Excitation inrush current suppression method for large power transformer

Abstract

The invention relates to an excitation inrush current suppression method for a large power transformer in a power supply and distribution circuit system, which is characterized in that a transfer branch for excitation, inrush current and overvoltage treatment is connected to a low-voltage winding side of a main transformer, and the branch comprises a power supply circuit, an inrush current treatment circuit and an overvoltage protection circuit. Through the transfer branch, the original direct closing of the main transformer and the generation of the magnetizing inrush current are converted into the generation of the magnetizing inrush current when the branch completes the closing and opening operation. And the main transformer is automatically magnetized, so that the burden of workers is reduced, and the intelligent degree of power supply and distribution is improved. In addition, a manual mode can be selected to magnetize the main transformer, so that a worker can conveniently carry out selective switching work according to actual conditions; when automatic magnetizing fails, workers adopt manual magnetizing, so that the purpose of switching can still be achieved.

Description

Technical Field

[0001] The invention relates to a circuit system for power supply and distribution, and in particular to a method for suppressing excitation surge current of a large power transformer. Background Art

[0002] In the power supply network system and distribution network system, when the main transformer of a newly built substation is put into operation or the transformer is put into operation after maintenance, a large excitation surge current will appear when the unloaded transformer is closed, which will have an impact on the power grid system.

[0003] For example, the excitation surge current generated by the closing of the transformer will cause the transformer winding to shift in the oil tank, which may put the connection between the coils and the connection between the coils and the terminals at risk of being damaged, ultimately leading to an open circuit in the winding.

[0004] For another example, the magnetizing inrush current generated when the transformer is switched on contains harmonics that cause the power grid system to resonate at a certain frequency, thereby adversely affecting the power supply quality of the power grid system.

[0005] In addition, the DC component of the magnetizing inrush current generated when the transformer is switched on will produce an oscillatory mechanical torque on the motor, thereby increasing the motor's oscillations and causing a reduction in the motor's life.

[0006] Therefore, controlling the transformer closing excitation inrush current is an important task in the industry. At present, the traditional strategies for controlling the transformer closing excitation inrush current are as follows:

[0007] 1. Pre-insertion resistance method. That is, a resistor is connected in series with the neutral point of the transformer, and the resistor is inserted in the process of closing the transformer to limit the closing current, and the resistor is removed after the power supply is completed. However, this method itself has certain limitations, such as: due to the heating and loss of the resistor, it is not suitable for occasions where it is frequently switched on and off; and the power supply reliability is reduced due to the need to maintain the resistor in the later stage of operation.

[0008] Second, demagnetization method. That is, AC or DC demagnetization method is used on site to eliminate the residual magnetic flux of the transformer. However, there is no standard operation process for this method, and the demagnetization effect is judged only by experience, so when the transformer is closed, a large excitation inrush current will still appear.

[0009] 3. Simple phase control method. That is, the circuit breaker selects different points on the voltage waveform to perform closing and opening actions, so as to achieve selective closing and opening operations. However, this method cannot accurately calculate the closing time due to the error in residual magnetism measurement. Secondly, the operating mechanism of the circuit breaker is mostly a spring mechanism or a hydraulic mechanism, which has a large dispersion in operation and is not conducive to accurate closing. In this way, large excitation inrush current events will still occur in actual applications. Summary of the invention

[0010] In order to solve the negative problems in the above background technology when using the pre-insertion resistance method, demagnetization method and simple phase control method to control the transformer closing excitation inrush current, the present invention provides the following technical solutions:

[0011] A method for suppressing excitation inrush current of a large power transformer is provided. The method is to connect a transfer branch for excitation, inrush current and overvoltage control to the low-voltage winding side of the main transformer, and the branch includes a power supply circuit, an inrush current control circuit and an overvoltage protection circuit.

[0012] in:

[0013] The power supply circuit excites the main transformer flux to a steady state before the main transformer is put into operation in the power system, clamping the inrush current peak value generated when the main transformer is put into operation. In the process of the power supply circuit exciting the main transformer, the inrush current control circuit suppresses the inrush current generated by the transfer branch, so that the main transformer is stably excited to a steady state before being put into operation, thereby achieving the control of the main transformer closing inrush current. In the operation of the transfer branch, the overvoltage protection circuit shunts the current of the power supply circuit and the inrush current control circuit, suppressing the overvoltage that occurs in the power supply circuit and the inrush current control circuit.

