Compressed air energy storage synchronous motor starting and operation control protection system

Abstract

The utility model relates to the field, and discloses a compressed air energy storage synchronous motor starting and operation control protection system, which comprises at least two sets of starting and operation control protection subsystems. Each set of start and operation control protection subsystem comprises a plurality of circuit breakers, a frequency conversion section, a tie line, a start / speed regulation frequency converter, a frequency conversion loop and a power frequency loop; the frequency conversion loop comprises a circuit breaker, a starting / speed regulation frequency converter and a frequency conversion section; the power supply side of the frequency conversion loop is connected to the working section, and the output side is respectively connected with a plurality of compressor motors through the frequency conversion section; the frequency conversion sections of the two subsystems are connected through a connecting line and an interconnecting circuit breaker and are used for standby starting and debugging; the input side of the power frequency loop is connected with the low-voltage side of the transformer, and the output side is connected with the multiple compressor motors through the frequency conversion section. According to the system, the switching sequence is optimized, and the configuration of the starting / speed regulation frequency converter is simplified, so that one standby starting / speed regulation frequency converter starts a plurality of compressor motors.

Description

TECHNICAL FIELD

[0001] The utility model relates to the field of electrical engineering, concretely relates to a compressed air energy storage synchronous motor starting and operation control protection system. BACKGROUND

[0002] As an energy storage technology with high efficiency and large capacity, compressed air energy storage technology has become one of the research hotspots in the field of energy storage technology.

[0003] Under the background of building a new energy system, to improve the comprehensive adjustment capability of the power system and accelerate the construction of flexible regulation power, the installation scale of new energy storage is also increasing year by year. Under the background of "double carbon", it is necessary to build a new power system with renewable energy as the main body, and large-scale energy storage technologies such as compressed air energy storage are important technical means to improve the renewable energy consumption capacity of the power grid. The commercialization of new compressed air energy storage systems is of great significance not only in terms of single machine scale, but also in terms of low carbon environmental protection and energy storage efficiency. The construction cost of compressed air energy storage power station is continuously decreasing. In the renewable energy consumption scene, the cost per kilowatt-hour of compressed air energy storage is lower than that of small and medium-sized pumped storage power stations, and is closer to the level of large-scale pumped storage power stations, and has certain economic competitiveness. There are problems such as poor water source conditions, high construction and operation costs of pumped storage power stations in the northwest region, and compressed air energy storage has certain development potential.

[0004] Large-capacity synchronous motors are often used as driving devices for air compressors in large-scale energy storage projects, which have the characteristics of large capacity and complex system. The starting of the motor has a great impact on the power grid, and its excitation system and protection configuration are more complex than conventional asynchronous motors. Large-capacity synchronous motors for large-scale, high-efficiency air compressors have the problems of large capacity and complex control and protection systems.

[0005] Usually, variable frequency starters are used to start multiple synchronous motors in sequence to achieve smooth starting. Variable frequency starter sequential starting refers to starting multiple motors in sequence through the control of the variable frequency starter to avoid simultaneous starting and impacting the power grid and the motor. At the same time, the variable frequency starter can also realize the speed regulation of the motor to meet the requirements of the production process. However, after the starting of each motor is completed, the last motor is operated at variable frequency, which may cause the delay of the starting of the last motor and affect the production efficiency. Secondly, when each motor operates at a power frequency, a protection device is configured for each power frequency circuit breaker, which may lead to complex protection configuration and complicated wiring, and is prone to misoperation, affecting the normal operation of the motor and equipment. Finally, the control mode of the variable frequency starter may not be flexible enough to meet the needs of different working conditions. Utility model content

[0006] To solve the existing problems, the utility model provides a kind of compressed air energy storage synchronous motor starting and running control protection system, by the switching of multiple circuit breakers and the control of frequency converter, reach the requirement of one frequency converter sequentially starting multiple synchronous motors and smooth starting, each level motor is started, and the last motor is operated by frequency conversion;When each motor is operated at power frequency, the protection device is configured in the power frequency circuit breaker respectively, the comprehensive protection device is not separately arranged in the circuit breaker loop in the speed regulation operation frequency conversion section, and the protection of synchronous motor dragged by high-voltage frequency conversion soft start and speed regulation device is realized;The corresponding protection configuration wiring is simple, the structure is clear, the scheme is simple and easy to implement, avoids the cooperation between each protection from leading to malfunction, and also reduces equipment cost.

