Seamless ring method for power distribution network based on load equivalent impedance

Abstract

The invention discloses a power distribution network suture-free ring method based on load equivalent impedance. The power distribution network suture-free ring method is characterized by comprising the following steps: step 1, processing a feeder line load; step 2, constructing a loop closing model of load equivalent impedance; step 3, calculating load equivalent impedance; 4, loop closing current is analyzed; step 5, a sutureless ring transfer process; according to the invention, power supply can be switched without power outage, the application range of loop-closing power supply is expanded, loop-closing impact current is eliminated, and the safety and reliability of loop-closing power supply are improved.

Description

Technical Field

[0001] The invention relates to the technical field of distribution network loop closing, in particular to a distribution network seamless loop closing method based on load equivalent impedance. Background Art

[0002] my country's distribution network follows the principle of "closed-loop design, open-loop operation". When the distribution network lines are running at high load for a long time or the equipment is planned for maintenance, the load needs to be transferred. At present, the "first on and then off" method is widely adopted, that is, the closed-loop power supply. The closed-loop power supply can realize load transfer under the condition of uninterrupted power supply, meeting the power supply reliability requirements of power users. However, large steady-state circulating currents and impulse currents may be generated during the closing process, resulting in false operation of relay protection and overload of power equipment, causing closing failure. Therefore, the calculation of closed-loop steady-state current and impulse current is the basis for closed-loop analysis, and the closed-loop model of the distribution network is the key to the calculation of closed-loop current.

[0003] In addition, ring-closing and power supply transfer is an effective measure to improve the reliability of power supply in the distribution network. When the upper power grid needs to be planned for maintenance, the load on the side to be shut down is directly transferred to the other side of the power grid by closing the tie switch between the two substations. If the ring-closing operation is performed between two different substations, the direct ring-closing operation will generate a large impact current due to the voltage difference between different power supply points. In order to avoid the risk of ring closing, ring-closing and power supply transfer is only implemented in a few power supply areas that meet the conditions for ring closing. Most areas still adopt the method of power outage first and then power transfer, which cannot meet people's growing demand for power supply reliability. Therefore, how to improve the safety and reliability of the ring-closing and power supply transfer of the distribution network and realize uninterrupted power supply to users has become an urgent problem to be solved in the current distribution network. Summary of the invention

[0004] The purpose of the present invention is to overcome the above-mentioned shortcomings and provide a distribution network seamless loop closing method based on load equivalent impedance, which can improve the safety and reliability of the loop closing.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for seamlessly closing a distribution network based on load equivalent impedance, which comprises the following steps:

[0006] Step 1: Processing of feeder load: Use the equivalent load model to treat all loads between a section of feeders as one load. If there is a branch feeder between feeders, it is equivalent to connecting a load at the branch node with a power equal to the head end of the branch feeder.

[0007] Step 2: Construct a closed-loop model of load equivalent impedance: treat the load on the feeder as an equivalent load and replace it with its equivalent impedance. Convert the power supply and impedance above 10kV to the 10kV side to obtain a closed-loop model taking into account the load equivalent impedance.

[0008] Step 3: Calculate the load equivalent impedance: According to the closed-loop model, find the Thevenin equivalent circuit of the port. The calculation of the equivalent impedance Zeq needs to include the load equivalent impedance.

[0009] Step 4: Analyze the closed-loop current: Perform transient analysis on the Thevenin equivalent circuit, write Zeq in the form of Req+jωLeq, write differential equations, analyze the decay time constant Ta, and solve the maximum effective value of the closed-loop current;

[0010] Step 5, seamless closing and power transfer process: Use a controllable voltage source to compensate for the voltage difference on both sides of the closing point to eliminate the closing impact, and control the closing current within the allowable range of the feeder. After impedance compensation, the size of the closing current is completely affected by the compensation impedance. Select a suitable compensation impedance to control the maximum value of the closing current within a smaller range.

[0011] Furthermore, in step 1, the phasor data of the distribution network nodes are obtained in real time by a micro-synchronous phasor measurement device μPMU; Figure 2 is the equivalent load diagram of feeder AB, where P K +jQ K is the power of equivalent load node K, I K ∠θ IK is the current phasor flowing through the equivalent load, I A ∠θ IA is the current phasor from node A to node K, I B ∠θ IB is the current phasor flowing from node K to node B, L1 and L2 are the lengths of the equivalent load node from nodes A and B respectively; to achieve load equivalence of feeder AB, μPMUs need to be configured at nodes A and B to obtain the voltage phasor of the distribution network nodes;

[0012] The principle of feeder AB load equivalence is that the equivalent load node voltage obtained by the node A voltage drop formula is equal to the equivalent load node voltage obtained by the node B voltage drop formula, where:

[0013]

[0014] Among them, U K is the equivalent load node voltage obtained from node A, U' K is the equivalent load node voltage obtained from node B, U A , U B are the voltages of the feeder head and end nodes A and B, respectively. L1 and L2 are the lengths of the equivalent load nodes from nodes A and B, respectively. The total length of feeder AB is L. The impedance of the feeder per unit length (km) is z=r+jx; P A ,P Bis the active power of the feeder head and end nodes A and B, Q A ,Q B The reactive power of the feeder head and end nodes A and B, L1 and L2 can be obtained by iteration. Given an error function and iteration step, the error function can meet the given accuracy ε by continuously correcting the length of L1, thereby determining the position K of the equivalent load.

[0015] Furthermore, in step 2, the impedance Z of the equivalent load is obtained by the power formula: K =R K +jX K :

[0016]

[0017] Among them, U K is the voltage of equivalent load node K, Z K is the impedance of the equivalent load node K, P K is the active power of equivalent load node K, Q K is the reactive power of equivalent load node K, R X is the resistance of the equivalent load node K, X K is the reactance of equivalent load node K;

[0018] The load equivalent impedance is based on the equivalent of real-time measurement information and can reflect the real-time load distribution of the feeder, thereby obtaining the equivalent circuit of feeder AB.

