A wide-range on-load voltage regulation compensation control method and system

Abstract

The application provides a wide-range on-load voltage regulation voltage compensation control method and system, and an AC / DC / AC converter is connected to the output of an on-load voltage regulation transformer. The converter mainly comprises a parallel side and a series side, and the output of the series side converter is connected to a series transformer. Output voltage regulation is combined with on-load voltage regulation distribution transformer gear adjustment and converter series compensation. Converter capacity configuration considers voltage compensation range, three-phase imbalance compensation, reactive power compensation, thermal effect caused by harmonic compensation and maximum current stress of the converter output. The series side converter output voltage compensation adopts a positive and negative sequence separation control algorithm. The scheme can realize wide-range output voltage stepless compensation regulation, can flexibly limit the current inner loop of the series side converter, avoids equipment overload, and improves equipment reliability.

Description

Technical Field

[0001] This invention relates to the fields of power electronics and flexible distribution network technology, and in particular to a wide-range on-load voltage compensation control method and system. Background Technology

[0002] With the integration of new power sources such as large-scale distributed photovoltaics and electric vehicle charging, the distribution network faces huge challenges such as heavy equipment overload, voltage exceeding limits, and three-phase imbalance. The power supply quality at the end of the weak grid deteriorates, affecting the safety of the power grid and electrical appliances, and reducing user service satisfaction.

[0003] Conventional on-load tap-changing transformers have limited tap life and cannot achieve stepless adjustment; AC / DC / AC power electronic converter technology can achieve flexible stepless voltage adjustment, but its overload capacity and reliability are poor. Summary of the Invention

[0004] Addressing the shortcomings and deficiencies of existing technologies, this invention proposes a wide-range on-load tap-changing voltage compensation control method and system. It combines the advantages of both on-load tap changing and power electronic converters to create an on-load tap-changing system, promoting the upgrading of distribution network architecture and enhancing the flexible interaction of resources and precise voltage compensation capabilities in urban centers, industrial parks, and rural areas. This enables the integration of large-scale distributed photovoltaic power and electric vehicle charging into existing distribution networks, improving the power supply quality at the end of weak power grids.

[0005] In the proposed scheme, the output of an on-load tap-changing transformer is connected to an AC / DC / AC converter. The converter mainly includes a parallel side and a series side, with the output of the series-side converter connected to the series transformer. Output voltage regulation integrates the on-load tap-changing transformer's tap adjustment with the series compensation of the converter. Converter voltage compensation regulation smooths out voltage discontinuities caused by on-load tap-changing transformer tap adjustments, achieving stepless adjustment of output voltage compensation over a wide compensation range. Converter capacity configuration considers the voltage compensation range, three-phase imbalance compensation, reactive power compensation, thermal effects caused by harmonic compensation, and the maximum output current stress of the converter. The output voltage compensation of the series-side converter employs a positive-negative sequence separation control algorithm. This scheme can achieve stepless compensation adjustment of output voltage over a wide range, flexibly limit the current inner loop of the series-side converter, avoid equipment overload, and improve equipment reliability.

[0006] The present invention specifically adopts the following technical solution:

[0007] A wide-range on-load tap-changing voltage compensation control method integrates on-load tap-changing distribution transformer tap adjustment and converter compensation; the converter voltage compensation adjustment is used to smooth the voltage discontinuity of on-load tap-changing distribution transformer tap adjustment, so as to achieve stepless adjustment of output voltage compensation over a wide compensation range.

[0008] Furthermore, the converter capacity configuration takes into account the voltage compensation range, three-phase imbalance compensation, reactive power compensation, thermal effects caused by harmonic compensation, and the maximum output current stress of the converter.

[0009] Furthermore, the output voltage compensation of the series-side converter adopts a positive and negative sequence separation control algorithm.

