Method for evaluating applicability of power grid strength index in continuous commutation failure

Abstract

The invention relates to a method for evaluating the applicability of a power grid strength index in continuous commutation failure. The method comprises the following steps: firstly, constructing a synchronous generator model, a direct current transmission model based on a power grid commutation converter, a load model and a common connection point power balance equation; calculating the bus voltage at the fault clearing moment according to the constructed model; judging whether continuous commutation failure occurs in the direct current power transmission system or not according to the bus voltage at the fault clearing moment; calculating a power grid strength index, and analyzing the relationship between the power grid strength index and the bus voltage at the fault clearing moment; the power grid strength index is evaluated according to the relation between the power grid strength index and the bus voltage at the fault clearing moment, the applicability of the continuous commutation failure is judged, reference can be provided for planning operation of the alternating-current and direct-current power grid, and the continuous commutation failure problem in actual high-voltage direct-current engineering can be avoided.

Description

Technical Field

[0001] The invention relates to a method for evaluating the applicability of a power grid strength index in continuous commutation failure, and belongs to the technical field of power systems. Background Art

[0002] At present, HVDC (high voltage direct current transmission) systems have been widely used in long-distance, large-capacity power transmission projects. HVDC systems have the advantages of fast power regulation, low loss, and small footprint. At present, most HVDC systems use grid-commutated converters (LCCs), so AC faults in the receiving grid are prone to commutation failures. Commutation failures will cause power transmission interruptions and threaten the stability of the AC system. In large-capacity HVDC systems, continuous commutation failures (CCF) will lead to long-term power surpluses in the sending grid and long-term power shortages in the receiving grid, which may lead to chain reactions such as unit shutdowns and load shedding. Since CCF will have a huge impact on the AC system, it is very important to assess the possibility of CCF.

[0003] According to the mechanism of CCF in HVDC system, scholars have proposed various CCF evaluation indicators and prediction methods. Among them, evaluating CCF with grid strength indicators is an important research direction. Grid strength is one of the basic concepts of power system. It is usually used to measure the strength of the interaction between the grid itself and the primary equipment connected to the grid, reflecting the influence of the bus voltage amplitude and frequency on the connected equipment. In previous studies, many different indicators were used to evaluate the grid strength of AC transmission systems, including short-circuit ratio, voltage stiffness, multi-infeed short-circuit ratio, etc. However, there are currently no tools and methods to analyze the applicability of these grid strength indicators in evaluating CCF, resulting in uncertainty in the accuracy of using these grid strength indicators to evaluate CCF. Summary of the invention

[0004] In order to solve the problems existing in the prior art, the present invention provides a method for evaluating whether a power grid strength index is suitable for determining continuous commutation failure.

[0005] In order to achieve the above object, the technical solution proposed by the present invention is: a method for evaluating the applicability of a power grid strength index in continuous commutation failure, specifically:

[0006] Construct synchronous generator model, DC transmission model based on grid-commutated converter and load model, and establish power balance equation at common connection point;

[0007] The synchronous generator model includes the output active power, reactive power and transient electromotive force of the synchronous generator; the DC transmission model based on the grid-commutated converter includes the active power, reactive power and DC current of the grid-commutated converter; the load model includes the active power and reactive power of the load;

[0008] The bus voltage at the time of fault clearing is calculated based on the synchronous generator model, the DC transmission model based on the grid-commutated converter, the load model and the power balance equation of the common connection point;

[0009] Judging whether the DC power transmission system will have continuous commutation failure based on the bus voltage at the time of fault clearing;

[0010] Calculate the grid strength index and establish the relationship between the grid strength index and the bus voltage at the time of fault clearing;

[0011] According to the relationship between the grid strength index and the bus voltage at the moment of fault clearing, the applicability of the grid strength index to judge the continuous commutation failure is evaluated.

