A method for grounding protection of generators and power plants for precise zoning in thermal power plants
By using the generator neutral point through arc suppression coil grounding method in the thermal power plant, the accuracy of grounding fault judgment and processing is solved, and the grounding fault detection and reduction of the thermal power plant is achieved quickly. The power outage range improves the reliability and economic benefits of the system.
Patent Information
- Application Number
- CN202111459419.X
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2021-12-01
- Publication Date
- 2024-12-20
- Estimated Expiration
- 2041-12-01
AI Technical Summary
It is difficult to accurately judge and quickly eliminate grounding faults in generators and factory power systems in thermal power plants in the prior art, resulting in the arc not being extinguished in time when single-phase grounding, causing equipment damage and power supply interruption, affecting system reliability.
The generator neutral point through arc suppression coil grounding method combined with flexible CT, PT, generator set protection and small current wiring selection device to form a factory-wide grounding protection solution. Through precise protection actions, the grounding fault points are distinguished and the power outage range is narrowed. , ensure the reliable operation of generators and factory electricity.
It realizes reliable identification and rapid processing of any grounding fault in the entire thermal power plant, narrows the fault range, ensures the continuous operation of generators and plant power, and improves power supply reliability and economic benefits.
Smart Images
Figure CN114336542B_ABST
Abstract
Description
Technical Field
[0001] The invention relates to a generator for accurate zoning of a thermal power plant and a plant power grounding protection method, belonging to the field of power generation electrical technology in the electric power industry. Background Art
[0002] There are three main ways to ground a generator: ungrounded neutral point, grounded neutral point via high resistance, and grounded neutral point via arc suppression coil. There are three main ways to ground the high voltage power system of a thermal power plant: ungrounded, grounded via high resistance, and grounded via low resistance.
[0003] When the capacitive current to ground is small, the neutral point ungrounded system is allowed to operate for a period of time, and the power supply reliability is guaranteed. However, once the capacitive current exceeds the allowable value, the arc cannot be extinguished automatically, and there is a risk of arc grounding overvoltage and burning equipment and cables. The neutral point is grounded through a low resistance system, which can directly eliminate the shortcomings of the neutral point ungrounded system, realize sensitive and selective grounding protection, and reduce the danger of arc grounding overvoltage. The neutral point is grounded through a high resistance system, which can limit the grounding fault current to a certain level and suppress transient overvoltage. However, the high resistance grounding system still fails to solve the problem of arc self-extinguishing when single-phase grounding occurs. In the factory power system, there are many cables. Once the cable is single-phase grounded, the arc cannot be extinguished in time, which will cause serious consequences. The neutral point is grounded through an arc suppression coil. This grounding method uses the inductive current of the arc suppression coil to compensate for the capacitive current, so that the fault current when single-phase grounding is very small, the arc can be extinguished by itself, and will not harm the equipment. It can also operate with faults for a period of time, and the power supply reliability is high.
[0004] Traditional small units at home and abroad mostly use the ungrounded generator neutral point and ungrounded plant power system operation mode. Large and medium-sized units abroad mostly use the generator neutral point through the grounding transformer high resistance grounding method. In the past few years, domestic hydropower generators mostly used the generator neutral point through the arc suppression coil grounding method. In recent years, it has gradually transitioned to the generator neutral point through the grounding transformer high resistance grounding method. Large and medium-sized units mostly use low resistance grounding for plant power.