[0007] In order to achieve the above purpose, the utility model provides the following technical scheme.

[0008] A kind of compressed air energy storage synchronous motor starting and running control protection system, including at least two sets of starting and running control protection subsystems;Each set of starting and running control protection subsystem includes several circuit breakers, frequency conversion section, tie line, starting / speed frequency converter, frequency conversion loop and power frequency loop;The frequency conversion loop includes circuit breaker, starting / speed frequency converter and frequency conversion section;The power supply side of the frequency conversion loop is connected into working section, and the output side is connected with multiple compressor motors respectively through frequency conversion section;The frequency conversion section of two sets of subsystems is connected by connecting line and tie circuit breaker, for standby starting and debugging use;The input side of the power frequency loop is connected with the low voltage side of transformer, and the output side is connected with multiple compressor motors respectively through frequency conversion section.

[0009] As a further improvement of the utility model, it further includes a mutual inductor and its connecting line;Mutual inductor and its connecting line are arranged on both sides of the power frequency loop.

[0010] As a further improvement of the utility model, it further includes several lightning arresters, and each power frequency loop is connected with circuit breaker and lightning arrester.

[0011] As a further improvement of the utility model, a circuit breaker is arranged on the connection branch of the frequency conversion section and each compressor, for starting different compressor motors respectively.

[0012] As a further improvement of the utility model, it further includes a metering and measuring element;The metering and measuring element is connected into the frequency conversion loop.

[0013] As a further improvement of the utility model, it further includes a comprehensive protection / fault filtering element;The comprehensive protection / fault filtering element is connected into the frequency conversion loop.

[0014] As a further improvement of the utility model, it further includes an overcurrent protection element;The overcurrent protection element is connected into the frequency conversion loop.

[0015] As a further improvement of the utility model, it further comprises a compressor differential circuit; the compressor differential circuit is connected to the frequency conversion loop.

[0016] As a further improvement of the utility model, it further comprises a mutual inductor and its connecting wire; the mutual inductor and its connecting wire are arranged on both sides of the frequency conversion loop, so as to realize the protection of the frequency conversion starting and operation of the synchronous motor.

[0017] As a further improvement of the utility model, it further comprises a plurality of standby circuits and differential protection elements; the standby circuits and differential protection elements are connected to the power frequency loop.

[0018] The utility model has the following beneficial effects:

[0019] The system has higher reliability and flexibility by designing at least two modules; when one module fails, the other module can be used as backup access, ensuring the continuous operation of the compressor motor and improving the overall stability and reliability of the system; by combining the frequency conversion and power frequency loop, flexible starting and operation control of the motor are realized; when the frequency conversion loop completes the starting stage, the power frequency loop can be accessed, ensuring the continuous and stable operation of the motor and improving the reliability and stability of the system; by switching the multiple circuit breakers and controlling the frequency converter, the requirements of sequentially starting multiple synchronous motors by one frequency converter and smoothly starting are met; after the starting of the motors at all levels is completed, the frequency operation of the last motor is carried out; the connection between the two sets of starting and operation control protection subsystems is realized through the frequency conversion section connection of each system; this connection mode facilitates standby starting and debugging operation; when a problem occurs in a subsystem, such as frequency conversion loop or starting / speed regulating frequency converter failure, the standby module can quickly access the system through the frequency conversion section communication line, ensuring seamless switching of the standby frequency conversion loop or starting / speed regulating frequency converter failure, and the circuit breaker on the frequency conversion section communication line provides additional safety protection and control.

[0020] Optionally, it further comprises a mutual inductor and its connecting wire; the mutual inductor and its connecting wire are arranged on both sides of the power frequency loop, so as to realize the protection of the power frequency operation of the synchronous motor.

[0021] Preferably, a circuit breaker and a lightning protection device are connected to each power frequency loop, which enhances the electrical safety performance of the system, especially the safety of power frequency current supply; the circuit breaker can quickly cut off the circuit in the case of short circuit or overload, preventing equipment damage; for the system in the wild, the lightning protection device can effectively prevent the damage of lightning impact to the system, protecting the safety of equipment and personnel; at the same time, this protection system also helps to protect the safety of gas compression gas storage equipment.

