CN111769583B - A Coordinated Control Method for Improving the Stability of Cascaded Hybrid DC Transmission System - Google Patents

Abstract

The invention discloses a coordinated control method for improving the stability of a cascading hybrid direct current transmission system. In the event of a fault, according to the line transmission power, the output power of the inverter side LCC and the steady state output power of the constant direct current voltage MMC, the control method of the multi-drop MMC is The active power command value is regulated to prevent the constant DC voltage station from changing from inverter to rectifier, and to prevent the occurrence of large-scale transfer of power on the AC side of the receiving end. At the same time, after the fault is cleared, the problem of large fluctuations in the system recovery process can still be alleviated, so that the system can quickly and smoothly recover to the rated operation state. The present invention can realize the smooth transition of the system under both AC and DC typical faults, and improve the stability of the DC receiving end system.

Description

A Coordinated Control Method for Improving the Stability of Cascaded Hybrid DC Transmission System

technical field

The invention relates to the technical field of direct current transmission, in particular to a coordinated control method for improving the stability of a cascaded hybrid direct current transmission system.

Background technique

For rectification using grid commutation converter LCC, inverter side using LCC and modular multi-level converter MMC in series with the receiving-end multi-drop cascade hybrid DC transmission system, due to the limitation of the manufacture of power devices level, it is usually necessary to use multiple MMCs in parallel to match the transmission capacity of the LCC, and to improve the transmission capacity of the entire system. The system topology is shown in Figure 1. The existence of the LCC and MMC parallel group enables the receiving end to have the conditions for decentralized access to the AC power grid, and can form a multi-drop form that meets the electricity demand of multi-load centers. At the same time, because the control of the MMC is very flexible, each MMC has different control methods and has a combination of various control methods. When the control methods of each MMC are different, the problem of unbalanced current distribution may occur, and in severe cases, it will lead to the phenomenon of power reverse transmission, which reduces the stability of the AC system at the receiving end. Therefore, it is of great significance to study the coordinated control strategy of the hybrid cascaded system for various possible faults.

Combined with the volt-ampere characteristics of the cascaded hybrid DC transmission system in Figure 2, the following theoretical analysis is carried out for common AC ground faults and DC faults that may occur in the system: When an AC ground fault occurs on the rectifier side, the rectifier side LCC switches to a fixed firing angle Control, the inverter side LCC is switched to constant DC current control or low voltage current limiting control, the system operating point moves to the left according to the inverter side current value, and the transmission power of the rectifier side and the inverter side is reduced; When the ground fault occurs, the inverter side LCC fails to commutate, and its control mode is switched to constant off-angle control, and the rectifier side LCC is switched to constant DC current control or low-voltage current limiting control, and the system operating point moves to the left according to the current value of the rectifier side. , the system transmission power is reduced; when a DC fault occurs, due to the single-phase conductivity of the LCC, there is no fault current, and the line DC voltage is 0, the system power transmission is interrupted. Therefore, the analysis shows that under these common faults, the transmission of line power will be reduced or interrupted. The next step is to analyze the fault on the rectifier side as an example. When a fault occurs, the system operating point moves to point M. It can be seen from Figure 2 that P _MMCB (that is, the power of the MMC parallel group) = P _mmc1 +P _mmc2 +P _mmc3 , and P _MMCB decreases. If the command value of the active MMC power is kept at a steady state, the command value remains unchanged (P _mmc2 and P _mmc3 remain unchanged), the reduction of MMCB (ie MMC parallel group) transmission power can only be balanced by the constant DC voltage MMC1 absorbing power, so MMC1 is changed from inverter to rectifier, and starts to absorb active power. The reverse transmission power phenomenon occurs in the system, and the stability of the AC system at the receiving end is greatly reduced.

Measure 1: Terminal wiring and control method of hybrid cascaded DC transmission system (Xu Zheng, Wang Shijia, Zhang Zheren, Xu Yuzhe, Xiao Huangqing. LCC-MMC hybrid cascade DC transmission system receiving terminal wiring and control method [J]. Electric Power Construction , 2018, 39(07): 115-122.), but this measure only studies the steady-state and fault characteristics of the hybrid cascaded DC transmission system with different terminal wiring and control methods, and does not propose the fault response characteristics. Coordinate control strategies.

Measure 2: DC Fault Recovery Control Strategy of Hybrid Cascade DC Transmission System (Yang Shuo, Zheng Anran, Peng Yi, Guo Chunyi, Zhao Chengyong. DC Fault Characteristics and Recovery Control Strategy of Hybrid Cascade DC Transmission System [J]. Electric Power Automation Device, 2019, 39(09):166-172+179.). This measure proposes a recovery control strategy for the hybrid cascaded DC transmission system during and after the DC fault is cleared, which alleviates the overcurrent phenomenon during the fault, but this control strategy is only for DC faults.