[0005] DL / T 5153-2014 "Technical Specifications for Design of Power Supply for Thermal Power Plants" does not provide a detailed description of the grounding operation mode for power supply of power plants through arc suppression coils, and few domestic power plants have applied it. As the reliability requirements for external heat supply of small back pressure units are getting higher and higher, the reliability requirements for power supply of power plants are also increasing. Traditional small pure condensing units usually use the direct wiring scheme of generators due to the small scale of the power supply system, and the neutral point of the generator is not grounded. After entering the 21st century, new coal-fired thermal power plants have small back pressure units, large boiler capacity, and large power supply system scale. When the direct wiring scheme of generators is selected, if the system single-phase to ground capacitance current is greater than 10A, in order to ensure that the power supply can continue to operate when single-phase grounding occurs, the generator and power supply must adopt the grounding scheme through arc suppression coils. After single-phase grounding occurs, arc suppression coil compensation is not allowed to operate for a long time. How to timely and accurately determine the location of the grounding point to eliminate the fault is particularly important. Summary of the invention
[0006] Purpose: In order to overcome the deficiencies in the prior art, the present invention provides a method for accurately zoning a generator and plant power grounding protection in a thermal power plant, which is used to distinguish between in-zone and out-of-zone grounding faults of the generator and each plant branch, and combines flexible CT, PT, generator-transformer group protection and small current wiring selection device to form an innovative plant-wide grounding protection scheme, which can reliably identify any grounding fault point in the whole plant, reduce the scope of power outages through precise protection actions, and ensure that the generator and plant power can continue to operate reliably.
[0007] Technical solution: To solve the above technical problems, the technical solution adopted by the present invention is:
[0008] A method for protecting a generator and power supply grounding for precise zoning of a thermal power plant comprises a generator, a branch outside the generator area, a power supply bus, and a power supply feeder. The generator end is provided with a CT1, a PT1, and a circuit breaker CB1. The bus outside the generator area is provided between the generator and the main transformer. The branch outside the generator area is connected to the power supply bus via a reactor. The power supply bus is connected with the power supply feeder. A PT2 is provided between the circuit breaker CB1 and the main transformer. A CT2 and a circuit breaker CB2 are provided on the incoming line of the power supply bus. A circuit breaker CB3 is provided on the power supply feeder. The method comprises the following steps:
[0009] When a ground fault occurs at K1 inside the generator, when the generator and the generator neutral point are operating normally through the arc suppression coil, the single-phase grounding capacitance current at the fault point is less than the generator allowable value, and the fundamental zero-sequence voltage stator grounding protection locked by the zero-sequence current direction alarms. When the generator is operating normally and the generator neutral point is under maintenance through the arc suppression coil, the single-phase grounding capacitance current at the fault point exceeds the generator allowable value, and the fundamental zero-sequence voltage stator grounding protection locked by the zero-sequence current direction trips the generator outlet circuit breaker CB1, demagnetizes, closes the steam valve, and shuts down.
[0010] When a ground fault occurs at the plant feeder K4, when the generator and the generator neutral point are operating normally through the arc suppression coil, the single-phase grounding capacitance current at the fault point is less than the allowable value, and the small current grounding line selection device gives an alarm. When the generator is operating normally, the generator neutral point is under maintenance through the arc suppression coil, or the generator is out of operation due to a fault or maintenance, the small current grounding line selection device will protect and trip the fault feeder outlet circuit breaker CB3.
[0011] When a ground fault occurs at the plant bus K3, when the generator and the generator neutral point are operating normally through the arc suppression coil, the single-phase grounding capacitance current at the fault point is less than the allowable value, and the small current grounding line selection device gives an alarm. When the generator is operating normally, the generator neutral point is under maintenance through the arc suppression coil, or the generator is out of operation due to a fault or maintenance, the small current grounding line selection device protection action trips the outlet circuit breaker CB2 on the lower side of the reactor, and the plant power switching must be locked at the same time. The protection action time must avoid the feeder grounding protection action time of the plant branch.
[0012] When a ground fault occurs at branch K2 outside the generator area, when the generator and the generator neutral point are operating normally through the arc suppression coil, the single-phase grounding capacitance current at the fault point is less than the allowable value, and the zero-sequence voltage protection on the low-voltage side of the main transformer gives a delayed alarm. When the generator is operating normally and the generator neutral point is under maintenance through the arc suppression coil, or the generator is out of operation due to a fault or maintenance, the single-phase grounding capacitance current flowing through the generator does not exceed the allowable value of the generator, but the single-phase grounding capacitance current at the fault point exceeds the allowable value of the high-voltage plant power grounding current. The zero-sequence voltage protection on the low-voltage side of the main transformer is activated at full stop, circuit breakers CB1 and CB2 are tripped, and the plant power switching is started at the same time. The protection action time needs to avoid the plant branch grounding protection action time and the selective generator stator grounding protection action time.