[0022] Preferably, a circuit breaker is arranged on the connection branch of each compressor in the variable frequency range, which provides flexibility for independent control of each compressor and can quickly cut off the power supply of the motor of a specific compressor when necessary to protect the compressor motor from damage.

[0023] Preferably, the variable frequency circuit is connected with the metering and measuring element, which allows the system to monitor and record the operating parameters of the compressor motor in real time, providing important data support for the maintenance and management of the equipment.

[0024] Optionally, the variable frequency circuit is connected with the comprehensive protection / fault filtering element: this enhances the fault detection and protection capability of the system. The comprehensive protection line can monitor the operating state of the system in real time and take measures immediately once an abnormality is found; the fault filtering line helps to accurately diagnose the cause of the fault and improve the efficiency of maintenance.

[0025] Optionally, the variable frequency circuit is connected with the overcurrent protection element, which can cut off the circuit in time limit or inverse time limit when the current exceeds the setting value, preventing the equipment from being damaged due to overload and improving the safety of the system.

[0026] Optionally, the variable frequency circuit is also connected with the compressor differential line, which can detect the current difference between the input and output of the compressor motor, and trigger a protection action once an abnormal difference is detected to prevent safety accidents such as equipment damage.

[0027] Optionally, it also includes a transformer and its wiring; the transformer and its wiring are arranged on both sides of the variable frequency circuit to realize protection for the variable frequency starting and operation of the synchronous motor.

[0028] Optionally, several standby lines and differential protection lines: the standby line provides additional redundancy for the compressor motor, enhancing the reliability of the system; the differential protection line further improves the protection level of the compressor motor, ensuring that the equipment can safely shut down in abnormal conditions. BRIEF DESCRIPTION OF DRAWINGS

[0029] The drawings described herein are for illustrative purposes only and do not limit the scope of the present disclosure in any way. In addition, the shapes and proportions of the components in the drawings are only illustrative and are used to help understand the present disclosure, and are not specific limitations on the shapes and proportions of the components. In the drawings:

[0030] Figure 1 The wiring principle diagram of the compressed air energy storage synchronous motor starting and operation control protection system device described in the present disclosure;

[0031] Figure 2The utility model discloses a kind of compressed air energy storage synchronous motor starting and running control protection system equipment motor side protection control schematic diagram.

[0032] Wherein, 1, 1A air compressor motor;2, 1B air compressor motor;3, 1C air compressor motor;4, SCB1-first subsystem;5, SCB2-first subsystem;6, SCB3-first subsystem;7, BCB-first subsystem;8, OCB-first subsystem;9, first starting / speed regulating frequency converter;10, ICB-first subsystem;11, RCB1-first subsystem;12, RCB2-first subsystem;13, 2A air compressor motor;14, 2B air compressor motor;15, 2C air compressor motor;16, SCB1-second subsystem;17, SCB2-second subsystem;18, SCB3-second subsystem;19, BCB-second subsystem;20, OCB-second subsystem;21, second starting / speed regulating frequency converter;22, ICB-second subsystem;23, RCB1-second subsystem;24, RCB2-second subsystem;25, metering, measuring element;26, comprehensive protection / fault filtering element;27, overcurrent protection element;28, compressor differential circuit;29, first differential protection circuit;30, first standby circuit;31, second standby circuit;32, second differential protection circuit;33, excitation power unit;34, excitation regulator. DETAILED DESCRIPTION

[0033] In order to make the person skilled in the art better understand the technical scheme in the utility model, the technical scheme in the embodiment of the utility model will be described clearly and completely below in conjunction with the drawings in the embodiment of the utility model. Obviously, the described embodiment is only a part of the embodiment of the utility model, not all the embodiments. Based on the embodiment in the utility model, all other embodiments obtained by the person skilled in the art without creative labor should belong to the scope of the utility model protection system.

[0034] It should be noted that when an element is referred to as "provided on" another element, it can be directly on the other element or there can be a middle element. When an element is referred to as "connected" to another element, it can be directly connected to the other element or there can be a middle element. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are for illustrative purposes only and are not the only embodiment.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description herein is for describing particular embodiments only and is not intended to be limiting of the application. The use herein of the terms "and / or" includes a set of one or more associated listed items.