[0019] Furthermore, in step 3, in the Thevenin equivalent circuit, ΔU ph is the phase voltage U on both sides of the tie switch before closing the loop aph , U bph The difference between the equivalent impedance Z eq The calculation formula is:

[0020]

[0021] Among them, Z eqa is the equivalent impedance of the tie switch a side; Z eqb is the equivalent impedance of the tie switch b side, Z a0 The impedance equivalent to the 10kV side above the 10kV busbar, Z ka is the impedance corresponding to the equivalent load on side a, Z al1 is the equivalent impedance of the total load of other feeders on bus 1, Z al2 is the load S at the end of the feeder on side a an The equivalent impedance, Z a1 is the impedance of the feeder L1 section on side a, Z a2 is the impedance of the feeder L2 section on side a; the relevant impedance parameters on side b are similar to those on side a.

[0022] Furthermore, the step 4 specifically includes the following steps:

[0023] 4.1、Calculation of the impulse current of the closed-loop branch: Perform transient analysis on the Thevenin equivalent circuit, write Zeq in the form of Req+jωLeq, and write the differential equation. Em, ω, α are the amplitude, angular frequency and initial phase angle of the Thevenin equivalent voltage source respectively; Req and Leq are the Thevenin equivalent resistance and inductance respectively; i c is the total current of the closed-loop branch ab;

[0024] 4.2 Decay time constant T a Calculation: For a certain moment after the loop is closed, take a specific frequency, that is, the optimal frequency f c , all rated frequencies f in the closed-loop network n The impedance under is converted to f c The impedance under the condition of , find the Thevenin equivalent impedance Zeq(fc)=Req(fc)+jXeq(fc) under the optimum frequency;

[0025] 4.3 Feeder inrush current calculation: For the inrush current flowing through the feeder switch, the superposition principle is applied to solve it; i c1 The total current of the feeder a before the loop is added to the total current of the feeder a before the loop is closed, and the total current i of the feeder a after the loop is closed can be obtained. a Expression: Subtract i from the total current of the feeder b before the loop is closed c2 The total current of feeder b after closing the loop can be obtained by b expression.

[0026] Furthermore, the step 5 includes the following steps:

[0027] 5.1. Seamless loop closing: In order to achieve seamless loop closing, adjust the compensation voltage output by the series compensation module until the loop closing condition is met, that is, ΔU max , δ<δ max , when it is detected that the closing loop condition is met, a closing loop instruction is sent to close switches K1 and K2;

[0028] 5.2. Load transfer: The seamless loop system works in the load transfer mode, and the load of bus 2 will be smoothly transferred to bus 1; the loop current is controlled to slowly increase from zero to the load current. This part of the current flowing through the series voltage source will generate active power and reactive power; this part of the power determines the capacity of the system, and the system must be able to provide active power to the system; after detecting that the load transfer is completed, the outgoing line circuit breaker CB2 of the distribution transformer T-2 is disconnected, and the distribution transformer T-2 seamlessly exits operation;

[0029] ​5.3. Live exit: Adjust the injected voltage so that the voltage amplitude and phase angle on both sides of the bypass switch remain consistent, and the bypass switch is closed in the zero voltage switching mode; during the exit process, the closed loop composed of the bypass switch and the series voltage source is regarded as a first-order RL circuit, and the current of the seamless loop transfer system decays according to the time constant τ=Lc / Rc. There is a commutation process between the two branches. After the commutation is completed, the load current is transferred to the bypass branch, and K1 and K2 are turned off in the zero current switching mode, thereby realizing the seamless exit of the system;

[0030] 5.4、System capacity analysis: Assume that the voltage of the power supply at both ends is constant during the power transfer process and the load is a constant power load;

[0031]

[0032] Among them, the phase angle of voltage U1 is set to δ, the phase angle of voltage U2 is set to 0, the phase angle of series compensation voltage is set to (δ+ρ), U1 and U2 are the voltages on both sides, U C is the compensation voltage;

[0033] After the ring is closed, the complex power of the tie line is:

[0034]

[0035] Among them, P CL is the active power of the tie line, Q CL is the tie line reactive power, X T is the transformer reactance, U1 and U2 are the voltages on both sides, and U C is the compensation voltage;

[0036] The active power and reactive power of the tie line are:

[0037]

[0038] Among them, P CL is the active power of the tie line, Q CL is the tie line reactive power, X T is the transformer reactance, U1 and U2 are the voltages on both sides, and U C For the compensation voltage, the phase angle of voltage U1 is set to δ, the phase angle of voltage U2 is set to 0, and the phase angle of the series compensation voltage is set to (δ+ρ);

[0039] After adding the series compensation voltage, the active power and reactive power transmitted to bus 1 by distribution transformer T-1 are:

[0040]

[0041] Among them, the phase angle of voltage U1 is set to δ, the phase angle of voltage U2 is set to 0, the phase angle of series compensation voltage is set to (δ+ρ), U1 and U2 are the voltages on both sides, U C is the compensation voltage, X T is the transformer reactance, P T1 is the active power of transformer T-1, Q T1 is the reactive power of transformer T-1, P L1 is the active power of the tie line L-1, Q L1 is the reactive power of tie line L-1;

[0042] The capacity of the seamless loop power transfer system mainly depends on the length of the compensation voltage phase and the tie line current. According to Kirchhoff's voltage law, the compensation voltage is:

[0043]

[0044] Among them, the phase angle of voltage U1 is set to δ, the phase angle of voltage U2 is set to 0, the phase angle of series compensation voltage is set to (δ+ρ), U1 and U2 are the voltages on both sides, U C is the compensation voltage, X T is the transformer reactance, I C is the tie line current;

[0045] Therefore, the system capacity is finally S SLTS =|Uc| max |Ic| max , where |Uc| max is the maximum compensation voltage, |Ic| max It is the maximum value of the tie line current, which is equal to the load current on the main supply side.