[0010] Specifically:

[0011] Under a certain on-load tap-changing transformer tap position, the voltage compensation of the series-side converter is achieved as follows: the coordinating controller decomposes the phase voltage compensation instructions UA_pk_t, UB_pk_t, and UC_pk_t based on the current effective values ​​of the three-phase grid voltages USA, USB, and USC, combined with the target effective value of the output voltage Uout_ref.

[0012] After receiving the phase voltage compensation command, the series-side converter further decomposes it into positive-sequence voltage, negative-sequence voltage, and zero-sequence voltage compensation commands.

[0013] A wide-range on-load voltage regulation compensation control system:

[0014] The output terminal of the on-load tap-changing transformer is connected to a converter, which includes a parallel-side converter and a series-side converter, wherein the output terminal of the series-side converter is connected to the series transformer.

[0015] The parallel-side converter and the series-side converter are connected through a DC bus; the DC bus then leads out a DC port through a DC circuit breaker to connect to a DC load.

[0016] Among them, parallel compensation for three-phase unbalanced current, parallel compensation for harmonic current, and DC power supply and distribution are achieved through parallel-side converters;

[0017] Output voltage compensation and regulation are achieved by combining on-load tap-changing distribution transformer tap adjustment and converter series compensation:

[0018] 1) On-load tap-changing distribution transformers only perform three-phase voltage average value compensation and tap adjustment; voltage compensation for unbalanced parts is entirely achieved by series-side converters.

[0019] 2) The voltage compensation regulation of the series-side converter is used to smooth the voltage discontinuity of the tap change of the on-load tap-changing distribution transformer, so as to achieve stepless adjustment of the output voltage compensation over a wide compensation range.

[0020] Furthermore, the capacity configuration method for the series-side converter is as follows:

[0021] Assuming the voltage regulation range on the low-voltage side of the series transformer is ±γ1%×UN, and the rated capacity of the distribution transformer is SN, then the capacity configuration of the series-side converter is as follows:

[0022] S_series = γ1% × SN

[0023] If the minimum tap size of the on-load tap-changing distribution transformer is δ%, then:

[0024] γ1≥δ.

[0025] Furthermore, the configuration of the parallel-side converter should be considered as follows:

[0026] 1) Provide a stable DC power supply and active power P1 for the series-side converter;

[0027] P1 = γ1% × SN

[0028] 2) To provide a stable DC power supply and active power P2 to the DC load, assuming the distribution transformer has a γ2% efficiency, then:

[0029] P2 = γ2% × SN

[0030] 3) Provide compensation for the three-phase unbalanced current of the AC load, including active power P3 and reactive power Q1. Assuming that the proportion of phase-specific active power compensation capacity to the single-phase capacity of the distribution transformer is γ3%, and the proportion of phase-specific reactive power compensation capacity to the single-phase capacity of the distribution transformer is γ4%, then:

[0031] P3 = γ3% × SN

[0032] Q1 = γ4% × SN

[0033] 4) Provide compensation for harmonic currents of AC loads, including 5th order compensation capacity H5, 7th order compensation capacity H7, and 9th order compensation capacity H9, compensated in order of increasing priority. Assuming the harmonic compensation current accounts for γ5% of the rated current of the distribution transformer, then the compensation capacity is:

[0034] H5 = γ5% × SN

[0035] H7 = γ7% × SN

[0036] H9 = γ9% × SN

[0037] Therefore, the capacity configuration of the parallel-side converter is calculated as follows:

[0038]

[0039] Furthermore, the capacity configuration of the parallel-side converter takes into account the instantaneous value of the maximum output current stress of the converter. In extreme cases, the peak value of the converter output current is:

[0040]

[0041] In the formula, IP is the peak value of active current, IQ is the peak value of reactive current, I5 is the peak value of the 5th harmonic, I7 is the peak value of the 7th harmonic, and I9 is ​​the peak value of the 9th harmonic.

[0042] When selecting switching devices, Imax should be designed within the safe operating range of the switching device.

