Capacitor series compensation device and overvoltage protection method thereof

Abstract

The application relates to the field of power transmission and distribution network voltage quality control, in particular to a capacitor series compensation device and an overvoltage protection method thereof, which is connected with a switching operation switch, a power supply and sensing unit, an overvoltage protection unit, a capacitor group C and a controller on a power transmission and distribution line. Compared with a traditional capacitor series compensation device, the overvoltage protection method of the capacitor group is innovated, a self-coupling transformer BK and a bypass circuit breaker CB are used to replace a traditional discharge gap, a zinc oxide voltage limiter, a discharge damping device, a power electronic switch or a fast circuit breaker. In addition, the third winding N3 of the self-coupling transformer BK is used as a voltage transformer and a discharge coil of the capacitor group, is used for monitoring internal faults of the capacitor group in a normal working state of the power grid, and is used for limiting the discharge current of the capacitor group and inhibiting high-frequency oscillation when the bypass circuit breaker CB is closed in a short-circuit state of a series compensation load.

Description

TECHNICAL FIELD

[0001] The application relates to the field of voltage quality control of power transmission and distribution networks, in particular to a capacitor series compensation device and an overvoltage protection method thereof, which is suitable for 750 kV and below AC power transmission and distribution networks. BACKGROUND

[0002] Due to the development of extra-high voltage power transmission of the power system, a capacitor series compensation device needs to be used to improve the transmission capacity and stability of long power transmission lines; at present, most high-capacity step-down transformers use high short-circuit impedance transformers, resulting in large voltage fluctuations of the next level power grid, and the use of capacitor series compensation can avoid the frequent adjustment of the on-load tap changer of the main transformer and the accompanying reactive power crossing; there is also a demand for using the capacitor series compensation device to compensate for voltage and reactive power loss for long-distance power transmission and distribution feeder lines.

[0003] In the power system, the overvoltage protection system of the traditional high-voltage, extra-high voltage or power distribution network level capacitor series compensation device is very complex. For example, the patents CN109066714A, CN109066720A, CN203536971U, CN204271652U, CN104882881A and the national standard GB / T6115.2-2017 (Series Capacitors for Electric Power Systems - Part 2: Protection Equipment for Series Capacitor Banks) describe the series compensation device, which shows that the overvoltage protection system generally includes a trigger conduction bypass gap, a discharge current limiting damping device, a zinc oxide voltage limiter, a bypass switch (a fast switch device, a contactor or a silicon controlled assembly) and the like.

[0004] When a short-circuit fault occurs at the load end of the series compensation line, it is required to limit the transient power frequency overvoltage several times the normal operating voltage at both ends of the series compensation capacitor bank, and the duration of the power frequency overvoltage can reach several seconds, which requires a very high withstand capability of the zinc oxide voltage limiter MOV in the overvoltage protector. Even if multiple groups of metal oxide valves are operated in parallel, they can only last for tens of milliseconds, so a discharge gap and a fast closing circuit breaker or a power electronic switch must also be configured.

[0005] Due to the negative temperature coefficient characteristic of the resistance of the metal oxide valve in the zinc oxide voltage limiter MOV, it is very difficult to achieve current sharing when multiple groups of valves are connected in parallel, and the consistency of the resistance characteristics of multiple groups of valves in parallel is extremely high, resulting in difficult quality control of the MOV, high manufacturing cost and high failure rate in operation. That is, even if it is a short-time support of tens of milliseconds, the MOV is not an ideal power frequency overvoltage limiter.

[0006] The discharge gap will be severely ablated under the action of the power frequency overvoltage, thereby increasing the operation and maintenance cost; at the same time, the outdoor weather conditions will also significantly affect the discharge characteristics of the discharge gap, thereby causing the overvoltage protection characteristics of the discharge gap to be unstable.

[0007] As the fast closing circuit breaker of power frequency overvoltage backup protection is not a regular product, it needs special manufacturing, and the strict requirement for closing time leads to the decrease of working reliability and stability of the circuit breaker. The power power switch can be used to replace the above-mentioned circuit breaker, but it will lead to the great increase of technical complexity and overall cost.

[0008] In summary, the discharge gap, zinc oxide voltage limiter MOV, the circuit breaker with fast closing characteristics or power electronic switch will greatly increase the cost of the series compensation device and reduce the reliability of the series compensation device. The high cost and high failure rate will also seriously affect the popularization and application of series compensation in power system. Therefore, it is urgent to innovate the traditional overvoltage protection method to greatly improve the reliability of the capacitor series compensation device and reduce the cost. SUMMARY

[0009] In view of the problems in the prior art, the application provides a capacitor series compensation device and an overvoltage protection method thereof.