[0046] Beneficial effects of the present invention:

[0047] 1) Aiming at the shortcomings of existing distribution network closing-loop models in load processing, the present invention considers the influence of feeder load on the steady-state and transient processes of closing-loop, and based on the real-time measurement information of a small number of μPMUs at the head and end of the feeder, uses an equivalent load model to equate the load on the feeder to a concentrated load, and then replaces it with the equivalent impedance corresponding to the equivalent load. A distribution network closing-loop analysis model taking into account the load equivalent impedance is proposed, and on the basis of this model, the closing-loop impulse current is calculated.

[0048] 2) The attenuation of the non-periodic component of the closed-loop impulse current is not only related to the Ta of the branch, but also affected by the Ta of other branches and the current distribution in the network; accurate calculation requires solving the differential equations of the entire network, and the solution process is complicated and cumbersome, which is not practical in engineering calculations; according to the closed-loop model of the present invention, an effective method for solving the impulse current based on the optimal frequency method is proposed, which can improve the accuracy of the impulse current calculation and does not require a complicated power flow calculation process.

[0049] 3) The seamless power transfer method proposed in the present invention utilizes the voltage amplitude and phase angle compensation functions provided by series compensation to achieve seamless loop-closing operation, and then utilizes the impedance compensation function to achieve load power transfer operation; this method can not only achieve non-stop power transfer, but also expand the scope of application of loop-closing power transfer, eliminate the loop-closing impact current, and improve the safety and reliability of loop-closing power transfer. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a typical closed loop schematic diagram;

[0051] Figure 2 is the equivalent load diagram of feeder AB;

[0052] Figure 3 Flowchart for solving L1 and L2;

[0053] Figure 4 This is a closed loop model diagram taking into account the equivalent impedance of the load;

[0054] Figure 5 A conceptual diagram of a seamless power supply closed-loop system;

[0055] Figure 6 The work flow chart of the seamless power supply closed-loop system;

[0056] Figure 7 It is a closed-loop steady-state circuit of the conventional closed-loop power transfer equivalent model;

[0057] Figure 8 It is the waveform details of voltage and current during the exit process of the closing device and two transient processes;

[0058] Fig. 9 The range of the tie line closing power changing with the phase angle difference;

[0059] Fig.10 It is the curve of the capacity of the seamless loop system changing with the phase angle difference;

[0060] Fig.11 is the closed-loop power curve;

[0061] Fig.12 It is a seamless loop power supply system based on series compensation;

[0062] Fig.13 It is a steady-state equivalent model of seamless ring power supply system. DETAILED DESCRIPTION

[0063] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0064] A method for seamlessly closing a distribution network based on load equivalent impedance comprises the following steps:

[0065] Step 1: Feeder load processing:

[0066] See also Figure 1 In the figure, S1 and S2 are the outgoing line switches of feeder a and b respectively, S is the tie switch, S ai , S bj (i, j = 1, 2, ···, n) are the loads carried by feeders a and b respectively. When bus 1 fails or is under maintenance, bus 2 is required to transfer the load of bus 1. The specific operation process is: first close the tie switch, then open S1, and transfer the load of feeder a to feeder b to ensure uninterrupted power supply.

[0067] After load equivalence, Figure 2 .P K +jQ K is the power of equivalent load node K, I K ∠θ IK is the current phasor flowing through the equivalent load, I A ∠θ IA is the current phasor from node A to node K, I B ∠θ IB is the current phasor from node K to node B, L1 and L2 are the lengths of the equivalent load node from nodes A and B, respectively. To achieve load equivalence of feeder AB, μPMUs need to be configured at nodes A and B to obtain the voltage phasors of the distribution network nodes.

[0068] The principle of load equivalence is that the equivalent load node voltage obtained by the voltage drop formula at node A is equal to the equivalent load node voltage obtained by the voltage drop formula at node B, where:

[0069]

[0070] Among them, U K is the equivalent load node voltage obtained from node A, U' K is the equivalent load node voltage obtained from node B, U A , U Bare the voltages of the feeder head and end nodes A and B, respectively. L1 and L2 are the lengths of the equivalent load nodes from nodes A and B, respectively. The total length of feeder AB is L. The impedance of the feeder per unit length (km) is z=r+jx. P A ,P B is the active power of the feeder head and end nodes A and B, Q A ,Q B is the reactive power of the feeder head and end nodes A and B. L1 and L2 can be obtained by iteration. Given an error function and iteration step, the error function can meet the given accuracy ε by continuously correcting the length of L1, thereby determining the position K of the equivalent load. The calculation process is as follows: Figure 3 shown.

[0071] Depend on Figure 2 It can be concluded that

[0072]

[0073] In the formula, S *A , S *B are the conjugate values ​​of the power of nodes A and B respectively; U. *A , U .*B are the conjugate values ​​of the voltage phasors at nodes A and B, respectively, and U K is the voltage of equivalent load node K, I A ,I B is the current at nodes A and B, I K is the current of the equivalent load node K, and z is the impedance per unit length of the feeder. If there is a branch feeder between feeder AB, it is equivalent to connecting a load with the same power as the head end of the branch feeder at the branch node. The load can also be equivalent according to the comb feeder method.

[0074] Step 2: Closed-loop model of load equivalent impedance:

[0075] After obtaining the equivalent load parameters, the impedance Z of the equivalent load can be obtained from the power formula. K =R K +jX K ,Right now

[0076]

[0077] Among them, U K is the voltage of equivalent load node K, Z K =R K +jX K is the impedance of the equivalent load node K, P K is the active power of equivalent load node K, Q K is the reactive power of equivalent load node K, R X is the resistance of the equivalent load node K, XK is the reactance of the equivalent load node K.

