Converter control method and device

Abstract

The invention relates to an island converter overload limit method and device for a bipolar flexible DC transmission system, and belongs to the field of DC transmission. During stable island operation, both bipolar converters adopt a voltage-frequency droop control strategy; and once it is detected that one converter is overloaded, the control mode of the overloaded converter is automatically switched to active-wattless power control from voltage-frequency droop control to fulfill an overload limit function. The method effectively avoids an overload of the bipolar flexible DC transmission system under island operation, effectively maintains the AC voltage and frequency stable, and has important guiding significance for applying island systems to DC power grids.

Claims

1. A converter control method, wherein two converters having AC sides connected in parallel constitute a bipolar system; when the system operates stably, the converters adopt a voltage-frequency droop control strategy; when one of the converters is overloaded first, a control mode of the overloaded converter is automatically switched to active-wattless power control from voltage-frequency droop control so as to limit an overload; the converter control method comprises the following steps: 1) acquiring measured active power P.sub.s1 absorbed by the AC side of one converter and determining whether or not the active power P.sub.s1 is out of limit by means of an overload limit logic, wherein if the active power P.sub.s1 is out of the limit, said one converter is determined to be overloaded, and entered into a control mode switching logic so as to switch the control mode of said one converter to active-wattless power control from voltage-frequency droop control; 2) obtaining, when it is detected that one converter is overloaded, an active power reference value and a wattless power reference value of said one converter by the overload limit logic through processing, and using an active overload limit reference value P.sub.ref_oll and a wattless overload limit reference value Q.sub.ref_oll which are automatically set by the overload limit logic as instruction inputs of the active-wattless power control; and 3) carrying out, when it is detected that the converter adopting the active-wattless power control is overloaded, phase locking on an AC-side voltage U.sub.so abc of the overloaded converter so as to obtain a system voltage phase control value θ.sub.ref controlled by an inner loop current.

2. The converter control method according to claim 1, wherein in Step 2), a maximum permissible active power to be absorbed by each said converter is set as P.sub.lim, and a maximum permissible wattless power to be absorbed by each said converter is set as Q.sub.lim; total active power P.sub.oll absorbed by the AC sides of the two converters is acquired by using the overload limit logic, and active power P.sub.s1 absorbed by the AC side of said one converter is acquired; and the overload limit logic is one of the following two control logics: i) when |P.sub.s1|>|P.sub.lim| and |P.sub.oll−P.sub.s1|≤|P.sub.lim|, switching the control mode of said one converter to the active-wattless power control from the voltage-frequency droop control, and setting the reference input limits P.sub.lim and Q.sub.lim of the active-wattless power control to meet a power circle and the following conditions:
P.sub.ref_oll=P.sub.set=P.sub.lim;
Q.sub.ref_oll=Q.sub.set=Q.sub.lim; ii) when |P.sub.s1|>|P.sub.lim| and |P.sub.oll−P.sub.s1|≤|P.sub.lim|, switching the control mode of said one converter to the active-wattless power control from the voltage-frequency droop control, and setting the reference input limits P.sub.lim and Q.sub.lim of the active-wattless power control to meet a power circle and the following conditions:
P.sub.ref_oll=P.sub.set, and |P.sub.set|<|P.sub.lim|;
Q.sub.ref_oll=Q.sub.set, and |Q.sub.set|<|Q.sub.lim|; where P.sub.set is a set value of the active power absorbed by said one converter, and Q.sub.set is a set value of the wattless power absorbed by said one converter.

3. The converter control method according to claim 2, wherein in the overload control logic i) of said one converter, the set value of the active power absorbed by said one converter meets P.sub.set=P.sub.lim when said one converter has been overloaded and been switched to the active-wattless power control; if the other converter connected in parallel to the AC side of said one converter is also overloaded, namely |P.sub.oll|>2|P.sub.lim|, a backup solution is adopted to make sure that the power of the other converter will not be out of limit.

