CN108809132B - Hybrid MMC half-bridge submodule capacitor voltage balancing method - Google Patents

Hybrid MMC half-bridge submodule capacitor voltage balancing method Download PDF

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CN108809132B
CN108809132B CN201810668992.3A CN201810668992A CN108809132B CN 108809132 B CN108809132 B CN 108809132B CN 201810668992 A CN201810668992 A CN 201810668992A CN 108809132 B CN108809132 B CN 108809132B
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bridge arm
current
bridge
reactive power
mmc
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CN108809132A (en
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苟锐锋
杨晓平
苏匀
李少华
姚东晓
马小婷
严冰
葛昭
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Xi'an Xd Digital Technology Co ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H02M7/4835—Converters with outputs that each can have more than two voltages levels comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage

Abstract

The invention discloses a hybrid MMC Half-Bridge submodule capacitor voltage balancing method.A converter station is formed by mixing a Full-Bridge submodule (FBSM) and a Half-Bridge submodule (HBSM). when the FBSM occupation ratio is lower and the voltage operation of a direct current system is lower, the direct current bias of Bridge arm current is improved by a method of increasing reactive power at an alternating current side, so that the Bridge arm current is positive or negative, the charging and discharging probability of the HBSM is increased, and the capacitor voltage balance is controllable.