[0078] Treat the load on the feeder as an equivalent load and replace it with its equivalent impedance. Convert the power supply and impedance above 10kV to the 10kV side to obtain the closed-loop model taking into account the equivalent impedance of the load as follows: Figure 4 As shown in the figure; U1 and U2 are the voltages of the voltage source above 10kV converted to the 10kV side, U3 and U4 are the 10kV bus voltages, and U a , U b is the voltage on both sides of the tie switch S. a0 The impedance equivalent to the 10kV side above the 10kV busbar, Z ka is the impedance corresponding to the equivalent load on side a, Z al1 is the equivalent impedance of the total load of other feeders on bus 1, Z al2 is the load S at the end of the feeder on side a an The equivalent impedance, Z a1 is the impedance of the feeder L1 section on side a, Z a2 is the impedance of the feeder L2 section on side a; the relevant impedance parameters on side b are similar to those on side a;

[0079] Step 3: Calculation of load equivalent impedance:

[0080] according to Figure 4 The closed-loop model can be used to calculate the Thevenin equivalent circuit of the port, and the equivalent impedance Z eq The calculation of the load equivalent impedance needs to be taken into account. The specific calculation formula is:

[0081]

[0082] Among them, Z eqa is the equivalent impedance of the tie switch a side; Z eqb is the equivalent impedance of the tie switch b side, Z a0 The impedance equivalent to the 10kV side above the 10kV busbar, Z ka is the impedance corresponding to the equivalent load on side a, Z al1 is the equivalent impedance of the total load of other feeders on bus 1, Z al2 is the load S at the end of the feeder on side a an The equivalent impedance, Z a1 is the impedance of the feeder L1 section on side a, Z a2 is the impedance of the feeder L2 section on side a; the relevant impedance parameters on side b are similar to those on side a;

[0083] Step 4: Analysis of closed loop current:

[0084] 4.1、Calculation of impact current of closed loop branch:

[0085] To solve the impact current during the closing process, it is necessary to perform transient analysis on the Thevenin equivalent circuit. ,ΔU ph is the phase voltage U on both sides of the tie switch before closing the loop aph , U bph The difference is

[0086] ΔUph= Uaph-Ubph (7)

[0087] Write Zeq in the form of Req+jωLeq and write the differential equation as

[0088]

[0089] In the formula, E m , ω, α are the amplitude, angular frequency and initial phase angle of the Thevenin equivalent voltage source respectively; R eq , L eq are the Thevenin equivalent resistance and inductance respectively; i c is the total current of the closed-loop branch ab. Assuming that the tie switch is closed at time t = 0, solve the periodic component and the non-periodic component in equation (8) and add them together to get i c for

[0090]

[0091] In the formula, i p 、i ap i c The periodic component and the non-periodic component of I c is the effective value of the periodic component; φ is the impedance angle of the Thevenin equivalent circuit; C is the initial value of the non-periodic component; Ta is the decay time constant.

[0092] 4.2. Calculation of decay time constant:

[0093] The optimal frequency method is to take a specific frequency (optimal frequency f) based on a certain time t of the closed loop. c ), the impedance of all components in the network (rated frequency f n ) is converted to this specific frequency f c The impedance under the frequency f is obtained by simplifying the network. c Equivalent impedance of the lower closing point

[0094] Z eq (f c )=R eq (f c )+jX eq (f c ) (10)

[0095] Then the decay time constant Ta of the closing point is

[0096]

[0097] Optimum frequency f c The selection is related to the time t. For a power grid with a rated frequency of 50Hz, when t = 0.01s, the optimal frequency f c Usually 20Hz is used. The key to the accuracy of calculating the closed-loop impulse current in this article lies in the impulse coefficient KM or Ta. As long as 0.005≤R / X≤5.0, the deviation of the impulse coefficient KM will not exceed ±5%.

[0098] If we assume that t = 0 is the most serious moment, we can get the full current expression of the closed-loop branch ab in the most serious case:

[0099]

[0100] Then the maximum value is the closed-loop impact current of branch ab.

[0101] 4.3. Feeder impulse current calculation:

[0102] For the inrush current flowing through the feeder switch, the superposition principle is applied to solve it. Add the total current of ic1 to the total current of feeder a before closing the loop, and you can get the expression of the total current ia of feeder a after closing the loop; subtract the total current of ic2 from the total current of feeder b before closing the loop, and you can get the expression of the total current ib of feeder b after closing the loop. Assuming that the tie switch is closed at t = 0, there is

[0103]

[0104] In the formula, I a0 ,θ a are the effective value and initial phase angle of the current in feeder a before closing the loop; I b0 ,θ b are the effective value and initial phase angle of the current in feeder b before closing the loop; I c1 ,θ c1 I .c Current I distributed to the head end of feeder a .c1 The effective value and initial phase angle of I c2 ,θ c2 I .c The current I distributed to the head end of feeder b. c2 The effective value and initial phase angle of C1 and C2 are i a 、i b Initial value of the non-periodic component.

[0105] Assuming that t = 0 is the most severe closing moment, consider the feeder closing impulse current under the most severe condition. Obviously, when the most severe circulating current ic flows through the closing branch, the expression in equation (12) has the most severe feeder closing impulse current. Then the full current expression of feeder a under the most severe condition is obtained as

[0106]

[0107] In the formula, I a0 ,θ a are the effective value and initial phase angle of the current in feeder a before closing the loop, I c1 ,θ c1 I .c Current I distributed to the head end of feeder a .c1 The effective value and initial phase angle of C1 is i a Initial value of the non-periodic component.

[0108] Similarly, the total current of feeder b in the worst case can be calculated as

[0109]

[0110] In the formula, I b0 ,θ b are the effective value and initial phase angle of the current in feeder b before closing the loop, I c2 ,θ c2 I .c Current I distributed to the head end of feeder b .c2 The effective value and initial phase angle of C2 is i b Initial value of the non-periodic component.