4. The converter control method according to claim 2, wherein in the overload control logic ii) of said one converter, the set value of the active power absorbed by said one converter meets |P.sub.set|<|P.sub.lim| when said one converter has been overloaded and been switched to the active-wattless power control; if the other converter connected in parallel to the AC side of the converter is also overloaded: (1) if |P.sub.oll|≤2|P.sub.lim|, the active overload limit reference value P.sub.ref_oll of said one converter is adjusted to:
P.sub.ref_oll≥P′.sub.set=P.sub.set+(P.sub.oll−P.sub.s1−P.sub.lim), and |P.sub.ref_oll|≤|P.sub.lim|; (2) if |P.sub.oll|>2|P.sub.lim|, the active overload limit reference value P.sub.ref_oll of said one converter is adjusted to the maximum value P.sub.lim; if the other converter is still overloaded, a backup solution is adopted to make sure that the power of the other converter will not be out of limit.

5. The converter control method according to claim 3, wherein the backup solution includes at least one of removing a part of fans, running an AC-side energy dissipation device, running a DC-side energy dissipation device, and blocking the converter and turning off an AC incoming switch.

6. The converter control method according to claim 1, wherein when exchange power between the one overloaded converter and an outside has been restored into a normal range and the other converter connected in parallel to the AC side of the one overloaded converter is not overloaded, one of the following two processing modes is adopted: a) exiting the one overloaded converter from the overload limit logic, maintaining the control mode of the one overloaded converter at the active-wattless power control, setting the active overload limit reference value P.sub.ref_oll of the one overloaded converter to be less than or equal to the maximum permissible active power P.sub.lim to be exchanged of the one overloaded converter, and setting the wattless overload limit reference value Q.sub.ref_oll of the one overloaded converter to be less than or equal to the maximum permissible wattless power Q.sub.lim to be exchanged of the one overloaded converter, that is:
|P.sub.ref_oll|≤|P.sub.lim|;
|Q.sub.ref_oll|≤|Q.sub.lim|; b) exiting the one overloaded converter from the overload limit logic, and switching the control mode of the one overloaded converter automatically or manually to the voltage-frequency droop control.

7. The converter control method according to claim 1, wherein in Step 3), an inner loop current controller adopts current vector control.

8. A converter control device, wherein two converters having AC sides connected in parallel constitutes a bipolar system, and the converter control device comprises a stable operation control unit, an overload determining unit, and a control mode switching unit, wherein: the stable operation control unit controls the converters to adopt a voltage-frequency droop control strategy when the system operates stably; the overload determining unit determines whether or not one converter is overloaded and, wherein if said one converter is determined to be overloaded, the overload determining unit enables the control mode switching unit; and the control mode switching unit automatically switches a control mode of the overloaded converter to active-wattless power control from voltage-frequency droop control; the overload determining unit comprises an active power acquisition sub-unit and an active power out-of-limit determining sub-unit, wherein: the active power acquisition sub-unit acquires measured active power P.sub.s1 absorbed by the AC side of each said converter and outputs the measured active power P.sub.s1 to the active power out-of-limit determining sub-unit; and the active power out-of-limit determining sub-unit determines whether or not the measured active power P.sub.s1 is out of limit by means of an overload limit logic and, wherein if the active power P.sub.s1 is out of the limit, the corresponding converter is determined to be overloaded and the control mode switching unit is entered into; the control mode switching unit comprises an active-wattless power reference value setting sub-unit and a system voltage phase control value calculation unit, wherein: the active-wattless power reference value setting sub-unit obtains, when it is detected that one converter is overloaded, active and wattless power reference values by means of the overload limit logic through processing, and uses an active overload limit reference value P.sub.ref_oll and a wattless overload limit reference value Q.sub.ref_oll which are automatically set by the overload limit logic as instruction inputs of the active-wattless power control; and the system voltage phase control value calculation unit carries out, when it is detected that the converter adopting the active-wattless power control is overloaded, phase locking on the AC-side voltage U.sub.so abc of the overloaded converter, so as to obtain a system voltage phase control value θ.sub.ref controlled by an inner loop current.