[0036] To meet the requirements of soft start and process speed regulation of synchronous motor in large capacity compressed air energy storage project, a compressed air energy storage synchronous motor starting and running control protection scheme is provided.

[0037] In large capacity compressed air energy storage project, each compression line is composed of three compressors, each of which is started by a synchronous motor, and the last stage compressor is speed-regulated by a frequency conversion device. Under the premise of reducing the cost, in order to successfully start each motor and meet the requirements of speed regulation of the last stage compressor, a set of high-voltage frequency conversion soft start and speed regulation device is matched for each compression line, and the capacity of the high-voltage frequency conversion soft start and speed regulation device is selected according to the capacity of simultaneously meeting the starting of the synchronous motor of the previous stage compressor and the speed regulation of the synchronous motor of the last stage compressor.

[0038] By using a set of high-voltage frequency conversion soft start and speed regulation device, the starting of several high-power synchronous motors and the speed regulation of the last stage synchronous motor are realized, which reduces the cost of equipment while meeting the requirements of process operation. In order to realize the step-by-step soft start of several high-power synchronous motors and the speed regulation of the last stage synchronous motor, a frequency conversion section is arranged at the outlet of the high-voltage frequency conversion soft start and speed regulation device to realize the step-by-step start switching of several synchronous motors. A set of high-voltage frequency conversion soft start and speed regulation device is used to start multiple synchronous motors and to frequency-regulate the speed of the last stage motor.

[0039] Embodiment 1

[0040] As shown in the accompanying drawings, Figure 1 The embodiment provides a compressed air energy storage synchronous motor starting and running control protection system, which includes at least two sets of starting and running control protection subsystems.

[0041] As Figure 1As shown, the left is the start and run control protection first subsystem, and the right is the start and run control protection second subsystem. The start and run control protection first subsystem includes 8 circuit breakers, 1 frequency conversion section, 1 start / speed regulating frequency converter, 3 lightning arresters, 1 frequency conversion circuit and 1 power frequency circuit, and a tie line for connecting the start and run control protection second subsystem. Among them, the power supply side of the frequency conversion circuit is connected to the working section, i.e. 10kV working section A, and the other side is connected to the air compressor motor; the rated current of the working section A is 3200A, and the rated operating short-circuit breaking capacity is 50kA~125kA; the frequency conversion circuit is connected in series with the circuit breaker ICB-first subsystem 10, the first start / speed regulating frequency converter 9 and the frequency conversion section A in turn; the frequency conversion section A is a 10kV frequency conversion section A; the frequency conversion section A is connected to 3 air compressor motors 1A, 1B and 1C and lightning arresters respectively; the frequency conversion section A is connected to the frequency conversion section B of the start and run control protection second subsystem through the tie line, and the tie line is provided with the circuit breaker BCB-first subsystem 7 and the circuit breaker BCB-second subsystem 19.

[0042] The input side of the power frequency circuit is connected to the low voltage side of the transformer, and the other side is connected to 2 air compressor motors 1A and 1B respectively; the branches connected to the air compressor motors 1A and 1B are provided with the circuit breakers RCB1-first subsystem 11 and RCB2-first subsystem 12 respectively; the power frequency circuit is connected with lightning arresters.

[0043] As shown, Figure 1 The start and run control protection second subsystem includes 8 circuit breakers, 1 frequency conversion section, 1 start / speed regulating frequency converter, 3 lightning arresters, 1 frequency conversion circuit and 1 power frequency circuit, and a tie line for connecting the start and run control protection first subsystem. Among them, the power supply side of the frequency conversion circuit is connected to the working section, i.e. 10kV working section B, and the other side is connected to the air compressor motor; the frequency conversion circuit is connected in series with the circuit breaker ICB-second subsystem 22, the second start / speed regulating frequency converter 21 and the frequency conversion section B in turn; the frequency conversion section B is a 10kV frequency conversion section A; the frequency conversion section B is connected to 3 air compressor motors 2A, 2B and 2C and lightning arresters respectively; the branches connected to the frequency conversion section B and the 3 air compressor motors 2A, 2B and 2C are provided with the circuit breakers SCB1-second subsystem 16, SCB2-second subsystem 17 and SCB3-second subsystem 18 respectively; the frequency conversion section B is connected to the frequency conversion section A of the start and run control protection first subsystem through the tie line, and the tie line is provided with the circuit breaker BCB-second subsystem 19 and the circuit breaker BCB-first subsystem 7.

