Split-phase current differential protection method based on inverter control strategy optimization

Abstract

A split-phase current differential protection method based on inverter control strategy optimization comprises an inverter and a filter which are arranged between photovoltaic power generation equipment and a power grid, an inner loop controller, an outer loop controller, an abc / dq module, a PWM controller, a power calculator, a dq / abc module and a phase-locked loop. The non-fault phase protection sensitivity is reduced while the protection action sensitivity in each phase differential current protection area is improved, maloperation can still be avoided when an external fault occurs, and high reliability is achieved. The method has better anti-transition resistance capability, has better performance when a grid-connected system is a strong power grid or a weak power grid, and has a wider application range compared with the situation that the grid-connected system is connected into the strong power grid only in the past.

Description

Technical Field

[0001] The present invention relates to a technology in the field of power control, in particular to a split-phase current differential protection method based on inverter control strategy optimization. Background Art

[0002] At present, the current differential protection of the transmission line of the inverter-type new energy station can operate reliably and sensitively when the grid-connected system is a strong or weak system due to the existence of zero-sequence current when a ground short circuit occurs; however, when a two-phase short circuit occurs, the current differential protection will be less sensitive in a strong system, and there is a risk of refusal to operate in a weak system. In order to improve the current differential protection, the existing technology either utilizes the limited amplitude of the fault current of the new energy power source, or utilizes the amplitude or phase characteristics of various sequence components of the fault current at both ends of the line to construct new protection action judgment criteria and setting methods. However, these technologies are all based on the scenario of the grid-connected system being a strong power grid, and their adaptability in the scenario of the grid-connected system being a weak power grid remains to be verified. In addition, the performance of the current differential protection is often seriously affected when there are unmeasurable branches in the line. In this regard, there is a related technology that uses impedance based on the mutation amount to calculate and set by analyzing the high-frequency impedance characteristics of the inverter, but measuring high-frequency signals requires upgrading the measurement hardware device, and high-frequency signals are susceptible to external interference. Summary of the invention

[0003] The present invention aims at the symmetrical and asymmetrical faults occurring on the transmission lines of the existing inverter-type new energy stations connected to the weak power grid, and proposes a phase-splitting current differential protection method based on the optimization of the inverter control strategy. While improving the protection action sensitivity in the differential current protection zone of each phase, it also reduces the protection sensitivity of the non-fault phase, and can still not malfunction when a fault occurs outside the zone, and has high reliability. It has good anti-transition resistance capability, and has good performance when the grid-connected system is a strong or weak power grid, and has a wider scope of application than the previous scenario of only considering the connection to the strong power grid.

[0004] The present invention is achieved through the following technical solutions:

[0005] The present invention relates to an inverter constant power control system, comprising: an inverter and a filter arranged between a photovoltaic power generation device and a power grid, an inner loop controller, an outer loop controller, an abc / dq module, a PWM controller, a power calculator, a dq / abc module and a phase-locked loop, wherein: the outer loop controller calculates the active power reference value P according to the input ref and reactive power reference value Q ref Calculate the positive and negative sequence reference current i d * and i q *, and transmits it to the inner loop controller; the inner loop controller is based on the reference current i d * and i q *With actual current id and i q The difference is calculated using the PI adjustment algorithm to calculate the current control command and output it to the dq / abc module; the dq / abc module receives the current control command and performs Park inverse transformation and transmits the control signal to the PWM controller; the PWM controller performs PWM modulation on the control signal and generates a semiconductor switch control signal and outputs it to the inverter; the inverter opens or closes the switch according to the control command, one end of which is connected to the photovoltaic DC power supply, and the current output at the other end is connected to the grid after filtering the harmonics through the filter; the abc / dq module collects the voltage and current on the transmission line and performs Park transformation, and its transformation result i d 、i q and U d , U q The outputs are sent to the power calculator and the inner loop controller respectively; the power calculator outputs the output to the power calculator and the inner loop controller respectively according to i d 、i q and U d , U q The real-time power size is calculated and output to the outer loop controller; the phase-locked loop is set on the grid side, and generates the output frequency f and the power frequency deviation Δω, as well as the grid voltage phase angle θ according to the three-phase voltage.