[0043] Furthermore, the series-side converter control strategy adopts a positive-negative sequence separation control algorithm, specifically as follows:

[0044] Under a certain on-load tap-changing transformer position, the voltage compensation of the series-side converter is achieved as follows: The coordinating controller, based on the current effective values ​​of the three-phase grid voltages USA, USB, and USC, and combined with the target effective value of the output voltage Uout_ref, decomposes the phase voltage compensation instructions UA_pk_t, UB_pk_t, and UC_pk_t:

[0045] UA_pk_t=(Uout_ref–USA)*1.414

[0046] UB_pk_t=(Uout_ref-USB)*1.414

[0047] UC_pk_t=(Uout_ref-USC)*1.414

[0048] After receiving the phase voltage compensation command, the series-side converter further decomposes it into positive-sequence voltage, negative-sequence voltage, and zero-sequence voltage compensation commands.

[0049] Furthermore, after receiving the phase voltage compensation command, the series-side converter further decomposes the positive-sequence voltage, negative-sequence voltage, and zero-sequence voltage compensation commands, specifically including:

[0050] 1) Calculation of positive sequence voltage compensation command

[0051] The positive sequence voltage compensation command is the average value U_pk_0 of the phase voltage compensation command, calculated as follows:

[0052] U_pk_0=(UA_pk_t+UB_pk_t+UB_pk_t) / 3;

[0053] In the dq rotating coordinate system, the positive sequence voltage compensation command is:

[0054] Ud_ref = U_pk_0;

[0055] Uq_ref = 0;

[0056] 2) Calculation of negative sequence voltage compensation command

[0057] Subtracting the average value of the phase-sequence voltage compensation commands from the phase-sequence voltage compensation commands yields a result containing only the negative-sequence and zero-sequence voltage compensation commands. This result further yields the three-phase unbalanced voltage compensation commands UA_pk, UB_pk, and UC_pk.

[0058] UA_pk = UA_pk_t - U_pk_0

[0059] UB_pk = UB_pk_t - U_pk_0

[0060] UC_pk = UC_pk_t - U_pk_0

[0061] Assuming the current grid phase angle is 0 degrees, then based on the three-phase unbalanced voltage compensation command, the target values ​​of the three-phase voltages of the current series-side converter are as follows:

[0062] As = UA_pk × cos0°

[0063] Bs = UB_pk × cos(0-120°)

[0064] Cs = UC_pk × cos(0 + 120°)

[0065] By using the Clark transformation, we obtain the values ​​of αβ in the stationary coordinate system:

[0066] Alpha = (2 × As - Bs - Cs) / 3;

[0067] Beta = 0.5774 × (Bs - Cs);

[0068] Through the Park transformation, the negative sequence voltage compensation command in the dq rotating coordinate system is obtained as follows:

[0069] Ud_N_ref = Alpha;

[0070] Uq_N_ref = Beta;

[0071] 3) Calculation of zero-sequence voltage compensation command

[0072] Let the current phase angle be θ. Based on the three-phase unbalanced voltage compensation command, the zero-sequence voltage compensation command is obtained as follows:

[0073] U0_ref=UA_pk×cos(θ)+UB_pk×cos(θ-120°)+UC_pk×cos(θ

[0074] +120°).

[0075] Compared with the prior art, the beneficial effects of the present invention and its preferred embodiments include at least the following:

[0076] 1) The tap position adjustment of the on-load tap-changing distribution transformer is integrated with the converter compensation to achieve stepless adjustment of the output voltage over a wide range.

[0077] 2) The converter capacity configuration not only takes into account the thermal effects (RMS value) caused by voltage compensation range, three-phase imbalance compensation, reactive power compensation, and harmonic compensation, but also the maximum output current stress (instantaneous value) of the converter.