[0044] The power frequency loop input side is connected with the low voltage side of the transformer, and the other side is connected with two air compressor motors 2A and 2B respectively, and the branch connected with the air compressor motors 2A and 2B is respectively provided with a circuit breaker RCB1-second sub-system 23 and a circuit breaker RCB2-second sub-system 24; and the power frequency loop is connected with a lightning protection device.

[0045] Figure 2 The protection control circuit of the display starting and running control protection first sub-system is shown, and the variable frequency loop is further connected with a metering and measuring element 25.

[0046] The variable frequency loop is further connected with a comprehensive protection / fault filtering element 26.

[0047] The variable frequency loop is further connected with an overcurrent protection element 27.

[0048] The variable frequency loop is further connected with a compressor differential line 28.

[0049] As shown in Figure 2 The compressed air energy storage synchronous motor starting and running control protection system device can further include an excitation power bus, three excitation power units and three excitation regulators; the excitation power bus is connected with the excitation power unit and the excitation regulator corresponding to each air compressor motor, such as the excitation power unit 33 and the excitation regulator 34 of the air compressor motor 1A, so as to control the excitation of the air compressor motor 1A.

[0050] As shown in Figure 2 The compressed air energy storage synchronous motor starting and running control protection system device can further include a plurality of standby lines and differential protection lines; each compressor motor is connected with the standby line and the differential protection line. The air compressor motor 1A is connected with a first differential protection line 29, a first standby line 30, a second standby line 31 and a second differential protection line 32 respectively.

[0051] The normal starting process of the synchronous motor starting and running control protection system is as follows:

[0052] In the normal starting process, the high-voltage cabinet is ready to receive the starting instruction of the coordinated control system (CCS), and the 10kV working section A works normally; then, the first starting / speed regulating frequency converter 9 performs self-checking, respectively closes the detection frequency power supply cabinet and the outgoing line cabinet, takes the first sub-system as an example, the OCB-first sub-system 8 and the ICB-first sub-system 10 on the frequency conversion loop are connected to work, and the air compressor motor is started step by step; 1A starting cabinet is closed, that is, SCB1-first sub-system 4 is closed, so that 1A air compressor motor is started; at the same time, when the synchronization conditions are met (such as voltage, frequency and phase are consistent), 1A running cabinet is closed; then, 1A starting cabinet is opened, and SCB1-first sub-system 4 is disconnected; then, the above process is repeated to start 1B air compressor motor (synchronous operation). The frequency conversion loop directly drives 1C air compressor motor to the process speed (frequency conversion operation).

[0053] In the fault emergency mode, when the single-sided starting and running control protection sub-system fails, for example, the first starting / speed regulating frequency converter 9 fails to be used, the breakers BCB-first sub-system 7 and BCB-second sub-system 19 are closed through the tie line, and the air compressor motor of the fault module is driven by the starting and running control protection sub-system on the other side; when the left and right modules cross the edge operation, it is necessary to ensure that the tie cabinet is closed, and the frequency conversion voltage provided by the second starting / speed regulating frequency converter 21 can be distributed across the edge.

[0054] The high-voltage frequency conversion soft starting and speed regulating device process of the synchronous motor starting and running control protection system is as follows:

[0055] In the starting stage, the power supply (auxiliary power supply, UPS) is checked, then the device is initialized, and then the air compressor motor is selected, and then the auxiliary / medium voltage system is started; the breakers OCB-first sub-system 8 (first starting / speed regulating frequency converter 9 input power-on), SCB1-first sub-system 4, SCB2-first sub-system 5, SCB3-first sub-system 6 (1A, 1B, 1C air compressor motor is connected to the first starting / speed regulating frequency converter 9), ICB-first sub-system 10 (system grid connection or synchronous operation) are closed, the excitation power supply bus provides excitation current to control the air compressor motor to accelerate to a speed >1%, to realize grid synchronization regulation; when the air compressor motor speed >99%, RCB1-first sub-system 11 and RCB2-first sub-system 12 are closed, and the soft starting is completed.