[0014] Furthermore, the circuit constituent unit of the transfer branch includes a main switch, a branch switch, a step-up transformer, a closing inrush current suppression device, and a power generation device. The wiring method of these circuit constituent units is that the low-voltage winding side of the main transformer is connected to the low-voltage bus of the power system through the main switch; the branch switch is set on the circuit connected to the load side of the low-voltage bus of the power system and serves as the main switch of the circuit; the high-voltage winding side of the step-up transformer is connected to the outgoing line side of the branch switch; the incoming line side of the closing inrush current suppression device is connected to the low-voltage winding side of the step-up transformer; and the output side of the power generation device is connected to the outgoing line side of the closing inrush current suppression device.

[0015] Furthermore, the main transformer is a transformer to be put into operation in the power system for receiving and distributing electricity.

[0016] Furthermore, the step-up transformer is a double-winding transformer, the high-voltage winding side of which is connected to the low-voltage busbar of the power system, and the low-voltage winding side of which is connected to the closing inrush current suppression device. The high-voltage winding side of the step-up transformer and the low-voltage winding side of the main transformer have the same nominal voltage, and the nominal voltage of the low-voltage winding side of the step-up transformer is not higher than 10KV.

[0017] Furthermore, the closing inrush current suppression device includes a phase-controlled switching main circuit, an inrush current suppression unit, a voltage limiting protection unit and an inrush current control unit. The phase-controlled switching main circuit is composed of a phase-controlled circuit breaker and a bypass switch in series, and is used to complete the switching of the power generation equipment. The inrush current suppression unit is connected in parallel to the bypass switch and in series with the phase-controlled circuit breaker to suppress the excitation inrush current of the circuit. The voltage limiting protection unit is connected in series to the phase-controlled circuit breaker, and the phase-controlled circuit breaker is grounded through the voltage limiting protection unit to suppress overvoltage. The inrush current control unit includes a transformer for collecting bus side voltage signals and a transformer for collecting load side voltage signals, a sensor for collecting load side current signals, and a phase-controlled controller for data processing and phase-controlled switching command processing.

[0018] Furthermore, the phase-controlled circuit breaker is a three-phase circuit breaker using an eddy current drive mode, the inrush current suppression unit is a silicon stack, and the voltage limiting protection unit is a nonlinear varistor.

[0019] Furthermore, in the transfer branch, the power generation equipment-phase-controlled switching main circuit-boosting transformer-branch switch-main switch are connected in series in sequence to form a power supply circuit in the transfer branch. In the transfer branch, the power generation equipment-inrush current suppression unit-phase-controlled circuit breaker-boosting transformer-branch switch-main switch are connected in series in sequence, or the power generation equipment-phase-controlled circuit breaker-inrush current suppression unit-boosting transformer-branch switch-main switch are connected in series in sequence to form an inrush current management circuit in the transfer branch. In the transfer branch, the power generation equipment-voltage limiting protection unit-grounding are connected in series in sequence, or the power generation equipment-phase-controlled circuit breaker-voltage limiting protection unit-grounding are connected in series in sequence to form an overvoltage protection circuit in the transfer branch.

[0020] Furthermore, the excitation method of the transfer branch to the main transformer adopts manual magnetization, and the specific method is as follows:

[0021] The switch status before commissioning, that is, before commissioning the main transformer, the main switch and branch switch are both in the closed state, and the phase-controlled circuit breaker and bypass switch are both in the open state;

[0022] Preparatory work before commissioning: start the power generation equipment for preheating so that the power generation equipment can transmit electricity to the step-up transformer in the subsequent process.

[0023] Inrush current suppression state, that is, the phase-controlled circuit breaker is used for phase selection and closing, so that the output side of the power generation equipment is connected to the main transformer through the inrush current control circuit, and the current of the transient process of excitation on the low-voltage winding side of the step-up transformer is suppressed within the rated current of the power generation equipment, so that the power generation equipment can operate stably and the low-voltage side winding of the main transformer is excited through the step-up transformer;

[0024] In the excitation state, after the inrush current suppression is completed, the bypass switch is closed to connect the output side of the power generation equipment to the main transformer through the power supply circuit, so that the power generation equipment can operate stably and the low-voltage side winding of the main transformer is excited through the step-up transformer;

[0025] In the overvoltage protection state, when overvoltage occurs at the inrush current suppression unit and the bypass switch, the voltage limiting protection unit automatically operates to provide overvoltage protection.

[0026] Furthermore, the excitation method of the transfer branch to the main transformer adopts automatic magnetization, and the specific method is:

[0027] In the initial state, the high-voltage winding side of the main transformer is connected to the high-voltage bus of the power system, and the medium-voltage windings are all connected to the medium-voltage bus of the power system. The main switch is in the closed state, while the branch switch, phase-controlled circuit breaker and bypass switch are all in the open state. The power generation equipment is in the hot standby state, and the inrush current control unit monitors the output side voltage of the power generation equipment in real time.

