CN107276103A - A kind of UPFC connection in series-parallel transverter coordination control strategies for improving ac bus voltage support intensity - Google Patents

Abstract

The invention discloses a coordinated control strategy for UPFC series-parallel converters that improves the supporting strength of the voltage of the AC bus, which monitors the amplitude of the voltage of the controlled AC bus during the operation of the power grid, and calculates and reflects the AC bus when the voltage of the AC bus exceeds the limit. The characteristic signal of the voltage cross-line degree, and automatically adjust the reactive power flow command value of the UPFC series converter and the reactive power command value of the parallel converter through the AC bus voltage coordination control algorithm, and maximize the use of the UPFC system-level adjustment capability. The AC voltage is controlled within the safe and stable operating range. It can be seen that the present invention provides an effective means for solving the problem of grid voltage support after UHV is connected to the system, and maximally improving the dynamic/static voltage support capability of the system.

Description

Coordinated control of UPFC series-parallel converters to improve AC bus voltage support strength control strategy

technical field

The invention belongs to the technical field of flexible power transmission and distribution of power systems, and in particular relates to a coordinated control strategy for UPFC series-parallel converters that improves the voltage support strength of an AC bus.

Background technique

The reverse distribution characteristics of my country's energy resources and loads determine the basic pattern of "West-to-East Power Transmission, North-to-South Power Transmission"; UHV transmission technology suitable for long-distance and large-capacity transmission, in order to realize the rational development and optimal allocation of my country's resources , Efficient utilization provides an effective solution. However, while realizing long-distance and large-capacity power transmission, the UHV power transmission project also brings new challenges to the safe and stable operation of the power grid; The power is up to 12000MW, and the centralized feed-in of large-scale power has brought great challenges to the power flow control and voltage support capabilities of the receiving end grid.

Unified PowerFlow Controller (UPFC), as the latest generation of Flexible AC Transmission System (FACTS) device, can not only realize the precise control of power flow, but also has the ability of voltage regulation. Power flow control and voltage support problems provide a comprehensive solution. UPFC is composed of two back-to-back voltage source converters. The two back-to-back converters share the DC bus and DC capacitors. Both of them are connected to the system through the converter transformer. The parallel converters are connected in parallel through the converter transformer. The series converters are connected in series through the converter transformers. As shown in Figure 1, UPFC usually has three system-level control dimensions, namely active/reactive power flow control of transmission lines and reactive power flow control of parallel converters. power control.

In the research on using FACTS devices to improve the voltage support capacity of the power grid, usually only parallel FACTS devices are considered, and there are few studies on the reactive power adjustment capability of series FACTS devices; in order to ensure the safe and stable operation of the power system, in addition to dynamic reactive power In addition to support, a large number of capacitor banks need to be configured for static reactive power compensation. The uncontrollability of reactive power flow and unbalanced reactive power configuration and demand in traditional power grid operation will lead to unbalanced distribution of reactive power in the power grid, resulting in the inability to fully utilize static reactive power compensation devices; UPFC’s reactive power flow control capability can be accurately Controlling the reactive power flow of the line provides a means to improve the unbalanced reactive power distribution of the system and solve the problem of insufficient reactive power support in local areas after N-1 faults. In order to make full use of the voltage regulation capability of UPFC and maximize the system voltage support strength, the reactive voltage regulation capability of UPFC series and parallel converters should be used at the same time, but at the same time, the UPFC series and parallel converters should be used to control the AC bus voltage. If the control does not consider the coordination of the two, it will cause the UPFC series and parallel converters to compete with each other for reactive power control, and the steady-state operating point cannot be determined.

From the above analysis, it can be seen that how to make full use of the reactive power control capabilities of UPFC series and parallel converters and coordinate the relationship between the reactive power control quantities of UPFC series and parallel converters can maximize the voltage support strength of the AC bus. It plays a very positive role in ensuring the safe and stable operation of the power system.

Contents of the invention

In view of the above, the present invention proposes a coordinated control strategy for UPFC series-parallel converters that improves the AC bus voltage support strength, and can automatically adjust the reactive power flow command value of the UPFC series converters and the parallel converters when the AC bus voltage exceeds the limit. The reactive power command value controls the AC voltage within the safe and stable operation range, and maximizes the voltage support strength of the AC busbar.

