CN111769583B - Coordination control method for improving stability of cascade hybrid direct-current transmission system - 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
- 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
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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
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
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Abstract
The invention discloses a coordination control method for improving the stability of a cascade type hybrid direct current transmission system. Meanwhile, the problem of large fluctuation in the system recovery process can be relieved after the fault is cleared, so that the system can be quickly and stably recovered to a rated operation state. The invention can realize the stable transition of the system under the typical faults of alternating current and direct current, and improve the stability of the direct current receiving end system.
Description
Technical Field
The invention relates to the technical field of direct current transmission, in particular to a coordination control method for improving the stability of a cascade hybrid direct current transmission system.
Background
Aiming at a receiving-end multi-drop cascade type hybrid direct-current power transmission system with a power grid commutation converter LCC adopted for rectification and a modular multilevel converter MMC adopted for an inversion side in series, due to the limitation of the manufacturing level of power devices, a plurality of MMCs are usually adopted to be connected in parallel to match the transmission capacity of the LCC, the transmission capacity of the whole system is improved, and the topological structure of the system is shown in figure 1. The existence of the LCC and MMC parallel group enables the receiving end to have the condition of decentralized access to the alternating current power grid, and a multi-drop point form meeting the power utilization requirement of a multi-load center can be formed. Meanwhile, as the MMC is controlled flexibly, the control modes of the MMCs have different choices and have various control mode combinations. When the MMC control modes are different, the problem of unbalanced current distribution can be caused, and the phenomenon of power back-sending can be caused when the problem is serious, so that the stability of a receiving end alternating current system is reduced. Therefore, the research on the coordination control strategy of the hybrid cascade system has important significance aiming at various faults which can occur.
In combination with a volt-ampere characteristic diagram 2 of a cascade type hybrid direct-current power transmission system, the following theoretical analysis is performed for common alternating-current ground faults and direct-current faults which may occur in the system: when an alternating current ground fault occurs on the rectification side, the LCC on the rectification side is switched to be controlled by a fixed trigger angle, the LCC on the inversion side is switched to be controlled by a fixed direct current or controlled by a low-voltage current limit, a system operation point moves leftwards according to the current value of the inversion side, and the transmission power of the rectification side and the transmission power of the inversion side are reduced; when an alternating current ground fault occurs on the inverter side, the inverter side LCC fails to change the phase, the control mode is switched to constant turn-off angle control, the rectifier side LCC is switched to constant direct current control or low-voltage current limiting control, the system operating point moves leftwards according to the current value of the rectifier side, and the transmission power of the system is reduced; when a direct current fault occurs, due to the single-phase conductivity of the LCC, no fault current exists, the direct current voltage of the line is 0, and the power transmission of the system is interrupted. Analysis has shown that the transmission of line power is reduced or interrupted under these several common faults. Next, the next analysis is performed by taking the rectification-side fault as an example. When a fault occurs, the system operating point moves to point M. From FIG. 2, P is shown MMCB(i.e., MMC parallel group power) Pmmc1+Pmmc2+Pmmc3，PMMCBDecreasing, if the power instruction value of the fixed active MMC is always kept unchanged (P) when the power instruction value is in a steady statemmc2And Pmmc3Invariable), MMCB (i.e. MMC parallel group) transmission power's reduction can only be balanced by the power absorbed by fixed direct current voltage MMC1, therefore MMC1 changes the rectification into by the contravariant, begins to absorb active power, and the reverse transmission power phenomenon appears in the alternating current system who connects with it, receives end alternating current system's stability greatly reduced.
The method comprises the following steps: the method comprises the steps of receiving end wiring and control modes of a hybrid cascade type direct current transmission system (Xuzheng, Wangshijia, Zhang taimen, Xurain taimen, Xiao Huan Qing, LCC-MMC hybrid cascade type direct current transmission system receiving end wiring and control mode [ J ] electric power construction, 2018, 39 (07): 115-.
And 2, measure 2: dc fault recovery control strategy for hybrid cascaded dc transmission systems (greater yang, zheng anhe, pengying, guo chun, zhao success). The measures provide a recovery control strategy of the hybrid cascade type direct current transmission system during the direct current fault period and after the fault is cleared, and relieve the over-current phenomenon during the fault period, but the control strategy only aims at the direct current fault.
