CN109787205B - Converter direct-current side fault current suppression method based on additional virtual inductance coefficient - Google Patents
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Abstract
The invention relates to a converter direct current side fault current suppression method based on an additional virtual inductance coefficient, and belongs to the technical field of flexible direct current transmission. Firstly, a calculation method for influence of submodule part removal on an alternating current system after outlet short circuit fault of a converter is provided, through-current capacity of an IGBT device and breaking capacity of a direct current breaker are comprehensively considered, a method for restraining direct current side short circuit fault current of the converter by reducing input proportion of submodules after fault is provided, meanwhile, a fault current restraining method for self-adaptively changing input proportion of the submodules after fault is mapped into a control system through virtual inductance coefficients is provided, and a setting method of the virtual inductance coefficients is provided. The method can effectively reduce the rising rate of the fault current and reduce the overcurrent stress of the fault device cut off by the direct current breaker when being applied to the MMC current converter, and has good economical efficiency and practicability compared with a method for inhibiting the fault current by adding current limiting equipment.
Description
Technical Field
The invention relates to the technical field of flexible direct current transmission, in particular to a direct current fault current suppression method of an MMC (modular multilevel converter), and particularly relates to a converter direct current side fault current suppression method based on an additional virtual inductance coefficient.
Background
In 2001, german scholars r.marquardt and a.leinicar proposed Modular Multilevel Converters (MMC), which promoted the development of high voltage direct current transmission (HVDC) technology. To date, the domestic MMC-HVDC engineering put into operation is as follows: shanghai Hui demonstration engineering, Nanao engineering, Zhoushan engineering, mansion engineering and the like. The projects adopt cables for power transmission, but compared with overhead lines, the cables are high in cost, most of faults are permanent, and the maintenance and the overhaul are inconvenient. Therefore, the expansion of the flexible direct current transmission technology to the overhead line transmission occasion is a trend of future development of the power grid.
The overhead line has a higher probability of short-circuit failure than the cable, and therefore, the problem of fault clearing and fault protection is particularly important. In the existing fault clearing mode, the direct current circuit breaker can cut off fault current in a short time, but in practical situations, the fault current is often very large, and due to technical limitations, the capacity of the direct current circuit breaker for cutting off the current is limited. Furthermore, the dc circuit breaker is costly due to the large number of power electronics employed. Another way is to use sub-modules with fail-over capability, such as full-bridge sub-modules, clamped dual sub-modules, etc. Such sub-modules are able to interrupt the fault current for a short time by generating a reverse voltage by themselves. However, compared with the half-bridge type sub-modules, the number and the loss of the power electronic devices of the sub-modules are increased, and the economical efficiency greatly limits the application of the power electronic devices in practical engineering. At present, for the direct current side fault of the converter, the adopted fault current suppression method still depends on changing the topological structure of the sub-module and adding external current limiting equipment to carry out fault current limiting, the current limiting measures have the defects of high on-state loss and addition of a control system, and no method for carrying out MMC converter fault current limiting by changing the control measure after the converter fault exists.
Disclosure of Invention
The invention aims to provide a converter direct-current side fault current suppression method based on an additional virtual inductance coefficient, and solves the problems in the prior art. The invention provides a method for selecting the lowest input proportion of a submodule comprehensively considering the voltage drop bearing capacity of an alternating current system and the on-off capacity of a direct current breaker after a converter fails, and introduces a method for calculating the safe interval of the input proportion of the submodule after the converter fails under the condition of ensuring that the converter is not locked according to the through-current capacity during IGBT.
The above object of the present invention is achieved by the following technical solutions:
the method for inhibiting the fault current at the direct current side of the converter based on the additional virtual inductance coefficient comprises the following steps:
step (1) equivalence of a fault discharge loop of the converter;
calculating alternating current after partial removal of the sub-modules in the step (2);
calculating the input proportion of the sub-modules after the fault;
and (4) setting the virtual inductance.
And (3) performing equivalence on the fault discharging circuit of the converter in the step (1), wherein a method for equalizing the discharging circuit after an interpolar short-circuit fault occurs on the direct-current side of the MMC converter is adopted.