[0111] Step 5: Seamless loop transfer method:

[0112] The seamless loop-closed power supply method uses a controllable voltage source to compensate for the voltage difference on both sides of the loop-closed point to eliminate the loop-closed impact, control the loop-closed current within the allowable range of the feeder, and reduce the impact of the loop-closed power supply on the original open-loop system. The controllable voltage source connected between bus 1 and bus 2 is actually a voltage source with an amplitude of U c (0≤U c ≤U cmax ), a synchronous sinusoidal voltage source with a phase angle of δ (-180°≤δ≤180°), which provides the main function of seamless loop transfer power supply, and the AC voltage source can be easily realized by using series compensation. Load transfer and loop current control mainly utilize the impedance adjustment function of the series voltage source. At this time, the voltage source can be equivalent to a series impedance Zeq=Req+jXeq that varies in the range of 0~∞, so the size of the loop current can be determined as

[0113]

[0114] In the formula, w is the synchronous rotation angular frequency, I C is the loop current, U1, U2 are the voltages of busbars 1 and 2, Z eq is the series impedance, R eq is the series resistance, L C is a series capacitor.

[0115] After adopting impedance compensation, the size of the closed-loop current is completely affected by the compensation impedance. By selecting a suitable compensation impedance, the maximum value of the closed-loop current can be controlled within a smaller range.

[0116] like Figure 5 As shown. It consists of a series compensation voltage source uc, series impedances Rc, Lc, access switches K1, K2 and a tie switch K, and is connected to a system with voltages u1 and u2 on both sides of the closing point through the access switch. The tie switch is used to bypass the SLTS and connect the two substations. The amplitude difference and phase angle difference of u1 and u2 are represented by ΔU and δ respectively. The currents flowing through the two distribution transformers are represented by is1 and is2 respectively, and the load currents are represented by il1 and il2 respectively. The current flowing through the SLTS is represented by ic. The resistor Rc simulates the active power loss of the SLTS, and the inductor Lc is mainly used to limit the current.

[0117] If the distribution transformer T-2 on the main supply side needs maintenance, the load L2 will be transferred to the backup supply side through SLTS. The operation of SLTS is divided into three stages, namely seamless loop closing, load transfer and live exit.

[0118] 5.1. Seamless loop closing: In order to achieve seamless loop closing, adjust the compensation voltage output by the series compensation module until the loop closing condition is met, that is, ΔU max , δ<δ max When it is detected that the closing loop condition is met, a closing loop instruction is sent to close switches K1 and K2. From an equivalent point of view, the series compensation voltage is the open circuit voltage of the closing loop port. Theoretically, as long as the voltage difference at the closing loop point is small enough, the current generated at the moment of closing the loop is approximately zero. Therefore, switches K1 and K2 work in zero voltage switching mode.

[0119] 5.2. Load transfer: SLTS works in load transfer mode, and the load of bus 2 will be smoothly transferred to bus 1. The loop current is controlled to slowly increase from zero to the load current. This part of the current flowing through the series voltage source will generate active power and reactive power. This part of the power determines the capacity of the system, and the system must be able to provide active power to the system. After detecting that the load transfer is completed, the outgoing line circuit breaker CB2 of the distribution transformer T-2 is disconnected, and the distribution transformer T-2 seamlessly exits operation.

[0120] ​5.3. Live exit: In order to ensure the live exit of the system and reduce the impact on the distribution network during exit, the operation mode of SLTS needs to be adjusted before closing the bypass switch. The injected voltage is adjusted so that the voltage amplitude and phase angle on both sides of the bypass switch are consistent, and the bypass switch is closed in zero voltage switching mode. During the exit process, the closed loop composed of the bypass switch and the series voltage source can be regarded as a first-order RL circuit. The current of SLTS decays according to the time constant τ = Lc / Rc. There is a commutation process between the two branches. After the commutation is completed, the load current is transferred to the bypass branch, and K1 and K2 are turned off in the zero current switching mode, thereby realizing the seamless exit of the system.

[0121] 5.4 Capacity Analysis Fig.13 For the steady-state equivalent model of the seamless loop power supply system, the series and parallel compensation modules are equivalent to two voltage sources with series impedances. In the following analysis, all electrical quantities are expressed in per unit.

[0122] Assume that the voltage of the power supply at both ends is constant during the power transfer process and the load is a constant power load. The phase angle of voltage U1 is set to δ, the phase angle of voltage U2 is set to 0, and the phase angle of the series compensation voltage is set to (δ+ρ).

[0123]

[0124] Among them, the phase angle of voltage U1 is set to δ, the phase angle of voltage U2 is set to 0, the phase angle of series compensation voltage is set to (δ+ρ), U1 and U2 are the voltages on both sides, U C is the compensation voltage.

[0125] After the ring is closed, the complex power of the tie line is:

[0126]

[0127] Among them, P CL is the active power of the tie line, Q CL is the tie line reactive power, X T is the transformer reactance, U1 and U2 are the voltages on both sides, and U C is the compensation voltage.

[0128] The active power and reactive power of the tie line are:

[0129]

[0130] Among them, P CL is the active power of the tie line, Q CL is the tie line reactive power, X T is the transformer reactance, U1 and U2 are the voltages on both sides, and U CTo compensate the voltage, the phase angle of voltage U1 is set to δ, the phase angle of voltage U2 is set to 0, and the phase angle of the series compensation voltage is set to (δ+ρ).

[0131] The tie line power consists of two parts. The first part is the power transmitted without series compensation determined by the phase angle difference δ, and the second part is the power added by series compensation. Since the angle ρ varies freely within the range of -180° to 180° under a given phase angle difference δ, the adjustment range of the tie line active power is:

[0132]

[0133] Among them, U1 and U2 are the voltages on both sides, U C The phase angle of voltage U1 is set to δ, the phase angle of voltage U2 is set to 0, and the phase angle of series compensation voltage is set to (δ+ρ). CL is the active power of the tie line, Q CL is the tie line reactive power, X T is the transformer reactance.