[0006] The present invention relates to a phase-splitting current differential protection method based on the inverter control strategy optimization of the above system. By constructing optimized inverter positive and negative sequence current control targets respectively, on the basis of the inverter adopting an optimized output current control strategy, a new phase-splitting braking current and differential current are set, and the differential current protection sensitivity of each sequence is calculated accordingly, and the judgment criteria are brought in to determine whether the fault location is within the zone. If it is an internal fault, a trip signal is sent to the protection on both sides, and the fault type and fault phase are determined. Technical Effects

[0007] For the transmission line scenario of an inverter-type new energy station connected to a weak power grid, the present invention improves the protection action sensitivity in the differential current protection zone of each phase, while also reducing the protection sensitivity of the non-fault phase, and can still not malfunction when a fault occurs outside the zone, thus having high reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 It is a flow chart of the present invention;

[0009] Figure 2 It is a structural diagram of the inverter constant power control system;

[0010] Figure 3 Schematic diagram of the model used in simulation experiment verification. DETAILED DESCRIPTION

[0011] like Figure 1As shown, this embodiment relates to an optimized phase-split current differential protection method based on inverter control strategy optimization, including:

[0012] Step 1: construct an optimized inverter positive sequence current control target, specifically including:

[0013] 1.1) The maximum reactive current cancellation is 1.05I N The limit of the inverter current amplitude k max I N , where: I N is the rated output current of the inverter, k max The maximum current multiple to protect the inverter from damage.

[0014] 1.2) Set the optimized inverter positive sequence current control target, that is, the positive sequence q-axis current output by the inverter and the positive sequence d-axis current Set the initial value k of the positive sequence reactive current proportionality coefficient q is 1.5, k p , k i are the amplification and integration coefficients in the PI controller respectively. The difference between the output line voltage frequency f and the power frequency is Δω. The adjusted reactive current proportional coefficient k q '=1.5+(k p +k i / s)Δω. According to k q 'The size of the change sets the reference current:

[0015] If 1.5≤k' q <3;

[0016] If k' q =3;

[0017] Among them: positive sequence reactive current proportionality coefficient k q The initial value is 1.5, k p , k i are the amplification and integration coefficients in the PI controller respectively, and the proportional coefficient of the reactive current after adjustment is k q '=1.5+(k p +k i / s)Δω. When a symmetrical fault occurs, the q-axis voltage u q <0, or when an asymmetric fault occurs, the phase-locked loop can eventually lock the phase correctly. In order to reduce the phase angle difference Δθ of the grid-connected point voltage mutation at the moment of the fault, the active current reference value i is not increased. d * Or reduce the reactive current reference value i q *, the original low voltage ride-through strategy remains unchanged. The subscript 1 represents the positive sequence component, and 0 represents the zero sequence component.

[0018] Step 2: construct an optimized inverter negative sequence current control target, including:

[0019] When photovoltaic low voltage ride-through control is adopted, a control strategy of suppressing negative sequence current is adopted to make the negative sequence current amplitude ratio of both sides λ 2 Theoretically, it is infinite. At this time, the negative sequence reference currents of the d-axis and q-axis satisfy When wind power low voltage ride-through control is used, the negative sequence impedance angle arg(Z IR2 ) and the negative sequence impedance angle arg(Z SCR ) are consistent, at this time the phase angle difference of the negative sequence current on both sides satisfies δ 2 =0, at this time, the d-axis and q-axis negative sequence reference currents are Where: k q2 k is the negative sequence reactive current compensation coefficient of the new energy station; u2 is the negative sequence voltage compensation coefficient of the new energy station; is the negative sequence voltage phase of the grid connection point; is the equivalent impedance angle of the power grid; when setting the positive sequence reactive current target i q1 * With negative sequence current target i q2 * After that, the inverter positive sequence d-axis current control target is updated accordingly: When the positive sequence active current is 0, the current amplitude still exceeds the limit. According to the grid connection guidelines, the negative sequence reactive current compensation coefficient k of the new energy station is reduced. q2 to avoid.

[0020] Step 3: The inverter positive sequence q-axis current control target i obtained in step 1 q1 * and the inverter d-axis and q-axis negative sequence reference current control target i obtained in step 2 d2 *、i q2 *、Updated inverter positive sequence d-axis current control target i d1 *、D-axis component U of inverter output voltage d and the q-axis component U q , calculate the active power reference value P that should be input into the inverter control module ref and reactive power reference value Q ref , and input the outer loop controller, specifically:

[0021] Step 4: Collect the three-phase current on the inverter output line, make a fault diagnosis and issue a corresponding trip command, including:

[0022] 4.1) Determine whether the fault point is located within the area, including:

[0023] i) Calculate the differential values ​​of the three-phase current and zero-sequence current of each sequence of the line and brake value Specifically: Where: subscript d represents differential, r represents braking; subscript 0, represents each phase current and zero sequence current respectively; subscript x = 1, 2, 0, represents positive sequence, negative sequence and zero sequence respectively; and They are the phase fault currents at the protection installations on the system side and the new energy station side of each sequence respectively.