Calculating the alternating current after the submodule part is cut off in the step (2), and specifically comprises the following steps:
DC voltage drop delta U generated by cutting off submodule after faultdcAnd the drop of the AC voltage is delta UdiffAC current component Δ IacRespectively as follows:
ΔUdc＝(1-kmin)Udc(1)
wherein: k is a radical ofminTo set the minimum value of the sub-module input ratio, UdcFor the converter valve to output DC voltage value, m is the modulation ratio, LeqIs an equivalent inductance, R, of an AC systemeqIs an equivalent resistance of an AC system, R0Is a bridge arm resistance, L0Is a bridge arm inductance.
And (4) calculating the input proportion of the sub-modules after the fault occurs in the step (3), and adopting a calculation method of the input proportion of the sub-modules.
And (4) setting the virtual inductance coefficient, namely a virtual inductance coefficient setting method according to the input proportion of the sub-modules.
The invention has the beneficial effects that: the method can effectively reduce the rising rate of the fault current and reduce the overcurrent stress of a fault device cut off by the direct current breaker when being applied to the MMC converter, and has good economical efficiency and practicability compared with a method for restraining the fault current by adding current limiting equipment.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the invention without limiting the invention.
FIG. 1 is a topological structure diagram of an MMC converter of the present invention;
FIG. 2 is a schematic diagram of the system modulation under steady state operating conditions of the present invention;
FIG. 3 is a sub-module discharge circuit diagram of the present invention;
FIG. 4 is a diagram of an equivalent circuit for sub-module discharge before control measures are taken in accordance with the present invention;
FIG. 5 is a diagram of a sub-module discharge circuit after control measures are taken in accordance with the present invention;
FIG. 6 is a schematic diagram of the system modulation after the improved control strategy of the present invention;
FIG. 7 is a schematic flow chart of the present invention.
Detailed Description
The details of the present invention and its embodiments are further described below with reference to the accompanying drawings.
Referring to fig. 1 to 7, the method for suppressing the fault current on the dc side of the converter based on the additional virtual inductance coefficient of the present invention solves the problem of the method for suppressing the short-circuit current on the dc side of the flexible dc power grid, and firstly provides a method for calculating the influence of the removal of the submodule part on the ac system after the short-circuit fault on the outlet of the converter, and provides a method for suppressing the short-circuit fault current on the dc side of the converter which reduces the input proportion of the submodule after the fault by comprehensively considering the through-current capacity of the IGBT device and the on-off capability of the dc circuit breaker, and also provides a method for suppressing the fault current which adaptively changes the input proportion of the submodule after the fault is mapped into the control system through the virtual inductance coefficient, and provides a method for setting the. The method can effectively reduce the rising rate of the fault current and reduce the overcurrent stress of the fault device cut off by the direct current breaker when being applied to the MMC current converter, and has good economical efficiency and practicability compared with a method for inhibiting the fault current by adding current limiting equipment.
1. And the equivalence of the fault discharge loop of the converter adopts a method of equivalence of the discharge loop after the short-circuit fault between electrodes occurs on the direct current side of the MMC converter.
As shown in fig. 1, in a normal operating state, the sub-modules are in an on or off operating state through the on/off of the power devices VT1 and VT 2. The submodule is in an input state, the capacitor charging or discharging is determined by the current direction of the bridge arm at the current moment, when the instantaneous value of the current flowing to the submodule is larger than 0, the submodule capacitor is charged, and otherwise, the submodule is in a discharging state. When the sub-modules are in a cut-off state, bridge arm currents form a loop through VD1 and VT 2. The three-phase bridge arms are always in a symmetrical state, the sum of submodules input by the upper bridge arm and the lower bridge arm of each phase is the same, the submodules are used for maintaining the stability of the direct-current voltage of the converter, the input quantity of the submodules of the upper bridge arm and the lower bridge arm is determined by the reference voltage generated by the station control system, the expected alternating-current side voltage is generated by modulation, and the modulation principle under the steady-state operation is shown in fig. 2.
The MMC converter direct current side generates an interelectrode short circuit, and a schematic diagram of a sub-module capacitor discharge loop is shown in figure 3. The number of sub-modules in the post-fault put-in state is the same as the number of sub-modules in the cut-out state. When the sub-module is in the on state, the sub-module capacitor passes VT1Discharging and passing the submodule VD in the cut-off state2A closed loop is formed, and the direct current fault current is rapidly increased due to the small loop damping.
