motors.
6. Automatic Air Circuit Breaker – An automatic air circuit breaker, also known as an automatic switch, is the most advanced low-voltage switch. It can not only interrupt the load current of a circuit but also break the short-circuit current, and is commonly used as a primary control device in low-voltage, high-power circuits.
7. Demagnetizing switch – a DC single-pole automatic air switch specifically designed for use in the generator excitation circuit.
8. Disconnecting switch – A disconnecting switch with a clearly visible break and no arc-extinguishing device. It can be used to connect or disconnect energized but unloaded lines, voltage transformers, and unloaded transformers of limited capacity. The main purpose of a disconnecting switch is to isolate the power supply voltage during electrical equipment maintenance.
9. High-voltage circuit breaker – also known as a high-voltage switch. It can not only interrupt or close the no-load current and load current in a high-voltage circuit, but also interrupt the short-circuit current through the action of relay protection devices when a system fault occurs. It has a fairly complete arc-extinguishing structure and sufficient breaking capacity.
10. Arc suppression coil – This is an adjustable inductor coil with an iron core, installed at the neutral point of a transformer or generator. When a single-phase ground fault occurs, it reduces the grounding current and suppresses the arc.
11. Reactor – A reactor is an inductor with very low resistance. The turns of the coil are insulated from each other, and the entire coil is insulated from the ground. A reactor is connected in series in a circuit to limit short-circuit current.
12. Eddy Current Phenomenon – Imagine a coil wrapped around a solid iron core. The iron core can be considered as being composed of many closed iron wires, with the plane formed by the closed wires perpendicular to the direction of the magnetic flux. Each closed iron wire can be considered a closed conductive loop. When an alternating current flows through the coil, the magnetic flux passing through the closed iron wires changes continuously, thus inducing an electromotive force and generating an induced current in each iron wire. In this way, induced currents flow in concentric circles around the axis of the iron core, much like eddies in water. This induced current generated in the iron core is called an eddy current.
13. Eddy current loss – Just as current flows through a resistor, eddy currents in the iron core consume energy and cause the iron core to heat up. This energy loss is called eddy current loss.
14. Low-current grounding system – the neutral point is not grounded or is grounded through an arc suppression coil.
15. High-current grounding system – a system with a directly grounded neutral point.
16. Armature reaction—When there is no armature current, the air gap main magnetic field is generated solely by the excitation current. When there is an armature current, the air gap main magnetic field is formed by the superposition of the magnetic field of the excitation current and the magnetic field of the armature current. This influence of the armature current on the main magnetic field is called armature reaction.
17. An asynchronous motor—also called an induction motor—operates based on two principles: the induced electromotive force generated when a conductor cuts magnetic lines of force, and the effect of permeability on a current-carrying conductor in a magnetic field. To maintain relative motion between the magnetic field and the rotor conductor, the rotor speed is always less than the speed of the rotating magnetic field; hence the name asynchronous motor.
18. Synchronous Speed – When a three-phase symmetrical current is passed through the three-phase symmetrical windings of an asynchronous motor, a rotating magnetic field is generated in the air gap of the motor. The speed of the rotating magnetic field varies depending on the number of poles in the motor; the more poles, the slower the speed. We call the speed of this rotating magnetic field the synchronous speed.
19. Slip – The difference between the synchronous speed n1 and the motor speed n (n1-n) is called the speed difference. The ratio of the speed difference to the synchronous speed is called the slip. The slip S is usually expressed as a percentage, i.e., S=(n1-n)/n1×100%.
20. Star-Delta Starting – If the stator windings of a motor are connected in delta configuration during normal operation, then connected in star configuration during startup, and finally connected in delta configuration after startup, this starting method is called star-delta starting. 21. Absorption Ratio – The ratio of the resistance of an insulating test sample after applying DC voltage for 60 seconds to 15 seconds.
22. Working grounding – In order to ensure the safe and reliable operation of electrical equipment under normal or fault conditions and to prevent high voltage caused by equipment failure, a certain point in the power system must be grounded, which is called working grounding.
23. Protective grounding – To prevent electric shock accidents caused by insulation damage to electrical equipment, the metal casing or frame of the electrical equipment, which is normally not energized, is connected to the earth. This is called protective grounding.
24. Protective grounding—in a power supply neutral point grounding system, this involves connecting the metal casing or frame of electrical equipment to the neutral wire drawn from the neutral point. This is also an important measure to protect personal safety.
25. Disconnecting switch – A switch with a clearly visible break and no arc-extinguishing device. It can be used to connect or disconnect energized but unloaded lines, voltage transformers, and unloaded transformers of limited capacity. The main purpose of a disconnecting switch is to isolate the power supply voltage during electrical equipment maintenance.
26. High-voltage circuit breaker – also known as a high-voltage switch. It can not only interrupt or close the no-load current and load current in a high-voltage circuit, but also interrupt the short-circuit current through the action of relay protection devices when a system fault occurs. It has a fairly complete arc-extinguishing structure and sufficient breaking capacity.
27. Arc suppression coil – This is an adjustable inductor coil with an iron core, installed at the neutral point of a transformer or generator. When a single-phase ground fault occurs, it reduces the grounding current and suppresses the arc.
28. Reactor – A reactor is an inductor with very low resistance. The turns of the coil are insulated from each other, and the entire coil is insulated from the ground. A reactor is connected in series in a circuit to limit short-circuit current.
