CN107039993B - Power band-turning control method for symmetrical bipolar flexible direct-current transmission converter - Google Patents
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
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- H02H7/268—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for dc systems
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
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Abstract
The invention relates to a power band-turning control method for a symmetrical bipolar flexible direct-current transmission converter. Firstly, when a flexible direct current transmission converter with symmetrical double-pole wiring has a single-pole fault, judging whether the converter adopts a single-end grounding operation mode of a double-pole metal return wire or not and a non-fault pole adopts a double-pole power control mode; if so, the power is determined according to the fault poleP aNon-fault pole powerP bUnipolar rated powerP NCalculating the amount of transferred band power to be equal toP aAndP N‑P bthe smaller of these; then, the fault pole converter is locked, and the fault pole metal return wire switch is disconnected, so that the system is converted into a single-pole single-end grounding operation mode with the metal return wires; then, a real-time simulation model is established according to the converter equipment parameters, and the power transfer belt speed fixed value is calculated through simulationP N/t1(ii) a Finally, the power of the non-fault electrode is increased to a fixed value according to the power conversion rateP b+ΔP. The invention aims to provide a power transfer control method and a theoretical basis for a symmetrical bipolar converter when a unipolar fault occurs.
Description
Technical Field
The invention relates to the technical field of flexible direct current transmission, belongs to the control technology of a modular multilevel converter adopting symmetrical bipolar wiring, and particularly relates to a control method for transferring the power of a fault pole to a non-fault pole after a symmetrical bipolar converter has a single-pole fault.
Background
The flexible direct-current power transmission system adopting the symmetrical bipolar connection has a flexible operation mode, and the poles I and II can integrally operate or independently operate. Under the normal operating mode, the system adopts the single-ended ground mode of bipolar belt metallic return wire, and the unbalanced current of two poles returns through the metallic return wire. When the single pole has a fault, the converter valve of the fault pole is locked and stopped, and simultaneously, all or part of power of the fault pole is transferred to the normal pole. The normal pole and the metal return wire are converted into a single pole operation mode with the metal return wire, and the working current returns through the metal return wire. At the moment, the project can still transmit 50% of rated capacity at most, the impact degree of load shedding on an alternating current power grid is reduced, and the power supply reliability is improved.
Currently, the research content for power transfer is lacked, but the essence of power transfer is the process of rapid power change. The work of high-voltage direct-current transmission technology based on a voltage source converter proposes that charging and discharging of a capacitor on a direct-current side can be completed within 2 ms; when a literature 'research on the operating characteristics of a symmetrical bipolar flexible direct-current transmission system' simulates the operating characteristics of a converter valve, power step from 200MW to 400MW can be realized within 30 ms; the flow reversal process of the converter valve simulated by the document 'design and operation characteristic experiment of a flexible direct current transmission system' can be completed within 40ms, and the direct current voltage has no fluctuation; the literature 'building door flexible direct current transmission project power mutation phenomenon and optimization strategy' analyzes the mechanism of reactive power mutation phenomenon caused by asynchronous control time of a direct current control system, and provides an optimization strategy. The research shows that the flexible direct-current transmission system can quickly realize active power change, step change and even reversal, but according to the test data of the domestic flexible direct-current transmission project, when the active power instruction changes quickly, the voltage of a direct-current bus can fluctuate, and even direct-current overvoltage protection action is caused to trip. In order to ensure the successful completion of the power transfer process, it is necessary to provide a power transfer control method and optimize related control parameters.
In view of the current situation of the power transfer control method deficiency, the invention provides a power transfer control method of a symmetrical bipolar flexible direct current transmission converter. The method firstly provides a feasible condition of power transfer; then, a calculation process of the switched power quantity is proposed according to the power values of the fault pole and the non-fault pole; establishing a simulation model by introducing actual equipment parameters such as a direct current cable and the like, and providing a setting method of a power transfer rate parameter according to a simulation result; and finally, providing a power transfer control flow. The control method provided by the invention has important significance for analyzing the power transfer process and setting related control parameters when the converter has a single-pole fault.
