Virtual synchronous control-based grid-construction type energy storage short-circuit current calculation method

Abstract

The invention provides a virtual synchronous control-based network construction type energy storage short-circuit current calculation method. The method comprises the following steps of: constructing an electrical equation based on fixed value virtual impedance control; constructing a control equation based on virtual synchronous control; based on the fault equation, solving an electrical equation by adopting Laplace transformation and substituting an initial value condition to obtain a current equation; the electrical equation and the control equation are combined to obtain an internal potential expression, the internal potential expression is substituted into the current equation to obtain an initial current expression, and pole solving and fractional expansion are performed on the initial current expression to obtain a full-time-domain short-circuit current expression so as to calculate the short-circuit current of the constructed network type energy storage. According to the virtual synchronous control-based short-circuit current calculation method for the network-building type energy storage, the influence of the virtual synchronous control on the power loop response under the fault and the influence of the fixed-value virtual impedance fault ride-through strategy on the short-circuit current are considered at the same time, and the short-circuit current of the network-building type energy storage calculated by the method is good in feasibility and high in accuracy.

Description

Technical Field

[0001] The present invention relates to the technical field of grid-type energy storage, and in particular to a method for calculating short-circuit current of grid-type energy storage based on virtual synchronous control. Background Art

[0002] Short-circuit current calculation is the basis for safe and stable operation of power grids. For example, protection setting and equipment selection are strongly dependent on the recognition and analysis of the fault characteristics of each device. However, with the construction of new power systems, the power supply characteristics are becoming more and more different from traditional synchronous motors, and the network characteristics of the grid-connected systems are becoming more and more diverse.

[0003] Among them, the grid-following power supply has problems such as the traditional phase-locked loop affecting the performance of the converter under the conditions of large fluctuations, many disturbances and small short-circuit ratio of the weak power grid. In order to meet the needs of new energy for voltage, frequency and inertia support, the grid-forming power supply has been developed. At present, energy storage devices are the mainstream of power supply units.

[0004] However, the research on the static and transient characteristics of grid-connected energy storage is still insufficient, and new control methods such as PQ control, VF control, droop control, matching control, and virtual oscillation control have been developed. In addition, there are fault-crossing strategies such as modifying the current reference value, inputting virtual impedance and voltage limiter. The fault characteristics of grid-connected energy storage depend on different control strategies, which directly causes the short-circuit current calculation method of traditional synchronous machines to be unable to be directly applied to grid-connected energy storage. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention provides a method for calculating short-circuit current of a grid-type energy storage system, which can simultaneously take into account the influence of virtual synchronous control on power loop response under fault and the influence of a fixed value virtual impedance fault ride-through strategy on short-circuit current.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows: a method for calculating short-circuit current of a grid-type energy storage based on virtual synchronous control, comprising the following steps:

[0007] Based on the grid-type energy storage, obtain the port filter parameters and control parameters;

[0008] Based on the fixed value virtual impedance control, an electrical equation is constructed according to the port filter parameters and the control parameters; the electrical equation is a relationship between the voltage at the common connection point and the equivalent internal potential of the grid-type energy storage;

[0009] Based on virtual synchronous control, constructing a control equation according to the port filter parameters and the control parameters;

[0010] Acquiring fault equations and initial value conditions based on the grid-type energy storage;

[0011] Based on the fault equation, the electrical equation is solved by Laplace transform and the initial value condition is substituted to obtain the current equation;

[0012] Based on the fault equation, the electrical equation and the control equation are combined to obtain an internal potential expression;

[0013] Substituting the internal potential expression into the current equation to obtain an initial current expression, wherein the initial current expression is a current expression in the complex domain during a fault period;

[0014] Obtaining a full time domain short-circuit current expression by performing pole solving and fraction expansion on the initial current expression;

[0015] The short-circuit current of the grid-type energy storage is obtained according to the full-time domain short-circuit current expression.

[0016] Furthermore, the electrical equation, the control equation, the fault equation and the initial value condition are obtained in a synchronously rotating coordinate system.

[0017] Furthermore, the fault equation includes an internal potential equation and a voltage equation; the internal potential equation is an expression of the instantaneous value of the equivalent internal potential of the grid-type energy storage, and the voltage equation is an expression of the first voltage, which is the instantaneous value of the voltage at the common connection point of the grid-type energy storage.

[0018] Furthermore, the steps of constructing the voltage equation are:

[0019] Obtaining the drop multiple and the voltage before the fault; the drop multiple is the multiple of the voltage drop after the fault to the voltage before the fault;

[0020] The voltage equation is constructed according to the drop multiple, the pre-fault voltage and the phase difference, wherein the phase difference is the phase difference between the equivalent internal potential and the common connection point voltage under the d-axis voltage directional control of the grid-type energy storage.