SUMMARY OF THE INVENTION

In view of the above problems, the purpose of the present invention is to provide a coordinated control strategy for improving the stability of the multi-drop cascaded hybrid DC transmission system at the receiving end, which can avoid changing the constant DC voltage station from inverter to rectification, and prevent the AC side of the receiving end. The phenomenon of large-scale power transfer occurs, and the system can achieve a smooth transition under typical AC and DC faults, improving the stability of the DC receiving-end system. Smooth return to rated operation. The technical solution is as follows:

A coordinated control method for improving the stability of a cascaded hybrid direct current transmission system, comprising the following steps:

S1: When the system is running normally, Ctrl=0, that is, the active power command value of the fixed active MMC is its given transmission value _Pref ;

S2: After the system determines that a fault has occurred, after the fault detection time of S ms, switch Ctrl=1, that is, the active power command value of the fixed active MMC is switched to the real-time calculated P′ _ref :

P' _ref =0.5×(P _dci -P _lcc -P _smmc1 )

In the formula, P _smmc1 is the constant value of power transmission during steady-state operation of constant DC voltage MMC1; P _dci is the real-time measurement value of the total power transmission at the inverter side; P _lcc is the real-time transmission measurement value of the LCC at the inverter side in the system; P′ _ref changes according to P _dci and P _lcc to constrain the fluctuation of P _smmc1 during the fault process, making it close to the steady-state operating value;

The _P'ref calculated in real time is passed through the limiter link to avoid exceeding the converter capacity, and considering that the MMC power adjustment is extremely fast, a slope limiter is set to improve the system operation stability;

And after the fault is cleared, Ctrl=1 is still maintained during system recovery;

S3: After the system returns to steady state operation, switch Ctrl=0, that is, the active power command value of the fixed active MMC is switched to the given transmission value P _ref , and the system returns to rated operation.

The beneficial effects of the present invention are: when the receiving end MMC adopts a combination of different control modes, the present invention can prevent the constant DC voltage station from changing from inverter to rectification, and prevent the occurrence of large-scale power transfer on the AC side of the receiving end. In the event of a fault, the system can achieve a smooth transition and improve the stability of the DC receiving-end system. At the same time, after the fault is cleared, the problem of large fluctuations in the system recovery process can still be alleviated, so that the system can quickly and smoothly return to the rated operating state. The control strategy is simple and easy to implement, which is beneficial to engineering practice.

Description of drawings

Figure 1 shows the topology of the multi-drop cascaded hybrid DC transmission system at the receiving end.

Figure 2 shows the volt-ampere characteristics of the cascaded hybrid DC transmission system.

Fig. 3 is a constant active MMC coordinated control strategy diagram.

Figure 4 shows the system response characteristics under the short-circuit fault of the rectifier side; (a) without coordinated control; (b) with coordinated control.

Figure 5 shows the system response characteristics under the short-circuit fault on the inverter side; (a) without coordinated control; (b) with coordinated control.

Figure 6 shows the response characteristics of the system under DC fault; (a) without coordinated control; (b) with coordinated control.

Detailed ways

The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The system coordination control strategy is shown in Figure 3:

According to the power balance formula on the inverter side (ignoring converter losses):

P _dci =P _lcc +P _mmc1 +P _mmc2 +P _mmc3

The active power command value can be set at fault:

P' _ref =0.5×(P _dci -P _lcc -P _smmc1 )

Among them, P _smmc1 is the power transmission value of the constant DC voltage MMC1 during steady-state operation; P′ _ref is equivalent to adjusting only according to the changes of P _dci and P _lcc , and the power change of the MMC parallel group is borne by the constant active power MMC, which restricts the The fluctuation of P _smmc1 (that is, the power of MMC1) during the fault process can make it close to the steady-state operating value to the greatest extent, and the phenomenon of power reversal will not occur. Therefore, during the fault period, the coordinated control strategy can prevent the constant DC voltage station from changing from inverter to rectifier, prevent the phenomenon of large-scale transfer of AC side power at the receiving end, and improve the stability of the receiving end system. At the same time, after the fault is cleared, it is beneficial to the system Quick and smooth return to steady state operation.

The coordinated control strategy scheme is:

1) During normal operation of the system, Ctrl=0, the active power command value of the fixed active MMC is a given transmission value _Pref ;

2) After the system determines that a fault occurs, after 2ms of fault detection time, switch Ctrl=1, and the active power command value of the fixed active MMC is switched to P' _ref calculated in real time, and after the fault is cleared, the system still maintains Ctrl=1 during recovery;

3) After the system returns to steady state operation, switch Ctrl=0.