[0024] ii) Calculate the differential values ​​of the three-phase current and zero-sequence current of the line respectively and brake value Specifically: Among them: Subscript They represent the phase current and zero-sequence current respectively; the subscripts 1, 2, and 0 represent the positive sequence, negative sequence, and zero sequence respectively.

[0025] iii) Calculate the sensitivity of differential current protection for each phase and sequence component separately and k senx =|I dx | / |I rx |, where: subscript They represent the phase current and the zero-sequence current respectively; the subscripts x=1, 2, 0 represent the positive sequence, negative sequence and zero sequence respectively. and k senx The action thresholds are and k x , the value is generally between 0.5 and 0.8.

[0026] iv) In order to prevent the negative sequence current differential protection from malfunctioning due to the negative sequence component of the unbalanced current generated on the system side when the negative sequence current suppression strategy δ = 0 is adopted by the new energy station, the magnitude of the negative sequence current on the system side should be judged. Therefore, the action conditions of each sequence component and each phase current differential protection are set as follows. If any one of them is met, it can be judged as an internal fault, specifically:

[0027] 4.2) Determine the fault phase and issue a trip command: When it is determined to be an internal fault in step 4.1, the type of fault is determined according to the sequence component, and finally the fault phase is determined according to the following ratio restraint current differential protection action criterion, specifically: Among them: Subscript Represent each phase current and zero-sequence current respectively; and They are the differential value and braking value of the three-phase current and zero-sequence current of the line, is the starting value, which is generally very small; k is the braking coefficient, which is typically in the range of 0.5 to 0.8. If a phase meets the action criteria, a trip command is issued to cut off the corresponding phase of the transmission line.

[0028] After specific experiments, the following simulation software was built in PSCAD / EMTDC: Figure 3 The single-feed inverter-type new energy station connected to the weak power grid system model shown in the figure has the same settings as shown in Table 1.

[0029] Table 1 Simulation experiment parameter settings

[0030] The rated capacity of the inverter-type renewable energy station is 150MW. The line adopts a π-shaped equivalent circuit model. The short-circuit ratio of the grid-connected system is SCR≈3. The positive and negative sequence equivalent impedance phase angle of the system is θ SCR is 80.3°, and the inverter operates at unity power factor PF=1 before the fault. The sampling rate in the simulation is 1kHz. Considering the most likely case of failure of the proposed protection scheme, the negative sequence reactive current compensation coefficient k after the fault is set q2 =1, braking coefficient k of each sequence x Take 0.8, the phase braking coefficient k φ Take 1. Next, a short-circuit fault is set in the output line in the model, and the influence of the above factors on the invented current differential protection method is explored by changing the fault point location, transition resistance, fault type, low voltage ride-through strategy, and system short-circuit ratio.

[0031] 1) Influence of fault transition resistance and fault location on the performance of the protection method: In the simulation model, the inverter adopts the wind power low voltage ride-through strategy, and sets the positive and negative sequence current optimization strategy. The transition resistance is 10Ω and 100Ω respectively, and the short circuit fault occurs at 10%, 90% and 101% in the transmission line area respectively. The fault occurrence time is 1s, and the protection criterion is selected to record the time of 1.02s after the fault. After the fault, the differential current criterion of each phase and the sequence component differential current criterion and the protection action of the present invention are shown in Tables 2 to 5.

[0032] Table 2 ABC fault protection action at different positions and transition resistances

[0033] Table 3 Protection action of Ag fault at different positions and transition resistance

[0034] Table 4 Fault protection actions at different positions and transition resistances BCg

[0035] Table 5 BC fault protection action at different positions and transition resistances

[0036] It can be seen from Tables 2 to 5 that under different fault locations and transition resistances, after an in-zone fault occurs, the sensitivity of the fault phase criterion is greater than 1, and the sensitivity of the non-fault phase is very small, and the protection method of the invention can act quickly and reliably. When an out-of-zone fault occurs, regardless of the fault type, the sensitivity of the phase criterion is very small, and the protection method of the invention can reliably refuse to act.

[0037] 2) The impact of different low voltage ride-through strategies on the performance of the protection method: The inverter in the simulation is set to adopt the photovoltaic low voltage ride-through strategy and the optimized current control target of the present invention, and different types of metallic short circuit faults are set to occur at different locations of the transmission line to verify the action performance of the current differential protection method of the invention when the inverter power supply adopts different control strategies. The fault location is set to 10% and 90% in the transmission line area respectively. After the fault, the differential current criterion of each phase and the zero-sequence differential current criterion of the present invention are shown in Table 6.