[0028] Early warning response, that is, after receiving the early warning, the power system disconnects the power system from the medium-voltage winding of the main transformer, and the entire station load is taken over by other transformers in the power system that run in parallel with the main transformer;

[0029] The step-up transformer is put into operation, i.e. the branch switch is closed, the step-up transformer is energized and operates at no load;

[0030] The commissioning of the power generation equipment, that is, when the dispatching center of the power system issues a command and executes the off-grid action of the high-voltage side winding of the main transformer, the low-voltage winding side of the step-up transformer loses pressure, the closing conditions of the phase-controlled circuit breaker are met and the phase-controlled circuit breaker is closed by phase selection, and the power generation equipment excites the low-voltage side winding of the main transformer through the step-up transformer;

[0031] In the excitation state, after the inrush current suppression is completed, the bypass switch is closed to connect the output side of the power generation equipment to the main transformer through the power supply circuit, so that the power generation equipment can operate stably and the low-voltage side winding of the main transformer is excited through the step-up transformer;

[0032] In the overvoltage protection state, when overvoltage occurs at the inrush current suppression unit and the bypass switch, the voltage limiting protection unit automatically operates to provide overvoltage protection.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1. The present invention transforms the original direct closing of the main transformer and the generation of excitation inrush current into the generation of excitation inrush current when the branch completes the closing and opening operation by transferring the branch. That is, the excitation inrush current is generated in the circuit on the low-voltage winding side of the step-up transformer. In this way, the magnitude of the impact current formed on the low-voltage winding side of the main transformer depends on the maximum allowable operating condition on the side of the power generation equipment, and the rated current of the power generation equipment is much smaller than the rated current on the low-voltage winding side of the main transformer, so that the peak value of the excitation inrush current can be clamped.

[0035] Second, the present invention transforms the original direct closing of the main transformer and the generation of excitation inrush current into the generation of excitation inrush current when the branch completes the closing and opening operation by transferring the branch. That is, the excitation inrush current is generated in the circuit on the low-voltage winding side of the step-up transformer. In this way, compared with directly managing the excitation inrush current on the low-voltage winding side of the main transformer, the present invention chooses to manage it on the side of the power generation equipment with a smaller excitation inrush current peak, which reduces the difficulty of management, and the use of the closing inrush current suppression device can further suppress the excitation inrush current of the branch.

[0036] 3. The present invention uses the closing and opening current suppression operation of the closing inrush current suppression device to enable the present invention to frequently perform switching operations. Since the present invention abandons the method of connecting a resistor in series at the neutral point of the transformer, the negative effects brought about by this method are avoided.

[0037] 4. The present invention can control the phase-controlled circuit breaker and the bypass switch to perform corresponding closing and opening actions within a preset closing and opening phase angle range through the inrush current control unit, without relying on calculating the residual magnetic flux of the transformer, thus avoiding the negative impact brought about by the empirical judgment method of the demagnetization method.

[0038] 5. The present invention can control the phase-controlled circuit breaker and bypass switch to perform corresponding closing and opening actions within the preset closing and opening phase angle range through the inrush current control unit, so that the inrush current and overvoltage at the closing and opening moment are relatively small, and then the inrush current suppression unit and the voltage limiting protection unit are used to further suppress the inrush current generated at this moment, thereby overcoming the problem that the simple phase control method cannot well suppress the inrush current due to the closing error. In addition, the phase-controlled circuit breaker of the present invention adopts an eddy current drive mode, which can realize rapid closing and opening actions, which is conducive to improving the closing accuracy.

[0039] 6. The present invention can use the power generation equipment to pre-magnetize the low-voltage winding side of the main transformer by transferring the branch circuit so that the main transformer can enter a steady state. In this way, a steady-state magnetic flux is established inside the iron core of the main transformer before it is put into operation, so that a large closing excitation inrush current will not be generated when the switch is closed for operation.

[0040] 7. The present invention achieves the purpose of controlling the main transformer closing excitation surge current by controlling the surge current of the transfer branch, thereby reducing the impact of the surge current on the power system, ensuring the operating stability and power supply reliability of the power system, and being conducive to improving the power supply quality.