[0010] The technical scheme adopted by the application to solve the technical problems is that a capacitor series compensation device is composed of a switching operation switch connected to a power transmission and distribution line, a power supply and sensing unit, an overvoltage protection unit, a capacitor bank C and a controller.

[0011] Specifically, the switching operation switch comprises a first disconnecting switch GK1, a second switch GK2 and a third disconnecting switch GK3, and the second switch GK2 can be a disconnecting switch or a conventional circuit breaker.

[0012] Specifically, the power supply and sensing unit comprises a voltage transformer TV1, a voltage measurement winding TV2 and a current transformer CT.

[0013] The overvoltage protection unit comprises an autotransformer BK and a bypass circuit breaker CB. The autotransformer BK is a special saturated reactor, and the core structure thereof is a closed core that is split into phases. Each phase comprises a first winding N1, a second winding N2 and a third winding N3 for measurement. The end of the first winding N1 and the start of the second winding N2 are connected, and the start of the first winding N1 and the start of the second winding N2 are the same name terminals. The start of the first winding N1 and the end of the second winding N2 are connected to the line-in end and the line-out end of the capacitor bank C respectively.

[0014] The bypass circuit breaker CB is a conventional circuit breaker, which is connected in parallel with the second winding N2.

[0015] The voltage measurement winding TV2 is the third winding N3 of the autotransformer BK.

[0016] Specifically, the capacitor bank C is a three-phase capacitor.

[0017] Specifically, the controller can be an intelligent control unit or an analog control unit.

[0018] An overvoltage protection method of a capacitor series compensation device, including three states of the capacitor series compensation device in an operation process;

[0019] First, when the power grid is in a normal operation state:

[0020] The first disconnector GK1 and the third disconnector GK3 are in a closed state, and the second switch GK2 and the bypass circuit breaker CB are in an open state;

[0021] The voltage Uc between the capacitor banks is less than or equal to Un, and Un is the rated voltage of the capacitor banks;

[0022] Under the action of the voltage Uc, the autotransformer BK works in section I of the curve and is in a non-saturated state, i.e., a high impedance state, and the excitation current flowing through the autotransformer BK and the first winding N1 and the second winding N2 is less than 1.0 A;

[0023] At this time, the capacitor banks C will automatically compensate the line voltage according to the change of the load current;

[0024] Second, a short-circuit state of the series compensation load side:

[0025] Once a short-circuit fault occurs on the load side of the series compensation device, the line current sharply increases, causing Uc to be higher than the rated voltage Un of the capacitor banks. At this time, the current i flowing through the autotransformer BK also synchronously rises with the rise of Uc. When i increases to Is, the autotransformer BK enters a saturated region and works in section II of the curve, i.e., a low impedance state. At this time, the voltage between the capacitor banks C no longer linearly rises, but slowly rises at a relatively gentle speed until it approaches the saturated voltage Upl corresponding to the maximum short-circuit current Imax of the line;

[0026] To ensure the safety of the capacitor banks, Upl should be significantly lower than the limit voltage Ulim that the capacitor banks can withstand.

[0027] In this process, the controller quickly judges whether a short circuit occurs on the series compensation load side by monitoring the current in the current transformer CT. If the current exceeds a preset threshold, the controller controls the bypass circuit breaker CB to close to bypass the second winding N2 of the autotransformer BK. At this time, only the first winding N1 is in an excitation state. Since the number of turns of the first winding N1 is much smaller than that of the second winding N2, the autotransformer BK will be pushed out of saturation and work in section III of the curve.

[0028] The use of the bypass circuit breaker CB can multiply the short-circuit current thermal stability requirement of the second winding N2, and relatively thin wires can be used for winding, thereby greatly reducing the manufacturing cost of the autotransformer BK.

[0029] The first winding N1 is used to damp the discharging current of the capacitor bank C when the bypass circuit breaker CB is closed, so as to eliminate the possible impact damage to the capacitor and the circuit breaker;

[0030] Third, the series compensation maintenance state:

[0031] First, the bypass circuit breaker CB is in the closed state, then the second switch GK2 is closed, and then the first disconnector GK1 and the third disconnector GK3 are opened;

[0032] The series compensation device exits the operation, and after the ground wire is connected, the series compensation device can be maintained.