[0078] 3) The output voltage control strategy of the series-side converter adopts a positive and negative sequence separation control algorithm, which can limit the positive and negative sequence currents in the inner loop of the series-side converter current, avoid equipment overload or overcurrent, improve the reliability of the converter, and make up for the shortcomings of the conventional proportional resonant controller for phase voltage control. Attached Figure Description

[0079] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0080] Figure 1 This is an electrical topology diagram of an on-load tap changer system according to an embodiment of the present invention;

[0081] Figure 2 This is a schematic diagram of wide-range stepless voltage adjustment according to an embodiment of the present invention;

[0082] Figure 3 This is a block diagram of the output voltage positive and negative sequence separation control algorithm in an embodiment of the present invention. Detailed Implementation

[0083] In the following, specific embodiments of this application will be described in detail with reference to the accompanying drawings. Based on these detailed descriptions, those skilled in the art will be able to clearly understand and implement this application. Without departing from the principles of this application, features from various embodiments can be combined to obtain new implementations, or certain features from some embodiments can be substituted to obtain other preferred implementations.

[0084] To make the features and advantages of this patent more apparent and understandable, specific embodiments are provided below, along with accompanying drawings, for detailed explanation:

[0085] 1. On-load tap changer system design

[0086] like Figure 1 As shown, in the on-load tap-changing system design provided by the present invention: the output terminal of the on-load tap-changing transformer is connected to the converter, and the converter includes a parallel-side converter and a series-side converter, wherein the output terminal of the series-side converter is connected to the series transformer.

[0087] The parallel-side converter and the series-side converter are connected via a DC bus. The DC bus then leads out a DC port through a DC circuit breaker to connect to a DC load.

[0088] The on-load tap changer system has functions such as output voltage compensation and regulation, three-phase unbalanced current parallel compensation (corresponding to active and reactive power of each phase), harmonic current parallel compensation, and DC power supply and distribution.

[0089] Among them, three-phase unbalanced current parallel compensation (corresponding to active and reactive power of each phase), harmonic current parallel compensation, and DC power supply and distribution are achieved through parallel-side converters.

[0090] Output voltage compensation regulation is achieved by combining two methods: tap adjustment of the on-load tap-changing distribution transformer and series compensation of the converter. The method is as follows:

[0091] 1) Since the independent phase tap change affects the magnetic circuit balance and lifespan of the on-load tap-changing distribution transformer, the on-load tap-changing distribution transformer only performs three-phase voltage average value compensation tap change, and the voltage compensation of the unbalanced part is all achieved by the series-side converter.

[0092] 2) such as Figure 2 As shown, the series-side converter voltage compensation regulation is used to smooth the voltage discontinuity during tap changing of the on-load tap-changing distribution transformer, and to achieve stepless adjustment of the output voltage compensation over a wide compensation range.

[0093] 2. Converter capacity configuration method

[0094] 2.1 The configuration method for the capacity of the series-side converter is as follows:

[0095] The series-connected converter primarily operates in VF (AC voltage frequency) mode, independently compensating for the voltages of phases A, B, and C. Assuming the voltage regulation range on the low-voltage side of the series transformer is ±γ1% × UN, and the rated current flowing through the low-voltage side is the same as the rated current IN of the distribution transformer, if the rated capacity of the distribution transformer is SN, then the capacity of the series transformer can be calculated as follows:

[0096] ST=3×γ1%×UN×IN=γ1%×SN (1)

[0097] The power balance between the primary and secondary sides of the series transformer leads to the following capacity configuration for the series-side converter:

[0098] Sseries = γ1% × SN(2)

[0099] As mentioned earlier, the series-side converter voltage compensation regulation is used to smooth out voltage discontinuities during tap changing in on-load tap-changing distribution transformers. The UPQC capacity does not need to be very large. Assuming the minimum tap size of the on-load tap-changing distribution transformer is δ%, then γ1 is generally preferred to be slightly larger than δ.

[0100] γ1≥ δ (3).

[0101] 2.2 The configuration method for the parallel-side converter is as follows:

[0102] The parallel-side converter mainly operates in UDC (Unified Voltage Control) mode, drawing energy from the 0.4kV AC grid to stabilize the DC bus voltage, and has the following four types of capacity requirements:

[0103] 1) Provide a stable DC power supply and active power P1 for the series-connected converter, with the maximum value being the same as that of the series-connected S mentioned above.

