Novel power system-oriented conventional direct current online power flow reversal method

Abstract

The invention discloses a conventional direct-current online power flow reversal method for a novel power system, and relates to the technical field of power flow reversal. Comprising the following steps: constructing a conventional direct-current bipolar structure, dividing the change process of direct-current power, voltage and current parameters generated by the conventional direct-current bipolar structure into five stages, setting power parameters in different stages, setting constraint conditions, and performing conventional direct-current online power flow inversion by using the set parameters and the constraint conditions. According to the invention, dead-zone-free rapid power control can be realized, the conventional direct current down-regulation capability is fully exerted, and the flexible regulation potential of conventional direct current is greatly released.

Description

Technical Field

[0001] The invention relates to the technical field of power flow reversal, and in particular to a conventional direct current online power flow reversal method for a new type of power system. Background Art

[0002] my country has formed a grid structure of seven regional power grids with asynchronous interconnection, and DC transmission is the main body of large-scale resource optimization allocation. The asynchronous inter-regional DC transmission system must not only play the role of a firewall, but also achieve flexible cross-regional adjustment and frequency stability support through "flexible interconnection" to support the long-distance development and wide-area consumption of new energy resources.

[0003] Conventional DC occupies a dominant position in my country's DC Internet architecture, with a transmission capacity of 178.2GW, accounting for 80% of the total DC capacity. Traditionally, conventional DC is mainly positioned for one-way power transmission. However, with the access of a high proportion of new energy, conventional DC limited to one-way power transmission is difficult to meet the increasing flexible regulation needs of the power grid. Therefore, releasing the online power flow reversal technology capabilities of conventional DC systems is the key to achieving "flexible interconnection" of the power grid.

[0004] Traditional conventional DC has an overvoltage caused by discontinuous low DC current, which can destroy the stability of thyristors. There is an inherent minimum operating power limit of 10% of the rated power. The power flow reversal will inevitably be locked first and then unlocked in the reverse direction. The downward regulation capacity is limited and the operation time is long. Continuous reversal is not possible, which limits the flexibility and regulation potential of conventional DC transmission systems across asynchronous partitions.

[0005] Therefore, it is an urgent problem for those skilled in the art to propose a conventional DC online power flow reversal method for new power systems to solve the problems existing in the prior art. Summary of the invention

[0006] In view of this, the present invention provides a conventional DC online power flow reversal method for new power systems, which can achieve fast power control without dead zone, give full play to the conventional DC down-regulation capability, and greatly release the flexible regulation potential of conventional DC.

[0007] In order to achieve the above object, the present invention adopts the following technical solution:

[0008] A conventional DC online power flow reversal method for a new power system comprises the following steps:

[0009] S1. Construct a conventional DC bipolar structure;

[0010] S2, the DC power, voltage and current parameter change process generated by the conventional DC bipolar structure is divided into five stages;

[0011] S3. Adjust the power parameters at different stages and set constraints;

[0012] S4. Perform conventional DC online power flow reversal using the adjusted parameters and constraints.

[0013] In the above method, optionally, the structure in S1 includes a converter station A and a converter station B, where converter station A is a rectifier station and converter station B is an inverter station.

[0014] In the above method, optionally, the five stages in S2 include: bipolar forward operation, unipolar forward operation, power cycle operation, unipolar reverse operation and bipolar reverse operation.

[0015] In the above method, optionally, the parameter setting in S3 includes the switching power P of the unipolar mode MPi , Power cycle mode switching power P RPi and adjustment rate K dc Setting.

[0016] The above method, optionally, the unipolar mode switching power P MPi To ensure a smooth transition between bipolar operation and power cycle operation, and At the same time | MPi |Less than the upper limit of DC transmission power under the minimum filter group.

[0017] The above method, optionally, the power cycle mode switching power P RPi Determine the coverage interval of the power cycle mode to meet

[0018] The above method optionally adjusts the rate K dc By P MPi , P RPi and the lockout waiting time t w Jointly determined, the expression is:

[0019] K dc =(|P MPi |-|P RPi |) / t w .