A coordinated control strategy for UPFC series-parallel converters that improves AC bus voltage support strength, comprising the following steps:

(1) Calculate the sensitivity A _ac of the voltage amplitude V _ac of the controlled AC bus node relative to the reactive power flow command value Q _L ^* of the UPFC series-side converter and the relative reactive power command value Q of the UPFC parallel-side converter _sh ^* sensitivity to change B _ac ;

(2) Calculate the voltage limit deviation signals ΔR _se and ΔR _sh corresponding to the UPFC series side and parallel side converters according to the sensitivities _Aac and _Bac ;

(3) When it is detected that ΔR _se ≠ 0 or ΔR _sh ≠ 0, that is, the voltage of the controlled AC bus node exceeds the limit, the reactive power flow regulation signal ΔQ _L of the UPFC series side converter and the UPFC The reactive power adjustment signal ΔQ _sh of the parallel side converter;

(4) Under normal working conditions, the reactive power flow command value Q _L0 of the UPFC series-side converter and the reactive power command value _Qsh0 of the UPFC parallel-side converter given by the control center are respectively related to the reactive power flow regulation signal ΔQ _L and The reactive power adjustment signals ΔQ _sh are superimposed to obtain the final reactive power flow command value Q _L ^* and reactive power command value Q _sh ^* ; then Q _L ^* and Q _sh ^* are respectively input to the UPFC series side converter The reactive power flow control module and the reactive power control module of the UPFC parallel side converter are used as the control instructions of the UPFC for control.

Further, in the step (1), the sensitivities A _ac and B _ac are calculated by the following formula:

A _ac ＝-j(Z _my -Z _mx )

B _ac ＝-jZ _mx

Among them: Z _mx is the element in row m and column x in matrix Z, Z _my is the element in row m and column y in matrix Z, x and y are the numbers of the busbar nodes at both ends of the line where UPFC is installed, and the positive direction of the current is from x flows to y, m is the number of the controlled AC bus node, Z is the inverse matrix of the grid system node admittance matrix, and j is the imaginary unit.

Further, in the step (2), the voltage over-limit deviation signals ΔR _se and ΔR _sh are calculated by the following formula:

ΔR _se =-ΔV _ac Sgn(A _ac )

ΔR _sh =ΔV _ac Sgn(B _ac )

ΔV _ac = max(V _ac -V _ac,max ,0)+min(V _ac -V _ac,min ,0)

Among them: V _ac,max and V _ac,min are the operating voltage upper limit and lower limit of the controlled AC bus node respectively, Sgn() is a sign function, that is, when the argument in () is ≥0, the function value is 1; when the argument in ()<0, the function value is -1.

Further, in the step (3), the reactive power flow regulation signal ΔQ _L and the reactive power regulation signal ΔQ _sh are calculated and generated by the following formula:

Among them: K _se and K _sh are the set amplification gain coefficients, D _se and D _sh are the set droop control coefficients, and s is the Laplacian operator.

Further, the ratio of the droop control coefficient D _se to D _sh is equal to the ratio of |B _ac | to |A _ac |.

The coordinated control strategy of the UPFC series-parallel converter of the present invention monitors the amplitude of the controlled AC bus voltage during the operation of the power grid, and calculates the characteristic signal reflecting the degree of the AC bus voltage crossing the line when the AC bus voltage exceeds the limit, and through the AC bus voltage The coordinated control algorithm automatically adjusts the reactive power flow command value of the UPFC series converter and the reactive power command value of the parallel converter, and maximizes the use of the UPFC system-level adjustment capability to control the AC voltage within a safe and stable operating range. It can be seen that the present invention provides an effective means for solving the problem of grid voltage support after UHV is connected to the system, and maximally improving the dynamic/static voltage support capability of the system.

Description of drawings

Figure 1 is a schematic diagram of the structure of the unified power flow controller UPFC.

Figure 2 is a schematic diagram of the structure of a power system containing UPFC.

Fig. 3 is a control block diagram of the AC bus voltage coordination controller of the present invention.

Fig. 4 is a simplified schematic diagram of an actual grid system.

Fig. 5 is a graph of the voltage response curve of the bus 3 under the natural power flow distribution and the control strategy of the present invention.