9. The converter control device according to claim 8, wherein the active-wattless power reference value setting sub-unit comprises a maximum active-wattless power setting unit, a total active power acquisition unit and a reference input limit unit, wherein: the maximum active-wattless power setting unit sets a maximum permissible active power to be absorbed by each said converter as P.sub.lim, and sets a maximum permissible wattless power to be absorbed by each said converter as Q.sub.lim; the total active power acquisition unit acquires total active power P.sub.all absorbed by the AC sides of the two converters and outputs the total active power P.sub.all to the reference input limit unit; the reference input limit unit is a first reference input limit unit or a second reference input limit unit; when P.sub.s1>|P.sub.lim| and |P.sub.all−P.sub.s1|≤|P.sub.lim|, the control mode of said one converter is switched by the first reference input limit unit to the active-wattless power control from the voltage-frequency droop control, and reference input limits P.sub.lim and Q.sub.lim of the active-wattless power control are set to meet a power circle and the following conditions:
P.sub.ref_oll=P.sub.set=P.sub.lim;
Q.sub.ref_oll=Q.sub.set=Q.sub.lim; when |P.sub.s1|>|P.sub.lim| and |P.sub.all−P.sub.s1|≤|P.sub.lim|, the control mode of said one converter is switched by the second reference input limit unit to the active-wattless power control from voltage-frequency droop control, and reference input limits P.sub.lim and Q.sub.lim of the active-wattless power control are set to meet a power circle and the following conditions:
P.sub.ref_oll=P.sub.set, and |P.sub.set|<|P.sub.lim|;
Q.sub.ref_oll=Q.sub.set, and |Q.sub.set|<|Q.sub.lim|; wherein, P.sub.set is a set value of the active power absorbed by said one converter, and Q.sub.set is a set value of the wattless power absorbed by said one converter.

10. The converter control device according to claim 9, wherein in the first reference input limit unit, the set value of the active power absorbed by said one converter meets P.sub.set=P.sub.lim when said one converter has been overloaded and been switched to the active-wattless power control; if the other converter connected in parallel to the AC side of said one converter is also overloaded, namely |P.sub.all|>2|P.sub.lim|, a backup solution is adopted to make sure that the power of the other converter will not be out of limit.

11. The converter control device according to claim 9, wherein in the second reference input limit unit, the set value of the active power absorbed by said one converter meets |P.sub.set|<|P.sub.lim| when said one converter has been overloaded and been switched to the active-wattless power control; if the other converter connected in parallel to the AC side of the converter is also overloaded: (1) if |P.sub.all|≤2|P.sub.lim|, the active overload limit reference value P.sub.ref_oll of said one converter is adjusted to:
P.sub.ref_oll≥P′.sub.set=P.sub.set+(P.sub.all−P.sub.s1−P.sub.lim), and |P.sub.ref_oll|≤|P.sub.lim|; (2) if |P.sub.all|>2|P.sub.lim|, the active overload limit reference value P.sub.ref_oll of said one converter is adjusted to the maximum value P.sub.lim; if the other converter is still overloaded, a backup solution is adopted to make sure that the power of the other converter will not be out of limit.

12. The converter control device according to claim 8, wherein further comprises: an overload restoration determining unit, used for determining whether or not exchange power between the overloaded converter and an outside has been restored into a normal range and whether or not the other converter connected in parallel to the AC side of this converter is not overloaded, and, if the exchange power between the overloaded converter and an outside has been restored into a normal range and the other converter connected in parallel to the AC side of this converter is not overloaded, the overload restoration determining unit triggers an over overload limit exit logic unit; and the overload limit exit logic unit, comprising a first overload limit exit logic unit and a second overload limit exit logic unit; the first overload limit exit logic unit enables this converter to exit from the one overloaded limit logic, maintains the control mode of the one overloaded converter at the active-wattless power control, and sets the active overload limit reference value P.sub.ref_oll of the one overloaded converter to be less than or equal to the maximum permissible active power P.sub.lim to be exchanged of the one overloaded converter and the wattless overload reference value Q.sub.ref_oll of this converter to be less than or equal to the maximum permissible wattless power Q.sub.lim to be exchanged of the one overloaded converter, that is:
|P.sub.ref_oll|≤|P.sub.lim|;
|Q.sub.ref_oll|≤|Q.sub.lim|; the second overload limit exit logic unit enables the one overloaded converter to exit from the overload limit logic, and automatically or manually switches the control mode of the one overloaded converter to the voltage-frequency droop control.