[0020] It can be seen from the above technical solutions that, compared with the prior art, the present invention provides a conventional DC online power flow reversal method for new power systems, which has the following beneficial effects: 1) The novel online power flow reversal method based on circulating power of the present invention can realize fast power control without dead zones, and provides a larger adjustment space for the operation of conventional DC under various working conditions, which is the key to realizing the "flexible interconnection" of asynchronous partitioned power grids; 2) The present invention retains the fault isolation characteristics of asynchronous interconnection and realizes "quasi-synchronous operation" between partitions, which can support the operation of cross-partition electric energy spot and frequency regulation auxiliary service markets, and is of great significance to supporting the long-distance development and wide-area consumption of new energy resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0022] Figure 1 A flow chart of a conventional DC online power flow reversal method for a new type of power system disclosed in the present invention;

[0023] Figure 2 It is a schematic diagram of a conventional DC bipolar structure disclosed in the present invention;

[0024] Figure 3a It is a DC power variation diagram disclosed in the present invention;

[0025] Figure 3b It is a diagram of the DC current and voltage changes disclosed in the present invention;

[0026] Figure 4 It is a schematic diagram of the timing switching of each stage of the novel conventional DC power flow reversal disclosed by the present invention;

[0027] Figure 5 A diagram of a novel conventional DC power flow reversal simulation system disclosed in an embodiment of the present invention;

[0028] Figure 6a A DC power variation diagram of a novel conventional DC power flow reversal simulation disclosed in an embodiment of the present invention;

[0029] Figure 6b This is a DC current and voltage variation diagram of a novel conventional DC power flow reversal simulation disclosed by the present invention. DETAILED DESCRIPTION

[0030] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] In this application, relational terms such as first and second, etc. are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. The terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of more restrictions, the elements defined by the sentence "comprise one..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0032] Reference Figure 1 As shown, the present invention discloses a conventional DC online power flow reversal method for a new power system, comprising the following steps:

[0033] S1. Construct a conventional DC bipolar structure;

[0034] S2, the DC power, voltage and current parameter change process generated by the conventional DC bipolar structure is divided into five stages;

[0035] S3. Adjust the power parameters at different stages and set constraints;

[0036] S4. Perform conventional DC online power flow reversal using the adjusted parameters and constraints.

[0037] Further, see Figure 2 As shown, the structure in S1 includes a converter station A and a converter station B, where converter station A is a rectifier station and converter station B is an inverter station.

[0038] Specifically, P dc is the total bipolar transmission power; U dc,i ,I dc,i are the single-pole voltage and current respectively; i is the pole number. Let U d ,I d is the normal operating voltage and current of a single pole, with U dc1 =-U dc2 =U d , I dc1 =Idc2 =I d The bipolar power is in the positive direction from converter station A to converter station B. Converter station A adopts constant power control; converter station B adopts constant voltage control.

[0039] Further, see Figure 3a and Figure 3b As shown, the five stages in S2 include: bipolar forward operation, unipolar forward operation, power cycle operation, unipolar reverse operation and bipolar reverse operation.

[0040] Furthermore, Figure 3a and Figure 3b Middle: P dc1 and P dc2 are the power transmitted by pole 1 and pole 2 respectively; K dc is the preset power change rate; P MP1 and P MP2 are the unipolar mode switching power; P RP1 and P RP2 Switching power for power cycle mode respectively; This is the minimum unipolar operating power.

[0041] Further, see Figure 4 As shown, the bipolar forward operation is to ensure P through bipolar constant current control dc At the preset rate K dc decline.

[0042] Monopolar (MP) forward operation is when P dc Down to unipolar mode switching power P MP1 When the non-dominant pole 2 voltage and current are set to zero and then locked. At this time, the pole 1 current rises, and the increment is equal to the absolute value of the pole 2 current change, so as to transfer the pole 2 power to the pole 1 and maintain P dc Then, the current of pole 1 continues to decrease, leading to P dc Change, wait for pole 2 lock time t w .