[0033] The beneficial effects of the application:

[0034] The series compensation device and the overvoltage protection method thereof provided by the application adopt the self-coupled transformer BK and the bypass circuit breaker CB to realize the power frequency overvoltage protection of the capacitor bank C. The no-load saturation characteristics of the self-coupled transformer with large energy capacity are used to replace the expensive and high-failure-rate discharge gap, the zinc oxide voltage limiter MOV, the damping device and the fast circuit breaker or the power electronic switch, and the electromagnetic voltage transformer usually connected in parallel at both ends of the capacitor for the discharge coil and the unbalanced voltage protection is omitted. The wiring is greatly simplified, the number of components is reduced, the cost of the power frequency overvoltage protection of the series compensation device is significantly reduced, and the reliability of the product operation is greatly improved. BRIEF DESCRIPTION OF DRAWINGS

[0035] The application will be further described below in combination with the drawings and embodiments.

[0036] Figure 1 The principle wiring diagram of the series compensation device of the capacitor provided by the application is provided.

[0037] Figure 2 The power frequency overvoltage protection characteristic diagram of the self-coupled transformer BK in the series compensation device of the capacitor provided by the application is provided.

[0038] In the figure: 1, switching operation switch; 11, first disconnector GK1; 12, second switch GK2; 13, third disconnector GK3; 2, power supply and sensing unit; 21, voltage transformer TV1; 22, voltage measurement winding TV2; 23, current transformer CT; 3, overvoltage protection unit; 31, self-coupled transformer BK; 32, bypass circuit breaker CB; 4, capacitor bank C; 5, controller. DETAILED DESCRIPTION

[0039] In order to make the technical means, creative features, purposes and effects realized by the application easy to understand, the application will be further described below in combination with specific embodiments.

[0040] As Figure 1 and Figure 2 The capacitor series compensation device comprises a switching operation switch 1 connected to a power transmission and distribution line, a power supply and sensing unit 2, an overvoltage protection unit 3, a capacitor bank C4 and a controller 5.

[0041] Specifically, the switching operation switch 1 comprises a first disconnector GK111, a second switch GK212 and a third disconnector GK313, and the second switch GK212 can be a disconnector or a conventional circuit breaker.

[0042] Specifically, the power supply and sensing unit 2 comprises a voltage transformer TV121, a voltage measurement winding TV222 and a current transformer CT23.

[0043] The overvoltage protection unit 3 comprises an autotransformer BK31 and a bypass circuit breaker CB32. The autotransformer BK31 is a special saturation reactor, and the core structure thereof is a closed core that is split into phases. Each phase comprises a first winding N1, a second winding N2 and a third winding N3 for measurement. The end of the first winding N1 and the start of the second winding N2 are connected, and the start of the first winding N1 and the start of the second winding N2 are the same name terminals. The start of the first winding N1 and the end of the second winding N2 are connected to the line-in and line-out terminals of the capacitor bank C4, respectively.

[0044] The bypass circuit breaker CB32 is a conventional circuit breaker, which is connected in parallel with the second winding N2.

[0045] The voltage measurement winding TV222 is the third winding N3 of the autotransformer BK31.

[0046] The winding can be used to monitor the voltage across the capacitor bank C4 during normal operation of the power grid.

[0047] Specifically, the capacitor bank C4 is a three-phase capacitor.

[0048] When the capacitor bank 4 is put into operation, the bypass circuit breaker CB32 is in an open state, and the autotransformer BK31 is in a no-load operation and works in a voltage transformer mode. When a short circuit occurs in the series compensation line, the power frequency overvoltage across the capacitor bank C4 exceeds the saturation voltage of the autotransformer, and the autotransformer works in a saturated reactance mode to limit the transient overvoltage across the capacitor bank. After a preset delay, the bypass circuit breaker CB32 is closed, and the short-circuit impedance of the autotransformer BK31 forms a current-limiting reactor and a damping resistance to limit the discharge current of the capacitor and high-frequency oscillation thereof.

[0049] Specifically, the controller 5 can be an intelligent control unit or an analog control unit.