[0056] In the stop and in the synchronization phase, the pulse is cut off, the field current is zero, the circuit breakers OCB-1st subsystem 8, SCB1-1st subsystem 4, SCB2-1st subsystem 5, SCB3-1st subsystem 6, ICB-1st subsystem 10 are opened, and then the auxiliary system is closed to complete the stop operation; after the fault is recovered, the field current is re-energized, the synchronization check and grid connection are performed to complete the resynchronization operation.

[0057] The practical advantages of the synchronous motor starting and running control protection system are as follows:

[0058] 1. Redundancy and flexibility: double-column independent design and tie-in switch support cross-column power supply and emergency failure; the first starting / speed regulating frequency converter 9 is used for soft starting (1A / 1B) and can also directly drive the frequency conversion load (1C).

[0059] 2. Protection mechanism: overcurrent protection is realized by the circuit breaker; there is also ground protection to ensure the safety of the equipment.

[0060] 3. Control logic layering: the main controller coordinates the starting / speed regulating frequency converter, the field system, and the circuit breaker action to realize full-automatic start-stop and synchronization.

[0061] The system can be used in industrial compressor units to support smooth starting and energy efficiency optimization of high-energy-consumption equipment; the system can be used for grid peak shaving to reduce start-stop impact by matching the load demand through frequency conversion and speed regulation; and the system can also be used for fault backup to provide continuous power guarantee for cross-column operation for key processes.

[0062] The above embodiment is only one of the implementation manners capable of realizing the technical scheme of the utility model, and the scope of the system claimed by the utility model is not limited by the embodiment, but also includes changes, substitutions and other implementation manners easily thought of by any person skilled in the art within the technical range disclosed by the utility model.

Claims

1. A compressed air energy storage synchronous motor start and run control protection system, characterized in that, It comprises at least two sets of starting and running control protection sub-systems; each set of starting and running control protection sub-systems comprises several circuit breakers, frequency conversion sections, tie lines, starting / speed regulating frequency converters, frequency conversion circuits and power frequency circuits; the frequency conversion circuit comprises circuit breakers, starting / speed regulating frequency converters and frequency conversion sections; the power supply side of the frequency conversion circuit is connected to the working section, and the output side is connected to several compressor motors through the frequency conversion sections; the frequency conversion sections of the two sets of sub-systems are connected through connecting wires and tie circuit breakers, which are used for standby starting and debugging.

2. A compressed air energy storage synchronous motor starting and running control protection system according to claim 1, characterized in that, It also comprises transformers and their connecting wires; the transformers and their connecting wires are arranged on both sides of the power frequency circuit.

3. A compressed air energy storage synchronous motor starting and running control protection system according to claim 1, characterized in that, It also comprises several lightning arresters, and each power frequency circuit is connected with a circuit breaker and a lightning arrester.

4. A compressed air energy storage synchronous motor starting and run control protection system as set forth in claim 1, wherein, A circuit breaker is arranged on the connecting branch of the frequency conversion section and each compressor, which is used for starting different compressor motors.

5. A compressed air energy storage synchronous motor starting and run control protection system as set forth in claim 1, wherein, It also comprises metering and measuring elements; the metering and measuring elements are connected to the frequency conversion circuit.

6. A compressed air energy storage synchronous motor starting and run control protection system as set forth in claim 1, wherein, It also comprises comprehensive protection / fault filtering elements; the comprehensive protection / fault filtering elements are connected to the frequency conversion circuit.

7. A compressed air energy storage synchronous motor starting and run control protection system as set forth in claim 1, wherein, It also comprises overcurrent protection elements; the overcurrent protection elements are connected to the frequency conversion circuit.

8. A compressed air energy storage synchronous motor starting and run control protection system as set forth in claim 1, wherein, It also comprises compressor differential lines; the compressor differential lines are connected to the frequency conversion circuit.

9. A compressed air energy storage synchronous motor starting and run control protection system as set forth in claim 1, wherein, It also comprises transformers and their connecting wires; the transformers and their connecting wires are arranged on both sides of the frequency conversion circuit, which realizes the protection of the frequency conversion starting and running of synchronous motors.

10. A compressed air energy storage synchronous motor starting and run control protection system as set forth in claim 1, wherein, It also comprises several standby lines and differential protection elements; the standby lines and differential protection elements are connected to the power frequency circuit.

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