A method for converting island operation to network operation in a flexible direct current transmission system

Abstract

The present invention discloses a method for switching from island operation to networking operation of a flexible direct current transmission system, and relates to the field of electric power. Two power sources with different frequencies can easily generate large power fluctuations in the system, thereby causing the flexible direct current system to trip due to overcurrent. The present invention includes the steps of: querying the permissible conditions for switching from island to networking of the flexible direct current system; obtaining the active power value and reactive power value during the island operation, and latching them; judging whether the AC power grid is connected to the power supply according to the real-time active power electrical quantity criterion, reactive power electrical quantity criterion, and voltage auxiliary criterion, so as to determine whether to continue the island mode operation or switch to networking mode operation; the island to networking constant value adopted in the present technical solution is obtained according to the actual load, and the reference value is slightly larger than the actual load. During the process of switching from island to networking, the control can be switched to networking control after the load changes, rather than switching from island to networking only when the load changes significantly, thereby avoiding overcurrent tripping and avoiding shutdown of the flexible direct current system.

Description

Technical Field

[0001] The present invention relates to the field of electric power, and in particular to a method for converting island operation to network operation of a flexible direct current transmission system. Background Art

[0002] The flexible direct current transmission system adopts switchable power electronic devices, which has the characteristics of flexible control, rapid dynamic response, and low harmonic content. It can quickly and independently adjust active power and reactive power, and has flexible control and coordination capabilities. It is widely regarded as one of the technical means for island power transmission, grid interconnection, and reliable access and effective utilization of new energy, and has broad application prospects.

[0003] The flexible DC transmission system can operate in both networking mode and island mode. When there is AC power in the AC grid area to which the flexible DC converter station is connected, the flexible DC transmission system operates in networking mode. When there is no AC power in the AC grid area, the flexible DC system needs to operate in island mode to provide stable AC voltage and frequency for the passive system. The existence of two operating modes inevitably requires mode conversion. At present, the way of switching from island to networking is mostly to close the switch synchronously with a fixed switch, and to determine whether to enter the networking state by detecting the switch position. However, this method requires complex wiring to connect to different switch states, resulting in increased costs; due to the hysteresis of the switch position, it is very easy to cause overcurrent tripping; and the switch position is not reliable, which causes the flexible DC to mistakenly switch from the island to the networking. When the flexible DC converter station is connected to the passive AC system, the flexible DC converter station needs to provide a stable AC voltage for the passive AC system in the island control mode. When the AC system is connected to the grid power supply, since both the flexible DC system and the grid power supply can control the frequency, two power supplies with different frequencies can easily generate large power fluctuations in the system, causing the flexible DC system to trip due to overcurrent. Therefore, it is necessary to quickly switch the flexible DC system from the island operation mode to the network operation mode to avoid causing large power disturbances to the system and avoid causing the flexible DC system to trip. Summary of the invention

[0004] The technical problem to be solved and the technical task proposed by the present invention are to improve and perfect the existing technical solutions and provide a method for switching from island operation to network operation of a flexible direct current transmission system, so as to achieve the purpose of balancing cost and operation stability. To this end, the present invention adopts the following technical solutions.

[0005] A method for switching from island operation to network operation of a flexible direct current transmission system, when the flexible direct current transmission system is in the island operation mode, when the flexible direct current transmission system needs to switch from island operation to network operation, a switching to network operation step is performed, and the steps include:

[0006] 1) Check the conditions for the flexible direct current system to allow the island to be connected to the network, and determine whether Ena = 1. If so, proceed to the next step. If not, wait for the operator to enable it;

[0007] 2) Obtain the active power value P during island operation ref And reactive power value Q ref , and latch it; determine whether: 0.2pu≤P ref ≤1.0pu and 0.2pu≤Q ref ≤1.0pu, if P ref , Q ref If it is less than 0.2pu, the corresponding minimum value is set to 0.2pu to avoid small power disturbances causing island networking errors; if P ref , Q ref If it is greater than 1.0pu, the corresponding maximum is 1.0pu. The normal operating power cannot be greater than 1.0pu. If it is greater than 1.0pu, it is considered as data abnormality and is directly limited to 1.0pu.

[0008] 3) Based on the real-time active power electrical quantity criterion, reactive power electrical quantity criterion, and voltage auxiliary criterion, determine whether the AC power grid is connected to the power supply, so as to determine whether to continue to operate in the island mode or switch to the network mode;

[0009] Among them, the voltage auxiliary criterion for island-to-grid operation is:

[0010] u rmsj ≥0.85U rmsN ,(j=a,b,c);

[0011] where u rmsj is the three-phase effective value of the grid-side voltage, U rmsN It is the rated value of the grid-side voltage effective value. Only when this voltage auxiliary criterion is met can the island-to-grid operation be allowed;

[0012] The criterion for active power electrical quantity of island-to-grid operation is:

[0013] P real ≥k p P ref ;

[0014] At the delay t≥t pset After that, it is allowed to switch from isolated island to network operation; among them, k p is the active power island transfer network setting value, k p ≥1.3, t pset It is the time setting value of active power island transfer to network, 10ms<t pset ≤200ms;

[0015] The reactive power electrical quantity criterion for island-to-grid operation is:

[0016] Q real ≥k q Q ref ;

[0017] At the delay t≥t qset After that, it is allowed to switch from isolated island to network operation; among them, k q is the active power island transfer network setting value, k q ≥1.3, t qset It is the time setting value of active power island transfer to network, 10ms<t qset ≤200ms;

[0018] 4) After switching from isolated island to networked operation, the isolated island to network enabling condition is automatically exited and Ena=0.