[0012] A further design of the above technical solution is that the output active power and reactive power of the synchronous generator are respectively:

[0013]

[0014] Among them, P SG and Q SG Respectively represent the output active power and reactive power of the synchronous generator; Z and They represent the amplitude and phase angle of the equivalent impedance of the AC system respectively; U is the voltage amplitude of the AC bus; δ is the voltage phase angle of the AC bus; E ′ q is the transient electromotive force of the synchronous generator.

[0015] The transient electromotive force differential equation is (3).

[0016]

[0017] Among them, T ′ d is the transient time constant of the synchronous generator; E fd is the excitation electromotive force of the synchronous generator; x d is the d-axis synchronous reactance of the synchronous generator; x ′ d is the d-axis transient reactance of the synchronous generator; i dg is the d-axis current of the synchronous generator.

[0018] The active power, reactive power and DC current of the DC system grid commutation converter are respectively:

[0019] P LCC =CU 2 [cos2γ-cos(2γ+2μ)]

[0020] Q LCC =CU2 [2μ+sin2γ-sin(2γ+2μ)]

[0021] I dc =KU[cosγ-cos(γ+μ)](6)

[0022]

[0023] Among them, P LCC and Q LCC Respectively represent the active power and reactive power of LCC; P dN is the rated active power of LCC; I dc is the DC current; γ is the LCC turn-off advance angle; μ is the LCC commutation overlap angle; S T is the capacity of the converter transformer; u k1 is the short-circuit ratio of the converter transformer; τ is the transformation ratio of the converter transformer; Q c It is the reactive power output by the compensation capacitor.

[0024] The reactive power output by the compensation capacitor is:

[0025] Q c =B c U 2

[0026] Among them, B c is the admittance of the compensation capacitor.

[0027] The load model adopts a constant impedance model.

[0028] The expressions of the active power and reactive power of the load are:

[0029] P load =U 2 G load

[0030] Q load =U 2 B load

[0031] Among them, P load and Q load Respectively represent the active power and reactive power of the load; G load is the conductivity of the load; B load is the susceptance of the load.

[0032] The power balance equation at the common connection point is:

[0033] P LCC +P SG -P load =0

[0034] Q SG +Q c -Q LCC -Q load =0.

[0035] The calculation method of the bus voltage at the time of fault clearing is:

[0036] The transient electromotive force when a fault occurs is calculated as:

[0037]

[0038] Among them, V Rmax is the maximum output limit of the excitation system;

[0039] Assume that the active power of the grid commutation converter is zero when the fault is cleared, and the DC current I dc is constant; thereby, the bus voltage at the moment of fault clearing is calculated by combining the synchronous generator model, the DC transmission model based on the grid-commutated converter, the load model and the power balance equation of the common connection point.

[0040] The beneficial effects of the present invention are:

[0041] The present invention constructs a DC transmission system model, and calculates the bus voltage at the time of fault clearing in combination with the special parameter value at the time of fault clearing, and judges whether continuous commutation failure will occur according to the bus voltage, and thus judges whether the power grid strength index is suitable for judging continuous commutation failure; the method for evaluating applicability proposed in the present invention can effectively evaluate the applicability of the power grid strength index when evaluating CCF, and can provide a reference for the planning and operation of AC and DC power grids, which is conducive to avoiding the problem of continuous commutation failure in actual high-voltage DC projects. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 This is the structural diagram of a single-infeed high-voltage direct current transmission system.

[0043] Figure 2 is the relationship between bus voltage and load active power.

[0044] Figure 3 is the relationship between bus voltage and SCR.

[0045] Figure 4 It is the grid structure near the high voltage direct current transmission project.

[0046] Figure 5 This is the simulation result of PCC short-circuit fault in Baihetan HVDC system.

[0047] Figure 6 This is the simulation result of PCC short-circuit fault in Jinsu HVDC system. DETAILED DESCRIPTION

[0048] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0049] Example

[0050] The method for evaluating the applicability of the grid strength index in continuous commutation failure of this embodiment specifically includes the following steps:

[0051] (1) Construct a synchronous generator model, a DC transmission model based on a grid-commutated converter, and a load model, and establish the power balance equation at the common connection point.