[0021] Furthermore, the initial value conditions include an initial value of the current before the fault and an initial value of the equivalent internal potential before the fault.

[0022] Further, the control parameters include a first control parameter and a second control parameter;

[0023] The first control parameter is a parameter based on the fixed value virtual impedance control;

[0024] The second control parameter is a parameter based on the virtual synchronous control.

[0025] Further, the first control parameter includes an equivalent total resistance and an equivalent total reactance;

[0026] The equivalent total resistance is the sum of a first resistance and a second resistance, the first resistance is a resistance value set in a fixed-value virtual impedance, and the second resistance is a filter resistance at an output port of a network-type energy storage;

[0027] The equivalent total reactance is the sum of a first reactance and a second reactance, wherein the first reactance is the reactance value set in the fixed virtual impedance, and the second reactance is the reactance value of the filter inductance at the output port of the grid-type energy storage at the grid frequency.

[0028] Furthermore, the second control parameters include a virtual synchronous control active loop control coefficient, a virtual synchronous control reactive loop control coefficient, an active reference value, a reactive reference value and a voltage reference value.

[0029] Furthermore, the steps of combining the electrical equation and the control equation to obtain the internal potential expression are:

[0030] Obtain the grid frequency and virtual synchronization control parameters of the grid-connected energy storage;

[0031] Constructing an initial expression of a control equation according to the grid frequency and the virtual synchronous control parameter;

[0032] Obtain the instantaneous power expression in the synchronous rotating coordinate system;

[0033] Substituting the instantaneous power expression into the initial expression of the control equation to obtain the control equation;

[0034] Based on the complex domain, a reactive power control equation in the complex domain is obtained according to the fault equation and the control equation;

[0035] The electrical equation and the reactive power control equation in the complex domain are combined to obtain the internal potential expression.

[0036] Furthermore, based on the three-phase short-circuit current formula under the symmetrical fault of the grid-forming energy storage, the short-circuit current of the grid-forming energy storage is obtained according to the full-time domain short-circuit current expression.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] The present invention can simultaneously take into account the influence of virtual synchronous control on the power loop response under fault conditions, as well as the influence of the fixed virtual impedance fault ride-through strategy on the short-circuit current, thereby realizing the analysis of the full-time domain short-circuit current of the grid-type energy storage, and providing the attenuation and frequency characteristics of each component of the short-circuit current, which is feasible and accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a schematic diagram of the flow of the method for calculating short-circuit current of a grid-type energy storage based on virtual synchronous control of the present invention;

[0040] Figure 2 It is a schematic diagram of the topological structure of the grid-type energy storage in the example of the present invention;

[0041] Figure 3 It is a schematic diagram of the control structure of the grid-type energy storage in the example of the present invention;

[0042] Figure 4 It is a schematic diagram of the equivalent circuit of the grid-type energy storage considering the control structure in the calculation example of the present invention;

[0043] Figure 5 It is a schematic diagram of the short-circuit current simulation results of the grid-type energy storage in the example of the present invention. DETAILED DESCRIPTION

[0044] In order to clearly illustrate the technical features of this solution, this solution is described below through a specific implementation method.

[0045] Considering the influence of control on short-circuit current characteristics, an embodiment of the present invention provides a method for calculating short-circuit current of a grid-type energy storage based on virtual synchronous control, comprising the following steps:

[0046] See also Figure 1 , based on the grid-type energy storage, obtain the port filter parameters and control parameters; the port filter parameters include the filter resistance R at the output port of the grid-type energy storage f The filter inductance L at the output port of the grid-type energy storage f ; The control parameters include a first control parameter and a second control parameter; the first control parameter is a parameter based on a fixed value virtual impedance control;

[0047] The first control parameter includes an equivalent total resistance R and an equivalent total reactance L; the equivalent total resistance R is the sum of the first resistance and the second resistance, that is:

[0048] R=R v +R f ;

[0049] In the formula, R v is the first resistor, that is, the resistance value set in the fixed virtual impedance, R f is the second resistor, i.e., the filter resistor at the output port of the network-type energy storage;

[0050] The equivalent total reactance L is the sum of the first reactance and the second reactance, that is:

[0051] L=L v +L f ;

[0052] Where, L v is the first reactance, that is, the reactance value set in the fixed virtual impedance, L fis the second reactance, which is the reactance value of the filter inductance at the output port of the grid-type energy storage at the grid frequency ω;