[0019] Flexible DC transmission system is referred to as flexible DC system; in step 1), Ena is a flag, through which we can clearly know that the task needs to be executed; for active power value P ref And reactive power value Q ref Latch, the latched value is used as the subsequent fixed value. After latching, the active power and reactive power at that time can be known, which is convenient for using active and reactive power as fixed values ​​to judge the island connection to the network. Active and reactive power directly reflect the load changes, and voltage is used as an auxiliary criterion. When the voltage is low, it is considered that a fault has occurred. At this time, the island connection should not be connected to the network. When the island connection is normally carried out by closing the switch, the AC voltage will not drop too much, thereby ensuring a smooth transition and reducing the impact on the system when the AC grid is connected.

[0020] As the preferred technical means: when the island is operating, it works in frequency and AC voltage control mode. After the island is connected to the network, it works in active power and reactive power control mode by default. The flexible DC system is equipped with a control mode modification module to modify the active power control mode to DC voltage control mode and the reactive power control mode to AC voltage control mode according to the command of the operator.

[0021] As a preferred technical means: in step 4), when the island is switched to network operation, its active power is calculated according to the active power instruction P ref Execute, reactive power follows reactive power instruction Q ref Execute. This keeps the power supplied to the load by the flexible DC unchanged before and after the operation mode is switched, reducing the impact on the AC system

[0022] Beneficial effects:

[0023] (1) The method for switching from island operation to grid operation of a flexible direct current transmission system provided by the present invention can reduce the demand of the flexible direct current system on the switching state of the alternating current grid, thereby reducing costs;

[0024] (2) The method for switching from island operation to interconnected operation of a flexible DC power transmission system provided by the present invention can set an island switching setting value according to real-time power, thereby reducing the impact on the system when the AC grid is connected;

[0025] (3) The island-to-network switching constant used in the present invention is obtained according to the actual load, and the reference value is slightly larger than the actual load. When the island is switched to the network, the control can be switched to the network after the load changes, rather than switching to the network only after a large load change. This can avoid overcurrent tripping and prevent the flexible DC system from shutting down. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is a schematic diagram of a two-terminal flexible direct current power transmission system of the present invention;

[0027] Figure 2 It is a flow chart of the isolated island operation to network operation of the present invention. DETAILED DESCRIPTION

[0028] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings.

[0029] like Figure 1 The figure shows a two-terminal flexible direct current transmission system (hereinafter referred to as flexible direct current system), which includes an island side pre-connected AC grid 1, an island side grid-side switch 2, an island side transformer 3, an island side converter valve 4, an overhead line or cable 5, a network side converter valve 6, a network side transformer 7, a network side switch 8, and a network side AC grid 9; the converter valve on one side of the two-terminal flexible direct current transmission system is in island operation, and the converter valve on the other side is in network operation.

[0030] like Figure 2 As shown in the figure, when the flexible DC transmission system is in island operation mode, the flexible DC transmission system needs to be switched from island operation to grid operation. The steps are as follows:

[0031] First, the operator enables the flexible DC system island switching networking permission condition Ena=1; Ena is a flag bit.

[0032] Secondly, the active power value stored in the flexible DC system at this time is P ref , the reactive power value is Q ref , where 0.2pu≤P ref ≤1.0pu, 0.2pu≤Q ref ≤1.0pu; if P ref , Q ref If it is less than 0.2pu, the corresponding minimum value is set to 0.2pu to avoid small power disturbances causing island networking errors; if P ref , Q refIf it is greater than 1.0pu, the corresponding maximum is 1.0pu. The normal operating power cannot be greater than 1.0pu. If it is greater than 1.0pu, it is considered to be data abnormality and is directly limited to 1.0pu. The latched value is used as the subsequent fixed value. After latching, the active power and reactive power at that time can be known, which is convenient for the subsequent use of active and reactive power as fixed values ​​to judge the island to network.

[0033] Thirdly, the flexible direct current system determines whether the AC grid is connected to the power supply through real-time active power electrical quantity judgment, reactive power electrical quantity judgment, and voltage auxiliary judgment, so as to determine whether to continue to operate in the island mode or switch to the network mode. Active and reactive power directly reflect the load changes, while voltage is used as an auxiliary judgment. When the voltage is low, it is considered that a fault has occurred, and the island should not be switched to the network. When the island is switched to the network normally by closing the switch, the AC voltage will not drop too much.