[0052] refer to Figure 1 In the single-infeed HVDC transmission system, the load is provided by LCC and synchronous generator (SG). The Thevenin equivalent model is used to simulate the SG, that is, a fixed impedance is connected in series with the voltage source, and the output power of the SG is obtained as Equation (1)-(2).

[0053]

[0054] The transient electromotive force E ′ q The differential equation is formula (3):

[0055]

[0056] To simplify the derivation, ignore the d-axis transient reactance and assume that the d-axis reactance is equal to the q-axis reactance, that is,

[0057] x q =x d =x ′ d

[0058] Among them, P SG and Q SG Respectively represent the output active power and reactive power of SG; Z and They represent the amplitude and phase angle of the equivalent impedance of the AC system respectively; U is the voltage amplitude of the AC bus; δ is the voltage phase angle of the AC bus; E ′ q is the transient electromotive force of SG; T ′ d is the transient time constant of SG; E fd is the excitation electromotive force of SG; x d is the d-axis synchronous reactance of SG; x ′ d is the d-axis transient reactance of SG; i dg is the d-axis current of SG.

[0059] The DC transmission model based on the grid-commutated converter (LCC) can be described as equations (4)-(9). The expressions of the active power and reactive power of the LCC are equations (4) and (5) respectively. The positive direction of the power is as follows: Figure 1 As shown. C and K are two constants related to the converter transformer parameters. Equation (6) is the expression of DC current. The output reactive power of the compensation capacitor is (9).

[0060] P LCC =CU 2 [cos2γ-cos(2γ+2μ)](4)

[0061] Q LCC =CU 2 [2μ+sin2γ-sin(2γ+2μ)](5)

[0062] I dc =KU[cosγ-cos(γ+μ)](6)

[0063]

[0064] Q c =B c U 2 (9)

[0065] Among them, P LCC and Q LCC Respectively represent the active power and reactive power of LCC; P dN is the rated active power of LCC; I dc is the DC current; γ is the LCC turn-off advance angle; μ is the LCC commutation overlap angle; S T is the capacity of the converter transformer; u k1 is the short-circuit ratio of the converter transformer; τ is the transformation ratio of the converter transformer; Q c B is the reactive power output by the compensation capacitor; c is the admittance of the compensation capacitor.

[0066] The load model uses a constant impedance model to simulate the load. The expressions of the active power and reactive power of the load are equation (10) and equation (11) respectively.

[0067] P load =U 2 G load (10)

[0068] Q load =U 2 B load (11)

[0069] Among them, P load and Q loadRespectively represent the active power and reactive power of the load; G load is the conductivity of the load; B load is the susceptance of the load.

[0070] The power balance equations at the point of common connection (PCC) are as follows: (12)-(13).

[0071] P LCC +P SG -P load =0 (12)

[0072] Q SG +Q c -Q LCC -Q load =0 (13)

[0073] (2) Calculate the bus voltage at the moment when the commutation failure fault is cleared based on the above model.

[0074] During a short-circuit fault, commutation failure occurs, causing the active / reactive power of the LCC to drop to zero, and the excitation system of the SG will try to increase the bus voltage. Since the bus voltage is very low during a fault, the potential output by the excitation system usually reaches the maximum value, as shown in (14). By solving equation (3), the transient electromotive force expression when a fault occurs can be obtained as (15).

[0075] E fd =V Rmax (14)

[0076]

[0077] Among them, V Rmax It is the maximum output limit of the excitation system.

[0078] In the early period after the fault is cleared, the active power of the LCC will gradually recover from zero. Therefore, assuming that the active power is zero at the moment of fault clearing, under this assumption, (16) can be obtained at the moment of fault clearing.