[0041] 8. The present invention can realize automatic magnetization of the main transformer, reduce the burden on the staff, and improve the intelligent level of power supply and distribution. In addition, the present invention can also choose to manually magnetize the main transformer, which is convenient for the staff to selectively switch on and off according to the actual situation; when the automatic magnetization fails, the staff uses manual magnetization, so that the present invention can still achieve the purpose of switching on and off. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 This is the wiring diagram of the power system of the present invention with the main transformer being 220KV / 110KV / 35KV;

[0043] Figure 2 It is the logic diagram of excitation and inrush current suppression of the present invention;

[0044] Figure 3 It is a wiring diagram of a closing inrush current suppression device of a phase-controlled circuit breaker of the present invention located at the incoming line side of a bypass switch;

[0045] Figure 4 It is a wiring diagram of a closing inrush current suppression device of a phase-controlled circuit breaker of the present invention located at the outgoing line side of a bypass switch;

[0046] Figure 5 It is a working logic diagram of the closing inrush current control unit of the present invention;

[0047] Figure 6 The present invention is a wiring diagram of switches and bypass switches of phase-controlled circuit breakers on the A, B and C three-phase lines. DETAILED DESCRIPTION

[0048] The preferred specific embodiments for implementing the present invention are described in detail below, and a clear and complete description is made in conjunction with the accompanying drawings.

[0049] See also Figure 1-Figure 6 The present invention provides a method for suppressing magnetizing inrush current of a large power transformer, which is applicable to the scenarios where main transformers of newly built substations at all levels are put into operation, and is also applicable to the scenarios where transformers are put into operation after maintenance. Figure 1 In the circuit, the main switch is configured in the main switch cabinet on the 35KV side, and the branch switch is configured in the branch switch cabinet on the 35KV side. Figure 3 and Figure 4 In the figure, ZK represents the inrush current control unit, K2 represents the phase-controlled switch, K1 represents the bypass switch, TL represents the inrush current suppression unit, and ZCD-B represents the voltage limiting protection unit. Figure 5 In the figure, Ua, Ub and Uc represent the three-phase lines A, B and C connected to the phase-controlled circuit breaker respectively, TV represents the mutual inductance for collecting the voltage signal on the bus side, and TA represents the current sensor for collecting the current signal on the load side. Figure 6 In the figure, A, B and C represent the three-phase lines A, B and C connected to the phase-controlled circuit breaker respectively. The figure takes the setting of bypass switches on the A-phase and C-phase lines as an example. In actual applications, bypass switches can also be set on the A-phase, B-phase and C-phase lines.

[0050] The present invention provides a method for suppressing excitation inrush current of a large power transformer. The method is to connect a transfer branch for excitation, inrush current and overvoltage control to the low-voltage winding side of the main transformer. That is, when the main transformer and main switch already exist in the power system, the construction of the present invention only needs to connect a branch consisting of a branch switch, a step-up transformer, a closing inrush current suppression device and a power generation equipment in the low-voltage side bus of the power system. The branch can reduce the difficulty of inrush current and overvoltage control and improve the control effect. That is, there is no need to change the working magnetic flux of the main transformer, nor the performance and working mode of the circuit breaker in the main switch, and there is no need to change the operating mechanism of power reception and distribution of the original power system.

[0051] In the large-scale power transformer excitation inrush current suppression method of the present invention, the circuit component unit of the transfer branch includes a main switch, a branch switch, a step-up transformer, a closing inrush current suppression device and a power generation device. Among them, the wiring method between each circuit component unit is:

[0052] The low-voltage winding side of the main transformer is connected to the low-voltage bus of the power system through the main switch. The branch switch is set on the circuit connected to the load side of the low-voltage bus of the power system and serves as the main switch of the circuit. The high-voltage winding side of the step-up transformer is connected to the outgoing line side of the branch switch. The incoming line side of the closing inrush current suppression device is connected to the low-voltage winding side of the step-up transformer. The output side of the power generation equipment is connected to the outgoing line side of the closing inrush current suppression device.

[0053] In addition, the closing inrush current suppression device includes a phase-controlled switching main line, an inrush current suppression unit, a voltage limiting protection unit and an inrush current control unit. The phase-controlled switching main line is composed of a phase-controlled circuit breaker and a bypass switch in series, and is used to complete the switching of the power generation equipment. The inrush current suppression unit is connected in parallel to the bypass switch and in series with the phase-controlled circuit breaker to suppress the excitation inrush current of the circuit. The voltage limiting protection unit is connected in series to the phase-controlled circuit breaker, and the phase-controlled circuit breaker is grounded through the voltage limiting protection unit to suppress overvoltage. The inrush current control unit includes a transformer for collecting bus side voltage signals and a transformer for collecting load side voltage signals, a sensor for collecting load side current signals, and a phase-controlled controller for data processing and phase-controlled switching command processing.