[0104] P1 = γ1% × SN (4)

[0105] 2) Provide a stable DC power supply and active power P2 for the DC load. The maximum value is related to the design capacity of the DC power supply and distribution system, and is assumed to be γ2% of the distribution transformer. Then:

[0106] P2 = γ2% × SN (5)

[0107] 3) Provide compensation for the three-phase unbalanced current of the AC load, including active power P3 and reactive power Q1. Assuming that the proportion of phase-specific active power compensation capacity to the single-phase capacity of the distribution transformer is γ3%, and the proportion of phase-specific reactive power compensation capacity to the single-phase capacity of the distribution transformer is γ4%, then:

[0108] P3 = γ3% × SN (6)

[0109] Q1 = γ4% × SN (7)

[0110] 4) Provide compensation for harmonic currents in AC loads, mainly including 5th order compensation capacity H5, 7th order compensation capacity H7, and 9th order compensation capacity H9, compensated in order of increasing priority. Assuming that the harmonic compensation current accounts for γ5% of the rated current of the distribution transformer, the compensation capacity is:

[0111] H5 = γ5% × SN (8)

[0112] H7 = γ7% × SN (9)

[0113] H9 = γ9% × SN (10)

[0114] Based on the above four types of compensation requirements, calculate the capacity of the parallel-side converter:

[0115]

[0116] Substituting equations (4) to (10) into (11), the final capacity configuration of the parallel-side converter is:

[0117]

[0118] 5) The capacity configuration of the parallel-side converters takes into account the maximum output current stress (instantaneous value) of the converters. The relative relationship between the harmonic phase and the power frequency phase is uncertain. In extreme cases, the peak output current of the converter is:

[0119]

[0120] In the above formula, IP is the peak value of active current, IQ is the peak value of reactive current, I5 is the peak value of the 5th harmonic, I7 is the peak value of the 7th harmonic, and I9 is ​​the peak value of the 9th harmonic.

[0121] When selecting switching devices, Imax should be designed within the SOA (Safe Operating Area) range of the switching device.

[0122] 3. The series compensation algorithm for the phase voltage of the series-connected converter is as follows:

[0123] like Figure 3 As shown, the series-side converter control strategy adopts a positive-negative sequence separation control algorithm.

[0124] Taking the voltage compensation of the series-side converter under a certain on-load tap-changing transformer tap as an example, the coordinating controller, based on the current effective values ​​of the three-phase grid voltage (USA, USB, USC) and the target effective value of the output voltage (Uout_ref), decomposes the phase voltage compensation commands UA_pk_t, UB_pk_t, and UC_pk_t:

[0125] UA_pk_t=(Uout_ref–USA)*1.414

[0126] UB_pk_t=(Uout_ref-USB)*1.414

[0127] UC_pk_t=(Uout_ref-USC)*1.414

[0128] After receiving the phase voltage compensation command, the series-side converter decomposes it into positive-sequence voltage, negative-sequence voltage, and zero-sequence voltage compensation commands.

[0129] 1) Calculation of positive sequence voltage compensation command

[0130] The positive sequence voltage compensation command is the average value U_pk_0 of the phase voltage compensation command, calculated as follows:

[0131] U_pk_0=(UA_pk_t+UB_pk_t+UB_pk_t) / 3;

[0132] In the dq rotating coordinate system, the positive sequence voltage compensation command is:

[0133] Ud_ref = U_pk_0;

[0134] Uq_ref = 0;

[0135] 2) Calculation of negative sequence voltage compensation command

[0136] Subtracting the average value of the phase-sequence voltage compensation commands from the phase-sequence voltage compensation commands yields a result containing only the negative-sequence and zero-sequence voltage compensation commands, resulting in the three-phase unbalanced voltage compensation commands UA_pk, UB_pk, and UC_pk.

