A Dispatching Automation Correction Method for Distance Section III Protection

Abstract

The present invention provides a method for distance segment III protection with automatic dispatching correction, which is a method for distance segment III protection integrating DC control information. The method first analyzes the problem of misoperation of AC side distance segment III protection caused by DC line removal; then proposes a flow transfer identification scheme integrating DC control information; finally, according to the flow transfer identification result, a revised distance segment III protection criterion is proposed. The present invention solves the problem of misoperation of existing AC distance protection caused by DC line removal, and reduces the probability of major power outages.

Description

Technical Field

[0001] The invention relates to a dispatching automation correction type distance III section protection method, belonging to the technical field of power system protection. Background Art

[0002] In the event of a large-scale power outage, the misoperation of distance protection caused by line overload contributed to the expansion of the accident. Line overload can be divided into normal overload and accident overload. Normal overload is when the current flowing through the line increases with the increase in capacitance and exceeds the normal operating level of the line. Accident overload is caused by a fault. When the faulty equipment is removed, the power flow is transferred, causing the current of the normally operating line to increase rapidly. During the power outage, the overload causes the protection to fail, the power flow is further transferred, and finally causes the protection to trip, expanding the scope of the accident.

[0003] As the most important backup protection for transmission lines, distance protection has been widely used in line protection at home and abroad. Distance protection has the advantages of not being affected by factors such as channels and data synchronization, having a stable protection range, and having little impact on the system operation mode. However, distance protection is easily affected by system oscillation and overload. When the line is overloaded, the load impedance continues to decrease as the load increases. When the load impedance enters the distance protection action area, the distance protection will fail. At present, overload identification methods can be divided into identification based on wide-area information and identification based on local information. The method proposed in an invention can improve the overload tolerance of protection. However, the identification process still relies on wide-area information. An invention proposes a configuration scheme based on local information, which uses substation fault information and second-order disturbance information to complete detection. An invention abandons the traditional distance protection impedance plane and proposes a new protection scheme based on the voltage plane criterion. An invention proposes an adaptive polygonal distance protection scheme based on impedance trajectory, which can prevent the overrunning phenomenon of distance protection during out-of-area faults. However, the above inventions do not consider the impact of DC line faults on the AC side. Summary of the invention

[0004] The purpose of the present invention is to provide a dispatching automation correction distance III section protection method, which can not only identify the flow transfer, but also does not lose the function of the distance III section backup protection.

[0005] In order to achieve the above object, the present invention is implemented through the following technical solutions:

[0006] 1) Use measuring elements to collect the protection control status information of the AC and DC sides, including the status information of the DC side converter, DC side circuit breaker, AC side main protection, and AC side backup protection;

[0007] 2) Calculate the power flow transfer identification result S according to the state information collected in step 1. The specific formula is as follows:

[0008] S=max[sum(A1,A2),sum(A1,A3),sum(A1,A4),sum(A2,A3),sum(A2,A4),sum(A3,A4)]

[0009] Where A1 represents the state of the DC side converter. In the unlocked state, its value is -1 to 0. In the locked state, its value is 0 to 1, which represents locking. The value of the failure state is 0.

[0010] Where A2 represents the state of the DC side circuit breaker. Its value is -1 to 0 in the non-actuated state, 0 to 1 in the acted state, representing blocking, and 0 in the failed state.

[0011] Where A3 represents the state of the main protection on the AC side. Its value is -1 to 0 in the non-actuated state, 0 to 1 in the acted state, representing blocking, and 0 in the failed state.

[0012] Where A4 represents the state of the AC side directional element. Its value is -1 to 0 in the non-actuated state, 0 to 1 in the acted state, representing blocking, and 0 in the failed state.

[0013] 3) If S satisfies 0~1, it is judged that power flow transfer occurs on the AC side and the next step is entered. If S satisfies -1~0, it is judged that a line fault occurs on the DC side and the process returns;

[0014] 4) The wide-area backup protection coordination device issues instructions for adjusting the action characteristics of distance protection section III according to the distance protection criterion of section III.

[0015] Preferably, the distance III protection criterion after a DC side fault occurs is as follows:

[0016]

[0017] Where S represents the result of the power flow transfer identification element and α represents the set phase angle.