[0134] The adjustment range of the tie line reactive power is:

[0135]

[0136] Among them, U1 and U2 are the voltages on both sides, U C The phase angle of voltage U1 is set to δ, the phase angle of voltage U2 is set to 0, and the phase angle of series compensation voltage is set to (δ+ρ). CL is the active power of the tie line, Q CL is the tie line reactive power, X T is the transformer reactance.

[0137] After adding the series compensation voltage, the active power transmitted by distribution transformer T-1 to bus 1 is:

[0138]

[0139] Among them, the phase angle of voltage U1 is set to δ, the phase angle of voltage U2 is set to 0, the phase angle of series compensation voltage is set to (δ+ρ), U1 and U2 are the voltages on both sides, U C is the compensation voltage, X T is the transformer reactance, P T1 is the active power of transformer T-1, Q T1 is the reactive power of transformer T-1, P CL is the active power of the tie line, Q CL is the reactive power of the interconnection line.

[0140] The capacity of the seamless loop power transfer system mainly depends on the length of the compensation voltage phase and the tie line current. According to Kirchhoff's voltage law, the compensation voltage is:

[0141]

[0142] Among them, the phase angle of voltage U1 is set to δ, the phase angle of voltage U2 is set to 0, the phase angle of series compensation voltage is set to (δ+ρ), U1 and U2 are the voltages on both sides, U C is the compensation voltage, X T is the transformer reactance, I C is the interconnection line current.

[0143] Therefore, the system capacity is finally SSLTS = |Uc|max|Ic|max. |Uc| max is the maximum compensation voltage, |Ic| max It is the maximum value of the tie line current, which is equal to the load current on the main supply side.

[0144] 5.5. Topology: Fig.12 A seamless ring-type power supply system based on partial power topology is described. It consists of a shunt compensation module, a series compensation module, and an industrial frequency transformer. The shunt compensation module is connected to bus 1 through the commutation inductor Lsh, and the AC terminal of the series compensation module is connected to an LC filter to prevent harmonics at the switching frequency from flowing into the power grid. The series compensation module is connected in series between two substations through an industrial frequency transformer. The industrial frequency transformer can provide electrical isolation between input and output and limit fault short-circuit current. It is worth noting that the series compensation module adopts a three-phase four-bridge arm topology to improve the system's ability to carry unbalanced loads. This topology adopts an AC-DC-AC conversion method and supports bidirectional power flow. The working principle is that the shunt compensation module works in rectification mode, obtains energy from the power grid and stores it in DC capacitors. The series compensation module acts as an inverter, converting DC into AC and returning it to the power grid, which is equivalent to Figure 5 In addition, both parallel and series compensation modules can exchange reactive power with the system independently.

[0145] Calculation example:

[0146] In order to analyze the advantages and disadvantages of conventional ring-closing power supply technology, a ring-closing power supply test was conducted on two low-voltage substations in a certain area. The two substations with ring-closing power supply come from two public transformer rooms on the same upper feeder line. The structural diagram is shown in the figure below. Figure 7 The two substations are connected through a fixed low-voltage ring-closing power supply device, the ring-closing switch K is closed, and the distribution transformer T-2 has been removed from maintenance.

[0147] Figure 8The waveforms of voltage and current during the exit of the closing device and the waveform details of the two transient processes are given. At T1, the circuit breaker CB2 on the T-2 side of the distribution transformer is closed to realize the closing operation, and there is no impact at the moment of closing; at T2, the contact switch K is disconnected to realize the opening operation. The voltage amplitude difference of phase A on both sides of the closing point is about 0.25V, the phase angle difference is 0.15°, and the fundamental amplitudes of the closing current of the three phases abc are 16.47A, 16.64A, and 28.88A respectively. The calculated closing current amplitude of phase A is 17.77A, and the error with the measured value is only 7.89%. The error is mainly due to the failure to consider the voltage amplitude difference and harmonic influence. FFT analysis of the grid voltage and load current shows that the THD value of the grid voltage is 2.17%, and the 5th harmonic content is the largest. The fundamental amplitudes of the three-phase load current are 73.84A, 84.4A, and 78.5A respectively. The load current is asymmetrical, with a maximum THD value of 28.78%, and the third and fifth harmonic current contents are the largest.

[0148] The power of phase A of the tie line is Fig.11 As shown in the figure. 15 seconds ago, the active power of the tie line was 12kW and the reactive power was 2.5kvar. 15 seconds later, the load power was redistributed between the two distribution transformers, and the active power of the tie line dropped to 2.5kW and the reactive power dropped to 1.2kvar. 95 seconds later, the tie switch was disconnected and the tie line power dropped to zero. The load power remained basically unchanged within the minute time scale.

[0149] According to the above analysis, the conventional ring-closing power supply has less obvious ring-closing impact when the angle difference is relatively small, and can achieve safe and reliable ring-closing power supply. Due to its simple structure, it has been applied in many adjacent low-voltage areas, but it is difficult to cover all low-voltage areas. The measured grid operation data shows that the operating environment of the ring-closing power supply device is very severe, and it needs to work reliably in the asymmetric and harmonic environment of the grid.

[0150] Assuming that the maximum compensation angle provided by the seamless loop power supply system is 30°, the maximum value of the compensation voltage is Uc max = 2sin15o UN = 0.52UN, X T Take 0.06, and take the rated capacity of the distribution transformer as the reference value to make the range of the tie line closing power changing with the phase angle difference as follows: Fig. 9 As shown in the figure. The blue area in the figure is the maximum adjustment range of active power and reactive power after adding series compensation, covering most of the operating range of the rated power of the forward or reverse transmission distribution transformer (red shaded area). Seamless ring-closing power supply realizes flexible control of the closing power of the tie line, solving the problem of uncontrollable closing power of conventional ring-closing power supply.