Description

Hybrid MMC half-bridge submodule capacitor voltage balancing method
Technical Field
The invention belongs to the technical field of direct current transmission, and particularly relates to a hybrid MMC half-bridge submodule capacitor voltage balancing method.
Background
The MMC is applied to high-capacity overhead line power transmission, is an objective requirement for realizing the optimization of energy resources and preventing commutation failure in China, combines the advantages of LCC and VSC, a rectifier station adopts LCC, and a hybrid high-voltage direct-current power transmission system of an inverter station adopting VSC becomes a hotspot of academic research. Because overhead lines are often adopted for large-capacity long-distance power transmission, temporary faults such as short circuit, flashover and the like easily occur on exposed lines. However, the current MMC based on a half-bridge sub-module (HBSM) cannot simply rely on the converter control to clear the dc side fault like the LCC, and even if the converter is locked, the ac system can still form an energy flow loop through the anti-parallel freewheeling diodes of the devices in the two-phase bridge arms inside the converter and the dc fault point, and thus is not suitable for the overhead line situation. MMCs based on full bridge sub-modules (FBSMs) can operate overmodulation to significantly reduce dc operating voltage, and are therefore particularly attractive. However, compared with a half-bridge MMC with the same capacity and voltage class, the power electronic devices used by the full-bridge MMC are almost doubled, so that not only is the investment cost increased, but also more operation loss is introduced. Therefore, the MMC converter station adopts a converter topology in which half-bridge and full-bridge sub-modules are mixed, as shown in detail in fig. 1. The main application occasions are as follows: the electric power of the energy base is sent out in a large scale and a long distance, and is generally sent to a load center within thousands of kilometers via an extra-high voltage direct current transmission line. How to optimize the hybrid MMC can reduce the usage of FBSMs as much as possible under the condition of meeting the performance of a system. About 50% of the submodules of the hybrid MMC must be full-bridge submodules. Under the condition, if the direct current system has a temporary fault, in the ride-through process of the direct current fault, the HBSM does not participate in the capacitor voltage balance control due to the bias of the bridge arm current, and the fault recovery operation of the system is not facilitated under the condition.
Disclosure of Invention
In order to solve the problems, the invention provides a hybrid MMC half-bridge submodule capacitor voltage balancing method, which improves the hybrid MMC half-bridge submodule capacitor voltage balancing and ensures the capacitor voltage balancing of HBSM when a direct current system runs at low voltage.
In order to achieve the purpose, the hybrid MMC comprises three phases ABC, each phase is divided into an upper bridge arm and a lower bridge arm, and each bridge arm is formed by cascading a plurality of half-bridge submodules and full-bridge submodules with the same structure and connecting the half-bridge submodules with a bridge arm inductor L0The upper bridge arm and the lower bridge arm are connected in series, and the upper bridge arm and the lower bridge arm in the same phase form a phase unit; the direct current bias of the bridge arm current is improved by increasing the reactive power of the alternating current side of the hybrid MMC converter, so that the bridge arm current has positive and negative.
Furthermore, the amplitude I of the phase current of the AC output end of the converter is increased by increasing the reactive power of the AC side of the hybrid MMC convertermLet ImSatisfy the relation
Figure GDA0002202936920000021
Wherein IdcIs a direct current.
Furthermore, in the process of passing through the direct-current fault, the detection of the direct-current bias of the bridge arm is started, and when a bridge is usedWhen the arm current is in a complete bias state, a reactive power increasing mechanism at the AC side is started, and the fixed value Q of the reactive power is continuously increased within the allowable power range of the MMC converterrefUntil the fully biased state disappears.
Furthermore, when the direct current fault ride-through is completed and the system is recovered to be normal, the reactive power increasing mechanism is exited, and the value Q is fixedrefAnd the constant value before the direct current fault is recovered.
Furthermore, in the hybrid MMC, the full-bridge submodule accounts for 45% -55%.
Compared with the prior art, the hybrid MMC has at least the following beneficial technical effects that when the FBSM proportion in the hybrid MMC is lower (about 50 percent) and the running voltage of a direct current system is lower (about 0.5pu), the direct current bias of the bridge arm current is improved by a method of increasing reactive power on the alternating current side, so that the bridge arm current has positive or negative, the charge and discharge probability of the HBSM is increased, and the capacitance-voltage balance of the hybrid MMC is controllable. The capacitance-voltage balance of the HBSM can be realized by only increasing the reactive power on the AC side without increasing the FBSM ratio. The HBSM improvement method is particularly suitable for occasions with low FBSM occupation ratio, can reduce the primary cost of a direct current system, and is good in economical efficiency. The method is only effective under the abnormal working condition of direct current fault ride-through or low-voltage operation, and the cost of the system is not increased.
Drawings
FIG. 1 is a detailed topology diagram of a hybrid MMC converter;
FIG. 2 is a MMC single line diagram;
FIG. 3 is a diagram of HBSM sub-module states and current paths;
fig. 4 is a schematic diagram of reactive power increase.
Detailed Description
The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.
Referring to fig. 1, the basic topology of the hybrid MMC is as shown in fig. 1, and it is composed of three phases of 6 bridge arms, each of which is composed of a number of HBSM and FBSM cascaded with the same structure and connected with a bridge arm inductor L0Formed by connecting in series, the upper and lower bridge arms with the same phase form a phase unit. HBSM submodule As shown in FIG. 3, there are two possibilities for HBSM submodule output voltage: when the upper IGBT T1 is turned on and the lower IGBT T2 is turned off, Usm ═ Uc; when the upper IGBT T1 is turned off and the lower IGBT T2 is turned on, Usm is 0.
Referring to fig. 2, since each HBSM of the MMC is a fully-controlled half-bridge, the SM sub-module can be divided into three operation states of latching, switching on and switching off according to the switching states of 2 IGBTs in each sub-module, as shown in detail in fig. 2:
the first operation state is as follows: IGBT T1 and IGBT T2 are both off: this state is a locked state of the inverter and is generally used during a fault period or at the initial charging period when the inverter is started. This state does not occur during normal power transmission.
And a second operation state: IGBT T1 turns on, IGBT T2 turns off: the sub-module port voltage is equal to the capacitor voltage in the sub-module regardless of the direction of the bridge arm current. The direction of the bridge arm current determines whether the sub-module capacitor is in a charging or discharging state, which is called an on state.
And a second operation state: IGBT T1 turns off, IGBT T2 turns on: regardless of the direction of the bridge arm current, the port voltage of the submodule is equal to 0, the capacitor in the submodule is bypassed, the capacitor voltage is kept stable, and the state is an off state.
Referring to fig. 1, it can be seen from the related studies that the duty requirement of FBSM can be expressed as:
Figure GDA0002202936920000031
in the above formula MacIs an ac modulation degree. From the constraint of the conventional direct current, since the minimum operating voltage of the module during the step-down operation is 0.7 times of the rated direct current voltage, about 50% of the submodules can be obtained and must be full-bridge submodules. The proportion of the full-bridge sub-modules and the half-bridge sub-modules is determined according to the actual engineering requirements, and generally, in order to reduce the engineering cost, the proportion of the FBSM is as low as possible. Under the condition, if the direct current system has temporary faults, such as the ride-through process of the direct current fault, the HBSM is not involved in the capacitance-voltage balance control due to the bias of bridge arm currentThis is not favorable for the system to operate after fault recovery.
Let the expression for the ac phase current:
iva＝Imcos(ωt+θia) (1)
in the above formula: im is the amplitude of the phase current of the AC output end of the converter; thetaiaIs the initial phase angle value of the phase current.
Taking the upper bridge arm current of the phase a as an example, neglecting the influence of the circulating current, as can be known from fig. 1, the expression of the bridge arm current is:
Figure GDA0002202936920000041
for HBSMs, the charging and discharging of the sub-module capacitors need to be done at different direction of the bridge arm current. That is, the necessary condition for the voltage of the HBSMs capacitors to be balanced is that the bridge arm current must have both positive and negative values. FBSMs can be charged or discharged in the same bridge arm current direction, so this is not required. Therefore, for HBSMs, to keep the capacitor voltage balanced, the following constraints must be introduced:
Figure GDA0002202936920000042
in the above formula, IdcRefers to direct current. The invention increases I by increasing the reactive power on the AC sidemA value of (A) to
Figure GDA0002202936920000043
The value of (3) is greater than 1/3 of the direct current, so that the bridge arm current can have a negative value, and the capacitance-voltage balance of the HBSM can be realized by changing the reactive power on the alternating current side under the condition of not increasing the proportion of the FBSM.
When the direct current fault ride-through is carried out, a reactive power increasing mechanism on the alternating current side is started, transient working conditions such as the direct current fault ride-through are ended, and the reactive power increasing mechanism is quitted when the system power voltage is recovered to be normal. As shown in fig. 4:
at present, the voltage source type direct current transmission system is commonly usedThe control method of (1) is direct current control, as shown in the above figure iq_refAnd d, decoupling the reactive current reference value for dq. In the transient working condition process such as the ride-through of the direct current fault, the bridge arm current direct current complete bias detection is started, and as long as the bridge arm current is in a complete bias state, the fixed value Q of the reactive power is increased all the time within the allowable power range of the hybrid MMC converterrefUntil the fully biased state disappears, i.e. the bridge arm current has both a positive and a negative value.
When the direct current fault ride-through is finished and the system is recovered to be normal, the reactive power increasing mechanism is quitted, and the value Q is fixedrefAnd the constant value before the direct current fault is recovered.
As can be seen from equation (3), when the amplitude of the ac voltage is increased, since the ac phase current of the bridge arm current 1/2 is superimposed on the dc current of 1/3, increasing the ac current can improve the dc offset of the bridge arm current, so that the bridge arm current has both positive and negative values, and thus the HBSM can better balance the capacitance voltage thereof. The method should avoid long-time operation as much as possible to reduce the reactive impact influence on the alternating current system.
The invention writes programs, debugs and verifies through a power system electromagnetic transient special simulation tool PSCAD/EMTDC. This tactics is fit for LCC + MMC mixed direct current's the HBSM capacitance voltage balance of receiving end hybrid MMC, and its control strategy is simple easy-to-use, can be in transient state in-process balance HBSM's capacitance voltage, has very big engineering practical value.