[0018] Preferably, the traditional distance III segment protection characteristic equation is:

[0019]

[0020] Among them, Z m Represents the measured impedance, Z set Represents the impedance threshold. The relationship between the impedance threshold and the measured impedance is Z set =0.75Z m .

[0021] The advantages of the present invention are: by constructing a single-element protection unit, the present invention conducts a comprehensive analysis of the DC side and AC side protection control information, which can identify the flow transfer without losing the function of the distance III section backup protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0023] Figure 1 It is a schematic diagram of the process structure of the present invention.

[0024] Figure 2 This is a schematic diagram of the protection action area for the modified distance III segment.

[0025] Figure 3 Schematic diagram of simulation results for the modified distance III protection

[0026] Figure 4 This is a schematic diagram of the action results of the unadjusted distance III protection

[0027] Figure 5 This is a structural diagram of the AC / DC hybrid system. DETAILED DESCRIPTION

[0028] 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.

[0029] Example

[0030] In order to better explain the technical concept of the present invention, the technical problems faced by the present invention are now explained:

[0031] After the DC line is removed due to a fault, the AC side distance backup protection may fail, resulting in a major power outage. The reason for the failure of the distance backup protection is the flow transfer. Therefore, it is necessary to study the various protection control information on the DC side and the AC side during the flow transfer.

[0032] like Figure 5As shown in the figure, DC side: After the DC line fails, the blocking strategy of the converter is as follows: The blocking strategy is executed for the fault on the rectifier side. When one pole or the last group of valves is blocked, the rectifier side immediately shifts the phase. After 50ms, the strategy is selected based on the DC current, and the converter on the rectifier side will be blocked after about 200ms. After the inverter side receives the protection action signal on the opposite side, it executes the normal blocking logic. The phase is shifted to 90°, and then the blocking command is executed. After the DC line fails, there are a large number of control signals in the converter. In addition, after the DC line fails, the DC circuit breaker trips and recloses within the subsequent 20-500ms. Therefore, the opening signal of the DC circuit breaker means the removal of the DC line. In summary, after a fault occurs on the DC side, there is a large amount of protection control information in the converter and circuit breaker. Such control information helps to identify power flow transfer.

[0033] AC side: Due to the presence of DC side protection devices, faults in the DC line will not spread to the AC side. There are many protection schemes for AC lines, such as differential main protection, three-stage distance backup protection, and directional elements. These protection schemes will only operate when the AC line fails. The flow transfer caused by the removal of the DC line will only increase the current in the AC line, without changing the direction and frequency of the current. Therefore, the differential main protection and directional elements will not operate. The protection status information on the AC side can also be used to identify the flow transfer.

[0034] In summary, the present invention proposes a power flow transfer identification solution that integrates DC side control information.

[0035] Step 1 uses the measuring element to collect the protection and control status information of the AC and DC sides, including the status information of the DC side converter, the DC side circuit breaker, the AC side main protection, and the AC side backup protection; the convergence of various types of information is shown in Table 1 below, where 0 represents that the element judges to be failed, including the following situations: sampling failure, channel failure, etc.; 0~1 represents that the protection control element judges that a fault has occurred based on the measurement information; -1~0 represents that the protection control element judges that no fault has occurred based on the measurement information.

[0036] Table 1

[0037]

[0038] Step 2 calculates the power flow transfer identification result S according to the state information collected in step 1. The specific formula is as follows:

[0039] S=max[sum(A1,A2),sum(A1,A3),sum(A1,A4),sum(A2,A3),sum(A2,A4),sum(A3,A4)]

[0040] Step 3: If S satisfies 0 to 1, it is determined that power flow transfer occurs on the AC side and the next step is entered. If S satisfies -1 to 0, it is determined that a line fault occurs on the DC side and the process returns.

[0041] Step 4: The wide-area backup protection coordination device issues an instruction to adjust the action characteristics of distance protection section III according to the distance protection criterion of section III.

[0042] Step 5: After the DC side fault ends, the revised distance III protection scheme is completed, and the wide area backup protection coordination device modifies the criterion to the traditional distance III protection criterion.

[0043] When the power flow transfer causes the line to be overloaded, the power flow transfer identification program is activated. When the calculated result of the line current meets the identification criteria of the power flow transfer, the wide-area backup protection coordination device issues an instruction to adjust the action characteristics of the distance protection section III to avoid erroneous disconnection of the line due to power flow transfer.