29. Eddy Current Phenomenon—Imagine a coil wrapped around a solid iron core, which can be considered as being composed of many closed iron wires, with the plane formed by the closed wires perpendicular to the direction of magnetic flux. Each closed wire can be considered a closed conductive loop. When an alternating current flows through the coil, the magnetic flux passing through the closed wires changes continuously, thus inducing an electromotive force and an induced current in each wire. In this way, induced currents flow in concentric circles around the axis of the iron core, much like eddies in water. This induced current generated in the iron core is called an eddy current.
30. Eddy current loss – Just as current flows through a resistor, eddy currents in the iron core consume energy and cause the iron core to heat up. This energy loss is called eddy current loss.
31. Low-current grounding system – the neutral point is not grounded or is grounded through an arc suppression coil.
32. High-current grounding system – a system with a directly grounded neutral point.
33. Armature reaction—When there is no armature current, the air gap main magnetic field is generated solely by the excitation current. When there is an armature current, the air gap main magnetic field is formed by the superposition of the magnetic field of the excitation current and the magnetic field of the armature current. This influence of the armature current on the main magnetic field is called armature reaction.
34. Electric arc – The accumulation of a large number of sparks forms an electric arc.
35. Phase sequence—the order in which sinusoidal quantities of each phase pass through the same value. Any set of asymmetrical three-phase sinusoidal AC voltage or current phasors can be decomposed into three sets of symmetrical components: one set is the positive sequence component, represented by the subscript "1", with the phase sequence consistent with the original asymmetrical sinusoidal quantity, i.e., the order of ABC, and each phase differing from the other by 120°; another set is the negative sequence component, represented by the subscript "2", with the phase sequence opposite to the original asymmetrical sinusoidal quantity, i.e., the order of ACB, and each phase differing from the other by 120°; the third set is the zero sequence component, represented by the subscript "0", with all three phases having the same phase. For example, asymmetrical two-phase operation will result in negative and zero sequence components.
36. Relay starting current – the minimum current value required to activate a relay.
37. Current relay – A relay whose operation is determined by the magnitude of the current flowing into its coil is called a current relay.
38. Voltage relay – A relay that determines whether to operate based on the applied voltage level.
39. Fast relay – generally refers to a relay whose operating time is less than 10 milliseconds.
40. Instantaneous overcurrent protection – protection that operates instantaneously as long as the current reaches the set value, without a time limit.
41. Differential protection – is a type of protection that utilizes the change in current when electrical equipment malfunctions to achieve activation.
42. Zero-sequence protection – protection against the zero-sequence current and zero-sequence voltage electrical quantities unique to grounding faults in power systems.
43. Distance protection – A protection device that reflects the distance from the fault point to the protection installation location.
44. Automatic Reclosing – A device that automatically recloses the circuit breaker after a line fault occurs and the circuit breaker trips, without manual operation. Reclosing is divided into single-phase reclosing and integrated reclosing.
45. Integrated reclosing – Its function is: single-phase fault trips single phase, and if unsuccessful, trips all three phases; phase-to-phase fault trips all three phases; three-phase reclosing, and if unsuccessful, trips all three phases.
46. Acceleration after reclosing: When a permanent fault occurs, the protection device trips the circuit breaker again without time limit and does not perform reclosing. This is called acceleration after reclosing.
47. Protection – Protection that meets the requirements of system stability and equipment safety, selectively and quickly disconnecting the protected equipment and the entire line of faults.
48. Backup protection – Protection used to isolate the fault when the main protection fails to operate or the circuit breaker fails to operate.
49. Power factor – the ratio of active power P to apparent power S.
50. Switching Operations – When electrical equipment changes from one state to another, or when the system's operating mode is altered, a series of operations are required. These operations are called switching operations for electrical equipment. Switching operations mainly include:
(1) Power outage of the transformer
(2) Power outage on power lines
(3) Generator starting, paralleling and disconnecting operations
(4) Loop formation and de-looping in networks
(5) Change of busbar connection method (i.e. busbar switching operation)
(6) Changes in neutral grounding method and adjustment of arc suppression coil
(7) Changes in the operating status of relay protection and automatic devices
(8) Installation and removal of grounding wire
51. No-load loss – This refers to the power absorbed by the transformer when a sinusoidal AC rated voltage at the rated frequency is applied to one coil of the transformer (at the rated tap position), while the other coils are open circuits. This power is used to supply the transformer core losses (eddy current and hysteresis losses).
52. No-load current – When a transformer is running under no-load conditions, the main magnetic flux is established by the no-load current, so the no-load current is the excitation current. The rated no-load current is the three-phase arithmetic average of the current drawn by the transformer when a rated positive weak AC voltage at the rated frequency is applied to one coil (at the rated tap position) while the other coils are open circuits. It is expressed as a percentage of the rated current.
53. Short-circuit loss – is the power absorbed by the transformer when the rated current at the rated frequency passes through one coil of the transformer and the other coil is short-circuited. It is the loss caused by the resistance of the transformer coil, i.e., copper loss (coil at the rated tap position, temperature 70℃).
54. Short-circuit voltage – This is the voltage at the rated frequency (at the rated tap position) applied to another coil to generate the rated current when one coil is short-circuited. It is expressed as a percentage of the rated voltage and reflects the transformer's impedance (resistance and leakage reactance) parameters. It is also called impedance voltage (at 70°C).
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