13. The converter control method according to claim 4, wherein the backup solution includes at least one of removing a part of fans, running an AC-side energy dissipation device, running a DC-side energy dissipation device, and blocking the converter and turning off an AC incoming switch.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of a true bipolar topological structure;

(2) FIG. 2 is a topological diagram of a bipolar flexible DC transmission system accessed to a new-energy island wind power plant;

(3) FIG. 3 is a block diagram of an overload control logic of an island converter;

(4) FIG. 4 is a structural diagram of a converter control system.

DETAILED DESCRIPTION OF EMBODIMENTS

(5) Embodiments of the invention are expounded below in combination with the accompanying drawings and are implemented on the basis of the technical solution of the invention, and a detailed implementation and a specific operation process are given. The protection scope of the invention is not limited to the following embodiments.

(6) According to a converter control method of the invention, the AC sides of two converters are connected in parallel, and the specific topological structure of the two converters is shown in FIG. 1. Each converter serves as a pole, and the two converters constitute a bipolar flexible DC system. In the island operation embodiment shown in FIG. 2, the AC bus sides of the two converters are connected with a new-energy island wind power plant. To guarantee stable operation of new-energy islands, each polar converter in the bipolar flexible DC system adopts voltage-frequency droop control, and a stable AC voltage magnitude and frequency are supplied to the new-energy island wind power plant.

(7) The converters in the bipolar flexible DC transmission system in FIG. 2 absorb active power provided by the wind power plant, and the two converters are respectively referred to as a converter 1 and a converter 2. In this embodiment, power absorbed by the converters from the AC sides is defined as positive power and is displayed by a per-unit value, the reference value of the per-unit value is the rated capacity of each converter, and the limit value of active power absorbed by the AC sides of the converters is set as P.sub.lim. During stable operation, the measured active power absorbed by the AC side of the converter 1 meets P.sub.s1<P.sub.lim, and the measured active power absorbed by the AC side of the converter 2 meets P.sub.s2<P.sub.lim.

(8) If the output of the wind power plant constantly increases, the converter 1 will be overloaded first, and at this moment, as illustrated by FIG. 3 which is an overload control block diagram, the control mode of the converter 1 is automatically switched to active-wattless power control from voltage-frequency droop control, and the converter 2 is stilled maintained at voltage-frequency droop control. In this case, the maximum active power P.sub.lim absorbed by the AC side of the converter 1 is the rated capacity of the converter 1, that is, P.sub.lim=1 pu, and the maximum permissible wattless power to be exchanged under the active power P.sub.lim is Q.sub.lim according to a power circle diagram. If the active overload limit reference value of the converter 1 is set as P.sub.ref_oll=P.sub.set1=0.5 pu<P.sub.lim and the wattless overload limit reference value is set as Q.sub.ref_oll=Q.sub.set1=0 pu, and the measured active power absorbed by the AC side of the converter 1 is finally stabilized to P.sub.s1=0.5 pu. At this embodiment, the converter 2 is still maintained at voltage-frequency droop control, and after the converter 1 is overloaded and an overload limit logic is executed, the converter 2 is made to work in any one of the following three conditions, and the control mode of the overload limit logic in this embodiment is introduced below:

(9) (1) The total active power absorbed by the AD sides meets P.sub.all=1.5 pu≤2 pu

(10) At this moment, the active power absorbed by the converter 2 is 1 pu, the converter 2 is not overloaded, the converter 1 may exit from the overload limit logic, and the control mode of the converter 1 may be switched to voltage-frequency droop control from active-wattless power control.

(11) (2) The converter 2 is overloaded and the total active power absorbed by the AD sides meets P.sub.all=1.9 pu≤2 pu

(12) At this moment, the active power absorbed by the converter 2 is 1.4 pu, the converter 2 is overloaded, the overload limit logic of the converter 1 is maintained, that is the converter 1 is maintained at active-wattless power control, the active overload limit reference value of the converter 1 is adjusted to meet P.sub.ref_oll=P.sub.set1+(P.sub.all−P.sub.s1−P.sub.lim)=0.5+(1.9−0.5−1)=0.9 pu<P.sub.lim, and the active power absorbed by the converter 2 is decreased to 1 pu. After the two converters become stable, the measured active power absorbed by the AC side of the converter 1 meets P.sub.s1=0.9 pu, and the measured active power absorbed by the AC side of the converter 2 meets P.sub.s2=1 pu. At this moment, the converter 1 may exit from the overload limit logic, and the control mode of the converter 1 may be switched to voltage-frequency droop control from active-wattless power control.