Disclosure of Invention
The invention aims to provide a power transfer control method of a symmetrical bipolar flexible direct current transmission converter, which effectively reduces the voltage fluctuation of a direct current bus when the converter generates a single-pole fault and quickly transfers the power of a fault pole to a non-fault pole. On one hand, the invention clarifies the feasibility condition of power transfer and provides a specific control flow; on the other hand, a method for calculating and setting parameters of the power quantity of the transferred band and the power transfer rate of the transferred band is provided. The invention provides a power transfer control method for the unipolar fault of the converter and provides theoretical basis and technical support for the setting of relevant control parameters.
In order to achieve the purpose, the technical scheme of the invention is as follows: a power band-turning control method for a symmetrical bipolar flexible direct-current transmission converter comprises the following steps,
step S1: judging whether the symmetrical bipolar flexible direct-current transmission converter can carry out power band conversion or not according to the power band conversion feasibility condition;
step S2: according to fault pole powerP aNon-fault pole powerP bUnipolar rated powerP NThe value of (A) is used to calculate the amount of transferred band power DeltaP；
Step S3: locking the fault pole converter and disconnecting the fault pole metal return wire switch to convert the system into a single-pole single-end grounding operation mode with the metal return wire;
step S4: increasing the non-fault electrode power to a fixed value according to the power conversion rateP b+ΔP。
Further, in step S1, the power conversion band feasibility condition is that the inverter uses a bipolar band metal loop single-ended ground operation mode, and the non-fault pole uses a bipolar power control mode.
Further, in the step S2, the amount of power Δ to be transferred isPThe calculation is as follows:
the power quantity of the transferred belt is smaller than the original power value of the fault pole:
the original power of the non-fault pole plus the power of the rotating band should be smaller than the rated power of a single pole:
the transferred power quantity should adopt the maximum value satisfying the above formula condition to reduce the influence of load shedding on the alternating current system, therefore, the transferred power quantity value is:
（1）。
further, in step S4, the power strip transfer rate fixed value is set according to the following steps:
step S41: establishing a real-time simulation model according to the equipment parameters of the symmetrical bipolar flexible direct current transmission converter;
step S42: the current converter in the model is set to be a bipolar metal return wire single-end grounding operation mode, and the fault pole is a monopolePower control, bipolar power control for non-fault electrode, and fault electrode powerP aRated power, non-fault pole powerP bIs zero;
step S43: setting non-fault pole power in modelP bFrom zero toP NTime t of1The power transfer rate in the simulation model is made to beP N/t1；
Step S44: under different tape rotating speeds, the simulation calculation obtains the retention time t that the direct current voltage exceeds 1.15 times of the rated voltage2；
Step S45: t is t2When the time is more than or equal to 20ms, the direct current overvoltage protection I section of the converter acts, and t is equal to1Increasing the delta t, and repeating the steps S43 and S44; t is t2When the time is less than 20ms, the I section of the direct-current overvoltage protection of the converter does not act, and the step S46 is carried out;
step S46: determining t1The value and the fixed value setting result of the power conversion band rate areP N/t1。
Further, in step S44, the dc voltage exceeds 1.15 times the rated voltage for the holding time t2The simulation calculation is carried out according to the following method:
the instantaneous value of the active power input to any one phase of upper and lower bridge arms of the non-fault pole converter can be represented as follows:
（2）
whereinU sIs the effective value of the alternating voltage;I sis the effective value of the alternating current; phi is the alternating voltage current phase difference;U dcis a direct current voltage;I dcis direct current;
in the power transfer process, the input power of the non-fault pole AC side is rapidly increased, but due to the current limiting effect of the equivalent reactance of the DC cable, the rising rate of the DC current is slow, the input power of the AC side of the converter valve is greater than the output power of the DC side, the voltage of a capacitor on a bridge arm is increased, and the energy variation of the active power of the bridge arm in unit time is equal to the energy increased by the capacitor:
（3）
in the formulaC 0Is the sub-module capacitance, and is,Nfor the number of sub-modules per bridge arm,U dcis the direct-current voltage at the current moment,ΔU dcis the DC voltage variation;
because one end of the flexible direct current transmission converter adopts a power control mode and the opposite end adopts a direct current voltage control mode, the direct current voltage of the opposite end can be considered to be kept constant, and thereforeΔU dcIs the amount of voltage change on the dc cable,ΔU dcminus the equivalent resistance of the DC cablerThe voltage of the DC cable is the voltage of the equivalent reactance of the DC cable, and the variation of the DC current in unit time is calculated：
（4）
WhereinLAn equivalent reactance of the dc cable;ris the equivalent resistance of the direct current cable;
the DC voltage value and the current value at the next moment are as follows:
（5）
the direct current voltage waveforms under different power transfer rates are obtained through iterative solution of the formula, and the holding time t when the direct current voltage exceeds 1.15 times of the rated voltage is calculated according to the waveforms2。
Compared with the prior art, the invention has the following beneficial effects: through experimental verification, the control method provided by the invention can smoothly complete the power transfer process when the converter has a single-pole fault, avoids tripping caused by direct-current overvoltage protection action due to overlarge voltage fluctuation of a direct-current bus, reduces the impact degree on an alternating-current power grid, and improves the power supply reliability.