Example: The cascading hybrid DC transmission system shown in Figure 1 is built as an example for verification, and its main parameters are shown in Table 1.

Table 1: Main Parameters of Cascade Hybrid DC Transmission System

The rectifier side LCC is controlled by constant DC current, and the inverter side LCC is controlled by constant DC voltage. The MMC control mode is shown in Table 2. MMC1 is constant DC voltage control, MMC2 and MMC3 are constant active power control, and their reactive power control adopts constant reactive power control, and the command value is set to 0.

Table 2: MMC Control Mode

Verification scheme 1: set a single-phase ground fault on the AC bus of the rectifier side, the fault starts from 4s and lasts for 0.5s

When the coordinated control strategy is not adopted, as shown in Figure 4(a) below, at the moment of 4s, the rectifier side fails, the AC bus voltage at the sending end drops, and the DC voltage of the line drops. Active power drops. The transmission power of the LCC on the inverter side is reduced, and the transmission power of the fixed DC voltage MMC is reduced due to the reduction of the transmission power at the sending end, and the power at both ends is unbalanced, resulting in a reduction in the DC voltage and fluctuations. The fixed active MMC power command value has been kept at the rated value of -620MW, and its DC current fluctuates. The power shortage needs to be transmitted by the fixed DC voltage MMC. Therefore, MMC1 switches from the inverter state to the rectifier state, and absorbs power from the AC system. The phenomenon of power transmission to the direction causes the bus voltage of the AC system AC ₂ connected to the MMC1 to fluctuate greatly, which reduces the stability of the AC system at the receiving end.

Figure 4(b) shows the simulation results after adopting the coordinated control strategy proposed in this paper. After the fault detection time of 2ms, the power command value of the fixed active MMC2 and MMC3 is switched to P' _ref , the output active power decreases rapidly, and is adjusted in real time according to the change of the system power transmission, which greatly restricts the power fluctuation of the fixed DC voltage MMC1 , so that its output active power is close to the rated operating value. It can be seen from the simulation diagram that the fluctuation of the DC voltage of MMC is small, and the unbalanced current distribution of the MMC parallel group is effectively alleviated, the fluctuation of the DC current of MMC1 is suppressed, and the phenomenon of power reverse transmission does not occur. Table 3 shows the comparison of the AC voltage and active power fluctuations of the system. It can be seen that the MMC1 power fluctuation ΔP is 180.9% without coordinated control, while the ΔP is 32.3% with coordinated control, and the power fluctuation is reduced by 151.6%. At the same time, the AC bus voltage fluctuation of the AC system AC2 connected to MMC1 is greatly reduced, and the voltage fluctuation value is reduced by 19.2%. Although the voltage and power of the AC system connected to MMC2 and MMC3 fluctuate to a certain extent, the fluctuation range is small, so The stability of the receiving side AC system is improved. After the fault is cleared, the system can also return to steady-state operation quickly and smoothly.

Table 3: Comparison of voltage and power fluctuations of MMC1 receiving system

Verification scheme 2: A single-phase ground fault is set on the BUS-1 AC bus on the inverter side, and the LCC on the inverter side fails to commutate. The fault starts from 4s and lasts for 0.5s.

The simulation result without the coordinated control strategy is shown in Fig. 5(a). Due to the failure of the inverter side LCC commutation, the power cannot be transmitted, the active power at both ends of the constant DC voltage MMC1 is unbalanced, and the DC voltage rises, resulting in large fluctuations. However, the power command value of active MMC2 and MMC3 has been maintained at the rated value of -620MW, and the power shortage needs to be transmitted by MMC1, so MMC1 needs to transmit power in reverse, and the uncontrollable DC current produces a large reverse shock, which is not conducive to The stability of the receiving end AC system. At the same time, because there is no better fault recovery strategy, the system fluctuates greatly after the fault is cleared.

After adopting the coordinated control strategy of the present invention, the simulation result is shown in Fig. 5(b). After the fault occurs, after the fault detection time of 2ms, the power command value of the fixed active MMC2 and MMC3 is switched to _P'ref , the active power command value decreases rapidly, and is adjusted in real time according to the system power transmission change. It can be seen from the simulation results that the fluctuation of the MMC DC voltage is small, which effectively alleviates the unbalanced current distribution of the MMC parallel group, suppresses the fluctuation of the DC current of the MMC1, and prevents the MMC1 from switching from the inverter state to the rectifier state, and there will be no reverse. transmission power phenomenon. Similarly, it can be seen from Table 4 that the power fluctuation ΔP of MMC1 is 199.1% without coordinated control, while ΔP is 32.7% with coordinated control, and the power fluctuation is reduced by 166.4%. At the same time, the AC bus voltage of the AC system AC2 connected to the MMC1 is small, and the voltage fluctuation value is reduced by 20.5%, which improves the stability of the AC system at the receiving end. After the fault is cleared, the inverter commands are coordinated and the system can quickly and smoothly return to steady-state operation.