(13) (3) The converter 2 is overloaded and the total active power absorbed by the AC sides meets P.sub.oll=2.1 pu>2 pu

(14) At this moment, the active power absorbed by the converter 2 is 1.6 pu, the converter 2 is overloaded, the overload limit logic of the converter 1 is maintained, that is, the converter 1 is maintained at active-wattless power control, and the active overload limit reference value of the converter 1 is adjusted to meet P.sub.ref_oll1=P.sub.lim−1 pu; after adjustment, the active power absorbed by the converter 2 is decreased to 1.1 pu, if the output of fans decreases in this period, the active power P.sub.s2 absorbed by the AC side of the converter 2 restores to a normal power range and is kept stable, then the converter 1 may exit from the overload limit logic, and the control mode of the converter 1 may be switched to voltage-frequency droop control from active-wattless power control; otherwise, a part of the fans arc removed after an AC-side energy dissipation device is nm, and then, so as to decrease the active power P.sub.all absorbed by the AC side of the converter to be less than 2 pu.

(15) As shown in FIG. 4, a control system for the converter 1 is designed in combination with the specific control method in the above embodiment. The control system comprises a stable operation control unit, an overload determining unit, a control mode switching unit and an overload restoration determining unit. The stable operation control unit realizes a voltage-frequency droop control strategy of the converters during stable operation. The overload determining unit detects whether or not the converters are overloaded and comprises an active power acquisition sub-unit and an active power out-of-limit determining sub-unit, wherein the active power acquisition sub-unit acquires the measured active power P.sub.s1 absorbed by the AC side of the converter 1, and when the active power out-of-limit determining sub-unit detects that P.sub.s1>P.sub.lim, the converter 1 is overloaded, and the control mode switching unit is enabled. The control mode switching unit consists of an active-wattless power reference value setting sub-unit and a system voltage phase control calculation sub-unit and is used for switching the converter 1 to an active-wattless power control strategy from the voltage-frequency droop control strategy. In this embodiment, the active-wattless power reference value setting sub-unit obtains an active-wattless reference value P.sub.ref_oll1=P.sub.set1=0.5 pu<P.sub.lim and a wattless overload limit reference value Q.sub.ref_oll1=Q.sub.set1=0 pu by processing of a second reference input limit sub-unit, and the measured active power absorbed by the AC side of the converter 1 is finally stabilized to P.sub.s1=0.5 pu. At this moment, the converter 2 is stilled maintained at voltage-frequency droop control; after the converter 1 is overloaded and the overload limit logic is executed, the total active power acquisition unit detects that the total active power absorbed by the AC sides meets P.sub.s1=1.9 pu, if the reference input limit unit detects that the active power absorbed by the converter 2 is overloaded, assume the active power absorbed by the converter 2 is 1.4 pu, the overload limit logic of the converter 1 is maintained, the reference input limit sub-unit adjusts the active load limit reference value of the converter 1 to meet P.sub.ref_oll1=P.sub.set1+(P.sub.all−P.sub.s1−P.sub.lim)=0.5+(1.9−0.5−1)=0.9 pu<P.sub.lim, and the active power absorbed by the converter 2 is decreased to 1 pu. After the two converters become stable, the measured active power absorbed by the AC side of the converter 1 meets P.sub.s1=0.9 pu, and the measured active power absorbed by the AC side of the converter 2 meets P.sub.s2=1 pu. The control system for the converter 1 enters into the overload restoration determining unit and executes a second unit of the overload limit exit logic unit, and the control mode of the converter 1 is switched to voltage-frequency droop control from active-wattless power control.

(16) The above embodiments are only used for explaining the technical concept of the invention, and are not intended to limit the protection scope of the invention. Any transformations made on the basis of the technical solution according to the technical concept of the invention should fall within the protection scope of the invention.