Disclosure of Invention
In view of the above problems, an object of the present invention is to provide a coordination control strategy for improving stability of a receiving-end multi-drop point cascaded hybrid dc power transmission system, which can avoid changing a fixed dc voltage station from inversion to rectification, prevent a large-scale power transfer phenomenon at the ac side of a receiving end, achieve stable system transition under typical ac and dc faults, improve stability of a dc receiving end system, and simultaneously alleviate a problem of large fluctuation in a system recovery process after a fault is cleared, so that the system can be quickly and stably recovered to a rated operation state. The technical scheme is as follows:
a coordination control method for improving stability of a cascade type hybrid direct current transmission system comprises the following steps:
s1: when the system normally operates, Ctrl is 0, namely the active power instruction value of the active MMC is determined as the given transmission value P thereofref；
S2: after the system judges that a fault occurs, Ctrl is switched to 1 after S ms fault detection time, namely the active power instruction value of the active MMC is switched to P 'calculated in real time'ref：
P′ref＝0.5×(Pdci-Plcc-Psmmc1)
In the formula, Psmmc1A fixed value is transmitted for the power of the constant direct current voltage MMC1 in the steady-state operation; pdciTransmitting a real-time measured value for the total power of the inversion side; plccTransmitting a measured value for the power of an inversion side LCC in the system in real time; p' refAccording to PdciAnd PlccMake changes, constrain P in the course of a faultsmmc1To approach a steady state operating value;
p 'calculated in real time'refThe method comprises the steps that through an amplitude limiting link, the capacity of a converter is prevented from being exceeded, meanwhile, the MMC power is adjusted very fast, and a slope limiter is arranged to improve the operation stability of a system;
and Ctrl is still kept as 1 during the system recovery after the fault is cleared;
s3: after the system is stably recovered to the steady state operation, Ctrl is switched to 0, that is, the active power instruction value of the active MMC is switched to the given transmission value PrefAnd the system is restored to the rated operation.
The invention has the beneficial effects that: when the receiving end MMC adopts different control modes for combination, the fixed direct-current voltage station can be prevented from being changed from inversion to rectification, the phenomenon that the power of the alternating-current side of the receiving end is transferred in a large range is prevented, the system can be stably transited under the typical alternating-current fault and the typical direct-current fault, the stability of the direct-current receiving end system is improved, and meanwhile, the problem of large fluctuation in the system recovery process can be relieved after the fault is cleared, so that the system can be quickly and stably recovered to a rated operation state. The control strategy is simple and easy to implement and is beneficial to engineering practice.
Drawings
Fig. 1 is a receiving-end multi-drop point cascade type hybrid direct-current power transmission system topology structure.
Fig. 2 is a current-voltage characteristic of a cascade type hybrid dc power transmission system.
Fig. 3 is a diagram of a fixed-active MMC coordination control strategy.
FIG. 4 is a system response characteristic under a rectification side short-circuit fault; (a) no coordination control is carried out; (b) there is coordinated control.
FIG. 5 shows the response characteristics of the system under the condition of short-circuit fault on the inverter side; (a) no coordination control is carried out; (b) there is coordinated control.
FIG. 6 is a system response characteristic under a DC fault; (a) no coordination control is performed; (b) there is a coordinated control.
Detailed Description
The invention is described in further detail below with reference to the figures and specific embodiments. The system coordination control strategy is shown in fig. 3:
according to the inversion side power balance (neglecting converter loss):
Pdci＝Plcc+Pmmc1+Pmmc2+Pmmc3
the active power command value can be set to be:
P′ref＝0.5×(Pdci-Plcc-Psmmc1)
wherein, Psmmc1The power transmission value is the power transmission value of the constant direct current voltage MMC1 in steady state operation; p'refIs equivalent to only according to PdciAnd PlccThe power change of the MMC parallel group is borne by the fixed active MMC, and the P in the fault process is restrainedsmmc1The fluctuation of the power (namely the power of the MMC 1) ensures that the power can be close to the steady-state operation value to the maximum extent, and the power back-off phenomenon can not occur. Therefore, during the fault period, the coordination control strategy can avoid the change of inversion of the fixed direct-current voltage station into rectification, prevent the phenomenon of large-range power transfer at the alternating-current side of the receiving end, improve the stability of the receiving end system, and simultaneously facilitate the system to quickly and stably recover to the steady-state operation after the fault is cleared.
The coordination control strategy scheme is as follows:
1) when the system normally operates, Ctrl is 0, and the active power instruction value of the active MMC is a given transmission value Pref；
2) After the system judges that a fault occurs, Ctrl is switched to 1 and the active power instruction value of the active MMC is switched to P 'calculated in real time within 2ms of fault detection time'refAfter the fault is cleared, Ctrl is still kept as 1 during the system recovery period;
3) and after the system is stably recovered to the steady operation, switching Ctrl to 0.
Example (b): the cascaded hybrid direct-current power transmission system shown in fig. 1 is set up as an example for verification, and the main parameters are shown in table 1.