Drawings
FIG. 1 is a flow chart of power transfer control according to the present invention.
Fig. 2 is a main connection circuit diagram of a symmetrical bipolar flexible direct current transmission converter.
FIG. 3 is a flow chart of a power transfer rate setting method of the present invention.
FIG. 4 is a DC voltage waveform during power transfer according to the present invention.
Detailed Description
The technical scheme of the invention is specifically explained below with reference to the accompanying drawings.
As shown in fig. 1, the power transfer control method of a symmetrical bipolar flexible dc transmission converter of the present invention includes the following steps,
step 1: judging whether the symmetrical bipolar flexible direct-current transmission converter can carry out power band conversion or not according to the power band conversion feasibility condition;
step 2: according to fault pole powerP aNon-fault pole powerP bUnipolar rated powerP NThe value of (A) is used to calculate the amount of transferred band power DeltaP；
And step 3: locking the fault pole converter and disconnecting the fault pole metal return wire switch to convert the system into a single-pole single-end grounding operation mode with the metal return wire;
and 4, step 4: increasing the non-fault electrode power to a fixed value according to the power conversion rateP b+ΔP。
In step 1, the feasible conditions of power conversion are that the converter adopts a bipolar metal return wire single-end grounding operation mode, and a non-fault pole adopts a bipolar power control mode.
The main connection circuit of the symmetrical bipolar flexible direct current transmission converter is shown in fig. 2, when the converter adopts a bipolar single-end grounding operation mode with a metal return wire, if a single pole fails, the control and protection system can lock the failed pole converter valve and disconnect the pole NBS switch, so that the system is converted into single-end grounding operation with the metal return wire. The power of the fault electrode is rapidly transferred to the non-fault electrode, the influence of load shedding on an alternating current system can be reduced, and the operation reliability of the flexible direct current transmission converter is improved.
The control mode of the pole I and II converter station is divided into two modes of bipolar power control and unipolar power control, and the two control modes can be flexibly combined. Taking the case where S1 is the sending-end converter station, S2 is the receiving-end converter station, and a unipolar fault occurs in pole i, the control method and the power transfer feasibility conditions that can be adopted are shown in table 1.
1. Bipolar power control is used for both dipoles
Active power is evenly distributed between the two poles, and when a single-pole fault occurs to the pole I, all or part of power can be transferred to the pole II.
2. The electrode I adopts unipolar power control, and the electrode II adopts bipolar power control
The unipolar power control command sets an active power value of pole i, and the bipolar power control command sets a total bipolar active power value, so that the active power value of pole ii is equal to the total power minus the unipolar power of pole i. When the single pole fault occurs to the pole I, all or part of the power can be transferred to the pole II.
3. Bipolar power control for pole I and unipolar power control for pole II
When utmost point I takes place the monopole trouble, because utmost point II power has been set for by monopole power control command, can not take utmost point I power to utmost point II, will take place to get rid of the load phenomenon.
4. Bipolar all using unipolar power control
When the monopole fault occurs to the pole I, the power can not be transferred to the pole II, and the load shedding phenomenon occurs.