[0079] P LCC =0(16)

[0080] In addition, assuming that the DC current I dc It is constant during the commutation failure recovery process. Combining (1)-(13) and (15)-(16), the bus voltage can be calculated. The normal range of bus voltage is determined according to the actual project. If the bus voltage at the time of fault clearing is within the normal range, it is considered that the HVDC system can recover normally from the commutation failure and CCF will not occur. On the contrary, if the bus voltage exceeds the normal range, the commutation failure will occur at least twice. The parameters used in the test system are shown in Table 1.

[0081] Table 1 Parameters of a simple test system

[0082]

[0083]

[0084] (3) Analyze the correlation between grid strength indicators and continuous commutation failure.

[0085] There are many indicators that reflect the strength of the AC system grid, such as short circuit ratio (SCR), voltage stiffness and multi-infeed short circuit ratio, among which the most widely used is the short circuit ratio (SCR). This embodiment takes SCR as an example to analyze the correlation between the grid strength index and the continuous commutation failure. SCR can be used to evaluate the strength of the AC system relative to the HVDC system. It is generally believed that when SCR is greater than 3, the AC system can be regarded as a strong system and the LCC can operate stably. When SCR is less than 2, the AC system is considered to be a weak system and the LCC is difficult to operate stably. When SCR is between 2 and 3, the AC system is a medium-strength system. The relationship between SCR and CCF will be analyzed in detail below to evaluate the applicability of SCR.

[0086] Assume that the reference voltage of the AC system is U N , reference power P dN is the rated power of the HVDC system Pd. According to the definition of SCR, the expression of SCR is as follows (17).

[0087]

[0088] Among them, S SC is the short-circuit capacity of the point of common connection (PCC); Z pu It is the per unit value of the system equivalent impedance.

[0089] By changing the system parameters, the change of bus voltage at the time of fault clearing can be obtained, such as Figure 2 , Figure 3 The relationship between bus voltage and load active power is shown in Figure 2 If the LCC attempts to recover from the commutation failure, the active power of the LCC will increase from zero and begin to absorb reactive power. At this time, the bus voltage at the time of fault clearing is as follows: Figure 2 As shown in a. Figure 2 a It can be seen that under the same SCR condition, the higher the load and the lower the bus voltage, the greater the possibility of CCF.

[0090] If SCR is used to assess the possibility of CCF, the probability of CCF at different load levels should be the same. However, the higher the load level, the greater the risk of CCF. If LCC is not restored, the bus voltage will be Figure 2 As shown in b. Figure 2 a and Figure 2 b It can be found that if the LCC does not recover from the commutation failure, the bus voltage will increase greatly when the fault is cleared. This shows that the temporary non-recovery of the commutation failure is conducive to the recovery of the bus voltage.

[0091] The relationship between bus voltage and SCR is as follows Figure 3 Similarly, if the LCC attempts to recover from a commutation failure, the bus voltage at the moment of fault clearing is Figure 3 As shown in a. Figure 3 In a, when the load level is high enough, even if the SCR is greater than 3, the bus voltage is still low at the time of fault clearing, and CCF will occur. Therefore, considering the impact of load, it is difficult for SCR to assess the risk of CCF. In addition, if the LCC does not recover from the commutation failure, the bus voltage at the time of fault clearing will be Figure 3 As shown in b. Figure 3 In b, the bus voltage increases significantly, and the LCC recovers after the bus voltage returns to the normal range.

[0092] In the above embodiment, the load adopts a constant impedance model. Therefore, the load does not provide short-circuit current and the impedance remains unchanged in dynamics. If the induction motor model is adopted, there are two main differences compared with the constant impedance model:

[0093] a. Induction motors can provide short-circuit current. Therefore, induction motors will increase the short-circuit capacity of PCC.

[0094] b. The equivalent impedance of the induction motor will change dynamically. Generally speaking, after a short circuit fault occurs, the motor slip rate will increase and the equivalent impedance of the motor will decrease, thereby reducing the bus voltage.