Table 1: main parameters of cascade type hybrid direct current transmission system
The LCC at the rectifying side is controlled by constant direct current, and the LCC at the inverting side is controlled by constant direct current voltage. The MMC control mode is shown in Table 2, MMC1 is constant direct current voltage control, MMC2 and MMC3 are constant active power control, reactive power control of the MMC2 and the MMC3 is constant reactive power control, and the instruction value is set to be 0.
Table 2: MMC control mode
A first verification scheme: setting single-phase earth fault on the AC bus of the rectification side, wherein the fault starts from 4s and lasts for 0.5s
When the coordination control strategy is not adopted, as shown in fig. 4(a), at the moment of 4s, a fault occurs on the rectifying side, the voltage of the alternating-current bus at the transmitting end drops, the direct-current voltage of the line drops, and the direct current is also reduced due to low-voltage current limiting, so that the active power transmitted by the line drops. Inverter side LC C transmission power is reduced, and the fixed direct-current voltage MMC has unbalanced power at two ends due to the fact that the transmission power at the sending end is reduced, so that direct-current voltage is reduced and fluctuation is generated. The power instruction value of the constant active power MMC is always kept at a rated value of-620 MW, direct current of the constant active power MMC fluctuates, and power shortage needs to be transmitted by the constant direct current voltage MMC, so that the MMC1 is switched from an inversion state to a rectification state, power is absorbed from an alternating current system, a reverse power transmission phenomenon occurs, and the alternating current system AC connected with the MMC1 is caused2The fluctuation of the bus voltage is large, and the stability of a receiving end alternating current system is reduced.
Fig. 4(b) shows the simulation result after the coordination control strategy provided in this document is adopted. After the fault detection time of 2ms, the power instruction values of the constant active MMC2 and MMC3 are switched to P'refThe output active power is rapidly reduced and is adjusted in real time according to the transmission change of the system power, so that the power fluctuation of the constant direct-current voltage MMC1 is greatly restrained, and the output active power is close to a rated operation value. As can be seen from the simulation diagram, the direct-current voltage fluctuation of the MMC is small, the unbalanced current distribution phenomenon of the parallel connection group of the MMC is effectively relieved, the fluctuation of direct current of the MMC1 is restrained, and the power back-sending phenomenon cannot occur. Table 3 shows the ac voltage and active power fluctuation of the system, and it can be seen that the power fluctuation Δ P of MMC1 is 180.9% without coordinated control, and the power fluctuation Δ P is 32.3% with coordinated control, which is reduced by 151.6%. Meanwhile, the voltage fluctuation of an alternating current bus of an alternating current system AC2 connected with the MMC1 is greatly reduced, the voltage fluctuation value is reduced by 19.2%, although the voltage and the power of the alternating current system connected with the MMC2 and the MMC3 have certain fluctuation, the fluctuation range is smaller, and therefore the stability of the receiving end alternating current system is improved. After the fault is cleared, the system can also quickly and smoothly resume steady state operation.
Table 3: voltage and power fluctuation comparison of MMC1 receiving end system
And a second verification scheme: and a single-phase earth fault is set on the BUS-1 alternating current BUS on the inversion side, the LCC on the inversion side fails to change phases, and the fault lasts for 0.5s from 4 s.
The simulation results without adopting the coordination control strategy are shown in fig. 5 (a). Because the LCC phase commutation of the inversion side fails, the power cannot be transmitted, the active power at the two ends of the fixed direct-current voltage MMC1 is unbalanced, the direct-current voltage rises, and large fluctuation is generated. The power command values of the fixed active power MMC2 and MMC3 are always kept at a rated value of-620 MW, the power shortage of the fixed active power MMC2 and the MMC3 needs to be transmitted by the MMC1, so that the MMC1 needs to transmit power reversely, and reverse large impact is generated due to uncontrollable direct current, and the stability of a receiving end alternating current system is not facilitated. Meanwhile, as no better fault recovery strategy exists, the system generates larger fluctuation after the fault is cleared.
After the coordination control strategy of the present invention is adopted, the simulation result is shown in fig. 5 (b). After 2ms fault detection time after the fault occurs, the power instruction values of the fixed active MMC2 and MMC3 are switched to P'refThe active command value is rapidly reduced and adjusted in real time according to the power transmission change of the system. According to the simulation result, the fluctuation of the direct-current voltage of the MMC is small, the unbalanced current distribution of the MMC parallel group is effectively relieved, the fluctuation of direct-current of the MMC1 is restrained, the MMC1 is prevented from being switched into a rectifying state from an inversion state, and the phenomenon of reverse power cannot occur. Similarly, from table 4, it can be seen that the power fluctuation Δ P of MMC1 was 199.1% without cooperative control, whereas the power fluctuation Δ P was 32.7% with cooperative control, which was reduced by 166.4%. Meanwhile, the voltage fluctuation of an alternating current bus of an alternating current system AC2 connected with the MMC1 is small, the voltage fluctuation value is reduced by 20.5%, and the stability of a receiving end alternating current system is improved. After the fault is cleared, the instructions of the current converters are coordinated, and the system can be quickly and stably recovered to the steady-state operation.