[0053] Based on the fixed value virtual impedance control, the electrical equation is constructed according to the port filter parameters and the control parameters; the electrical equation is the relationship between the voltage at the common connection point and the equivalent internal potential of the grid-type energy storage;

[0054] When the grid-type energy storage is operating normally or fails, the voltage at the common connection point and the equivalent internal potential of the grid-type energy storage satisfy the three-phase coordinate electrical equation, which is:

[0055]

[0056] In the formula, e abc is the column vector composed of the instantaneous values ​​of the three-phase equivalent internal potential of the grid-type energy storage, u abc is the column vector composed of the instantaneous values ​​of the three-phase voltages at the common connection point of the grid-type energy storage, i abc It is a column vector composed of the instantaneous values ​​of the three-phase current between the grid-type energy storage and the common connection point;

[0057] In the synchronous rotating coordinate system, the electrical equation can be obtained by introducing the fixed value virtual impedance control. The electrical equation is:

[0058]

[0059] In the formula, e d is the instantaneous value of the d-axis component of the equivalent internal potential of the grid-type energy storage, e q is the q-axis component of the instantaneous value of the equivalent internal potential of the grid-type energy storage, u d is the d-axis component of the instantaneous voltage at the common connection point of the grid-type energy storage, u q is the q-axis component of the instantaneous voltage value at the common connection point of the grid-type energy storage, i d is the instantaneous d-axis component of the current at the common connection point of the grid-type energy storage, i q is the q-axis component of the instantaneous value of the current at the common connection point of the grid-type energy storage;

[0060] The second control parameter is a parameter based on virtual synchronous control, namely, a virtual synchronous control parameter; the second control parameter includes a virtual synchronous control active loop control coefficient m, a virtual synchronous control reactive loop control coefficient n, an active reference value P ref , reactive power reference value Q ref And voltage reference value U n .

[0061] Based on virtual synchronous control, the control equation is constructed according to the port filter parameters and control parameters; the initial expression of the control equation is:

[0062]

[0063] In the formula, θ ref is the transformation phase of the three-phase stationary coordinate system to the two-phase rotating coordinate system, E ref is the internal potential reference value calculated by the reactive loop, which can be regarded as the actual equivalent internal potential value, P is the actual active power output by the grid-forming energy storage, Q is the actual reactive power output by the grid-forming energy storage, and ω is the grid frequency;

[0064] Among them, the actual active power output by the grid-building energy storage and the actual reactive power output by the grid-building energy storage are both instantaneous power, and the expression of instantaneous power is:

[0065]

[0066] Substituting the expression of instantaneous power into the initial expression of the control equation, we obtain the control equation; the control equation is:

[0067]

[0068]

[0069] In the formula, m1, m2, n1, and n2 are all constants;

[0070] Based on the grid-type energy storage, a fault equation and initial value conditions are obtained, wherein the initial value conditions include an initial value of the current before the fault and an initial value of the equivalent internal potential before the fault;

[0071] The fault equation is the condition that the voltage satisfies after the fault. The fault equation includes the internal potential equation and the voltage equation. The internal potential equation is the expression of the instantaneous value of the equivalent internal potential of the grid-type energy storage. The voltage equation is the expression of the first voltage, which is the instantaneous value of the voltage at the common connection point of the grid-type energy storage. The steps of constructing the voltage equation are:

[0072] Obtain the drop multiple h and the voltage U before the fault; the drop multiple is the multiple of the voltage drop after the fault to the voltage before the fault;

[0073] The voltage equation is constructed based on the drop multiple, the voltage before the fault and the phase difference δ. The phase difference is the phase difference between the equivalent internal potential and the voltage at the common connection point under the d-axis voltage directional control of the grid-type energy storage.

[0074] The fault equation is:

[0075]

[0076] Based on the fault equation, the electrical equation is solved by Laplace transform and the current equation is obtained by substituting the initial value conditions;

[0077] Substituting the fault equation into the electrical equation, we get the intermediate expression of the electrical equation:

[0078]

[0079] Let the variable to be determined in the intermediate expression of the electrical equation be the instantaneous value of the d-axis component i of the current at the common connection point of the grid-type energy storage d The instantaneous value of the current at the common connection point of the grid-type energy storage is the q-axis component i q , the intermediate expression of the electrical equation is Laplace transformed and substituted into the initial value conditions to obtain the current equation; the current equation is:

[0080]

[0081] Which meets:

[0082]

[0083] Where s is the complex variable in Laplace transform, i d0 is the initial value of the d-axis current before the fault, i q0 is the initial value of the q-axis current before the fault, and E0 is the initial value of the equivalent internal potential before the fault;

[0084] Based on the fault equation, the internal potential expression is obtained by combining the electrical equation and the control equation;