Fig. 6 is a response curve of 2-1 line reactive power flow and UPFC parallel converter output reactive power under natural power flow distribution and under the control strategy of the present invention.

detailed description

In order to describe the present invention more specifically, the technical solutions of the present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

The coordinated control strategy of the UPFC series-parallel converter for improving the voltage support strength of the AC bus in the present invention comprises the following steps:

(1) Calculate the sensitivity of the controlled AC bus voltage amplitude V _ac to the change of the reactive power flow command value Q _L ^* of the UPFC series side converter and the reactive power command value Q _sh ^* of the parallel side converter, the specific method is as follows :

For a power system with a AC bus, suppose the number of the controlled AC bus in the system is m, the UPFC parallel side is connected to bus 1, and the UPFC series side is installed between bus 1 and 2, as shown in Figure 2. The matrix Z is the inverse matrix of the admittance matrix of the electric network node, and its i-th row and j-th column element is Z _ij (i,j=1,2,...,a); according to the electric network theory, the controlled bus voltage The calculation formulas of the sensitivity A _ac and B _ac of the amplitude V _ac to the change of the reactive power flow command value Q _L ^* of the UPFC series side converter and the reactive power command value Q _sh ^* of the parallel side converter are as follows:

A _ac ＝-j(Z _m2 -Z _m1 )

B _ac = -jZ _m1

In the actual transmission network, the resistance value of the overhead line is much smaller than its reactance value. Compared with the imaginary part, the real part of Z _ij can be ignored. At the same time, the phase angles of the voltage phasors of the AC buses near the UPFC installation line are relatively close, and the amplitudes are all around 1.0pu. Under this condition, the relationship between the voltage amplitude V _ac of the controlled bus and the variation of the reactive power flow command value Q _L ^* of the UPFC series-side converter and the variation of the reactive power command value Q _sh ^* of the parallel-side converter is as follows: :

It can be seen that A _ac and B _ac reflect the sensitivity of the controlled bus voltage amplitude V _ac to the change of the reactive power flow command value Q _L ^* of the UPFC series side converter and the reactive power command value Q _sh ^* of the parallel side converter.

(2) Monitor the controlled AC bus voltage amplitude V _ac online and calculate the voltage over-limit deviation signals ΔR _se and ΔR _sh used to generate the reactive power command value of the UPFC series and parallel side converters; when it is detected that ΔR _se ≠0 Or ΔR _sh ≠ 0, that is, when the controlled AC bus voltage exceeds the limit, the AC bus voltage coordinating controller generates the adjustment signal ΔQ _L of the reactive power flow command value Q _L ^* of the UPFC series-side converter and the reactive power of the parallel-side converter An adjustment signal ΔQ _sh of the power command value Q _sh ^* .

The AC bus voltage coordinating controller is shown in Fig. 3, where K _se and K _sh are the amplifier gains of the AC bus voltage controllers of the UPFC series and parallel converters respectively, and D _se and D _sh are the AC voltages of the series and parallel converters respectively. Droop control coefficient of bus voltage control, s is the Laplacian operator. The error signal ΔR _se and the feedback signal D _se ΔQ _L of the output signal ΔQ _L are superimposed, and after the integration link, the adjustment signal ΔQ _L of the reactive power flow command value of the UPFC series side is obtained; the error signal ΔR _sh and the feedback signal D _sh of the output signal ΔQsh After ΔQ _sh is superimposed, the adjustment signal ΔQ _sh of the reactive power command value of the UPFC parallel side is obtained after the integral link, namely:

The distribution of the reactive voltage control quantity between the series and parallel converters can be distributed according to the predetermined distribution principle by changing the droop control coefficients D _se and D _sh of the AC bus voltage control of the series and parallel converters. In this embodiment, according to the voltage amplitude of the controlled bus, the sensitivities to changes in the reactive power flow command value of the UPFC series-side converter and the reactive power command value of the parallel-side converter are allocated, that is, D _se : D _sh =|B _ac |:|A _ac |.

The error signals ΔR _se and ΔR _sh in the AC bus voltage coordination controller are calculated by the following formula:

ΔR _se =-ΔV _ac Sgn(A _ac )

ΔR _sh =ΔV _ac Sgn(B _ac )

Where: ΔV _ac = max(V _ac -V _ac,max ,0)+min(V _ac -V _ac,min ,0), V _ac,max and V _ac,min are respectively controlled by the upper limit of the operating voltage of the AC bus value and the lower limit value, Sgn(x) is a sign function, and the transformation law of its value with the independent variable x is:

(3) Add the adjustment signal _{ΔQ L} to the reactive power flow setting value Q _L0 under normal working conditions to obtain the final reactive power flow command value Q _L ^* of the UPFC series side converter; In this case, the reactive power setting value Q _sh0 is added to get the final reactive power command value Q _sh ^* of the UPFC series side converter; the command value is input to the reactive power flow control module and the UPFC series side converter respectively. The reactive power control module of the parallel side converter is used as the control reference value of UPFC and controlled to realize the maximum utilization of UPFC reactive power control capability.