[0038] Table 6 Protection actions of different fault locations for improved PV control strategy

[0039] It can be seen that the photovoltaic control strategy is a special case of the wind power control strategy with a negative sequence current amplitude of 0, and the invented protection method can act accurately and quickly when a fault occurs in the area.

[0040] 3) The influence of different system short-circuit ratios on the performance of the protection method: In the simulation, the inverter is set to adopt the wind power low voltage ride-through strategy and the optimized current control target of the present invention. The grid-connected capacity of the inverter-type new energy station corresponding to the grid-connected system short-circuit ratio SCR=9 and SCR=3 is 50MW and 150MW respectively. Different types of metallic short-circuit faults are set to occur at the midpoint of the transmission line to verify the action performance of the current differential protection method of the invention at different short-circuit ratios. The fault occurred at 50% of the transmission line area. After the fault, the differential current criterion of each phase and the zero-sequence differential current criterion of the present invention are shown in Table 7.

[0041] Table 7 Fault protection action at the midpoint in different short-circuit ratio areas

[0042] It can be seen from Table 7 that the invented current differential protection method can effectively judge the fault in the area under different grid strengths. When the short-circuit ratio SCR of the grid-connected system is low and the new energy station is connected to the weak grid, the short-circuit current amplitude on both sides is smaller than λ, and the sensitivity of the differential current protection is higher when a symmetrical short-circuit fault occurs in the line.

[0043] To sum up, compared with the prior art, the present invention has extremely high sensitivity, reliability and strong adaptability in the scenario of weak AC power grid transmission lines connected to inverter-type new energy stations, and is not affected by the size of transition resistance, fault type, fault location, low voltage ride-through strategy, grid strength, etc.

[0044] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principle and purpose of the present invention. The protection scope of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. Each implementation scheme within its scope shall be subject to the constraints of the present invention.

Claims

1. An inverter constant power control system, characterized in that: include: The inverter and filter, inner loop controller, outer loop controller, abc / dq module, PWM controller, power calculator, dq / abc module and phase-locked loop are arranged between the photovoltaic power generation equipment and the power grid, wherein: the outer loop controller is based on the input active power reference value P ref and reactive power reference value Q ref Calculate the positive and negative sequence reference current i d * and i q *, and transmits it to the inner loop controller; the inner loop controller is based on the reference current i d * and i q *With actual current i d and i q The difference is calculated using the PI adjustment algorithm to calculate the current control command and output it to the dq / abc module; the dq / abc module receives the current control command and performs Park inverse transformation and transmits the control signal to the PWM controller; the PWM controller performs PWM modulation on the control signal and generates a semiconductor switch control signal and outputs it to the inverter; the inverter opens or closes the switch according to the control command, one end of which is connected to the photovoltaic DC power supply, and the current output at the other end is connected to the grid after filtering the harmonics through the filter; the abc / dq module collects the voltage and current on the transmission line and performs Park transformation, and its transformation result i d 、i q and U d , U q The outputs are sent to the power calculator and the inner loop controller respectively; the power calculator outputs the output to the power calculator and the inner loop controller respectively according to i d 、i q and U d , U q The real-time power size is calculated and output to the outer loop controller; the phase-locked loop is set on the grid side, and generates the output frequency f and the power frequency deviation Δω, as well as the grid voltage phase angle θ according to the three-phase voltage.

2. A phase-splitting current differential protection method based on inverter control strategy optimization of the system of claim 1, characterized in that: By constructing optimized inverter positive and negative sequence current control targets respectively, and on the basis of the inverter adopting optimized output current control strategy, a new type of phase braking current and differential current are set, and the differential current protection sensitivity of each sequence is calculated accordingly. The judgment criteria are used to determine whether the fault location is within the zone, and when a fault occurs within the zone, a tripping signal is sent to the protection on both sides to further determine the fault type and fault phase.