[0151] Fig.10The curves of SLTS capacity (with distribution transformer capacity as the reference value) changing with phase angle difference when the main supply side distribution transformer has different load rates are described. Fig.10 It can be seen that the capacity of SLTS shows an asymmetric distribution with the phase angle difference. This is because when the phase angle difference is negative, the series compensation voltage needs to absorb a large reactive power. When facing the 30° distribution network ring closing and power supply scenario, under the most demanding conditions, the maximum capacity of SLTS is only 58% of the rated capacity of the distribution transformer. The cost of achieving seamless ring closing and expanding the power supply scenario is only about half of the rated capacity of the distribution transformer. This is only considered when the distribution transformer is fully loaded. The actual distribution network distribution transformer is generally under a 1 / 4 load rate condition. At this time, the capacity of SLTS is about 12% of the rated capacity of the distribution transformer. If a modular design concept is adopted, it can not only effectively reduce the capacity of the system, but also achieve the effect of capacity expansion by combining multiple modules.

[0152] The present invention considers the influence of load on the steady-state and transient process of the closed loop, performs equivalent processing on the load based on the real-time measurement information of a small number of micro-synchronous phasor measurement devices at the head and end of the feeder, and proposes a closed loop power supply analysis model that takes into account the equivalent impedance of the load. On the basis of the model, the calculation of the closed loop steady-state current and the impulse current is analyzed in detail using the superposition principle. In view of the shortcomings of the current solution of the closed loop impulse current, an effective method for solving the closed loop impulse current based on the optimal frequency method is proposed. In addition, the present invention proposes a new method for seamless closed loop power supply of the distribution network based on series compensation. The voltage amplitude and phase angle compensation functions provided by the series compensation are used to realize the seamless closed loop operation, and the impedance compensation function is used to realize the load transfer operation. This method can not only realize non-stop power supply transfer, but also expand the scope of application of closed loop power supply, eliminate the closed loop impulse current, and improve the safety and reliability of closed loop power supply. In addition, the basic working principle of the system is analyzed, including the three operating stages of seamless closed loop, load transfer and live exit, and the closed loop power and system capacity of the seamless closed loop power supply system. The analysis results show that the proposed method can control the interconnection line power within the safe operation area within the phase angle difference range of -30°~30°.

[0153] The above embodiments are only preferred technical solutions of the present invention and should not be regarded as limiting the present invention. The protection scope of the present invention shall be the technical solutions recorded in the claims, including equivalent replacement solutions of the technical features in the technical solutions recorded in the claims. That is, equivalent replacement improvements within this scope are also within the protection scope of the present invention.

Claims

1. A method for seamlessly closing a distribution network based on load equivalent impedance, characterized in that: It includes the following steps: Step 1: Processing of feeder load: Use the equivalent load model to treat all loads between a section of feeders as one load. If there is a branch feeder between feeders, it is equivalent to connecting a load at the branch node with a power equal to the head end of the branch feeder. Step 2: Construct a closed-loop model of load equivalent impedance: treat the load on the feeder as an equivalent load and replace it with its equivalent impedance. Convert the power supply and impedance above 10kV to the 10kV side to obtain a closed-loop model taking into account the load equivalent impedance. Step 3: Calculate the load equivalent impedance: According to the closed-loop model, find the Thevenin equivalent circuit of the port. The calculation of the equivalent impedance Zeq needs to include the load equivalent impedance. Step 4: Analyze the closed-loop current: Perform transient analysis on the Thevenin equivalent circuit, write Zeq in the form of Req+jωLeq, write differential equations, analyze the decay time constant Ta, and solve the maximum effective value of the closed-loop current; Step 5, seamless closing and power transfer process: Use a controllable voltage source to compensate for the voltage difference on both sides of the closing point to eliminate the closing impact, and control the closing current within the allowable range of the feeder. After impedance compensation, the size of the closing current is completely affected by the compensation impedance. Select a suitable compensation impedance to control the maximum value of the closing current within a smaller range.

2. The method for seamlessly closing a distribution network based on load equivalent impedance according to claim 1 is characterized in that: In step 1, the phasor data of the distribution network nodes are obtained in real time through the micro-synchronous phasor measurement device μPMU; FIG2 is an equivalent load diagram of feeder AB, where P K +jQ K is the power of equivalent load node K, I K ∠θ IK is the current phasor flowing through the equivalent load, I A ∠θ IA is the current phasor from node A to node K, I B ∠θ IB is the current phasor flowing from node K to node B, L1 and L2 are the lengths of the equivalent load node from nodes A and B respectively; to achieve load equivalence of feeder AB, μPMUs need to be configured at nodes A and B to obtain the voltage phasor of the distribution network nodes; The principle of feeder AB load equivalence is that the equivalent load node voltage obtained by the node A voltage drop formula is equal to the equivalent load node voltage obtained by the node B voltage drop formula, where: Among them, U K is the equivalent load node voltage obtained from node A, U ’ K is the equivalent load node voltage obtained from node B, U A , U B are the voltages of the feeder head and end nodes A and B, respectively. L1 and L2 are the lengths of the equivalent load nodes from nodes A and B, respectively. The total length of feeder AB is L. The impedance of the feeder per unit length (km) is z=r+jx; P A ,P B is the active power of the feeder head and end nodes A and B, Q A ,Q B The reactive power of the feeder head and end nodes A and B, L1 and L2 can be obtained by iteration. Given an error function and iteration step, the error function can meet the given accuracy ε by continuously correcting the length of L1, thereby determining the position K of the equivalent load.

3. The method for seamlessly closing a distribution network based on load equivalent impedance according to claim 1 is characterized in that: In step 2, the impedance Z of the equivalent load is obtained by the power formula. K =R K +jX K : Among them, U K is the voltage of equivalent load node K, Z K is the impedance of the equivalent load node K, P K is the active power of equivalent load node K, Q K is the reactive power of equivalent load node K, R X is the resistance of the equivalent load node K, X K is the reactance of equivalent load node K; The load equivalent impedance is based on the equivalent of real-time measurement information and can reflect the real-time load distribution of the feeder, thereby obtaining the equivalent circuit of feeder AB.

