A black start method for offshore wind farm based on diesel-storage combined system

Abstract

The present invention discloses a black start method for an offshore wind farm based on a diesel-storage combined system, comprising: step S1, the energy storage device adopts a zero-voltage start control strategy to reduce the excitation inrush current; step S2, starting the diesel generator set, and using the auxiliary load to assist the diesel generator set in starting; step S3, putting in a balanced load and supplying power to the auxiliary machine of the wind turbine set; step S4, the wind turbine sets are started one by one and capture wind energy for power generation; step S5, after the start of the units suitable for transmitting active power is completed, the wind turbine sets operating in the phase adjustment mode are put in; step S6, the adjustable load is reduced according to the rated power of the motor load to be put in, and the motor load is put in after the system enters a steady state to realize the black start. The present invention adopts a reasonable wind farm black start operation method and control strategy. The wind turbine phase adjustment operation mode allows the grid-side converter of the non-starting unit to participate in reactive power support, improves the system voltage stability during the black start process, and is conducive to the smooth black start.

Description

Technical Field

[0001] The invention relates to the technical field of power grid black start, and in particular to an offshore wind farm black start method based on a diesel-storage combined system. Background Art

[0002] At present, the black start power supply of the power system is mainly conventional power plants, but the cold start of traditional power plants relies on the backup power supply to start the auxiliary units. With the increase in the proportion of new energy, wind power has gradually become a new choice for starting auxiliary units. Wind farms connected to the grid through high-voltage DC lines rely on obtaining frequency and voltage references from the grid during the startup process. Therefore, when the high-voltage DC transmission line fails or the grid fails and energy cannot be obtained from the grid, an auxiliary power supply is required as the starting power supply for wind turbines and auxiliary equipment.

[0003] Wind turbines equipped with auxiliary power supplies have a self-starting function and a short start-up time, which enables wind farms to promote the recovery of the system grid. The use of auxiliary power supplies can not only achieve the self-starting of wind turbines, but also the load power shortage can be supplemented by energy storage devices when wind power fluctuates. Through reasonable operation steps and control strategies, wind turbines can start the units in the wind farm in sequence, provide long-term stable power for the load and improve the system voltage quality. However, the system robustness is low during the black start of the wind farm, and the voltage quality will affect the stable operation of the system and the smooth progress of the black start. Summary of the invention

[0004] In order to solve the above problems, the present invention proposes a reasonable black start method for offshore wind farms based on a diesel-storage combined system. The present invention adopts a reasonable wind farm black start operation method and control strategy. The wind turbine phase adjustment operation mode enables the grid-side converter of the non-starting unit to participate in reactive power support, thereby improving the system voltage quality.

[0005] In order to achieve the above object, the present invention is implemented by the following technical solutions:

[0006] The present invention is a black start method for an offshore wind farm based on a diesel-storage combined system, comprising the following steps:

[0007] Step S1, closing the energy storage line boost line circuit breaker, the energy storage device is started at zero voltage to reduce the excitation inrush current, the energy storage system charges the high-voltage side bus of the wind farm through the boost transformer and the submarine cable, and the zero-voltage start operation ends after the rated voltage is reached;

[0008] Step S2, start the diesel generator set, wait for the diesel generator outlet voltage to reach the rated value, put the diesel generator start-up load into operation, wait for the diesel generator to return to a steady state again, perform the synchronous grid-connected operation to close the diesel generator set port circuit breaker, and after the grid connection is completed, it will serve as a balancing power source for the isolated grid system together with the energy storage system, gradually reduce the diesel generator start-up load, and put the energy storage system in a charging state;

[0009] Step S3, when the energy storage system charge meets the startup requirements, the adjustable load is put into use as a balancing load, and then the auxiliary load circuit breaker of the wind turbine is closed to supply power to the auxiliary machine of the wind turbine. When the wind speed meets the startup conditions, the wind turbine starts to capture wind energy, and the outlet port circuit breaker of the wind turbine is closed. The speed of the permanent magnet synchronous generator is controlled by the machine-side converter and the DC side capacitor is charged, and power is transmitted to the high-voltage side bus through the grid-side converter.