[0137] UA_pk = UA_pk_t - U_pk_0

[0138] UB_pk = UB_pk_t - U_pk_0

[0139] UC_pk = UC_pk_t - U_pk_0

[0140] To simplify the calculation, assuming the current grid phase angle is 0 degrees, the target three-phase voltage of the series-side converter is obtained based on the three-phase unbalanced voltage compensation command:

[0141] As = UA_pk × cos0°

[0142] Bs = UB_pk × cos(0-120°)

[0143] Cs = UC_pk × cos(0 + 120°)

[0144] By using the Clark transformation, we obtain the values ​​of αβ in the stationary coordinate system:

[0145] Alpha = (2 × As - Bs - Cs) / 3;

[0146] Beta = 0.5774 × (Bs - Cs);

[0147] Through the Park transformation, the negative sequence voltage compensation command in the dq rotating coordinate system is obtained as follows:

[0148] Ud_N_ref = Alpha;

[0149] Uq_N_ref = Beta;

[0150] 3) Calculation of zero-sequence voltage compensation command

[0151] Assuming the current phase angle is θ, the zero-sequence voltage compensation command is obtained based on the three-phase unbalanced voltage compensation command:

[0152] U0_ref=UA_pk×cos(θ)+UB_pk×cos(θ-120°)+UC_pk×cos(θ

[0153] +120°).

[0154] The above description is a preferred embodiment of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

[0155] This patent is not limited to the above-described preferred embodiments. Anyone can derive other forms of wide-range on-load tap changer voltage compensation control methods and systems based on the teachings of this patent. All equivalent changes and modifications made within the scope of the claims of this invention shall fall within the scope of this patent.

Claims

1. A wide-range on-load tap-changing voltage compensation control system, characterized in that: The output terminal of the on-load tap-changing transformer is connected to a converter, which includes a parallel-side converter and a series-side converter, wherein the output terminal of the series-side converter is connected to the series transformer. The parallel-side converter and the series-side converter are connected through a DC bus; the DC bus then leads out a DC port through a DC circuit breaker to connect to a DC load. Among them, parallel compensation for three-phase unbalanced current, parallel compensation for harmonic current, and DC power supply and distribution are achieved through parallel-side converters; Output voltage compensation and regulation are achieved by combining on-load tap-changing distribution transformer tap adjustment and converter series compensation: 1) On-load tap-changing distribution transformers only perform three-phase voltage average value compensation and tap adjustment; voltage compensation for unbalanced parts is entirely achieved by series-side converters. 2) The voltage compensation regulation of the series-side converter is used to smooth the voltage discontinuity of the tap change of the on-load tap-changing distribution transformer, so as to achieve stepless adjustment of the output voltage compensation over a wide compensation range. The capacity configuration method for the series-side converter is as follows: Assuming the voltage regulation range on the low-voltage side of the series transformer is ±γ1%×UN, and the rated capacity of the distribution transformer is SN, then the capacity configuration of the series-side converter is as follows: Sseries = γ1 % × SN If the minimum tap size of the on-load tap-changing distribution transformer is δ%, then: γ1 ≥ δ; Parallel-side converter configuration considerations: 1) Provide a stable DC power supply and active power P1 for the series-side converter; P1 = γ1 %×SN 2) To provide a stable DC power supply and active power P2 to the DC load, assuming the distribution transformer has a ratio of γ2%, then: P2 = γ2 %×SN 3) Provide compensation for the three-phase unbalanced current of the AC load, including active power P3 and reactive power Q1. Assuming that the proportion of phase-specific active power compensation capacity to the single-phase capacity of the distribution transformer is γ3%, and the proportion of phase-specific reactive power compensation capacity to the single-phase capacity of the distribution transformer is γ4%, then: P3 = γ3 %×SN Q1 = γ4 %×SN 4) Provide compensation for harmonic currents in AC loads, including 5th order compensation capacity H5, 7th order compensation capacity H7, and 9th order compensation capacity H9, compensated in order of increasing priority. Assuming the harmonic compensation current accounts for γ5% of the rated current of the distribution transformer, then the compensation capacity is: H5 = γ5 %×SN H7 = γ7 %×SN H9 = γ9 %×SN Therefore, the capacity configuration of the parallel-side converter is calculated as follows: 。 2. The wide-range on-load tap changer voltage compensation control system according to claim 1, characterized in that: The capacity configuration of the parallel-side converter takes into account the instantaneous value of the maximum output current stress of the converter. In extreme cases, the peak value of the converter output current is: （13） In the formula, IP is the peak value of active current, IQ is the peak value of reactive current, I5 is the peak value of the 5th harmonic, I7 is the peak value of the 7th harmonic, and I9 is ​​the peak value of the 9th harmonic. When selecting switching devices, Imax should be designed within the safe operating range of the switching device.