[0085] The steps to obtain the internal potential expression by combining the electrical equations and the governing equations are:

[0086] Obtain the grid frequency and virtual synchronization control parameters of the grid-connected energy storage;

[0087] Constructing the initial expression of the control equation according to the grid frequency and the virtual synchronous control parameters;

[0088] Obtain the instantaneous power expression in the synchronous rotating coordinate system;

[0089] Substitute the instantaneous power expression into the initial expression of the control equation to obtain the control equation;

[0090] Based on the complex domain, the reactive power control equation in the complex domain is obtained according to the fault equation and the control equation;

[0091] Since the active loop of virtual synchronous control has a certain inertia and the active loop is locked during the fault to ensure transient stability, it can be considered that the transformation phase θ of the three-phase stationary coordinate system to the two-phase rotating coordinate system during the fault is ref The power loop is unchanged, that is, the reactive loop only acts on the size of the internal potential. The reactive control equation in the complex domain is:

[0092]

[0093] The internal potential expression is obtained by combining the electrical equations and the reactive power control equation in the complex domain; the internal potential expression is:

[0094]

[0095] Which meets:

[0096]

[0097] Substitute the internal potential expression into the current equation to obtain the initial current expression, which is the current expression in the complex domain during the fault period; the initial current expression is:

[0098]

[0099] Which meets:

[0100]

[0101]

[0102] The full time domain short-circuit current expression is obtained by performing pole solving and fraction expansion on the initial current expression; the fraction expansion is a partial fraction expansion, and the full time domain short-circuit current expression is:

[0103]

[0104] Which meets:

[0105]

[0106]

[0107] In the formula, G′(s) is the function obtained by taking the derivative of function G(s) with respect to s, c dq is the real root of the cubic equation in F(s)=0, α1+jω1 is the conjugate complex root of the cubic equation in F(s)=0, α1 is the real part of the conjugate complex root of the cubic equation in F(s)=0, ω1 is the imaginary part of the conjugate complex root of the cubic equation in F(s)=0, α2+jω2 is the conjugate complex root of the equation formed by the quadratic polynomial in function G(s), α2 is the real part of the conjugate complex root of the equation formed by the quadratic polynomial in function G(s), ω2 is the imaginary part of the conjugate complex root of the equation formed by the quadratic polynomial in function G(s);

[0108] The short-circuit current of the grid-type energy storage is obtained based on the full-time domain short-circuit current expression.

[0109] Based on the three-phase short-circuit current formula under symmetrical fault of grid-forming energy storage, the short-circuit current of grid-forming energy storage is obtained according to the full-time domain short-circuit current expression; the three-phase short-circuit current formula is:

[0110]

[0111] In the formula, i a is the a-phase short-circuit current of the grid-type energy storage considering the constant virtual impedance and virtual synchronous machine control, i b In order to consider the short-circuit current of phase b controlled by the fixed virtual impedance and virtual synchronous machine, i c In order to consider the c-phase short-circuit current of the fixed virtual impedance and virtual synchronous machine control, i dq =i d +ji q In order to consider the stator current in the synchronous rotating coordinate system under the constant virtual impedance control and virtual synchronous machine control, θ0 is the angle between the d-axis and the axis of the a-phase winding at the time of fault occurrence, j is the imaginary unit, Re represents the operation of the complex real part, and t is the time.

[0112] Calculation example

[0113] This example is based on PSCAD / EMTDC. Figure 2 The grid-type energy storage simulation model, the control structure of the simulation model is as follows Figure 3 As shown, the grid-type energy storage parameters in the simulation model are shown in Table 1:

[0114] Table 1 Electrical and control parameters of the grid-connected energy storage simulation model

[0115]

[0116] The power reference value meets:

[0117]

[0118] Where P0 is the output before the fault.

[0119] The grid-type energy storage is fully output before the fault. Assume that the fault occurs at time 0 and the voltage drop is h. The equivalent circuit topology after the fault is as follows: Figure 4 In this fault condition, the simulation value and calculated value of the short-circuit current of the grid-type energy storage are as follows: Figure 5 shown.

[0120] Figure 5 In the figure, the red solid line is the short-circuit current simulation value, and the black dotted line is the short-circuit current calculation value. In comparison, the simulation value is consistent with the calculated value, which verifies the feasibility and accuracy of the proposed calculation method.

[0121] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, rather than to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions of the technical solution of the present invention by ordinary technicians in this field do not deviate from the essence and scope of the technical solution of the present invention.