[0095] As mentioned above, when there are a large number of loads near the LCC, even if the SCR judges the system as a strong system, CCF may still occur. Therefore, the SCR, a grid strength indicator, cannot effectively reflect the occurrence of CCF. Considering that the motor will increase the short-circuit capacity and deteriorate the dynamic characteristics of the bus voltage, compared with the constant impedance load, the use of the induction motor load model will affect the accuracy of the SCR and other grid strength indicators.

[0096] Verify the instance

[0097] In order to verify the above method, this verification example simulates and analyzes the Jiangsu power grid, and still uses SCR for illustration. The data used is based on the peak operation in the summer of 2023. The simulation software is Power System Simulator / Engineering (PSS / E). Two LCC-HVDC projects were selected as research objects. One is the Jinsu HVDC project, and the other is the Baihetan HVDC project. The Jiangsu power grid is the receiving power grid of these two HVDC projects. The Jinsu HVDC project is a high-voltage DC system based on traditional LCC, and both the rectifier and inverter are composed of LCC. The Baihetan DC transmission project is a low-voltage-multi-voltage hybrid DC transmission system. The rectifier station of the Baihetan HVDC system is composed of LCC, and the inverter station is composed of 1LCC+3VSCs cascaded. The grid structure near the access point of the two HVDC projects is as follows: Figure 4 As shown. It is worth noting that Jiangsu power grid is a multi-infeed HVDC system. Therefore, SCR cannot be directly used for stability assessment of HVDC system. In order to apply SCR in Jiangsu power grid, when a certain HVDC system is selected as the research object, other HVDC systems are replaced by constant impedance systems.

[0098] Table 2 SCR of Baihetan HVDC system

[0099]

[0100] First, the Baihetan DC project is selected as the target DC system, and the Jinsu DC system is replaced by a constant impedance; the SCR of the Baihetan HVDC system under different working conditions is shown in Table 2. VSCs do not have commutation failure problems, so VSCs are not the focus of this paper. The SCRs shown in Table 2 are all LCCs.

[0101] As can be seen from Table 2, the SCR of the Baihetan DC system is relatively large under normal conditions. However, under the LCC-Doushan N-2 condition, the SCR will drop to a very low value. If the SCR is used to evaluate the risk of CCF, the CCF of the Baihetan DC transmission system should occur under the LCC-Doushan N-2 condition. However, through the simulation under the LCC-Doushan N-2 condition, it can be seen that there is no CCF after a serious short-circuit fault occurs in the PCC. The simulation results are shown in Figure 5 As shown. Figure 5 a. After the fault is cleared, the voltage of PCC can be restored to the normal range. Figure 5 b It can be seen that the LCC DC voltage is zero when the commutation fails, and the LCC can recover from the commutation failure after the fault is cleared.

[0102] Secondly, the Jinsu DC project is selected as the target DC system, and the Baihetan DC system is replaced by a constant impedance. The SCR of the Jinsu HVDC system is 4.43. Therefore, the Jiangsu power grid is a strong power grid compared with the Jinsu HVDC system. If the SCR evaluates the risk of CCF, CCF will not occur. However, it can be seen from the simulation that CCF will occur after a short-circuit fault occurs in the PCC. The simulation results are shown in Figure 2. Figure 6 As shown. Figure 6 b It can be seen that the LCC DC voltage is zero when the commutation fails, and the commutation failure lasts for 300ms.

[0103] The responses of the two systems show that even if the SCR is small, CCF may not occur, and even if the SCR is large enough, CCF may occur. The SCR, a grid strength indicator, cannot effectively reflect the occurrence of CCF in the DC system. The reason for this phenomenon is that the PCC of the Jinsu DC transmission system is located at the load center, while there are fewer loads near the PCC of the Baihetan DC transmission system.

[0104] The technical solutions of the present invention are not limited to the above-mentioned embodiments, and any technical solutions obtained by equivalent replacement methods fall within the scope of protection required by the present invention.