[0054] In the transfer branch, the power generation equipment-phase-controlled switching main circuit-step-up transformer-branch switch-main switch are connected in series in sequence to form a power supply circuit in the transfer branch.

[0055] In the transfer branch, the power generation equipment-inrush current suppression unit-phase-controlled circuit breaker-boosting transformer-branch switch-main switch are connected in series in sequence, or the power generation equipment-phase-controlled circuit breaker-inrush current suppression unit-boosting transformer-branch switch-main switch are connected in series in sequence to form an inrush current control circuit in the transfer branch.

[0056] In the transfer branch, the power generation equipment-voltage limiting protection unit-grounding are connected in series in sequence, or the power generation equipment-phase-controlled circuit breaker-voltage limiting protection unit-grounding are connected in series in sequence to form an overvoltage protection circuit in the transfer branch.

[0057] Specifically:

[0058] The power supply circuit excites the main transformer flux to a steady state before the main transformer is put into operation in the power system, clamping the inrush current peak value generated when the main transformer is put into operation. In the process of the power supply circuit exciting the main transformer, the inrush current control circuit suppresses the inrush current generated by the transfer branch, so that the main transformer is stably excited to a steady state before being put into operation, thereby achieving the control of the main transformer closing inrush current. In the operation of the transfer branch, the overvoltage protection circuit shunts the current of the power supply circuit and the inrush current control circuit, suppressing the overvoltage that occurs in the power supply circuit and the inrush current control circuit.

[0059] Preferably, the main transformer is the main transformer to be put into operation in the power system for receiving and distributing power. For example, in the application of the distribution network system, the main transformer in the power grid system in the substation can be selected as the main transformer; or a new transformer is added to the power grid system in the substation to replace the original transformer, and the new transformer is used as the main transformer; or a new transformer is added to the power grid system in the substation, and the new transformer is used as the main transformer.

[0060] More preferably, the main transformer is a three-winding transformer or a two-winding transformer. If the main transformer is a three-winding transformer, its high-voltage winding side is connected to the high-voltage bus of the power system, its medium-voltage winding side is connected to the medium-voltage bus of the power system, and its low-voltage winding side is connected to the low-voltage bus of the power system; if the main transformer is a two-winding transformer, its high-voltage winding side is connected to the high-voltage bus of the power system, and its low-voltage winding side is connected to the low-voltage bus of the power system.

[0061] Preferably, the step-up transformer is a double-winding transformer, whose high-voltage winding side is connected to the low-voltage busbar of the power system, and whose low-voltage winding side is connected to the closing inrush current suppression device. The high-voltage winding side of the step-up transformer and the low-voltage winding side of the main transformer have the same nominal voltage, and the nominal voltage of the low-voltage winding side of the step-up transformer is not higher than 10KV. Since the closing inrush current suppression device is on the low-voltage winding side of the step-up transformer, the closing inrush current suppression device controls the inrush current and overvoltage here, and the lower the nominal voltage on the low-voltage winding side of the step-up transformer, the correspondingly limited inrush current peak value, the lower the difficulty of inrush current control, and the more conducive it is for the closing inrush current suppression device to control the inrush current.

[0062] Preferably, the phase-controlled circuit breaker is a three-phase circuit breaker using an eddy current drive mode, which can perform phase-selective closing operations. The surge suppression unit is a silicon stack, which is composed of a number of diodes connected in series in the same direction, and the cathode of the diode is in the same direction as the outgoing line side of the closing surge suppression device. When the bypass switch is in the open state, the surge suppression unit can cooperate with the phase-controlled closing of the phase-controlled circuit breaker to perform secondary clamping of the surge current. The voltage-limiting protection unit is a nonlinear varistor. After the charge accumulates on the negative side of the surge suppression unit and reaches the turn-on voltage of the voltage-limiting protection unit, the voltage-limiting protection unit is turned on, and the current is shunted to the voltage-limiting protection unit, and the voltage-limiting protection unit converts the current flowing therein into heat, thereby consuming the charge to suppress overvoltage.

[0063] Embodiment 1

[0064] In this embodiment, the excitation method of the transfer branch to the main transformer adopts manual magnetization, and the specific method is:

[0065] The switch status before commissioning, that is, before the staff commissions the main transformer, the main switch and branch switch are both in the closed state, and the phase-controlled circuit breaker and bypass switch are both in the open state.

[0066] The preparatory work before commissioning is that the staff turns on the power generation equipment and puts the power generation equipment into a preheating state so that the power generation equipment can transmit electricity to the step-up transformer in the subsequent process.