Therefore, the feasible conditions for power transfer are: the converter station adopts a bipolar metal return wire single-end grounding operation mode, and a non-fault electrode adopts a bipolar power control mode;
in step 2, the amount of power to be transferred is calculated as follows:
the power quantity of the transferred belt is smaller than the original power value of the fault pole:
the original power of the non-fault pole plus the power of the rotating band should be smaller than the rated power of a single pole:
the transferred power quantity should adopt the maximum value satisfying the above formula condition to reduce the influence of load shedding on the alternating current system, therefore, the transferred power quantity value is:
（1）。
in step 4, as shown in fig. 3, the power transfer rate constant value is set according to the following steps:
step 41: establishing a real-time simulation model according to the equipment parameters of the symmetrical bipolar flexible direct current transmission converter;
step 42: the current converter in the model is set to be a bipolar metal return wire single-end grounding operation mode, fault electrode is unipolar power control, non-fault electrode is bipolar power control, fault electrode powerP aRated power, non-fault pole powerP bIs zero;
step 43: setting non-fault pole power in modelP bFrom zero toP NTime t of1The power transfer rate in the simulation model is made to beP N/t1；
Step 44: under different tape rotating speeds, the simulation calculation obtains the retention time t that the direct current voltage exceeds 1.15 times of the rated voltage2；
Step 45: t is t2When the time is more than or equal to 20ms, the direct current overvoltage protection I section of the converter acts, and t is equal to1Increasing delta t, and repeating the steps 43 and 44; t is t2When the time is less than 20ms, the I section of the direct-current overvoltage protection of the current converter does not act, and the step is enteredStep 46;
step 46: determining t1The value and the fixed value setting result of the power conversion band rate areP N/t1。
In step 44, the DC voltage exceeds 1.15 times the rated voltage for a holding time t2The simulation calculation is carried out according to the following method:
the instantaneous value of the active power input to any one phase of upper and lower bridge arms of the non-fault pole converter can be represented as follows:
（2）
whereinU sIs the effective value of the alternating voltage;I sis the effective value of the alternating current; phi is the alternating voltage current phase difference;U dcis a direct current voltage;I dcis direct current;
in the power transfer process, the input power of the non-fault pole AC side is rapidly increased, but due to the current limiting effect of the equivalent reactance of the DC cable, the rising rate of the DC current is slow, the input power of the AC side of the converter valve is greater than the output power of the DC side, the voltage of a capacitor on a bridge arm is increased, and the energy variation of the active power of the bridge arm in unit time is equal to the energy increased by the capacitor:
（3）
in the formulaC 0Is the sub-module capacitance, and is,Nfor the number of sub-modules per bridge arm,U dcis the direct-current voltage at the current moment,ΔU dcis the DC voltage variation;
because one end of the flexible direct current transmission converter adopts a power control mode and the opposite end adopts a direct current voltage control mode, the direct current voltage of the opposite end can be considered to be kept constant, and thereforeΔU dcIs the amount of voltage change on the dc cable,ΔU dcminus the equivalent resistance of the DC cablerThe voltage on the DC cable equivalent reactance is calculated to obtain the DC current sheetAmount of change in bit time：
（4）
WhereinLAn equivalent reactance of the dc cable;ris the equivalent resistance of the direct current cable;
the DC voltage value and the current value at the next moment are as follows:
（5）
the direct-current voltage waveforms under different power transfer band rates are obtained through iterative solution of the formula, and are shown in fig. 4. According to the waveform of FIG. 4, the holding time t of the DC voltage exceeding 1.15 times of the rated voltage can be obtained2。
The objects, technical solutions and advantages of the present invention have been described in detail, it should be understood that the above-mentioned embodiments are only preferred embodiments of the present invention, and should not be construed as limiting the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (4)
1. A power band-turning control method for a symmetrical bipolar flexible direct current transmission converter is characterized by comprising the following steps: comprises the following steps of (a) carrying out,
step S1: judging whether the symmetrical bipolar flexible direct-current transmission converter can carry out power band conversion or not according to the power band conversion feasibility condition;
step S2: according to fault pole power PaNon-fault pole power PbUnipolar rated power PNCalculating the amount of transferred band power delta P by the numerical value of (1);
step S3: locking the fault pole converter and disconnecting the fault pole metal return wire switch to convert the system into a single-pole single-end grounding operation mode with the metal return wire;
step S4: increasing the power of the non-fault electrode to P according to the fixed value of the power conversion rateb+ Δ P; in step S4, the power strip transfer rate fixed value is set according to the following steps:
step S41: establishing a real-time simulation model according to the equipment parameters of the symmetrical bipolar flexible direct current transmission converter;
step S42: the current converter in the model is set to be a bipolar metal return wire single-end grounding operation mode, the fault electrode is unipolar power control, the non-fault electrode is bipolar power control, and the fault electrode power PaRated power, non-fault pole power PbIs zero;
step S43: setting non-fault pole power P in modelbFrom zero to PNTime t of1Making the power transfer rate in the simulation model be PN/t1；
Step S44: under different tape rotating speeds, the simulation obtains the retention time t that the direct current voltage exceeds 1.15 times of the rated voltage2；
Step S45: t is t2When the time is more than or equal to 20ms, the direct current overvoltage protection I section of the converter acts, and t is equal to1Increasing the delta t, and repeating the steps S43 and S44; t is t2When the time is less than 20ms, the I section of the direct-current overvoltage protection of the converter does not act, and the step S46 is carried out;
step S46: determining t1The value of the fixed value setting result of the power conversion band rate is PN/t1。
2. The power transfer control method of the symmetrical bipolar flexible direct current transmission converter according to claim 1, characterized by comprising the following steps: in step S1, the feasible condition of power conversion is that the inverter uses a bipolar metal return line single-ended ground operation mode, and the non-fault pole uses a bipolar power control mode.