[0010] Step S4, starting all wind turbines in sequence according to step S3 to expand the startup scale and maintain the power balance of the isolated grid system, and increasing the size of the adjustable load as more wind turbines are put into use;

[0011] Step S5, after the active power transmission unit is started, the wind turbine unit operating in the phase adjustment mode is put into operation;

[0012] Step S6, reducing the size of the adjustable load according to the rated power of the motor to be put into operation, closing the load port circuit breaker after the system enters a steady state, and starting the motor load to realize a black start.

[0013] A further improvement of the present invention is that: the zero-voltage start-up of the energy storage system in S1 is specifically: the zero-voltage start-up control strategy makes the outer loop reference voltage u dref According to the grid voltage u g With the network reference voltage u nref The difference is integrated until the difference between the energy storage output voltage and the rated voltage enters the regulation dead zone. The PI link is used to time the time of entering the dead zone. After being in the dead zone for more than the set time, the PI timing link is set to a high level and no longer changes, latching u dref The signal determines the voltage outer loop reference value of the initial state, and the zero-voltage starting process ends here.

[0014] The further improvement of the present invention is that: the specific operation of maintaining the power balance of the isolated grid system in step S4 is: the power balance is specifically the balance of active power and reactive power. When the diesel generator sets in the system are running at rated state, the system active power balance formula is as follows:

[0015] P BESS +P oil +P w =P load +P loss (1)

[0016] Where: P BESS is the active power of the energy storage system; P w is the active power of wind turbine; P load is the load power; P oil is the active power of the diesel generator set; P lossis the system power loss;

[0017] The system reactive power balance formula is as follows:

[0018] Q BESS +Q oil +Q WSVG =Q load +Q loss (2)

[0019] Where: Q BESS is the reactive power of the energy storage system; Q WSVG Reactive power compensation for wind turbines; Q load is the load reactive power; Q oil is the reactive power of the diesel generator set; Q loss Reactive power loss of the system.

[0020] A further improvement of the present invention is that: the phase adjustment operation mode of the wind turbine set in step S5 is specifically divided into:

[0021] a. The voltage droop control method is used when the wind turbine is in normal operation mode;

[0022] b. Set the reactive power compensation mode when the wind turbine is not transmitting active power.

[0023] A further improvement of the present invention is that the specific operation of the voltage droop control method is as follows: the grid-side controller adopts a grid voltage-oriented vector control strategy, and introduces the grid-side voltage information into the q-axis, obtains a reactive reference value through a proportional link by passing the difference between the voltage reference value and the actual value, and sends the q-axis current reference value obtained through a proportional-integral link after subtracting the actual reactive output into the control link to realize voltage droop control of the grid-side controller.

[0024] A further improvement of the present invention is that the operation of setting the reactive compensation mode is as follows: first, the machine-side controller module is removed, then the DC-side capacitor is adjusted to an appropriate value, then the reactive compensation control loop is switched and connected to the grid, and finally, a 2.0uF capacitor filter is added on the high-voltage side of the transformer to smooth out fluctuations.

[0025] A further improvement of the present invention is that when operating in reactive power compensation mode, according to the output reactive power Q g The grid voltage v g After voltage drop compensation, the voltage at the PCC measurement point is obtained and then compared with the reference value v ref Make a difference, calculate the trigger angle through the lead-lag compensation link and the PI link, and use the trigger angle to control the grid-side voltage phase shift. When the grid voltage is higher than the reference voltage, the trigger angle is negative, the output voltage leads the voltage at the collection point, and the controller absorbs reactive power. Otherwise, the controller will emit reactive power.