3. The phase-splitting current differential protection method according to claim 2 is characterized in that: The inverter positive sequence current control target with optimized structure specifically includes: 1.1) The maximum reactive current cancellation is 1.05I N The limit of the inverter current amplitude k max I N , where: I N is the rated output current of the inverter, k max The maximum current multiple to ensure the inverter is not damaged; 1.2) Set the optimized inverter positive sequence current control target, that is, the positive sequence q-axis current output by the inverter and the positive sequence d-axis current Set the initial value k of the positive sequence reactive current proportionality coefficient q is 1.5, k p , k i are the amplification and integration coefficients in the PI controller respectively. The difference between the output line voltage frequency f and the power frequency is Δω; the adjusted reactive current proportional coefficient k q '=1.5+(k p +k i / s)Δω; according to k q 'The size of the change sets the reference current: like If k' q =3; where: positive sequence reactive current proportionality coefficient k q The initial value is 1.5, k p , k i are the amplification and integration coefficients in the PI controller respectively, and the proportional coefficient of the reactive current after adjustment is k q '=1.5+(k p +k i / s)Δω; when a symmetrical fault occurs, the q-axis voltage u q <0, or when an asymmetric fault occurs, the phase-locked loop can eventually lock the phase correctly. In order to reduce the phase angle difference Δθ of the grid-connected point voltage mutation at the moment of the fault, the active current reference value i is not increased. d * Or reduce the reactive current reference value i q * , the original low voltage ride-through strategy remains unchanged; the subscript 1 represents the positive sequence component, and 0 represents the zero sequence component.

4. The phase-splitting current differential protection method according to claim 3 is characterized in that: The optimized inverter negative sequence current control target includes: when photovoltaic low voltage ride-through control is adopted, a control strategy for suppressing negative sequence current is adopted, so that the negative sequence current amplitude ratio λ2 on both sides is theoretically infinite, and at this time, the d-axis and q-axis negative sequence reference currents respectively satisfy When wind power low voltage ride-through control is adopted, the negative sequence impedance angle arg(Z IR2 ) and the negative sequence impedance angle arg(Z SCR ), the negative sequence current phase difference on both sides satisfies δ2=0, and the negative sequence reference currents of d-axis and q-axis are respectively Where: k q2 k is the negative sequence reactive current compensation coefficient of the new energy station; u2 is the negative sequence voltage compensation coefficient of the new energy station; is the negative sequence voltage phase of the grid connection point; is the equivalent impedance angle of the power grid; when setting the positive sequence reactive current target i q1 * With negative sequence current target i q2 * After that, the inverter positive sequence d-axis current control target is updated accordingly: When the positive sequence active current is 0, the current amplitude still exceeds the limit. According to the grid connection guidelines, the negative sequence reactive current compensation coefficient k of the new energy station is reduced. q2 to avoid.

5. The phase-splitting current differential protection method according to claim 3 is characterized in that: The optimized output current control strategy is: according to the inverter positive sequence q axis current control target i q1 * and inverter d-axis and q-axis negative sequence reference current control target i d2 *、i q2 *、Updated inverter positive sequence d-axis current control target i d1 *、D-axis component U of inverter output voltage d and the q-axis component U q , calculate the active power reference value P that should be input into the inverter control module ref and reactive power reference value Q ref , and input the outer loop controller, specifically:

6. The phase-splitting current differential protection method according to claim 3 is characterized in that: The differential current protection sensitivity is calculated in the following way: i) Calculate the differential values ​​of the three-phase current and zero-sequence current of each sequence of the line and brake value Specifically: Where: subscript d represents differential, r represents braking; subscript Represent the phase current and zero-sequence current respectively; subscript x=1, 2, 0, represent positive sequence, negative sequence and zero sequence respectively; and They are the phase fault currents at the protection installations on the system side and the new energy station side of each sequence respectively; ii) Calculate the differential values ​​of the three-phase current and zero-sequence current of the line respectively and brake value Specifically: Among them: Subscript Represent each phase current and zero sequence current respectively; subscripts 1, 2, and 0 represent positive sequence, negative sequence, and zero sequence respectively; iii) Calculate the sensitivity of differential current protection for each phase and sequence component separately and k senx =|I dx | / |I rx |, where: subscript Represent the phase current and zero-sequence current respectively; subscript x=1, 2, 0, represent positive sequence, negative sequence and zero sequence respectively; and k senx The action thresholds are and k x .

7. The phase-splitting current differential protection method according to claim 6 is characterized in that: The determination of whether the fault location is within the zone refers to: setting the action conditions of each sequence component and each phase current differential protection as follows. If any one of them is met, it can be determined as an within-zone fault, specifically:

8. The split-phase current differential protection method according to claim 7 is characterized in that: The further determination of the fault type and fault phase after sending a trip signal to the protection on both sides means: when it is determined to be an internal fault, the fault type is determined according to the sequence component, and finally the fault phase is determined according to the following ratio restraint current differential protection action criterion, specifically: Among them: Subscript Represent each phase current and zero-sequence current respectively; and They are the differential value and braking value of the three-phase current and zero-sequence current of the line, is the starting value, which is generally very small; k is the braking coefficient, and its typical value range is 0.5~0.

8. If a phase meets the action criterion, a tripping command is issued to cut off the corresponding phase of the transmission line.

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