[0013] As a preferred solution, the CT1, PT1, and PT2 use flexible optical CT and PT.
[0014] As a preferred solution, the fundamental zero-sequence voltage stator ground fault protection action delay time is set to t1, t1=1s.
[0015] As a preferred solution, the protection action delay of the small current wiring selection device is set to t2, t2=2s.
[0016] As a preferred solution, the action delay of the output circuit breaker CB2 on the lower side of the reactor that trips the small current grounding line selection device protection is set to t3=t2+0.5s, t3=2.5s.
[0017] As a preferred solution, the zero-sequence voltage protection on the low-voltage side of the main transformer is activated when the transformer is completely stopped, tripping circuit breakers CB1 and CB2, and simultaneously starting the factory power switching with an action delay setting of t4=t3+0.5, t4=3s.
[0018] Beneficial effects: The present invention proposes a protection design scheme when single-phase grounding protection occurs when the generator and the plant power are grounded through arc suppression coils. By designing zoning, the grounding faults inside and outside the generator area, the grounding faults at the plant power incoming line, busbar and feeder are accurately distinguished, the faults are accurately removed, the fault range is reduced, and the coordinated actions of automatic devices such as fast cutting are combined to ensure the continuous operation of the plant power.
[0019] The present invention can improve the power supply reliability of the power supply system of the thermal power plant, thereby improving the reliability of heat supply, reducing the significant losses of heat users caused by heat supply interruption, and also improving the economic benefits of the thermal power plant. BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Figure 1 This is a primary wiring diagram of the present invention under the generator and plant power, main transformer and plant power operation modes.
[0021] Figure 2 This is a primary wiring diagram of the present invention in the start-up and standby operation mode.
[0022] Figure 3 The figure is a schematic diagram of the circuit structure of the application scenario of the method of the present invention. DETAILED DESCRIPTION
[0023] The present invention will be further described below in conjunction with specific embodiments.
[0024] Example 1
[0025] The primary wiring of a thermal power plant in an industrial park in Yantai, Shandong Province is as follows Figure 1-2 As shown, the 50MW steam turbine generator set is transmitted to the existing 110kV bus section of the 110KV main substation in the industrial park through a 10 / 110kV step-up transformer and a 110kV cable line. The factory starting standby power supply is equipped with a 35 / 10kV high-voltage starting / standby transformer, which is led from the 35kV bus section of the industrial park substation using a 35kV cable line.
[0026] In terms of primary system configuration: In order to provide the reliability of plant power and not affect the stable operation of plant power when the generator fails, a generator outlet circuit breaker is installed between the generator and the main transformer. Since the capacity of the boiler used for heat supply to the outside of the thermal power plant is large, the scale of the plant power system is large. After calculation, the total capacitance current of the plant power system is 28A. Adding the capacitance current of the generator and other equipment, the capacitance current of the entire system is about 32A. According to the specifications and the data of the generator factory, the maximum allowable grounding current of the generator is 3A, so only arc suppression coil grounding or high resistance grounding can be selected. Since the thermal power plant has high requirements for heat supply reliability, the neutral point of the generator of the thermal power plant is equipped with an arc suppression coil grounding. Each boiler is equipped with 2 sections of 10kV high-voltage plant busbars, and no high-voltage plant transformer is set. In order to limit the short-circuit current, high-voltage plant current limiting reactors are set on the high-voltage plant branches. The high-voltage plant power supply branch is connected to the low-voltage side of the main transformer. A 10kV standby section is set on the low-voltage side of the high-voltage start / standby transformer to provide standby power for the whole plant.
[0027] In terms of secondary system configuration, independent voltage transformers PT are installed at the generator end, the low-voltage side of the main transformer and the 10kV bus section, flexible optical CT is installed at the generator outlet, and zero-sequence CT is installed on each incoming line and feeder of the 10kV bus section. Independent generator protection devices and main transformer protection devices are installed to protect the generator and main transformer. Flexible optical CT1 and PT1 are connected to the generator protection device, and PT2 is connected to the main transformer protection device. Each section of 10kV plant bus is equipped with a small current grounding line selection device for plant power grounding protection. The incoming line circuit zero-sequence CT2, each feeder circuit zero-sequence CT3 and each section of plant bus PT3 are connected to the small current grounding line selection device. Each section of 10kV plant bus is equipped with a microcomputer-type plant power fast switching device for plant power accident switching.