[0026] The beneficial effects of the present invention are as follows: the present invention adopts a reasonable wind farm black start operation method and control strategy, and the wind turbine phase-shifting operation mode enables the grid-side converter of the non-starting unit to participate in reactive support, thereby improving the stability of the system voltage during the black start process, reducing the reactive power demand of the rotating load during the startup phase, which causes the system node voltage to drop, leading to failures of the energy storage system and the wind turbine, and improving the load-carrying capacity of the wind-storage-diesel combined system when used as a black start power source. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is the wiring structure diagram of the offshore wind power system in the black start scenario;

[0028] Figure 2 This is the voltage simulation waveform of PCC point during the self-starting process of the wind farm;

[0029] Figure 3 This is the voltage simulation waveform of the PCC point during the motor load starting process. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention, so that those skilled in the art can implement them according to the text of the specification.

[0031] like Figure 1-Figure 3 As shown, the present invention is a black start method for an offshore wind farm based on a diesel-storage combined system under isolated grid conditions, and the specific steps are as follows:

[0032] Step S1, close the energy storage line boost line breakers KM1 and KM2, the energy storage device adopts a zero-voltage startup control strategy to reduce the excitation inrush current, and the energy storage system charges the high-voltage side bus of the wind farm through the boost transformer and the submarine cable. After the rated voltage is reached, the zero-voltage startup operation ends. After the startup is completed, the energy storage system will serve as the main reference source in the black start process to provide stable voltage and frequency support for the isolated grid system;

[0033] Step S2, close circuit breakers KM5 and KM3 to start the diesel generator set, and after the diesel generator outlet voltage reaches the rated value, close circuit breaker KM4 to start the diesel generator load, and after the diesel generator returns to a steady state, perform synchronous grid-connected operation to close the grid-connected port circuit breaker of the diesel generator set. After the grid connection is completed, it will serve as a balancing power source for the isolated grid system together with the energy storage. Gradually reduce the diesel generator start-up load to put the energy storage system in a charging state;

[0034] Step S3, when the energy storage system charge meets the startup requirements, the adjustable load is put into use as a balancing load. Then the auxiliary load circuit breaker of the wind turbine is closed to supply power to the auxiliary machine of the wind turbine of about 100KW and 50KVar. When the wind speed meets the startup conditions, the wind turbine starts to capture wind energy, the outlet port circuit breaker of the wind turbine is closed, and the speed of the permanent magnet synchronous generator is controlled by the machine-side converter and the DC-side capacitor is charged, and power is transmitted to the high-voltage bus through the grid-side converter;

[0035] Step S4, start other wind turbines in sequence according to step S3 to expand the startup scale. At this time, the system does not drive the black start load and the energy storage system is in a charging state. On the one hand, the wind turbine needs to limit the unit power through variable pitch control to maintain the power balance of the isolated grid system and avoid overcharging of the energy storage system and power reverse transmission of the diesel generator set, which may lead to overheating and damage of the coil; on the other hand, the size of the adjustable load should be increased as more wind turbines are put into use, so that the output power of the wind turbine is far away from the power lower limit;

[0036] Step S5, when the wind turbines suitable for capturing wind energy are started up, the grid-side controller of the wind turbines that are not started adopts a phase-shifting operation control strategy to perform reactive power compensation on the system to improve the system voltage quality;

[0037] Step S6, reducing the size of the adjustable load according to the rated power of the motor to be put into operation, closing the load port circuit breaker after the system enters a steady state, and starting the motor load to realize a black start.

[0038] Step S1 The zero-voltage startup method of the energy storage system is: The zero-voltage startup control strategy sets the outer loop reference voltage u dref According to the grid voltage u g With the network reference voltage u nref The difference is integrated to achieve a gradual increase in the control signal and output voltage. When the difference between the energy storage output voltage and the rated voltage enters the regulation dead zone, the PI link is used to time the time of entering the dead zone. After being in the dead zone for more than the set time, the PI timing link is set to a high level and no longer changes, latching u dref The signal determines the voltage outer loop reference value of the initial state, and the zero-voltage starting process ends here.