Figure 4 is a simplified schematic diagram of an actual power grid system. _UPFC is installed on the side of line 1-2 close to AC bus 1 ^. Table 1 shows the comparison between the simulated value and the analytical value of the reactive power command value Q _sh ^* of the converter. The simulated values in Table 1 are The change of the voltage amplitude of each busbar (unit pu) when changing 1pu separately, the analytical value is the result calculated by using the formula given in this embodiment; ΔV ₁ , ΔV ₃ , and ΔV ₄ are respectively The amount of change in the voltage amplitude.

Table 1

It can be seen from the data in Table 1 that the simulated value of the sensitivity of the AC bus voltage amplitude to the reactive power flow command value Q _L ^* of the UPFC series-side converter and the reactive power command value Q _sh ^* of the parallel-side converter is consistent with The difference between the analytical values is small, and the calculation method given in this embodiment can be used to judge the effect of the AC bus voltage amplitude on the reactive power flow command value Q _L ^* of the UPFC series-side converter and the reactive power command value Q _sh of the parallel-side converter ^* Change the size of the sensitivity level. At the same time, in the UPFC control strategy given in this embodiment, only the signs of A _ac and B _ac are used in the process of feedback control of the cross-line voltage, and the values do not need to be exactly the same. The control effect of the coordinated control strategy of this implementation is verified below. The steady-state voltage level and transient voltage support strength of the AC bus of the DC converter station are crucial to the consumption of UHV DC and the safe and stable operation of the system. Therefore, the AC busbar (busbar 3) of the DC converter station is selected as the controlled AC busbar.

The response process of the coordinated control strategy of the UPFC series-parallel converter in this embodiment is shown in Figure 5 and Figure 6. The dynamic simulation process is: a non-metallic ground fault occurs on the side 1 of the 1-2 double-circuit line at 1.0s, and the UPFC protection Action, the series converter is out of operation, the faulty circuit is cut off in 1.1s, and the UPFC series converter on the non-faulty circuit is put into operation again in 3.1s. In the dynamic simulation, when the UPFC series-parallel converter coordination controller of this embodiment is installed in the UPFC, the parameters used are as follows: V _ac,min =0.96pu, V _ac,max =1.04pu, K _se =1000, D _se =0.00062, K _sh =1000, D _sh =0.00056, Q _L0 =5.94pu, Q _sh0 =0.

It can be seen from Figures 5 and 6 that the transient voltage support strength and steady-state voltage regulation capability of the AC busbar (busbar 3) of the DC converter station are improved after the control strategy of this embodiment is adopted. At the same time, it can be seen from the partially enlarged part in Figure 6 that if there is no reactive power flow adjustment of the series converter, when the reactive power output of the parallel converter increases, the natural power flow distribution will be transmitted to the DC drop point area through the 2-1 line The reactive power will decrease after the accident, which is not conducive to the recovery of the bus voltage of the DC converter station after the accident. After the series converters are put into operation, the reactive power flow on lines 2 to 1 is adjusted by changing the UPFC reactive power flow command value to further increase the recovery voltage after the bus failure of the DC converter station, improve the reactive power flow distribution of the system after the accident, and reduce the Capacity requirements for DC capacitors. It can be seen that the coordinated control strategy of the UPFC series-parallel converter of the present invention has better dynamic characteristics, and the above analysis also verifies the effectiveness of the coordinated control strategy of the UPFC series-parallel converter of the present invention.

The above description of the embodiments is for those of ordinary skill in the art to understand and apply the present invention. It is obvious that those skilled in the art can easily make various modifications to the above-mentioned embodiments, and apply the general principles described here to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art according to the disclosure of the present invention should fall within the protection scope of the present invention.