[0067] Inrush current suppression state, the staff issues a closing command to the surge control unit to select and close the phase-controlled circuit breaker, so that the output side of the power generation equipment is connected to the main transformer through the surge current management circuit, and the excitation transient process current on the low-voltage winding side of the step-up transformer is suppressed within the rated current of the power generation equipment. In this way, the power generation equipment can operate stably and stably excite the low-voltage winding side of the main transformer through the step-up transformer. Among them: the phase-controlled split-phase closing method is adopted so that after the phase-controlled circuit breaker receives the closing command, the switches on the three-phase lines A, B and C complete the corresponding closing action within the respective preset closing phase angle ranges. Since the phase-controlled circuit breaker is controlled to close in phase, the size of the surge current can be suppressed during the closing process, so that the peak value of the surge current can be reduced. The phase-controlled circuit breaker adopts an eddy current drive method, which makes its closing and opening speed better than that of traditional mechanical circuit breakers, thereby improving the closing accuracy. In addition, after the phase-controlled circuit breaker is closed, the current of the power generation equipment flows to the low-voltage winding side of the step-up transformer through the phase-controlled circuit breaker and the inrush current suppression unit. During this period, the inrush current is suppressed again by the inrush current suppression unit, which improves the effect of inrush current suppression and overcomes the problem that the simple phase control method cannot suppress the inrush current well due to the closing error. It can even limit the inrush current to less than one times the rated current of the transformer, thereby eliminating the false operation of the differential protection and improving the power quality.

[0068] In the excitation state, when the inrush current is suppressed, the staff closes the bypass switch to connect the output side of the power generation equipment to the main transformer through the power supply circuit. Since the inrush current has been suppressed, the power generation equipment can operate stably and stably excite the low-voltage winding side of the main transformer through the step-up transformer. Among them: after the inrush current generated by the closing of the phase-controlled circuit breaker is suppressed, the current in the circuit is stable, and the bypass switch can be closed to exit the inrush current suppression unit. At this time, the current of the power generation equipment flows to the low-voltage winding side of the step-up transformer through the phase-controlled circuit breaker and the bypass switch. After being stepped up, it flows to the low-voltage winding side of the main transformer through the main switch and the branch switch, realizing the stable magnetization of the main transformer.

[0069] In the overvoltage protection state, when overvoltage occurs at the inrush current suppression unit and the bypass switch, the voltage limiting protection unit automatically operates to perform overvoltage protection. Among them: if the voltage on the negative side of the inrush current suppression unit exceeds the threshold of the voltage limiting protection unit, the resistance of the voltage limiting protection unit decreases rapidly and turns on, causing a large amount of current to flow into the voltage limiting protection unit, and the voltage limiting protection unit converts these inrush currents into heat, thereby achieving the purpose of consuming the inrush current. That is, the voltage limiting protection unit can both play the role of shunting and suppressing overvoltage.

[0070] In this embodiment, before the main transformer is put into operation, the main transformer is first magnetized through the transfer branch, so that the main transformer enters steady-state operation before it is put into operation, thereby avoiding the situation where the surge current in the power system increases due to the transient magnetic flux change caused by the direct operation of the main transformer. Since the main transformer is excited through the transfer branch before entering the steady state, it is equivalent to transferring the excitation surge current generated by the direct closing of the main transformer to the low-voltage winding side circuit of the transfer branch step-up transformer for management. That is, the closing surge current suppression device clamps the surge current appearing in the low-voltage winding side circuit of the step-up transformer to suppress its peak value.

[0071] Embodiment 2

[0072] In this embodiment, the transfer branch adopts automatic magnetization to excite the main transformer, and the specific method is as follows:

[0073] In the initial state, the high-voltage winding side of the main transformer is connected to the high-voltage bus of the power system, and the medium-voltage winding side is connected to the medium-voltage bus of the power system; the main switch is in the closed state, while the branch switch, phase-controlled circuit breaker and bypass switch are all in the open state; the power generation equipment is in the hot standby state, and the inrush current control unit monitors the output side voltage of the power generation equipment in real time.

[0074] The advance response state means that before the power system needs to put the main transformer into operation, a response signal is sent to the inrush current control unit in advance. The interval between the time when the response signal is sent and the time when the main transformer is closed is t, and the time when the response signal is sent is before the time when the main transformer is closed. After receiving the response signal, the inrush current control unit performs the following actions:

[0075] S1, step-up transformer starts operation. That is, the inrush current control unit commands the branch switch to close.