[0039] Step S4: The system power balance method is: During the operation of the wind-storage-diesel system, the active power and reactive power of the system need to be balanced. As the wind speed fluctuates, the maximum output power and actual output power of the wind turbine also change. The power shortage caused by the power fluctuation of the wind turbine needs to be provided by the energy storage system. Assuming that the diesel generator set in the system is operating at rated state, the main active support equipment in the system is the diesel generator set, the energy storage system and the wind turbine set. The active power balance formula is as follows:

[0040] P BESS +Poil +P w =P load +P loss (1)

[0041] Where: P BESS is the active power of the energy storage system; P w is the active power of wind turbine; P load is the load power; P oil is the active power of the diesel generator set.

[0042] Under the traditional control strategy, the grid-side control loop of the wind turbine sets the reactive reference value to zero, but the power electronic device conversion process and the step-up transformer will still consume a certain amount of reactive power; when the wind turbine sets are put into the voltage droop strategy, the wind turbine sets will provide a certain amount of reactive support when the voltage drops; the reactive compensation mode of the wind turbine sets operates similarly to the static VAR compensator. The main devices that provide reactive power in the system are diesel generator sets, energy storage systems and phase-shifting units, so the system reactive power balance formula is as follows:

[0043] Q BESS +Q oil +Q WSVG =Q load +Q loss (2)

[0044] Where: Q BESS is the reactive power of the energy storage system; Q WSVG Reactive power compensation for wind turbines; Q load is the load reactive power; Q oil is the reactive power of the diesel generator set.

[0045] Step S5 The phase adjustment operation mode of the wind turbine set is specifically as follows: In the normal operation mode of the wind turbine set, the voltage droop control method is adopted, the grid-side controller will adopt the grid voltage directional vector control strategy, and introduce the grid-side voltage information into the q-axis, and the difference between the voltage reference value and the actual value is obtained through the proportional link to obtain the reactive reference value, which is subtracted from the actual reactive output and then sent to the control link through the q-axis current reference value of the proportional integral link to realize the voltage droop control of the grid-side controller; when the wind turbine set does not perform active transmission, the reactive compensation mode can be set to make the grid-side controller operate in a reactive compensation mode similar to SVG, which is called WSVG mode. At this time, the following operations need to be performed: first, the machine-side controller module is removed, then the DC side capacitor size is adjusted to an appropriate value, and then the reactive compensation control loop is switched to and connected to the grid. Finally, a 2.0uF capacitor filter can be added on the high-voltage side of the transformer to smooth out fluctuations. When operating in reactive compensation mode, according to the output reactive power Q g The grid voltage v g After voltage drop compensation, the voltage at the PCC measurement point is obtained and then compared with the reference value v refMake a difference, calculate the trigger angle through the lead-lag compensation link and the PI link. Use the trigger angle to control the grid voltage phase shift. When the grid voltage is higher than the reference voltage, the trigger angle is negative, the output voltage is ahead of the collection point voltage, and the controller absorbs reactive power. Otherwise, the controller will emit reactive power.

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents, and any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention should be included in the scope of the present claims.