Claims (4)

1. The method for balancing the capacitance and the voltage of the hybrid MMC half-bridge submodule is characterized in that the hybrid MMC comprises ABC three phases, each phase is divided into an upper bridge arm and a lower bridge arm, and each bridge arm is formed by cascading a plurality of half-bridge submodules and full-bridge submodules which are the same in structure and is connected with a bridge arm inductor L in a cascaded mode0The upper bridge arm and the lower bridge arm are connected in series, and the upper bridge arm and the lower bridge arm in the same phase form a phase unit; improving the direct current bias of the bridge arm current by increasing the reactive power of the alternating current side of the hybrid MMC converter to enable the bridge arm current to have positive or negative;
in the process of passing through the direct-current fault, starting the direct-current bias detection of the bridge arm current, and when the bridge arm current is in a bias stateStarting a reactive power increasing mechanism at the AC side, and continuously increasing the constant value Q of the reactive power within the allowable power range of the MMC converterrefUntil the fully biased state disappears; in the mixed MMC, the full-bridge submodule accounts for 45% -55%.
2. The hybrid MMC half-bridge submodule capacitor voltage balancing method of claim 1, wherein an amplitude I of a phase current at an AC output terminal of the converter is increased by increasing a reactive power at an AC side of the convertermLet ImSatisfy the relation
Figure FDA0002202936910000011
Wherein IdcIs a direct current.
3. The hybrid MMC half-bridge submodule capacitive voltage balancing method of claim 1, wherein the reactive power increase mechanism is exited when the system returns to normal after the DC fault ride through is completed.
4. The hybrid MMC half-bridge submodule capacitor voltage balancing method of claim 1, wherein a constant Q is obtained when a system is recovered to normalrefAnd the constant value before the direct current fault is recovered.
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CN111509741B (en) * 2020-04-20 2023-01-03 兰州理工大学 Interphase power balance control method for micro-grid with MMC half-bridge series structure
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CN105656070A (en) * 2016-03-11 2016-06-08 特变电工新疆新能源股份有限公司 Grid fault ride-through control method for flexible direct-current transmission system
CN106026163A (en) * 2016-05-27 2016-10-12 南京工程学院 MMC-based low-voltage ride through control method and system of photovoltaic grid-connected inverter
CN106160545A (en) * 2016-07-06 2016-11-23 清华大学 A kind of brachium pontis hybrid bipolar modular multi-level converter
CN107659192A (en) * 2017-09-26 2018-02-02 许继集团有限公司 A kind of current conversion station and its valve group, which are thrown, moves back process Neutron module pressure equalizing control method

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN104901524A (en) * 2015-05-26 2015-09-09 清华大学 DC bipolar short-circuit fault crossing method for modular multilevel converter
CN105656070A (en) * 2016-03-11 2016-06-08 特变电工新疆新能源股份有限公司 Grid fault ride-through control method for flexible direct-current transmission system
CN106026163A (en) * 2016-05-27 2016-10-12 南京工程学院 MMC-based low-voltage ride through control method and system of photovoltaic grid-connected inverter
CN106160545A (en) * 2016-07-06 2016-11-23 清华大学 A kind of brachium pontis hybrid bipolar modular multi-level converter
CN107659192A (en) * 2017-09-26 2018-02-02 许继集团有限公司 A kind of current conversion station and its valve group, which are thrown, moves back process Neutron module pressure equalizing control method

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