Under the condition that the direct current side is short-circuited and the control strategy is not changed, the number of the submodules input by each bridge arm is still N, and the other N submodules are in a cutting-off state. Because the effect of voltage-sharing link, every submodule piece all can participate in the discharge process after the trouble, supposes submodule piece voltage under the initial condition to be Uc, and the principle based on the unchangeable and bearing voltage of capacitor energy storage has:
in the formula: cphIs an equivalent capacitance of a discharge circuit, C0For a single sub-module capacitor, UdcIs DC voltage of converter in steady state operation, U0The rated voltage of a single sub-module, and N is the number of the sub-modules.
When a bipolar short-circuit fault occurs on the direct current side and the converter is not locked, the sub-module capacitor is discharged through a loop formed by the bridge arm inductors through a short-circuit point, and the equivalent circuit is shown in fig. 4.
2. Calculation of AC current after partial removal of submodule
Reducing the dc side outlet voltage of the converter can suppress the dc short circuit current rise rate, but the number of submodules to be cut is limited in various aspects. The minimum value k of the input proportion of the sub-modules needs to be set by comprehensively considering the bearing capacity of the voltage drop on the alternating current side, the configuration capacity requirement of the direct current breaker and other factorsmin。
Defining the modulation ratio of the system:
wherein: u shapediffmFor amplitude of AC fundamental voltage, UdcAnd outputting a direct current voltage value for the converter valve.
DC voltage drop delta U generated by cutting off submodule after faultdcAnd the drop of the AC voltage is delta UdiffAC current component Δ IacRespectively as follows:
ΔUdc＝(1-kmin)Udc(2-2)
wherein: k is a radical ofminTo set the minimum value of the sub-module input ratio, UdcFor the converter valve to output DC voltage value, m is the modulation ratio, LeqIs an equivalent inductance, R, of an AC systemeqIs an equivalent resistance of an AC system, R0Is a bridge arm resistance, L0Is a bridge arm inductance.
Neglecting the DC voltage drop caused by the capacitor discharge of the submodule before the fault is removed, setting the AC side current to be composed of the AC current component and the normal working component generated by the voltage drop caused by the submodule removal, and setting the AC current amplitude I under the steady state operation stateacCan be expressed as:
p, Q are respectively the real and reactive power, U, of the system transmission under steady state operationNThe rated operating voltage of the alternating current system.
The ac current on the ac side is:
Iac_max＝ΔIac+Iac(2-6)
3. calculation of bridge arm current after partial removal of submodule
The loop current of the second-order circuit shown in FIG. 4 is calculated as equation (3-1).
In the formula: 1/is the discharge current decay time constant; omega0The natural angular frequency of the discharge loop, i.e. the resonance angular frequency, omega is the angular frequency of the discharge circuit current, β is the initial phase angle of the current caused by the initial current, the above four parameters are determined by the circuit parameters,
wherein the content of the first and second substances,
in general, Rst is much less than
Therefore, omega is approximately equal to omega0I (t) can be simplified to:
order:
comprises the following steps:
i(t)＝i′(t)+i″(t) (3-4)
wherein: i '(t) is a fault current component caused by discharge of the sub-module capacitor, and i' (t) is a fault current component generated by an initial value of direct current.
Bridge arm current iarmCan be expressed as
Bridge arm current peak value i at fault clearing momentarm_maxCan be expressed as
In the formula:
the magnitude of the direct current fault current at the outlet of the direct current converter at the fault clearing moment (such as 6ms after the fault occurs),
the magnitude of the fundamental frequency current in the fault bridge arm at the moment of cutting off.
According to the formula (3-1), the discharge voltage of the converter after the fault can be reduced by reducing the input number of the submodules after the fault of the converter, so that the rising rate of the fault current is reduced. When the number of the converter-input submodules is changed after the fault, the equivalent capacitance value in the equivalent discharge circuit is also changed according to the formula (1-2). The number of the input sub-modules is changed to k (k is less than or equal to 1) times under the normal working state after the regulation and control measures are taken, and a discharge equivalent circuit diagram is shown in figure 5.
As can be seen from the formula (3-4), the fault current consists of a fault component i '(t) caused by the discharge of the sub-module capacitor and an initial current component i' (t) obtained on the inductor, only the fault current component of the sub-module capacitor discharge is taken into account, and the oscillation frequency of the discharge loop is very low after the current limiting reactor is connected in series in the discharge loop, namely omega0Very small, so there are several ms after the failure: sin (omega)0t)＝ω0t, can be simplified for i' (t):
derivation of i' (t):
due to the value of 0, the current rising rate in a few ms after the fault happens can be simplified to be
The rate of rise of the fault current before control measures are taken is
When U is turned0＝kUdcAt a rate of rise of the fault current of
According to the formula (3-11), the outlet voltage of the converter after the fault is changed into k times under the normal operation state, and the fault current caused by the discharge of the sub-module capacitor is k (k is less than or equal to 1) times of that of the traditional regulation strategy.