[0076] S2, the power generation equipment starts to move. That is, after the branch switch is closed, the information is fed back to the inrush current control unit, and then the inrush current control unit commands the phase-controlled circuit breaker to select the phase and close the circuit, and the power generation equipment excites the main transformer through the inrush current control circuit;

[0077] S3, bypass switch operation. That is, after the inrush current suppression is completed, the circuit monitored by the inrush current control unit is in a stable state. At this time, the inrush current control unit commands the bypass switch to close, so that the output side of the power generation equipment is connected to the main transformer through the power supply circuit, so that the power generation equipment can operate stably and the low-voltage side winding of the main transformer is excited through the step-up transformer;

[0078] In the overvoltage protection state, when overvoltage occurs at the inrush current suppression unit and the bypass switch, the voltage limiting protection unit automatically operates to perform overvoltage protection. Among them: if the voltage on the negative side of the inrush current suppression unit exceeds the threshold of the voltage limiting protection unit, the resistance of the voltage limiting protection unit decreases rapidly and turns on, causing a large amount of current to flow into the voltage limiting protection unit, and the voltage limiting protection unit converts these inrush currents into heat, thereby achieving the purpose of consuming the inrush current. That is, the voltage limiting protection unit can both play the role of shunting and suppressing overvoltage.

[0079] Embodiment 3

[0080] This embodiment provides a method for cutting off power generation equipment and transferring branches.

[0081] When it is necessary to cut off the power generation equipment, after the phase-controlled controller receives the opening command, it sends an opening command to the phase-controlled circuit breaker and the bypass switch, and opens the bypass first. The phase switches of the phase-controlled circuit breaker then complete their respective opening actions within their respective preset opening phase angle ranges, and finally complete the cutting off of the transfer branch at the opening branch switch.

[0082] Based on the above content and the accompanying drawings, those skilled in the art can understand and implement the present invention. In addition, any non-creative modifications made to the present invention by those skilled in the art without creative work still fall within the protection scope of the present invention.

Claims

1. A method for suppressing magnetizing inrush current of a large power transformer, characterized in that: Connect the transfer branch for excitation, inrush current and overvoltage control to the low-voltage winding side of the main transformer; The transfer branch includes a power supply circuit, which excites the main transformer flux to a steady state before the main transformer is put into operation in the power system, and clamps the inrush current peak value generated when the main transformer is put into operation; The transfer branch includes an inrush current control circuit. When the power supply circuit excites the main transformer, the inrush current control circuit suppresses the inrush current generated by the transfer branch, so that the main transformer is stably excited to a steady state before being put into operation, thereby achieving control of the closing inrush current of the main transformer; The transfer branch includes an overvoltage protection circuit. During the operation of the transfer branch, the overvoltage protection circuit shunts the current of the power supply circuit and the surge current management circuit to suppress the overvoltage occurring in the power supply circuit and the surge current management circuit.

2. The method for suppressing magnetizing inrush current of a large power transformer according to claim 1, characterized in that: The circuit structure and wiring method of the transfer branch are as follows: Main switch: The low-voltage winding side of the main transformer is connected to the low-voltage busbar of the power system through the main switch; The branch switch is installed on the circuit connected to the load side of the busbar on the low-voltage side of the power system and serves as the main switch of the circuit; The high-voltage winding side of the step-up transformer is connected to the outgoing line side of the branch switch; The closing inrush current suppression device has its incoming line side connected to the low-voltage winding side of the step-up transformer; The power generation equipment has its output side connected to the outgoing line side of the closing inrush current suppression device.

3. The method for suppressing magnetizing inrush current of a large power transformer according to claim 1 or 2, characterized in that: The main transformer is a transformer that is to be put into operation in the power system for receiving and distributing electricity.

4. The method for suppressing magnetizing inrush current of a large power transformer according to claim 2, characterized in that: The step-up transformer is a double-winding transformer, with its high-voltage winding side connected to the low-voltage busbar of the power system, and its low-voltage winding side connected to the closing inrush current suppression device; The nominal voltage of the high voltage winding side of the step-up transformer and the low voltage winding side of the main transformer are the same, and the nominal voltage of the low voltage winding side of the step-up transformer is not higher than 10KV.

5. The method for suppressing magnetizing inrush current of a large power transformer according to claim 2, characterized in that: The closing inrush current suppression device includes: The phase-controlled switching main circuit is composed of a phase-controlled circuit breaker and a bypass switch in series, and is used to complete the switching of power generation equipment; The inrush current suppression unit is connected in parallel with the bypass switch and in series with the phase-controlled circuit breaker to suppress the excitation inrush current of the circuit; A voltage limiting protection unit is connected in series to the phase-controlled circuit breaker, and the phase-controlled circuit breaker is grounded through the voltage limiting protection unit, for suppressing overvoltage; The inrush current control unit includes a transformer for collecting bus side voltage signals and a transformer for collecting load side voltage signals, a sensor for collecting load side current signals, and a phase control controller for performing data processing and phase control switching command processing.