3. The wide-range on-load tap changer voltage compensation control system according to claim 1, characterized in that: The series-side converter control strategy employs a positive-negative sequence separation control algorithm, specifically: Under a certain on-load tap-changing transformer position, the voltage compensation of the series-side converter is achieved as follows: The coordinating controller, based on the current effective values ​​of the three-phase grid voltages USA, USB, and USC, and combined with the target effective value of the output voltage Uout_ref, decomposes the phase voltage compensation instructions UA_pk_t, UB_pk_t, and UC_pk_t: UA_pk_t = (Uout_ref–USA)*1.414 UB_pk_t = (Uout_ref - USB)*1.414 UC_pk_t = (Uout_ref - USC)*1.414 After receiving the phase voltage compensation command, the series-side converter further decomposes it into positive-sequence voltage, negative-sequence voltage, and zero-sequence voltage compensation commands.

4. A wide-range on-load tap changer voltage compensation control system according to claim 3, characterized in that: After receiving the phase voltage compensation command, the series-side converter further decomposes it into positive-sequence voltage, negative-sequence voltage, and zero-sequence voltage compensation commands, specifically including: 1) Calculation of positive sequence voltage compensation command The positive sequence voltage compensation command is the average value U_pk_0 of the phase voltage compensation command, calculated as follows: U_pk_0 = (UA_pk_t + UB_pk_t + UB_pk_t) / 3; In the dq rotating coordinate system, the positive sequence voltage compensation command is: Ud_ref = U_pk_0; Uq_ref = 0; 2) Calculation of negative sequence voltage compensation command Subtracting the average value of the phase-sequence voltage compensation commands from the phase-sequence voltage compensation commands yields a result containing only the negative-sequence and zero-sequence voltage compensation commands. This result further yields the three-phase unbalanced voltage compensation commands UA_pk, UB_pk, and UC_pk. UA_pk = UA_pk_t - U_pk_0 UB_pk = UB_pk_t - U_pk_0 UC_pk = UC_pk_t - U_pk_0 Assuming the current grid phase angle is 0 degrees, then based on the three-phase unbalanced voltage compensation command, the target values ​​of the three-phase voltages of the current series-side converter are as follows: As = UA_pk×cos0° Bs = UB_pk × cos(0-120°) Cs = UC_pk×cos(0+120°) By using the Clark transformation, we obtain the values ​​of αβ in the stationary coordinate system: Alpha = (2×As - Bs - Cs) / 3; Beta = 0.5774 × (Bs - Cs); Through the Park transformation, the negative sequence voltage compensation command in the dq rotating coordinate system is obtained as follows: Ud_N_ref = Alpha; Uq_N_ref = Beta; 3) Calculation of zero-sequence voltage compensation command Let the current phase angle be θ. Based on the three-phase unbalanced voltage compensation command, the zero-sequence voltage compensation command is obtained as follows: U0_ref = UA_pk×cos(θ) + UB_pk×cos(θ-120°) + UC_pk×cos(θ+120°).

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