The bridge arm current can then be expressed as:
4. calculation of sub-module input proportion after fault
Setting the rated working current of the IGBT as INAccording to the safe working area of the IGBT device, the maximum reliable blocking current is 2 times of the rated current value, namely 2INWhen the bridge arm current exceeds 2INIn time, the inverter will not be able to be locked reliably and the power electronics will burn out.
In order to ensure the safety of the IGBT device, the converter is under the condition of no locking during the fault removal:
Iarm_max≤2IN(4-1)
the vertical type (3-6) comprises:
order:
the initial value of the direct current and the rising rate of the direct fault current can be obtained as follows:
in the formula: i isdc0The value of the direct current is under the condition of steady-state operation.
The united type (4-3) and (4-4) can obtain the maximum value k of the input proportion of the submodule under the condition of ensuring that the converter is not locked for ensuring the fault removal devicemax。
The number of the submodules which are put into by the upper bridge arm and the lower bridge arm is equal to
Nup＝round[(0.5+0.5Vref)*kmax*N](4-5)
Ndown＝round[(0.5-0.5Vref)*kmax*N](4-6)
5. And setting the virtual inductance coefficient, namely a virtual inductance coefficient setting method according to the input proportion of the sub-modules.
Virtual inductance L is introduced into a control systemCLet us order
The control block diagram of the modulation section of the control system is shown in fig. 6.
As can be seen from fig. 6, the virtual inductance in the control system does not play a role in the steady-state operation state, and only fails at the dc side, following the dc current idcAnd the modulation ring section can automatically reduce the input number of the submodules during the fault period so as to reduce the outlet voltage of the converter to inhibit the fault current after the fault occurs. The occurrence of the interelectrode short circuit at the outlet of the converter belongs to the most serious fault condition, and the input proportion k of the sub-module after the fault is set under the fault condition. Since the time from the occurrence of the fault to the fault removal period of the circuit breaker is short, the rising slope of the fault current can be regarded as a constant in the period, namely the current change rate is a constant, and therefore the virtual inductance L can be adjusted through the setting value kcAnd (6) setting.
Order:
the virtual inductance parameter is
And L is the total inductance value in the direct current discharge loop when the outlet of the converter is in short circuit.
The above description is only a preferred example of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, improvement and the like of the present invention shall be included in the protection scope of the present invention.
Claims (1)
1. A converter direct current side fault current suppression method based on an additional virtual inductance coefficient is characterized by comprising the following steps: the method comprises the following steps:
step (1) equivalence of a fault discharge loop of the converter;
calculating alternating current after partial removal of the sub-modules in the step (2);
calculating the input proportion of the sub-modules after the fault;
setting a virtual inductance coefficient;
the equivalence of the fault discharging circuit of the converter in the step (1) adopts a method of equivalence of the discharging circuit after an interpolar short-circuit fault occurs on the direct current side of the MMC converter;
calculating the alternating current after the submodule part is cut off in the step (2), and specifically comprises the following steps:
DC voltage drop delta U generated by cutting off submodule after faultdcAnd the drop of the AC voltage is delta UdiffAC current component Δ IacRespectively as follows:
ΔUdc＝(1-kmin)Udc(1)
wherein: k is a radical ofminTo set the minimum value of the sub-module input ratio, UdcFor the converter valve to output DC voltage value, m is the modulation ratio, LeqIs an equivalent inductance, R, of an AC systemeqIs an equivalent resistance of an AC system, R0Is a bridge arm resistance, L0Bridge arm inductance;
calculating the sub-module investment proportion after the fault in the step (3), and adopting a calculation method of the sub-module investment proportion;
and (4) setting the virtual inductance coefficient, namely a virtual inductance coefficient setting method according to the input proportion of the sub-modules.
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Effective date of registration: 20201229 Address after: No. 169, Changchun Road, Jilin City, Jilin Province, 132000 Patentee after: NORTHEAST DIANLI University Patentee after: SONGYUAN POWER SUPPLY COMPANY OF STATE GRID JILINSHENG ELECTRIC POWER SUPPLY Co. Address before: No. 169, Changchun Road, Jilin City, Jilin Province, 132000 Patentee before: NORTHEAST DIANLI University |