Table 4: Comparison of voltage and power fluctuations of MMC1 receiver system

Verification scheme 3: At 4s, set the DC ground fault on the overhead line, the fault lasts for 0.5s, after the 2ms fault detection time, the LCC delay trigger angle of the rectifier side is phase-shifted to 150°, and after the fault is cleared, the line goes through the 0.2s deionization process, The system begins a failback restart.

Figure 6(a) shows the DC fault transient characteristics of the system. Since the inverter side of the system is composed of LCC and half-bridge MMC in series, the forced phase shift of the LCC can be used to clear the DC fault, so it has the DC fault ride-through capability. When the fault occurs, the DC voltage rapidly drops to 0, the power transmission is interrupted, and the MMC DC voltage drops to 303kV and continues until the fault is cleared. The power command value of MMC2 and MMC3 is maintained at the rated value of -620MW, and the power shortage can only be transmitted by MMC1. Therefore, MMC1 needs to switch from the inverter state to the rectifier state, and reverse power transmission occurs, causing the AC system connected to MMC1. The bus voltage of ₂ fluctuates greatly, which reduces the stability of the AC system at the receiving end.

After adopting the coordinated control strategy of the present invention, the transient characteristics of the system are shown in Fig. 6(b). After the fault occurs, after the fault detection time of 2ms, the power command value of the fixed active MMC2 and MMC3 is switched to _P'ref , the active power command value decreases rapidly, and is adjusted in real time according to the system power transmission change. It can be seen from the simulation results that the DC voltage fluctuation of the MMC parallel group is small, and quickly recovers to the rated value. At the same time, the coordinated control strategy effectively alleviates the current distribution imbalance of the MMC, and the constant DC voltage MMC1 does not appear to reverse the power phenomenon. . Although the power of MMC1 and the voltage of the receiving-end AC system still fluctuate greatly under the coordinated control, they soon recover to near the rated value, reducing the continuous impact on the receiving-end system, and the MMC1 will not be in the rectification state for a long time. The power is sent back, which improves the stability of the system. After the fault is cleared, the inverter commands are coordinated and the system can quickly and smoothly return to steady-state operation.

Through comparative analysis, it can be seen that the coordinated control strategy of the present invention can effectively solve the problem of unbalanced current distribution of the MMC parallel group during the fault period, avoid changing the constant DC voltage station from inverter to rectifier, and prevent the phenomenon of large-scale transfer of AC side power at the receiving end. , which improves the stability of the receiver system.

Claims (1)

1. A coordinated control method for improving the stability of a cascaded hybrid direct current transmission system, characterized in that, for the receiving end multi-drop cascade hybrid direct current transmission system topology structure, the rectifier side LCC is controlled by a constant direct current, and the inverter is controlled by a constant direct current. The side LCC is controlled by constant DC voltage, MMC1 is controlled by constant DC voltage, MMC2 and MMC3 are controlled by constant active power, and their reactive power control adopts constant reactive power control, and the command value is set to 0; the control method includes the following steps: S1: When the system is running normally, let Ctrl=0, that is, the active power command value of the active MMC is set to its given transmission value _Pref ; S2: After the system determines that a fault has occurred, after the fault detection time of S ms, switch Ctrl=1, that is, the active power command value of the fixed active MMC is switched to the real-time calculated P′ _ref : P' _ref =0.5×(P _dci -P _lcc -P _smmc1 ) In the formula, P _smmc1 is the constant value of power transmission during steady-state operation of constant DC voltage MMC1; P _dci is the real-time measurement value of the total power transmission at the inverter side; P _lcc is the real-time transmission measurement value of the LCC at the inverter side in the system; P′ _ref changes according to P _dci and P _lcc to constrain the fluctuation of P _smmc1 during the fault process, making it close to the steady-state operating value; The _P'ref calculated in real time is passed through the limiter link to avoid exceeding the converter capacity, and considering that the MMC power adjustment is extremely fast, a slope limiter is set to improve the system operation stability; And after the fault is cleared, Ctrl=1 is still maintained during system recovery; S3: After the system returns to steady state operation, switch Ctrl=0, that is, the active power command value of the fixed active MMC is switched to the given transmission value P _ref , and the system returns to rated operation.

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