[0044] When no power flow transfer occurs, the distance III protection of the AC line adopts the directional circle characteristic, and its action characteristic equation is:

[0045]

[0046] Among them, Z m Represents the measured impedance, Z set Represents the impedance threshold. The relationship between the impedance threshold and the measured impedance is Z set =0.75Z m .

[0047] The modified distance III protection criteria after a DC side fault occurs are as follows:

[0048]

[0049] Among them, S represents the result of the power flow transfer identification element, and α represents the setting phase angle. Therefore, the action area (gray part) of the distance III protection criterion is as follows Figure 3 Obviously, the modified distance III protection can be adaptively adjusted according to the result of the power flow transfer identification element, thus avoiding its erroneous action.

[0050] A simulation model was built in PSCAD according to the structural diagram of the AC / DC hybrid system. The detailed parameters of the AC side are shown in Table 1.

[0051] PART OF THE PARAMETERS OF THE 10-MACHINE 39-NODE SYSTEM

[0052]

[0053] The DC line is set to be cut off a few milliseconds after the fault. At this time, the fault current of the 28-29 node line on the AC side will undergo a flow transfer. The fault current of this line increases rapidly, meeting the traditional distance protection start-up criteria. If the three-stage protection without adjusting the distance is used, the simulation results of the protection device are shown in the figure below. Obviously, the flow transfer makes the measured impedance enter the distance protection characteristic circle, and the protection malfunctions.

[0054] If the modified distance III protection scheme of the present invention is configured in the AC system, the simulation process is as follows:

[0055] First, the removal of the DC side line causes the fault current of the 28-29 node line on the flow side to increase, which meets the starting criterion and the scheme of the present invention is started.

[0056] Subsequently, the power flow transfer identification component collects control and protection information, which is summarized in the following table. Since the simulation is in an ideal state, the state of each component is an ideal result, that is, 1.

[0057]

[0058] The power flow transfer identification result shows that power flow transfer has occurred at this time. The wide-area backup protection coordination device adjusts the AC side distance protection to the modified distance III protection criterion. The simulation result of the distance protection criterion is shown in the figure below. Obviously, the characteristic circle of the modified distance III protection can avoid its false operation.

[0059] In summary, the proposed scheme can prevent the erroneous operation of the AC side distance backup protection due to power flow transfer and reduce the probability of major power outages in the system.

Claims

1. A dispatching automation correction distance III section protection method, characterized in that: The following steps are involved: 1) Use measuring elements to collect the protection and control status information of the AC and DC sides, including the status information of the DC side converter, DC side circuit breaker, AC side main protection, and AC side directional elements; 2) Calculate the power flow transfer identification result S according to the state information collected in step 1. The specific formula is as follows: In the formula Indicates the state of the DC side converter. In the unlocked state, its value is -1~0. In the locked state, its value is 0~1, indicating locking. The value of the failed state is 0. In the formula Indicates the state of the DC side circuit breaker. Its value is -1~0 in the non-actuated state, 0~1 in the acted state, indicating locking, and 0 in the failed state. In the formula Indicates the state of the main protection on the AC side. Its value is -1~0 in the non-actuated state, 0~1 in the acted state, indicating blocking, and 0 in the failed state. In the formula Indicates the state of the AC side directional element. Its value is -1~0 in the non-actuated state. Its value 0~1 in the acted state represents blocking. The value of the failure state is 0. 3) If S satisfies 0~1, it is judged that power flow transfer occurs on the AC side and the next step is entered. If S satisfies -1~0, it is judged that a line fault occurs on the AC side and the process returns; 4) The wide-area backup protection coordination device issues instructions to adjust the operating characteristics of distance protection segment III according to the distance segment III protection criteria. 5) After the DC side fault ends, the revised distance III protection scheme ends, and the wide-area backup protection coordination device modifies the criterion to the traditional distance III protection criterion; The judgment criteria for distance III protection after a DC side fault occurs are as follows: Where S represents the result of the power flow transfer identification element, represents the setting phase angle; The traditional distance III protection characteristic equation is: in, represents the measured impedance, represents the impedance threshold, and the relationship between the impedance threshold and the measured impedance is =0.75 .

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