Claims

1. A black start method for an offshore wind farm based on a diesel-storage combined system, characterized in that: The steps include: Step S1, closing the energy storage line boost line circuit breaker, the energy storage device is started at zero voltage to reduce the excitation inrush current, the energy storage system charges the high-voltage side bus of the wind farm through the boost transformer and the submarine cable, and the zero-voltage start operation ends after the rated voltage is reached; Step S2, start the diesel generator set, wait for the diesel generator outlet voltage to reach the rated value, put the diesel generator start-up load into operation, wait for the diesel generator to return to a steady state again, perform the synchronous grid-connected operation to close the diesel generator set port circuit breaker, and after the grid connection is completed, it will serve as a balancing power source for the isolated grid system together with the energy storage system, gradually reduce the diesel generator start-up load, and put the energy storage system in a charging state; Step S3, when the energy storage system charge meets the startup requirements, the adjustable load is put into use as a balancing load, and then the auxiliary load circuit breaker of the wind turbine is closed to supply power to the auxiliary machine of the wind turbine. When the wind speed meets the startup conditions, the wind turbine starts to capture wind energy, and the outlet port circuit breaker of the wind turbine is closed. The speed of the permanent magnet synchronous generator is controlled by the machine-side converter and the DC side capacitor is charged, and power is transmitted to the high-voltage side bus through the grid-side converter. Step S4, starting all wind turbines in sequence according to step S3 to expand the startup scale and maintain the power balance of the isolated grid system, and increasing the size of the adjustable load as more wind turbines are put into use; Step S5, after the active power transmission unit is started, the wind turbine unit operating in the phase adjustment mode is put into operation; Step S6, reducing the size of the adjustable load according to the rated power of the motor to be put into operation, closing the load port circuit breaker after the system enters a steady state, and starting the motor load to realize a black start; The wind turbine phase adjustment operation mode in step S5 is specifically divided into: a. The voltage droop control method is used when the wind turbine is in normal operation mode; b. Set the reactive power compensation mode when the wind turbine is not transmitting active power; The specific operation of the voltage droop control method is as follows: the grid-side controller will adopt the grid voltage oriented vector control strategy, and introduce the grid-side voltage information into the q-axis, and obtain the reactive power reference value by passing the difference between the voltage reference value and the actual value through the proportional link, and then send the q-axis current reference value obtained by the proportional integral link after making a difference with the actual reactive output into the control link to realize the voltage droop control of the grid-side controller; The operation of setting the reactive power compensation mode is as follows: first, remove the machine-side controller module, then adjust the DC side capacitor size to an appropriate value, then switch to the reactive power compensation control loop and connect it to the grid, and finally, to smooth out fluctuations, a 2.0uF capacitor filter can be added on the high-voltage side of the transformer.

2. According to claim 1, a method for black start of an offshore wind farm based on a diesel-storage combined system is characterized in that: The zero-voltage start-up of the energy storage system in S1 is specifically as follows: the zero-voltage start-up control strategy sets the outer loop reference voltage u dref According to the grid voltage u g With the network reference voltage u nref The difference is integrated until the difference between the energy storage output voltage and the rated voltage enters the regulation dead zone. The PI link is used to time the time of entering the dead zone. After being in the dead zone for more than the set time, the PI timing link is set to a high level and no longer changes, latching u dref The signal determines the voltage outer loop reference value of the initial state, and the zero-voltage starting process ends here.

3. According to claim 1, a method for black start of an offshore wind farm based on a diesel-storage combined system is characterized in that: The specific operation of maintaining the power balance of the isolated grid system in step S4 is: the power balance is specifically the balance of active power and reactive power. When the diesel generator sets in the system are running at rated state, the system active power balance formula is as follows: P BESS +P oil +P w =P load +P loss (1) Where: P BESS is the active power of the energy storage system; P w is the active power of wind turbine; P load is the load power; P oil is the active power of the diesel generator set; P loss is the system power loss; The system reactive power balance formula is as follows: Q BESS +Q oil +Q WSVG =Q load +Q loss (2) Where: Q BESS is the reactive power of the energy storage system; Q WSVG Reactive power compensation for wind turbines; Q load is the load reactive power; Q oil is the reactive power of the diesel generator set; Q loss Reactive power loss of the system.

4. According to claim 1, a method for black start of an offshore wind farm based on a diesel-storage combined system is characterized in that: When operating in reactive power compensation mode, according to the output reactive power Q g The grid voltage v g After voltage drop compensation, the voltage at the PCC measurement point is obtained and then compared with the reference value v ref Make a difference, calculate the trigger angle through the lead-lag compensation link and the PI link, and use the trigger angle to control the grid-side voltage phase shift. When the grid voltage is higher than the reference voltage, the trigger angle is negative, the output voltage leads the voltage at the collection point, and the controller absorbs reactive power. Otherwise, the controller will emit reactive power.

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