3. The power transfer control method of the symmetrical bipolar flexible direct current transmission converter according to claim 1, characterized by comprising the following steps: in step S2, the transferred power amount Δ P is calculated as follows:
the power quantity of the transferred belt is smaller than the original power value of the fault pole:
ΔP≤Pa
the original power of the non-fault pole plus the power of the rotating band should be smaller than the rated power of a single pole:
Pb+ΔP≤PN
the transferred power quantity should adopt the maximum value satisfying the above formula condition to reduce the influence of load shedding on the alternating current system, therefore, the transferred power quantity value is:
4. the power transfer control method of the symmetrical bipolar flexible direct current transmission converter according to claim 1, characterized by comprising the following steps: in step S44, the dc voltage exceeds 1.15 times the rated voltage for the holding time t2The simulation calculation was carried out as follows,
the instantaneous value of the active power input to any one phase of upper and lower bridge arms of the non-fault pole converter is represented as follows:
wherein U issThe effective value of the alternating current voltage at any cross current side of the non-fault pole current converter is obtained; i issThe effective value of the alternating current at any cross current side of the non-fault pole converter is obtained;the phase difference of alternating voltage and current at any cross current side of the non-fault pole converter is obtained; u shapedcThe direct current voltage is the direct current voltage of the connecting point of the non-fault pole converter and the direct current cable; i isdcIs the direct current on the direct current cable;
in the power transfer process, the input power of the non-fault pole AC side is rapidly increased, but due to the current limiting effect of the equivalent reactance of the DC cable, the rising rate of the DC current is slow, the input power of the AC side of the converter valve is greater than the output power of the DC side, the voltage of a capacitor on a bridge arm is increased, and the energy variation of the active power of the bridge arm in unit time is equal to the energy increased by the capacitor:
in the formula C0Is a sub-module capacitor, N is the number of sub-modules per bridge arm, Udc(t) is the DC voltage of the connection point of the non-fault pole converter and the DC cable at the current moment, delta UdcIs the DC voltage variation;
one end of the flexible direct current transmission converter adopts a power control mode, the opposite end adopts a direct current voltage control mode, and the direct current voltage of the opposite end is considered to be constant, so that the delta UdcThat is the voltage variation, Δ U, on the DC cabledcSubtracting the voltage variation on the equivalent resistance r of the direct current cable, namely the voltage variation on the equivalent reactance of the direct current cable, and further calculating to obtain the variation delta I of the direct current in unit timedc：
Wherein L is the equivalent reactance of the dc cable; r is the equivalent resistance of the direct current cable;
the DC voltage value and the current value at the next moment are as follows:
the direct current voltage waveforms under different power transfer rates are obtained through iterative solution of the formula, and the holding time t when the direct current voltage exceeds 1.15 times of the rated voltage is calculated according to the waveforms2。
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| CN111769581B (en) * | 2020-06-09 | 2021-08-27 | 中国南方电网有限责任公司超高压输电公司检修试验中心 | Three-terminal direct-current transmission project fault power coordination control method |
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