[0028] Thermal power plants with generator neutral point grounded via arc suppression coils generally need to achieve the following protection conditions:
[0029] (1) The generator and the generator neutral point are operating normally through the arc suppression coil, the generator provides power to the plant, and the thermal power plant supplies heat and electricity normally.
[0030] (2) The generator operates normally and the neutral point of the generator is under maintenance through the arc suppression coil. The generator provides power to the plant and the thermal power plant supplies heat and electricity normally.
[0031] (3) The generator stops running due to failure or maintenance, and the main transformer provides power for the plant. The thermal power plant provides heat normally but does not generate electricity.
[0032] In the above working condition (1), when a single-phase grounding fault occurs, the generator can continue to operate with the fault. However, since the neutral point of the generator is put into operation through the arc suppression coil, when a single-phase grounding fault occurs, the arc suppression coil will generate an inductive current under the action of the zero-sequence voltage. The inductive current will compensate for the capacitive current of the single-phase grounding, limiting the single-phase grounding current to within the allowable value of the single-phase grounding fault current of the generator stator winding. Since the single-phase grounding current after compensation is within 3A, a highly sensitive flexible optical CT is installed at the generator end to ensure the accuracy of the zero-sequence current. At the same time, a small current grounding line selection device using multiple methods such as the comprehensive transient phase comparison method, the transient power direction method, and the transient amplitude comparison method is selected to accurately determine the single-phase grounding line selection of the arc suppression coil grounding system, ensuring the accuracy of the line selection.
[0033] 1) Install flexible optical CT and PT at the generator outlet, and cooperate with the generator protection device to distinguish between ground faults inside and outside the generator area.
[0034] 2) Install a through-type zero-sequence CT in the high-voltage plant power incoming cabinet and each feeder cabinet, and install a PT at the high-voltage plant power busbar, and cooperate with the small current grounding line selection device to distinguish whether the feeder grounding fault is or the busbar grounding fault.
[0035] 3) Through the delay setting coordination of the main transformer protection device, the generator protection device and the small current grounding line selection device, it was identified that a grounding fault occurred in the section from the generator outlet to the factory power incoming cabinet.
[0036] When the arc extinguishing coil is put into operation, a ground fault will only cause an alarm, and the fault will be eliminated manually; when the arc extinguishing coil is out of operation, a ground fault will cause a protection trip, and the auxiliary power switching will be started or locked according to the fault situation. The distinction between the arc extinguishing coil in operation and the out-of-operation state is achieved by sending the auxiliary contact position of the arc extinguishing coil disconnector to the generator protection device, the small current grounding line selection device and the main transformer protection device. The R&D personnel of the generator protection device, the small current grounding line selection device and the main transformer protection device manufacturers modify the program logic according to this requirement.
[0037] Now, combined with working conditions (1), (2), (3) and Figure 3 The protection logic coordination scheme of the present invention is implemented as follows:
[0038] 1) When a ground fault occurs at K1 inside the generator, the zero-sequence voltage of the generator end PT1 and the zero-sequence current of the generator outlet flexible optical CT1 are collected through the generator protection device. When the operating condition (1) occurs, the single-phase grounding capacitance current at the fault point is less than the allowable value of the generator, and the fundamental zero-sequence voltage stator grounding protection locked by the zero-sequence current direction alarms, and the fault can be eliminated manually. When the operating condition (2) occurs, the single-phase grounding capacitance current at the fault point exceeds the allowable value of the generator, and the fundamental zero-sequence voltage stator grounding protection locked by the zero-sequence current direction trips the generator outlet circuit breaker CB1, demagnetizes, closes the steam valve, shuts down, etc. The fundamental zero-sequence voltage stator grounding protection action delay time can be set to t1, and t1=1s is recommended. Generator protection device program to be developed content: The generator protection must add a selective stator grounding protection function based on flexible optical CT, where the zero-sequence voltage is taken from Figure 1 In PT1, the zero-sequence current is taken from CT1 (flexible optical CT). It should be noted that at this time, the high-voltage plant power supply is obtained through the main transformer to ensure that the boiler will not stop when the machine is shut down, and the reliability of the heat supply of the thermal power plant is guaranteed.