4. The method for seamlessly closing a distribution network based on load equivalent impedance according to claim 1 is characterized in that: In step 3, in the Thevenin equivalent circuit, ΔU ph is the phase voltage U on both sides of the tie switch before closing the loop aph , U bph The difference between the equivalent impedance Z eq The calculation formula is: Among them, Z eqa is the equivalent impedance of the tie switch a side; Z eqb is the equivalent impedance of the tie switch b side, Z a0 The impedance equivalent to the 10kV side above the 10kV busbar, Z ka is the impedance corresponding to the equivalent load on side a, Z al1 is the equivalent impedance of the total load of other feeders on bus 1, Z al2 is the load S at the end of the feeder on side a an The equivalent impedance, Z a1 is the impedance of the feeder L1 section on side a, Z a2 is the impedance of the feeder L2 section on side a; the relevant impedance parameters on side b are similar to those on side a.

5. The method for seamlessly closing a distribution network based on load equivalent impedance according to claim 1 is characterized in that: The step 4 specifically comprises the following steps: 4.1、Calculation of the impulse current of the closed-loop branch: Perform transient analysis on the Thevenin equivalent circuit, write Zeq in the form of Req+jωLeq, and write the differential equation. Em, ω, α are the amplitude, angular frequency and initial phase angle of the Thevenin equivalent voltage source respectively; Req and Leq are the Thevenin equivalent resistance and inductance respectively; i c is the total current of the closed-loop branch ab; 4.2 Decay time constant T a Calculation: For a certain moment after the loop is closed, take a specific frequency, that is, the optimal frequency f c , all rated frequencies f in the closed-loop network n The impedance under is converted to f c The impedance under the condition of , find the Thevenin equivalent impedance Zeq(fc)=Req(fc)+jXeq(fc) under the optimum frequency; 4.3 Feeder inrush current calculation: For the inrush current flowing through the feeder switch, the superposition principle is applied to solve it; i c1 The total current of the feeder a before the loop is added to the total current of the feeder a before the loop is closed, and the total current i of the feeder a after the loop is closed can be obtained. a Expression: Subtract i from the total current of the feeder b before the loop is closed c2 The total current of feeder b after closing the loop can be obtained by b expression.

6. The method for seamlessly closing a distribution network based on load equivalent impedance according to claim 1, characterized in that: The step 5 includes the following steps: 5.

1. Seamless loop closing: In order to achieve seamless loop closing, adjust the compensation voltage output by the series compensation module until the loop closing condition is met, that is, ΔU max , δ<δ max , when it is detected that the closing loop condition is met, a closing loop instruction is sent to close switches K1 and K2;​ 5.

2. Load transfer: The seamless loop system works in the load transfer mode, and the load of bus 2 will be smoothly transferred to bus 1; the loop current is controlled to slowly increase from zero to the load current. This part of the current flowing through the series voltage source will generate active power and reactive power; this part of the power determines the capacity of the system, and the system must be able to provide active power to the system; after detecting that the load transfer is completed, the outgoing line circuit breaker CB2 of the distribution transformer T-2 is disconnected, and the distribution transformer T-2 seamlessly exits operation; 5.

3. Live exit: Adjust the injected voltage so that the voltage amplitude and phase angle on both sides of the bypass switch remain consistent, and the bypass switch is closed in the zero voltage switching mode; during the exit process, the closed loop composed of the bypass switch and the series voltage source is regarded as a first-order RL circuit, and the current of the seamless loop transfer system decays according to the time constant τ=Lc / Rc. There is a commutation process between the two branches. After the commutation is completed, the load current is transferred to the bypass branch, and K1 and K2 are turned off in the zero current switching mode, thereby realizing the seamless exit of the system; 5.4、System capacity analysis: Assume that the voltage of the power supply at both ends is constant during the power transfer process and the load is a constant power load; Among them, the phase angle of voltage U1 is set to δ, the phase angle of voltage U2 is set to 0, the phase angle of series compensation voltage is set to (δ+ρ), U1 and U2 are the voltages on both sides, U C is the compensation voltage; After the ring is closed, the complex power of the tie line is: Among them, P CL is the active power of the tie line, Q CL is the tie line reactive power, X T is the transformer reactance, U1 and U2 are the voltages on both sides, and U C is the compensation voltage; The active power and reactive power of the tie line are: Among them, P CL is the active power of the tie line, Q CL is the tie line reactive power, X T is the transformer reactance, U1 and U2 are the voltages on both sides, and U C For the compensation voltage, the phase angle of voltage U1 is set to δ, the phase angle of voltage U2 is set to 0, and the phase angle of the series compensation voltage is set to (δ+ρ); After adding the series compensation voltage, the active power and reactive power transmitted to bus 1 by distribution transformer T-1 are: Among them, the phase angle of voltage U1 is set to δ, the phase angle of voltage U2 is set to 0, the phase angle of series compensation voltage is set to (δ+ρ), U1 and U2 are the voltages on both sides, U C is the compensation voltage, X T is the transformer reactance, P T1 is the active power of transformer T-1, Q T1 is the reactive power of transformer T-1, P L1 is the active power of the tie line L-1, Q L1 is the reactive power of tie line L-1; The capacity of the seamless loop power transfer system mainly depends on the length of the compensation voltage phase and the tie line current. According to Kirchhoff's voltage law, the compensation voltage is: Among them, the phase angle of voltage U1 is set to δ, the phase angle of voltage U2 is set to 0, the phase angle of series compensation voltage is set to (δ+ρ), U1 and U2 are the voltages on both sides, U C is the compensation voltage, X T is the transformer reactance, I C is the tie line current; Therefore, the system capacity is finally S SLTS =|Uc| max |Ic| max , where |Uc| max is the maximum compensation voltage, |Ic| max It is the maximum value of the tie line current, which is equal to the load current on the main supply side.

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