6. The method for suppressing magnetizing inrush current of a large power transformer according to claim 5, characterized in that: The phase-controlled circuit breaker is a three-phase circuit breaker using eddy current drive; The surge suppression unit is a silicon stack; The voltage limiting protection unit is a nonlinear varistor.

7. The method for suppressing magnetizing inrush current of a large power transformer according to claim 5, characterized in that: In the transfer branch, the power generation equipment-phase-controlled switching main circuit-boosting transformer-branch switch-main switch are connected in series in sequence to form a power supply circuit in the transfer branch; In the transfer branch, the power generation equipment-inrush current suppression unit-phase-controlled circuit breaker-boosting transformer-branch switch-main switch are sequentially connected in series, or the power generation equipment-phase-controlled circuit breaker-inrush current suppression unit-boosting transformer-branch switch-main switch are sequentially connected in series to form an inrush current control circuit in the transfer branch; In the transfer branch, the power generation equipment-voltage limiting protection unit-grounding are connected in series in sequence, or the power generation equipment-phase-controlled circuit breaker-voltage limiting protection unit-grounding are connected in series in sequence to form an overvoltage protection circuit in the transfer branch.

8. The method for suppressing magnetizing inrush current of a large power transformer according to claim 5 or 7, characterized in that: The excitation method of the transfer branch to the main transformer adopts manual magnetization, the specific method is: The switch status before commissioning, that is, before commissioning the main transformer, the main switch and branch switch are both in the closed state, and the phase-controlled circuit breaker and bypass switch are both in the open state; Preparatory work before commissioning: start the power generation equipment for preheating so that the power generation equipment can transmit electricity to the step-up transformer in the subsequent process. Inrush current suppression state, that is, the phase-controlled circuit breaker is used for phase selection and closing, so that the output side of the power generation equipment is connected to the main transformer through the inrush current control circuit, and the current of the transient process of excitation on the low-voltage winding side of the step-up transformer is suppressed within the rated current of the power generation equipment, so that the power generation equipment can operate stably and the low-voltage side winding of the main transformer is excited through the step-up transformer; In the excitation state, after the inrush current suppression is completed, the bypass switch is closed, so that the output side of the power generation equipment is connected to the main transformer through the power supply circuit, so that the power generation equipment can operate stably and the low-voltage winding of the main transformer is excited through the step-up transformer; In the overvoltage protection state, when overvoltage occurs at the inrush current suppression unit and the bypass switch, the voltage limiting protection unit automatically operates to provide overvoltage protection.

9. The method for suppressing magnetizing inrush current of a large power transformer according to claim 5 or 7, characterized in that: The transfer branch uses automatic magnetization to excite the main transformer. The specific method is as follows: In the initial state, the high-voltage winding side of the main transformer is connected to the high-voltage bus of the power system, and the medium-voltage winding side is connected to the medium-voltage bus of the power system. The main switch is in the closed state, while the branch switch, phase-controlled circuit breaker and bypass switch are all in the open state. The power generation equipment is in the hot standby state, and the inrush current control unit monitors the output side voltage of the power generation equipment in real time. In the early response state, that is, before the power system is connected to the main transformer that needs to be put into operation, the connection between the power system and the medium-voltage winding side of the main transformer is disconnected, and the entire station load is carried by other transformers in the power system that run in parallel with the main transformer; The step-up transformer is put into operation, i.e. the branch switch is closed, the step-up transformer is energized and operates at no load; The commissioning of the power generation equipment, that is, when the dispatching center of the power system issues a command and executes the off-grid action of the high-voltage winding side of the main transformer, the low-voltage winding side of the step-up transformer loses pressure, the closing conditions of the phase-controlled circuit breaker are met and the phase-controlled circuit breaker is closed by phase selection, and the power generation equipment excites the low-voltage winding of the main transformer through the step-up transformer; In the excitation state, after the inrush current suppression is completed, the bypass switch is closed, so that the output side of the power generation equipment is connected to the main transformer through the power supply circuit, so that the power generation equipment can operate stably and the low-voltage winding of the main transformer is excited through the step-up transformer; In the overvoltage protection state, when overvoltage occurs at the inrush current suppression unit and the bypass switch, the voltage limiting protection unit automatically operates to provide overvoltage protection.

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