[0034] The voltage auxiliary criterion for island-to-network operation is as follows:

[0035] u rmsj ≥0.85U rmsN ,(j=a,b,c), where u rmsj is the three-phase effective value of the grid-side voltage, U rmsN It is the rated value of the grid-side voltage RMS. The flexible DC system must meet this voltage auxiliary criterion before it is allowed to switch from an isolated island to a grid.

[0036] The active power electrical quantity criterion for island-to-network operation is as follows:

[0037] When P real ≥k p P ref When the delay t≥t pset , the flexible DC system is transformed from an isolated island to a networked operation. p is the active power island transfer network setting value, k p ≥1.3, t pset It is the time setting value of active power island transfer to network, 10ms<t pset ≤200ms; using k p ≥1.3, which can prevent the island from being mistakenly switched to the network due to a small change in the island load. If the real switch is switched from the island to the network, the power will gradually increase until it exceeds the fixed value; the delay is also to prevent the power change caused by the disturbance alone, thereby avoiding false operation. In this embodiment, k p Take 1.3, t pset Take 100ms.

[0038] The reactive power electrical quantity criterion for island-to-network operation is as follows:

[0039] When Q real ≥kq Q ref When the delay t≥t qset , the flexible DC system is transformed from an isolated island to a networked operation. q is the active power island transfer network setting value, k q ≥1.3, t qset It is the time setting value of active power island transfer to network, 10ms<t qset ≤200ms; using k q ≥1.3 can prevent the island from switching to the network due to a small change in the island load. If the real switch switches from the island to the network, the power will gradually increase until it exceeds the set value. The delay is also to prevent the power change caused by the disturbance, thereby avoiding false operation. In this embodiment, k q Take 1.3, t qset Take 100ms.

[0040] Finally, after the flexible direct current system switches from isolated island operation to networked operation, it automatically exits the isolated island to network enabling condition, and Ena=0.

[0041] In order to take into account both the reliability and flexibility of control, the island operates in frequency and AC voltage control mode. After the island is connected to the network, it works in active power and reactive power control mode by default. The operator can change the active power control mode to DC voltage control mode and the reactive power control mode to AC voltage control mode.

[0042] In order to reduce the impact of the system when the AC grid is connected, when the flexible DC system is switched from an isolated island to a grid-connected system, its active power is adjusted according to the active power instruction P ref Execute, reactive power follows reactive power instruction Q ref Keep the power supplied to the load by the flexible DC unchanged before and after the operation mode is switched, and reduce the impact on the AC system

[0043] The method for switching from island operation to grid operation of a flexible DC transmission system shown above is a specific embodiment of the present invention, which has embodied the substantial characteristics and progress of the present invention. According to actual use needs and under the guidance of the present invention, equivalent modifications in shape, structure, etc. can be made to the method, which are all within the protection scope of this scheme.

Claims

1. A method for switching from island operation to network operation in a flexible DC transmission system, characterized in that: When the flexible direct current transmission system is in the island operation mode, when the flexible direct current transmission system needs to be switched from the island operation to the grid operation, the grid operation switching steps are performed, and the steps include: 1) Check the conditions for allowing the flexible direct current system to switch to the network and determine whether If yes, go to the next step, if not, wait for the operator to enable; 2) Obtaining the active power value during island operation and reactive power value , and latch it; determine whether: and ,like , Less than 0.2 , then the corresponding minimum is set to 0.2 , to avoid small power disturbances causing island networking errors; if , Greater than 1.0 , then the corresponding maximum is 1.0 , normal operating power cannot be greater than 1.0 , greater than 1.0 It is considered as data anomaly and is directly limited to 1.0 ; 3) Based on the real-time active power electrical quantity criterion, reactive power electrical quantity criterion, and voltage auxiliary criterion, determine whether the AC power grid is connected to the power supply, so as to determine whether to continue to operate in the island mode or switch to the network mode; Among them, the voltage auxiliary criterion for island-to-grid operation is: ； in is the three-phase effective value of the grid-side voltage, It is the rated value of the grid-side voltage effective value. Only when this voltage auxiliary criterion is met can the island-to-grid operation be allowed; The criterion for active power electrical quantity of island-to-grid operation is: ； In Delay After that, it is allowed to switch from isolated island to network operation; among them, Set the value for active power island transfer to network. , It is the setting value of active power island transfer time. ; The reactive power electrical quantity criterion for island-to-grid operation is: ； In delay After that, it is allowed to switch from isolated island to network operation; among them, Set the value for active power island transfer to network. , It is the setting value of active power island transfer time. ; 4) After switching from isolated island to networked operation, the isolated island to network enabling condition will be automatically exited and the .

2. The method for switching from island operation to network operation of a flexible DC transmission system according to claim 1, characterized in that: When operating in an isolated island, it works in frequency and AC voltage control mode. After the island is connected to the network, it works in active power and reactive power control mode by default. The flexible DC system is equipped with a control mode modification module to modify the active power control mode to DC voltage control mode and the reactive power control mode to AC voltage control mode according to the operator's command.

3. The method for switching from island operation to network operation of a flexible DC transmission system according to claim 2, characterized in that: In step 4), when the island is switched to network operation, its active power is adjusted according to the active power instruction. Execute, reactive power follows reactive power instruction implement.

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