[0039] 2) When a ground fault occurs at the plant feeder K4, the voltage of the plant power bus PT3 and the zero-sequence current of CT2 and each feeder CT3 are collected by the small current grounding line selection device. At this time, the zero-sequence current value of CT3 is the largest (when the arc suppression coil is put into operation, the transient amplitude of the current is the largest and the transient phase is different from the non-fault line; when the arc suppression coil is not put into operation, the steady-state amplitude of the current is the largest). When the working condition (1) is met, the single-phase grounding capacitance current at the fault point is less than the allowable value, and the small current grounding line selection device gives an alarm, which can be manually eliminated. When the working conditions (2) and (3) are met, the small current grounding line selection device protects and trips the fault feeder outlet circuit breaker CB3. The protection action delay of the small current wiring selection device can be set to t2, and it is recommended that t2=2s. It should be noted that only one power supply circuit is affected at this time, which can ensure the normal operation of the machine and boiler and the reliability of the heating supply of the thermal power plant.
[0040] 3) When a ground fault occurs at the plant bus K3, the voltage of the plant power bus PT3 and the zero-sequence current of CT2 and each feeder CT3 are collected by the small current grounding line selection device. At this time, the polarity of all zero-sequence CTs is from the bus to the line. When the working condition (1) is met, the single-phase grounding capacitance current at the fault point is less than the allowable value, and the small current grounding line selection device gives an alarm, which can be manually eliminated. When the working conditions (2) and (3) are met, the small current grounding line selection device protection action trips the outlet circuit breaker CB2 on the lower side of the reactor, and the plant power switching must be locked at the same time. This protection action time must avoid the feeder grounding protection action time of the plant branch. The step difference is 0.5s, and it is recommended to set t3=t2+0.5=2+0.5=2.5s. It should be noted that the machine can be kept running after the fault is removed. If the boiler process configuration is a double set of auxiliary machines, the boiler can also be kept running to ensure the reliability of the thermal power plant's heating.
[0041] 4) When a ground fault occurs at K2 outside the generator area, the main transformer protection device collects the zero-sequence voltage of PT2 on the low-voltage side of the main transformer. When operating condition (1), the single-phase grounding capacitance current at the fault point is less than the allowable value, and the zero-sequence voltage protection on the low-voltage side of the main transformer (not through the direction) gives a delayed alarm, and the fault can be eliminated manually. When operating conditions (2) and (3), the single-phase grounding capacitance current flowing through the generator is provided for itself and does not exceed the allowable value of the generator, but the single-phase grounding capacitance current at the fault point exceeds the allowable value of the high-voltage plant power grounding current. The zero-sequence voltage protection on the low-voltage side of the main transformer (not through the direction) is activated at full stop, CB1 and CB2 are tripped, and the plant power switching is started at the same time. The action time of this protection needs to avoid the action time of the plant branch grounding protection and the action time of the selective generator stator grounding protection. It is recommended that this protection action time be t4=t3+0.5=3s. The main transformer protection device program needs to be developed: The zero-sequence voltage protection function on the low-voltage side of the main transformer must be added to the transformer protection, which can be activated at full stop by tripping, where the zero-sequence voltage is taken from Figure 1 It should be noted that at this time, the high-voltage plant power supply is obtained from the standby transformer through the action of the quick-cut device, ensuring that the boiler will not be stopped during shutdown, and ensuring the reliability of the heat supply of the thermal power plant.
[0042] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.