[0043] The round power (RP) operation is when P dc > 0, the bipolar power drops to the power cycle mode switching power P RP1 >0, pole 2 is unlocked in reverse, and the voltage and current are increased to the rated value and the minimum value respectively, so that the operation is carried out at the minimum single-pole operation power. During this phase, the constant current control of the pole 1 continues to maintain P dc Downward trend until P dc = 0. After that, the dominant pole is switched to pole 2, and the total power is increased in the reverse direction through constant current control.

[0044] Unipolar reverse operation is when P dc Reverse to reach power cycle mode switching power P RP2 After that, the voltage and current of pole 1 are set to zero and enter the locked state. After that, pole 2 maintains P through constant current control. dc Reverse growth trend.

[0045] Bipolar reverse operation is when P dc =P MP2 When P1 is unlocked, the voltage and current are increased to the rated value and the minimum value respectively. After that, the two poles increase the power synchronously until P dc =-P d .

[0046] Furthermore, the parameter setting in S3 includes the switching power P MPi , Power cycle mode switching power P RPi and adjustment rate K dc Setting.

[0047] Furthermore, the unipolar mode switching power P MPi To ensure a smooth transition between bipolar operation and power cycle operation, and At the same time | MPi |Less than the upper limit of DC transmission power under the minimum filter group.

[0048] Specifically, the unipolar mode corresponds to the unipolar operation of the system. First, the process of switching power from power drop to power cycle mode should completely cover the lockout waiting time; secondly, the DC down-regulation capacity should meet the DC frequency regulation standby requirements. When the DC operates in the low power area in the A→B direction, it is easy to be restricted in down-regulation. Therefore, it is necessary to enter the unipolar operation in time to avoid low-frequency load reduction in the sending-end synchronous network A when the large unit trips; for conventional DC with fewer AC filters, the filter input and output during the reversal period should be minimized.

[0049] Furthermore, the power cycle mode switching power P RPi Determine the coverage interval of the power cycle mode to meet

[0050] Specifically, since the power of the two poles is opposite in this stage, part of the power forms inter-pole circulating current loss, and the bipolar net power makes the DC equivalent operation within the minimum power range. In order to reduce the loss, P RPi Should be as small as possible, the power cycle process takes time t RP t RP =|P RPi | / K dc ,It should be pointed out that due to the opposite power directions of the two poles, both are in low power operation state, and bidirectional ,frequency regulation standby is not restricted.

[0051] Furthermore, the adjustment rate K dc By P MPi , P RPi and the lockout waiting time t w Jointly determined, the expression is:

[0052] K dc =(|P MPi |-|P RPi |) / t w .

[0053] Specifically, the adjustment rate K dc It should meet the requirements for stable operation under various operating conditions, especially avoiding excessive values ​​that affect the frequency control quality on both sides.

[0054] Furthermore, the constraints set in S3 include the problem of setting the blocking waiting time, the problem of reactive power adaptation and the problem of control system modification.

[0055] Furthermore, in the problem of setting the lockout waiting time, since oil-paper insulated equipment such as converter transformers and wall bushings have critical field strength problems during polarity reversal, the deionization time from pole locking to pole reverse unlocking must take into account the equipment's tolerance. According to the relevant provisions in the current IEC and GB standards, polarity reversal tests are carried out during type tests of oil-paper insulated equipment. The test voltage is much higher than the rated voltage during normal operation, and the reversal process is completed within 2 minutes. Further combined with factors such as the equipment's service life, filter discharge time, and regulation rate, t w It should be slightly longer than 2 minutes.

[0056] In the reactive power adaptation problem, since the conventional DC is positioned for unidirectional power transmission in the initial design, the inverter side ( Figure 2 The reactive power consumption of converter station B) is greater than that of the rectifier side ( Figure 2 Converter station A), the number of filter groups configured in the former is greater than that in the latter, and the switching thresholds of the two are also significantly different. The reactive capacity configuration and switching logic need to be updated to ensure voltage and power stability during the power flow reversal process.