Claims

1. A method for evaluating the applicability of a grid strength index in continuous commutation failures, characterized in that: Construct synchronous generator model, DC transmission model based on grid-commutated converter and load model, and establish power balance equation at common connection point; The synchronous generator model includes the output active power, reactive power and transient electromotive force of the synchronous generator; the DC transmission model based on the grid-commutated converter includes the active power, reactive power and DC current of the grid-commutated converter; the load model includes the active power and reactive power of the load; The bus voltage at the time of fault clearing is calculated based on the synchronous generator model, the DC transmission model based on the grid-commutated converter, the load model and the power balance equation of the common connection point; Judging whether the DC power transmission system will have continuous commutation failure based on the bus voltage at the time of fault clearing; Calculate the grid strength index and establish the relationship between the grid strength index and the bus voltage at the time of fault clearing; According to the relationship between the grid strength index and the bus voltage at the moment of fault clearing, the applicability of the grid strength index to judge the continuous commutation failure is evaluated.

2. The method for evaluating the applicability of the grid strength index in continuous commutation failure according to claim 1 is characterized in that: The output active power and reactive power of the synchronous generator are respectively: Among them, P SG and Q SG Respectively represent the output active power and reactive power of the synchronous generator; Z and They represent the amplitude and phase angle of the equivalent impedance of the AC system respectively; U is the voltage amplitude of the AC bus; δ is the voltage phase angle of the AC bus; E' q is the transient electromotive force of the synchronous generator.

3. The method for evaluating the applicability of the grid strength index in continuous commutation failure according to claim 2 is characterized in that: The transient electromotive force differential equation is: Among them, T' d is the transient time constant of the synchronous generator; E fd is the excitation electromotive force of the synchronous generator; x d is the d-axis synchronous reactance of the synchronous generator; x' d is the d-axis transient reactance of the synchronous generator; i dg is the d-axis current of the synchronous generator.

4. The method for evaluating the applicability of the grid strength index in continuous commutation failure according to claim 3 is characterized by: The active power, reactive power and DC current of the grid-commutated converter of the DC system are: P LCC =CU 2 [cos2γ-cos(2γ+2μ)] Q LCC =CU 2 [2μ+sin2γ-sin(2γ+2μ)] I dc =KU[cosγ-cos(γ+μ)] (6) Among them, P LCC and Q LCC Respectively represent the active power and reactive power of LCC; P dN is the rated active power of LCC; I dc is the DC current; γ is the LCC turn-off advance angle; μ is the LCC commutation overlap angle; S T is the capacity of the converter transformer; u k1 is the short-circuit ratio of the converter transformer; τ is the transformation ratio of the converter transformer; Q c It is the reactive power output by the compensation capacitor.

5. The method for evaluating the applicability of the grid strength index in continuous commutation failure according to claim 4 is characterized in that: The reactive power output by the compensation capacitor is: Q c =B c U 2 Among them, B c is the admittance of the compensation capacitor.

6. The method for evaluating the applicability of the grid strength index in continuous commutation failure according to claim 5 is characterized by: The load model adopts a constant impedance model.

7. The method for evaluating the applicability of the grid strength index in continuous commutation failure according to claim 6 is characterized by: The expressions of the active power and reactive power of the load are: P load =U 2 G load Q load =U 2 B load Among them, P load and Q load Respectively represent the active power and reactive power of the load; G load is the conductivity of the load; B load is the susceptance of the load.

8. The method for evaluating the applicability of the grid strength index in continuous commutation failure according to claim 6 is characterized by: The power balance equation at the common connection point is: P LCC +P SG -P load =0 Q SG +Q c -Q LCC -Q load =0。 9. The method for evaluating the applicability of the grid strength index in continuous commutation failure according to claim 6, characterized in that: The calculation method of the bus voltage at the time of fault clearing is: The transient electromotive force when a fault occurs is calculated as: Among them, V Rmax is the maximum output limit of the excitation system; Assume that the active power of the grid commutation converter is zero when the fault is cleared, and the DC current I dc is constant; the bus voltage at the moment of fault clearing is calculated by combining the synchronous generator model, the DC transmission model based on the grid-commutated converter, the load model and the power balance equation of the common connection point.

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