[0050] The device uses a self-coupled transformer BK31 and a bypass circuit breaker CB32 to realize the power frequency overvoltage protection of the capacitor bank C4. The self-coupled transformer BK31 is composed of N1, N2 and N3 three windings, wherein N1 and N2 are power frequency overvoltage protection windings, and N3 is a measurement winding for monitoring the unbalanced voltage during internal fault of the capacitor. The bypass circuit breaker CB32 can use a conventional circuit breaker, and does not need to use a fast circuit breaker. It is controlled by the controller 5, and the voltage transformer TV121 supplies power to the controller 5 and the operating mechanism of the bypass circuit breaker CB32;

[0051] Figure 2 Ulim: the limit peak value of the transient overvoltage that the capacitor bank can withstand;

[0052] Ulim: the limit peak value of the transient overvoltage that the capacitor bank can withstand;

[0053] Us: the initial value of the saturation voltage of the self-coupled transformer when it is in an unloaded state;

[0054] Is: the saturation excitation current of the self-coupled transformer when it is in an unloaded state;

[0055] Imax: the maximum short-circuit fault current at the installation point of the capacitor series compensation device;

[0056] I: the voltage-ampere characteristic line segment of the self-coupled transformer in an unloaded working mode without short-circuit fault;

[0057] II: the voltage-ampere characteristic line segment of the self-coupled transformer in a saturated working mode when it is in an unloaded state under short-circuit fault;

[0058] III: the voltage-ampere characteristic line segment of the self-coupled transformer in a short-circuit working mode when the bypass circuit breaker is closed;

[0059] P: the initial saturation point of the excitation voltage of the self-coupled transformer, OP is the V-A characteristic of the voltage transformer mode.

[0060] When the power grid is in a normal operating state:

[0061] The first disconnector GK111 and the third disconnector GK313 are in a closed state, and the second switch GK212 and the bypass circuit breaker CB32 are in an open state. The voltage Uc between the capacitor banks is ≤Un, and Un is the rated voltage of the capacitor bank. Under the action of the Uc voltage, the self-coupled transformer BK31 works in the blue curve I segment and is in a non-saturated state, i.e., a high impedance state, and only a small excitation current <10A flows through the N1 and N2 windings of the self-coupled transformer BK31. At this time, the capacitor bank 4 will realize automatic compensation of the line voltage according to the change of the load current.

[0062] Short-circuit state of the series compensation load side:

[0063] Once the short-circuit fault occurs at the load side of the series compensation device, the line current increases sharply, resulting in Uc being higher than the rated voltage Un of the capacitor bank 4. At this time, the current i flowing through the autotransformer BK31 also increases synchronously with the rise of Uc. When i increases to Is, the autotransformer BK31 enters the saturation region and works in the low impedance state, i.e., the II section of the middle curve. At this time, the voltage across the capacitor bank C4 no longer increases linearly, but slowly increases at a relatively gentle speed until it approaches the saturation voltage Upl corresponding to the maximum short-circuit current Imax of the line. To ensure the safety of the capacitor bank, Upl should be significantly lower than the limit voltage Ulim that the capacitor bank can withstand. In this process, the controller 5 quickly determines whether a short circuit occurs at the load side of the series compensation device by monitoring the current in the current transformer CT23. If the current exceeds the preset threshold, the bypass circuit breaker CB32 is closed to bypass the N2 winding of the autotransformer BK31. At this time, only the N1 winding is in the excitation state. Since the number of turns of the N1 winding is much smaller than that of the N2 winding, the autotransformer BK31 will be pushed out of saturation and work in the III section of the curve, i.e., the III section. Figure 2 The bypass circuit breaker CB32 has the advantages of reducing the short-circuit current thermal stability requirement of the N2 winding by several times, allowing the N2 winding to be wound with thinner wires, and thus greatly reducing the manufacturing cost of the autotransformer BK31. The N1 winding has the advantage of damping the discharge current of the capacitor bank C4 when the bypass circuit breaker CB32 is closed to eliminate the possible impact damage to the capacitor and the circuit breaker. Figure 2

[0064] The series compensation device is in the maintenance state.

[0065] The operation steps are as follows: first, the bypass circuit breaker CB32 is placed in the closed state, then the second switch GK212 is closed, and then the first disconnector GK111 and the third disconnector GK313 are opened. At this time, the series compensation device exits the operation, and after the ground wire is connected, the series compensation device can be maintained and repaired.

[0066] It can be seen that the use of the autotransformer BK31 and the bypass circuit breaker CB32 realizes the protection of the power frequency overvoltage across the capacitor bank C4. The advantages are that the no-load saturation characteristics of the autotransformer with a relatively large energy capacity are used to replace the expensive and high-failure-rate discharge gap, zinc oxide voltage limiter MOV, damping device, and fast circuit breaker or power electronic switch, and the electromagnetic voltage transformer usually connected in parallel across the capacitor for discharge coil and unbalanced voltage protection is also omitted. This greatly simplifies the wiring and reduces the number of components, significantly reduces the cost of the power frequency overvoltage protection of the series compensation device, and greatly improves the reliability of the product operation.