[0057] In the control system modification problem, the existing conventional DC pole control in China requires the bipolar power to be set at the same time and in the same direction, while the power circulation mode requires unipolar control. The DC station control distributes the bipolar power according to the idea of ​​power averaging, but the DC power change during the power circulation process is borne by the dominant pole. Therefore, the power adjustment and distribution logic of the pole control and station control need to be adaptively modified.

[0058] In one embodiment, the present application is compared with the traditional offline power flow reversal time. Table 1 shows the comparison between online power flow reversal and traditional offline power flow reversal in long-distance overhead line DC and back-to-back DC. In traditional power flow reversal, DC needs to be shut down and then power reversal is carried out. After the bipolar power is set to reverse, the DC is unlocked. This method requires the bipolar operation to cold standby, reset the power direction, and then operate the bipolar to the unlocked state, which actually takes about 1-2 hours.

[0059] Table 1 Comparison between online power flow reversal and traditional offline power flow reversal

[0060]

[0061]

[0062] In contrast, this application only needs to carry out power reversal in the single-pole locked state, without the need to operate the single pole to cold standby, and only needs to consider the equipment's tolerance. According to the above content, the equipment polarity reversal time can be controlled within 2 minutes. Therefore, in actual engineering applications, the reversal time can be set to more than 2 minutes, for example, about 5 minutes, so that online power flow reversal can be achieved while ensuring equipment safety.

[0063] If online power flow reversal is applied to long-distance overhead line DC and back-to-back DC systems, the impact of DC line length on reversal time needs to be considered. For overhead lines, DC lines need to undergo a de-ionization time after blocking, which is generally around 500ms. For back-to-back DC, the DC line length can be considered to be 0, and no de-ionization time is required. Therefore, the de-ionization time of the line is not a dominant factor. It can be considered that during the online power flow reversal process, the time consumption of power flow reversal has nothing to do with the line, but is only related to the complexity of the wiring of the converter station and the polarity reversal time of the equipment. Therefore, there is almost no difference between power flow reversal of long-distance overhead lines and back-to-back DC.

[0064] In another embodiment, referring to Figure 5 As shown in Figure 2, an asynchronous interconnected electromagnetic transient simulation benchmark system is set up. The two areas are connected via a conventional DC line of ±500kV / 1000MW. The basic parameters of the system are set as shown in Table 2. In order to analyze the reversal process at the electromagnetic transient scale, the experiment t w , K dc The smaller and larger values ​​were taken respectively.

[0065] Table 2 Parameter settings of conventional DC power flow reversal simulation system

[0066]

[0067] Figure 6a and Figure 6bThe power, voltage and current changes at each stage of the online power flow reversal of DC power from 1000MW to -1000MW are given, where the voltage and current are per unit values ​​based on the bipolar operation rated value:

[0068] 1) Bipolar forward operation. The initial bipolar voltage and current are both running at rated values. At 0s, the bipolar current and power maintain the same downward trend.

[0069] 2) Unipolar forward operation. At 1.5s, the current of both poles dropped to about 0.31pu, and the bipolar power reached the unipolar mode switching threshold of 625MW, entering the unipolar operation mode. The current of pole 2 dropped to 0 instantly, and the voltage dropped to 0 after about 0.2s, and then pole 2 was locked. At the same time, the current of pole 1 suddenly increased to about 0.625pu, and as the dominant pole, the bipolar power remained unchanged, and then the current decreased at a rate of 0.25pu / s.