Claims (5)

1. A kind of UPFC series-parallel converter coordinated control strategy that improves AC bus voltage support strength, comprises the steps: (1) Calculate the sensitivity A _ac of the voltage amplitude V _ac of the controlled AC bus node relative to the reactive power flow command value Q _L ^* of the UPFC series-side converter and the relative reactive power command value Q of the UPFC parallel-side converter _sh ^* sensitivity to change B _ac ; (2) Calculate the voltage limit deviation signals ΔR _se and ΔR _sh corresponding to the UPFC series side and parallel side converters according to the sensitivities _Aac and _Bac ; (3) When it is detected that ΔR _se ≠ 0 or ΔR _sh ≠ 0, that is, the voltage of the controlled AC bus node exceeds the limit, the reactive power flow regulation signal ΔQ _L of the UPFC series side converter and the UPFC The reactive power adjustment signal ΔQ _sh of the parallel side converter; (4) Under normal working conditions, the reactive power flow command value Q _L0 of the UPFC series-side converter and the reactive power command value _Qsh0 of the UPFC parallel-side converter given by the control center are respectively related to the reactive power flow regulation signal ΔQ _L and The reactive power adjustment signals ΔQ _sh are superimposed to obtain the final reactive power flow command value Q _L ^* and reactive power command value Q _sh ^* ; then Q _L ^* and Q _sh ^* are respectively input to the UPFC series side converter The reactive power flow control module and the reactive power control module of the UPFC parallel side converter are used as the control instructions of the UPFC for control. 2. UPFC series-parallel converter coordinated control strategy according to claim 1, is characterized in that: in described step (1), calculate sensitivity A _ac and B _ac by following formula: A _ac ＝-j(Z _my -Z _mx ) B _ac ＝-jZ _mx Among them: Z _mx is the element in row m and column x in matrix Z, Z _my is the element in row m and column y in matrix Z, x and y are the numbers of the busbar nodes at both ends of the line where UPFC is installed, and the positive direction of the current is from x flows to y, m is the number of the controlled AC bus node, Z is the inverse matrix of the grid system node admittance matrix, and j is the imaginary unit. 3. The UPFC series-parallel converter coordinated control strategy according to claim 1, characterized in that: in the step (2), the voltage over-limit deviation signals ΔR _se and ΔR _sh are calculated by the following formula: ΔR _se =-ΔV _ac Sgn(A _ac ) ΔR _sh =ΔV _ac Sgn(B _ac ) ΔV _ac = max(V _ac -V _ac,max ,0)+min(V _ac -V _ac,min ,0) Among them: V _ac,max and V _ac,min are the operating voltage upper limit and lower limit of the controlled AC bus node respectively, Sgn() is a sign function, that is, when the argument in () is ≥0, the function value is 1; when the argument in ()<0, the function value is -1. 4. The coordinated control strategy of UPFC series-parallel converters according to claim 1, characterized in that: in the step (3), the reactive power flow regulation signal ΔQ _L and the reactive power regulation signal ΔQ _sh are calculated and generated by the following formula : <mfenced open = "" close = ""><mtable><mtr><mtd><mrow><msub><mi>&amp;Delta;Q</mi><mi>L</mi></msub><mo>=</mo><mfrac><mrow><msub><mi>&amp;Delta;R</mi><mrow><mi>s</mi><mi>e</mi></mrow></msub></mrow><mrow><mfrac><mi>s</mi><msub><mi>K</mi><mrow><mi>s</mi><mi>e</mi></mrow></msub></mfrac><mo>+</mo><msub><mi>D</mi><mrow><mi>s</mi><mi>e</mi></mrow></msub></mrow></mfrac></mrow></mtd><mtd><mrow><msub><mi>&amp;Delta;Q</mi><mrow><mi>s</mi><mi>h</mi></mrow></msub><mo>=</mo><mfrac><mrow><msub><mi>&amp;Delta;R</mi><mrow><mi>s</mi><mi>h</mi></mrow></msub></mrow><mrow><mfrac><mi>s</mi><msub><mi>K</mi><mrow><mi>s</mi><mi>h</mi></mrow></msub></mfrac><mo>+</mo><msub><mi>D</mi><mrow><mi>s</mi><mi>h</mi></mrow></msub></mrow></mfrac></mrow></mtd></mtr></mtable></mfenced> Among them: K _se and K _sh are the set amplification gain coefficients, D _se and D _sh are the set droop control coefficients, and s is the Laplacian operator. 5. The coordinated control strategy for UPFC series-parallel converters according to claim 4, wherein the ratio of the droop control coefficient D _se to D _sh is equal to the ratio of |B _ac | to |A _ac |.