Claims
1. A method for accurately partitioning a generator and a plant power grounding protection for a thermal power plant, comprising a generator, a branch outside the generator area, a plant bus, and a plant feeder, wherein the generator end is provided with a CT1, a PT1, and a circuit breaker CB1, the branch outside the generator area is provided between the generator and the main transformer, the branch outside the generator area is connected to the plant bus via a reactor, the plant feeder is connected to the plant bus, a PT2 is provided between the circuit breaker CB1 and the main transformer, a CT2 and a circuit breaker CB2 are provided on the incoming line of the plant bus, and a circuit breaker CB3 is provided on the plant feeder, characterized in that: The steps include: When a ground fault occurs at K1 inside the generator, when the generator and the generator neutral point are operating normally through the arc suppression coil, the single-phase grounding capacitance current at the fault point is less than the allowable value of the generator, and the fundamental zero-sequence voltage stator grounding protection locked by the zero-sequence current direction gives an alarm; when the generator is operating normally and the generator neutral point is under maintenance through the arc suppression coil, the single-phase grounding capacitance current at the fault point exceeds the allowable value of the generator, and the fundamental zero-sequence voltage stator grounding protection locked by the zero-sequence current direction trips the generator outlet circuit breaker CB1, demagnetizes, closes the steam valve, and shuts down; When a ground fault occurs at the plant feeder K4, when the generator and the generator neutral point are operating normally through the arc suppression coil, the single-phase grounding capacitance current at the fault point is less than the allowable value, and the small current grounding line selection device gives an alarm; when the generator is operating normally, the generator neutral point is under maintenance through the arc suppression coil, or the generator is out of operation due to a fault or maintenance, the small current grounding line selection device will protect and trip the fault feeder outlet circuit breaker CB3; When a ground fault occurs at the plant bus K3, when the generator and the generator neutral point are operating normally through the arc suppression coil, the single-phase grounding capacitance current at the fault point is less than the allowable value, and the small current grounding line selection device gives an alarm; when the generator is operating normally and the generator neutral point is under maintenance through the arc suppression coil, or the generator is out of operation due to a fault or maintenance, the small current grounding line selection device will trip the outlet circuit breaker CB2 on the lower side of the reactor, and the plant power switch must be locked at the same time. The protection action time must avoid the feeder grounding protection action time of the plant branch; When a grounding fault occurs at branch K2 outside the generator area, when the generator and the generator neutral point are operating normally through the arc suppression coil, the single-phase grounding capacitance current at the fault point is less than the allowable value, and the zero-sequence voltage protection on the low-voltage side of the main transformer gives a delayed alarm; when the generator is operating normally and the generator neutral point is under maintenance through the arc suppression coil, or the generator is out of operation due to a fault or maintenance, the single-phase grounding capacitance current flowing through the generator does not exceed the allowable value of the generator, but the single-phase grounding capacitance current at the fault point exceeds the allowable value of the high-voltage plant power grounding current; the zero-sequence voltage protection on the low-voltage side of the main transformer is activated at full stop, circuit breakers CB1 and CB2 are tripped, and the plant power switching is started at the same time. The protection action time needs to avoid the plant branch grounding protection action time and the selective generator stator grounding protection action time.
2. A method for grounding protection of generators and power supply for thermal power plants for precise zoning according to claim 1, characterized in that: The CT1, PT1 and PT2 adopt flexible optical CT and PT.
3. A method for grounding protection of generators and power supply for thermal power plants for precise zoning according to claim 1, characterized in that: The fundamental zero-sequence voltage stator ground fault protection action delay time is set to t1, t1=1s.
4. A method for grounding protection of generators and power supply for thermal power plants for precise zoning according to claim 1, characterized in that: The protection action delay setting of the small current wiring selection device is t2, t2=2s.
5. A method for grounding protection of generators and power supply for thermal power plants for precise zoning according to claim 4, characterized in that: The action delay setting of the output circuit breaker CB2 on the lower side of the reactor that trips when the protection of the small current grounding line selection device is activated is t3=t2+0.5s, t3=2.5s.
6. A method for grounding protection of generators and power supply for thermal power plants for precise zoning according to claim 5, characterized in that: The zero-sequence voltage protection on the low-voltage side of the main transformer is activated when the transformer stops completely, tripping circuit breakers CB1 and CB2. At the same time, the action delay setting of starting the auxiliary power switching is t4=t3+0.5, t4=3s.