[0067] ​The above shows and describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above-mentioned embodiments, and the above-mentioned embodiments and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A capacitor series compensation device, characterized in that: The entire device is composed of a switching operation switch (1) connected to a transmission and distribution line, a power supply and sensing unit (2), an overvoltage protection unit (3), a capacitor bank C (4) and a controller (5); The switching operation switch (1) comprises a first isolating switch GK1 (11), a second switch GK2 (12) and a third isolating switch GK3 (13), and the second switch GK2 (12) is an isolating switch or a conventional circuit breaker; The power supply and sensing unit (2) comprises a voltage transformer TV1 (21), a voltage measuring winding TV2 (22) and a current transformer CT (23); The overvoltage protection unit (3) includes an autotransformer BK (31) and a bypass circuit breaker CB (32). The autotransformer BK (31) is a specially made saturated reactor. Its iron core structure is a phase-separated closed iron core. Each phase includes a first winding N1, a second winding N2, and a third winding N3 for measurement. The end of the first winding N1 is connected to the beginning of the second winding N2. The beginning of the first winding N1 and the beginning of the second winding N2 are the same end. The beginning of the first winding N1 and the end of the second winding N2 are connected to the input terminal and the output terminal of the capacitor bank C (4) respectively. The bypass circuit breaker CB (32) is a conventional circuit breaker, which is connected in parallel with the second winding N2; The voltage measurement winding TV2 (22) is the third winding N3 of the autotransformer BK (31).

2. The capacitor series compensation device according to claim 1, characterized in that: The capacitor group C (4) is a three-phase capacitor.

3. The capacitor series compensation device according to claim 1, characterized in that: The controller (5) is an intelligent control unit or an analog control unit.

4. An overvoltage protection method for a capacitor series compensation device according to any one of claims 1 to 3, characterized in that: Including three states of the capacitor series compensation device during operation; First, when the power grid is operating normally: The first disconnector GK1 (11) and the third disconnector GK3 (13) are in the closed state, and the second disconnector GK2 (12) and the bypass circuit breaker CB (32) are in the open state; The voltage across the capacitor bank is Uc≤Un, where Un is the rated voltage of the capacitor bank; Under the action of the Uc voltage, the autotransformer BK (31) operates in section I of the curve, in a non-saturated state, i.e., a high impedance state, and the excitation current flowing through the autotransformer BK (31), the first winding N1, and the second winding N2 is less than 1.0A; At this time, the capacitor bank C (4) will automatically compensate the line voltage according to the change of load current; Second, the series compensation load side short circuit state: Once a short-circuit fault occurs on the load side of the series compensation device, the line current increases sharply, causing Uc to be higher than the rated voltage Un of the capacitor bank (4). At this time, the current i flowing through the autotransformer BK (31) also increases synchronously with the increase of Uc. When i increases to Is, the autotransformer BK (31) enters the saturation region and operates in the second section of the curve, that is, the low impedance state. At this time, the voltage across the capacitor bank C (4) no longer increases linearly, but slowly increases at a relatively slow speed until it approaches the saturation voltage Upl corresponding to the maximum short-circuit current Imax of the line. To ensure the safety of the capacitor bank, the Upl here is significantly lower than the limit voltage Ulim that the capacitor bank can withstand; During this process, the controller (5) quickly determines whether a short circuit occurs on the series compensation load side by monitoring the current in the current transformer CT (23). If the current exceeds a preset threshold, the bypass circuit breaker CB (32) is controlled to close and bypass the second winding N2 of the autotransformer BK (31). At this time, only the first winding N1 is in an excited state. Since the number of turns of the first winding N1 is much smaller than the number of turns of the second winding N2, the autotransformer BK (31) will be pushed out of saturation and operate in section III of the curve. By adopting the bypass circuit breaker CB (32), the short-circuit current thermal stability requirement of the second winding N2 can be reduced by several times, and the manufacturing cost of the autotransformer BK (31) can be greatly reduced by adopting thin wire winding; The first winding N1 is used to damp the discharge current of the capacitor bank C (4) when the bypass circuit breaker CB (32) is closed to eliminate possible impact damage to the capacitor and the circuit breaker; Third, the maintenance status of series compensation: First, the bypass circuit breaker CB (32) is placed in the closed state, and then the second switch GK2 (12) is closed, and then the first disconnector GK1 (11) and the third disconnector GK3 (13) are opened; The series compensation device can be inspected and maintained after the series compensation device is shut down and the ground wire is connected.

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