[0070] 3) Power cycle operation. At 2s, the current of pole 1 is 0.5pu, and the monopolar power reaches 500MW. Pole 2 is unlocked in reverse, and the current reaches the preset minimum value of 0.125pu after about 0.16s, and the voltage stabilizes at the reverse rated value after 0.25s. In this process, the current growth trend of pole 1 is consistent with that of pole 2, until it reaches 0.625pu, causing the DC system to enter the power cycle mode. Pole 1 still maintains the bipolar net power downward adjustment until the current reaches the minimum value of 0.125pu at 5.1s, and the bipolar net power is 0. At this time, the dominant pole is switched from pole 1 to pole 2, the bipolar net power is maintained by pole 2, and the current begins to increase.

[0071] 4) Unipolar reverse operation. At 7s, the current of pole 2 rises to 0.625pu, and the bipolar net power reaches the power threshold of -500MW for exiting the power mode. Similarly, pole 1 is locked with similar voltage and current changes, and the current of pole 2 quickly drops to 0.5pu, entering unipolar operation, and the current continues to increase.

[0072] 5) Bipolar reverse operation. At 7.6s, the current of pole 2 is 0.625pu, and the bipolar power reaches the power threshold of -625MW for exiting the unipolar mode. In the process of adjusting the voltage to the reverse rated value, the current of pole 1 suddenly increases to 0.125pu, and the DC enters bipolar operation after reverse unlocking. The bipolar current and power growth remain consistent until the power reaches the reverse preset value of -1000MW at 8.1s, and the power flow reversal ends.

[0073] From the above simulation results and analysis, it can be found that by using the new power flow reversal method, the conventional DC system can break through the minimum operating power limit and achieve smooth control within the full power range, solving the system shutdown problem caused by the low power limit of the traditional power flow reversal method. Compared with the traditional method, the new power flow reversal method based on power circulation is suitable for online power flow reversal, which greatly taps the flexible regulation capability of conventional DC.

[0074] Each embodiment in this specification is described in a progressive manner, and the same or similar parts between the embodiments can refer to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the system or system embodiment, since it is basically similar to the method embodiment, the description is relatively simple, and the relevant parts can refer to the partial description of the method embodiment. The system and system embodiments described above are merely schematic, wherein the units described as separate components may or may not be physically separated, and the components displayed as units may or may not be physical units, that is, they may be located in one place, or they may be distributed on multiple network units. Some or all of the modules may be selected according to actual needs to achieve the purpose of the scheme of this embodiment. Ordinary technicians in this field can understand and implement it without creative work.

[0075] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A conventional DC online power flow reversal method for a new power system, characterized in that: The following steps are involved: S1. Construct a conventional DC bipolar structure; S2, the DC power, voltage and current parameter change process generated by the conventional DC bipolar structure is divided into five stages; S3. Adjust the power parameters at different stages and set constraints; S4. Perform conventional DC online power flow reversal using the adjusted parameters and constraints.

2. A conventional DC online power flow reversal method for a new power system according to claim 1, characterized in that: The structure of S1 includes converter station A and converter station B. Converter station A is a rectifier station, and converter station B is an inverter station.

3. A conventional DC online power flow reversal method for a new power system according to claim 1, characterized in that: The five stages in S2 include: bipolar forward operation, unipolar forward operation, power cycle operation, unipolar reverse operation and bipolar reverse operation.

4. A conventional DC online power flow reversal method for a new power system according to claim 1, characterized in that: The parameter setting in S3 includes the switching power P MPi , Power cycle mode switching power P RPi and adjustment rate K dc Setting.

5. A conventional DC online power flow reversal method for a new power system according to claim 4, characterized in that: Unipolar mode switching power P MPi To ensure a smooth transition between bipolar operation and power cycle operation, and At the same time | MPi |Less than the upper limit of DC transmission power under the minimum filter group.

6. A conventional DC online power flow reversal method for a new power system according to claim 4, characterized in that: Power cycle mode switching power P RPi Determine the coverage interval of the power cycle mode to meet 7. A conventional DC online power flow reversal method for a new power system according to claim 4, characterized in that: Adjustment rate K dc By P MPi , P RPi and the lockout waiting time t w Jointly determined, the expression is: K dc＝ ( P MPi |-| P RPi |) / t w 。

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