Table 4: comparison of voltage and power fluctuation conditions of MMC1 receiving-end system
And (3) a third verification scheme: and in 4s, setting the overhead line to generate direct-current ground fault, wherein the fault lasts for 0.5s, delaying the trigger angle to shift to 150 degrees by the LCC at the rectification side after 2ms of fault detection time, removing the dissociation process by a 0.2s line after the fault is cleared, and starting the fault recovery restart of the system.
Fig. 6(a) shows the dc fault transient characteristics of the system. Because the inverter side of the system is formed by connecting the LCC and the half-bridge MMC in series, the direct current fault can be cleared by using the forced phase shift of the LCC, and the system has the direct current fault ride-through capability. When a fault occurs, the direct current voltage is rapidly reduced to 0, power transmission is interrupted, the MMC direct current voltage is reduced to 303kV, and the fault is continuously cleared. The power instruction values of the MMC2 and the MMC3 are kept at a rated value of-620 MW, the power shortage can be only conveyed by the MMC1, so that the MMC1 needs to be switched from an inversion state to a rectification state, a reverse power transmission phenomenon occurs, and an alternating current system AC connected with the MMC1 is caused2The fluctuation of the bus voltage is large, and the stability of a receiving end alternating current system is reduced.
After the coordinated control strategy of the present invention is adopted, the transient characteristics of the system are shown in fig. 6 (b). After 2ms fault detection time after the fault occurs, the power instruction values of the fixed active MMC2 and MMC3 are switched to P' refThe active command value is rapidly reduced and adjusted in real time according to the power transmission change of the system. As can be seen from simulation results, direct-current voltage of the MMC parallel group has small fluctuation and is rapidly recovered to a rated value, meanwhile, the phenomenon of unbalanced current distribution of the MMC is effectively relieved by the coordination control strategy, and the phenomenon of reverse power cannot occur to the fixed direct-current voltage MMC 1. Although the fluctuation of the power of the MMC1 and the voltage of the receiving-end alternating-current system is still large under the coordination control, the power and the voltage of the receiving-end alternating-current system can be quickly recovered to be close to the rated value, the continuous influence on the receiving-end system is reduced, the MMC1 cannot be in a rectification state for a long time to carry out back power transmission, and the stability of the system is improved. After the fault is cleared, the instructions of the converters are coordinated and matched, and the system can be quickly and stably recovered to the steady-state operation.
Through comparative analysis, the coordination control strategy of the invention can effectively solve the problem of unbalanced current distribution of the MMC parallel group during the fault period, avoid the change of inversion of the fixed direct-current voltage station into rectification, prevent the phenomenon of large-range power transfer of the alternating-current side of the receiving end and improve the stability of the receiving end system.
Claims (1)
1. A coordination control method for improving stability of a cascade type hybrid direct current transmission system is characterized in that for a receiving-end multi-drop cascade type hybrid direct current transmission system topological structure, a rectification side LCC is used for constant direct current control, an inversion side LCC is used for constant direct current voltage control, an MMC1 is used for constant direct current voltage control, an MMC2 and an MMC3 are used for constant active power control, reactive control of the two is realized by constant reactive power control, and an instruction value is set to be 0; the control method comprises the following steps:
S1: when the system normally operates, Ctrl is made 0, that is, the active power instruction value of the active MMC is set as the given transmission value Pref；
S2: after the system judges that a fault occurs, Ctrl is switched to 1 after S ms fault detection time, namely the active power instruction value of the active MMC is switched to P 'calculated in real time'ref：
P′ref＝0.5×(Pdci-Plcc-Psmmc1)
In the formula, Psmmc1A fixed value is transmitted for the power of the constant direct current voltage MMC1 in the steady-state operation; p isdciTransmitting a real-time measured value for the total power of the inversion side; plccTransmitting a measured value for the power of an LCC (lower control center) at an inversion side in a system in real time; p'refAccording to PdciAnd PlccMake changes, constrain P in the course of a faultsmmc1To approach a steady state operating value;
p 'calculated in real time'refThe method comprises the steps that through an amplitude limiting link, the capacity of a converter is prevented from being exceeded, meanwhile, the MMC power is adjusted very fast, and a slope limiter is arranged to improve the operation stability of a system;
and Ctrl is still kept as 1 during the system recovery after the fault is cleared;
s3: after the system is stably recovered to the steady state operation, Ctrl is switched to 0, that is, the active power instruction value of the active MMC is switched to the given transmission value PrefAnd the system is restored to the rated operation.
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