Claims

1. A method for calculating short-circuit current of grid-type energy storage based on virtual synchronous control, characterized in that: The following steps are involved: Based on the grid-type energy storage, obtain the port filter parameters and control parameters; Based on the fixed value virtual impedance control, an electrical equation is constructed according to the port filter parameters and the control parameters; the electrical equation is a relationship between the voltage at the common connection point and the equivalent internal potential of the grid-type energy storage; Based on virtual synchronous control, constructing a control equation according to the port filter parameters and the control parameters; Acquiring fault equations and initial value conditions based on the grid-type energy storage; Based on the fault equation, the electrical equation is solved by Laplace transform and the initial value condition is substituted to obtain the current equation; Based on the fault equation, the electrical equation and the control equation are combined to obtain an internal potential expression; Substituting the internal potential expression into the current equation to obtain an initial current expression, wherein the initial current expression is a current expression in the complex domain during a fault period; Obtaining a full time domain short-circuit current expression by performing pole solving and fraction expansion on the initial current expression; The short-circuit current of the grid-type energy storage is obtained according to the full-time domain short-circuit current expression.

2. The method for calculating short-circuit current of grid-type energy storage based on virtual synchronous control according to claim 1 is characterized in that: The electrical equation, the control equation, the fault equation and the initial value condition are obtained in a synchronously rotating coordinate system.

3. The method for calculating short-circuit current of a grid-type energy storage based on virtual synchronous control according to claim 2 is characterized in that: The fault equation includes an internal potential equation and a voltage equation; the internal potential equation is an expression of the instantaneous value of the equivalent internal potential of the grid-type energy storage, and the voltage equation is an expression of the first voltage, which is the instantaneous value of the voltage at the common connection point of the grid-type energy storage.

4. The method for calculating short-circuit current of grid-type energy storage based on virtual synchronous control according to claim 3 is characterized in that: The steps to construct the voltage equation are: Obtaining the drop multiple and the voltage before the fault; the drop multiple is the multiple of the voltage drop after the fault to the voltage before the fault; The voltage equation is constructed according to the drop multiple, the pre-fault voltage and the phase difference, wherein the phase difference is the phase difference between the equivalent internal potential and the common connection point voltage under the d-axis voltage directional control of the grid-type energy storage.

5. The method for calculating short-circuit current of grid-type energy storage based on virtual synchronous control according to claim 2 is characterized in that: The initial value conditions include an initial value of the current before the fault and an initial value of the equivalent internal potential before the fault.

6. The method for calculating short-circuit current of grid-type energy storage based on virtual synchronous control according to claim 5 is characterized in that: The control parameters include a first control parameter and a second control parameter; The first control parameter is a parameter based on the fixed value virtual impedance control; The second control parameter is a parameter based on the virtual synchronous control.

7. The method for calculating short-circuit current of grid-type energy storage based on virtual synchronous control according to claim 6 is characterized in that: The first control parameter includes an equivalent total resistance and an equivalent total reactance; The equivalent total resistance is the sum of a first resistance and a second resistance, the first resistance is a resistance value set in a fixed-value virtual impedance, and the second resistance is a filter resistance at an output port of a network-type energy storage; The equivalent total reactance is the sum of a first reactance and a second reactance, wherein the first reactance is the reactance value set in the fixed virtual impedance, and the second reactance is the reactance value of the filter inductance at the output port of the grid-type energy storage at the grid frequency.

8. The method for calculating short-circuit current of grid-type energy storage based on virtual synchronous control according to claim 6 is characterized in that: The second control parameters include a virtual synchronous control active loop control coefficient, a virtual synchronous control reactive loop control coefficient, an active reference value, a reactive reference value and a voltage reference value.

9. The method for calculating short-circuit current of grid-type energy storage based on virtual synchronous control according to claim 1 is characterized in that: The steps of combining the electrical equation and the control equation to obtain the internal potential expression are: Obtain the grid frequency and virtual synchronization control parameters of the grid-connected energy storage; Constructing an initial expression of a control equation according to the grid frequency and the virtual synchronous control parameter; Obtain the instantaneous power expression in the synchronous rotating coordinate system; Substituting the instantaneous power expression into the initial expression of the control equation to obtain the control equation; Based on the complex domain, a reactive power control equation in the complex domain is obtained according to the fault equation and the control equation; The electrical equation and the reactive power control equation in the complex domain are combined to obtain the internal potential expression.

10. The method for calculating short-circuit current of a grid-type energy storage based on virtual synchronous control according to any one of claims 1 to 9, characterized in that: Based on the three-phase short-circuit current formula under the symmetrical fault of the grid-forming energy storage, the short-circuit current of the grid-forming energy storage is obtained according to the full-time domain short-circuit current expression.

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