CN101202445A

CN101202445A - Method for double feed-in d.c. power transmission

Info

Publication number: CN101202445A
Application number: CN 200710185454
Authority: CN
Inventors: 赵成勇
Original assignee: North China Electric Power University
Current assignee: North China Electric Power University
Priority date: 2007-12-21
Filing date: 2007-12-21
Publication date: 2008-06-18
Anticipated expiration: 2027-12-21
Also published as: CN101202445B

Abstract

The invention provides a dual feed-in DC transmission method, belonging to the distribution technical field. The invention is used for solving the operation reliability of the high-voltage DC transmission. The technical proposal is that an HVDC subsystem and a VSC-HVDC subsystem are connected on the same generatrix of a transmission AC system in parallel or respectively connected with two substations which are very closely connected with each other electrically; the HVDC subsystem consists of an AC system S1 at a sending end, a system resistance Zs1, a converter transformer Ts1, an HVDC converter and a transmission line DC1, and a converter transformer (Tr1); the VSC-HVDC subsystem consists of an AC system S2 at the sending end, a system resistance Zs2, a converter transformer Ts, a VSC-HVDC converter and a transmission line DC2, and a converter transformer (Tr2). Compared with the existing HVDC system, the invention can effectively reduce the possibility that steering failure occurs to the HVDC sub-system inverter, and improves the operation reliability of the HVDC subsystem.

Description

Double-fed direct current power transmission method

Technical Field

The invention relates to a high-voltage direct-current power transmission method, and belongs to the technical field of power transmission and distribution.

Background

High Voltage Direct Current (HVDC) transmission has been developed in recent years due to advantages of large transmission capacity, low line cost, asynchronous operation of ac grids at two ends of the HVDC transmission line, and the like, and has become one of the most successful application models of modern power electronic technology in power systems. However, the power transmission method has disadvantages that the operation reliability is affected by the alternating current grids at two ends, because the HVDC converter generally adopts a common thyristor without self-turn-off capability as a converter element, the HVDC converter needs an alternating current system to provide a phase-change current during the operation, the current is actually a short-circuit current between phases, and when the alternating current potential drops due to the failure of the alternating current grid or serious asymmetry of three phases and the like, the converter overlap angle of the HVDC converter increases, which easily causes the phase-change failure. Therefore, to ensure reliable commutation, the receiving ac system must have sufficient capacity, i.e. a sufficient short-circuit ratio, otherwise the HVDC is susceptible to commutation failure, resulting in reduced operational reliability of the HVDC.

Voltage Source Converter based High Voltage direct Current (VSC-HVDC) adopts a fully controlled device as a Converter element, and compared with HVDC, the VSC-HVDC has the following characteristics:

(1) when in normal operation, the VSC can simultaneously and independently control active power and reactive power, and the control is more flexible and convenient. In HVDC, the control quantity only has a trigger angle, only active power can be controlled, and the regulation capacity for reactive power is very weak.

(2) The VSC does not need an alternating current side to provide reactive power, can play a role of the STATCOM, dynamically compensates the reactive power of an alternating current bus and stabilizes the voltage of the alternating current bus. This means that when the VSC capacity permits at the time of a fault, the VSC-HVDC can provide emergency support of both active power and reactive power to the faulty system, and thus can improve both the frequency characteristics of the system and the voltage characteristics of the receiving ac bus.

(3) The VSC current can be automatically turned off and can work in a passive inversion mode, so that additional phase-change voltage is not needed, a receiving end system can be a passive network, the fundamental defect that an HVDC receiving end must be an active network is overcome, and the VSC-HVDC power transmission method for the remote isolated load becomes practical.

Despite the above advantages of VSC-HVDC, there are disadvantages of relatively small transmission capacity (currently VSC-HVDC can reach about one tenth of HVDC in capacity) and relatively high transmission costs, and thus are not ideal transmission methods.

How to overcome the defects of the HVDC, and finding a power transmission method which can enable the existing HVDC to be more reliable in operation is an important and urgent subject faced by the current power workers.

Disclosure of Invention

The invention aims to provide a double-feed-in direct-current power transmission method which is large in power transmission capacity and high in operation reliability.

The problem is realized by the following technical scheme:

a double-feed direct current transmission method is characterized in that an HVDC subsystem and a VSC-HVDC subsystem are connected to the same bus of a transmission alternating current system in parallel or are respectively connected to two substations which are electrically connected tightly; the HVDC subsystem is composed of a transmitting end alternating current system S₁System impedance Z_s1Converter transformer T_s1HVDC converter, power transmission line DC1 and converter transformer T_r1Composition is carried out; the VSC-HVDC subsystem is composed of a transmitting end alternating current system S₂System impedance Z_s2Converter transformer T_s2VSC-HVDC converter, transmission line DC2 and converter transformer T_r2Composition is carried out; the VSC-HVDC subsystem is connected with the same bus of the receiving end alternating current system of the HVDC subsystem inverter or is connected with the receiving end alternating current system of the HVDC subsystem inverter through an alternating current transmission line L₁₂Is connected with a receiving end alternating current system bus of an HVDC subsystem inverter, and a line L₁₂The distance is shorter.

According to the double-feed-in direct current transmission method, the rectification side of the HVDC subsystem adopts a constant direct current control mode, and the inversion side adopts constant arc-quenching angle control; the VSC-HVDC subsystem adopts constant direct current voltage and constant reactive power control on the rectification side, and adopts a constant direct current and constant reactive power control or constant alternating current voltage control mode on the inversion side.

According to the double-feed-in direct current transmission method, the voltage of the alternating current system at the inverter side of the HVDC subsystem is detected in real time, and if the voltage of the alternating current system at the inverter side of the HVDC subsystem is suddenly reduced, the control circuit enables the inverter side of the VSC-HVDC subsystem to adopt constant alternating current voltage control.

In the above double-feed direct-current transmission method, the modulation degree m of the VSC-HVDC subsystem inverter is determined by the following formula:

fundamental phasor of voltage of bus at alternating current side of VSC (voltage source converter)

Fundamental phasor of side bus voltage of lagging AC systemIs determined by:

<math><mrow> <mi>δ</mi> <mo>=</mo> <mfenced open='{' close=''> <mtable> <mtr> <mtd> <mi>arctg</mi> <mrow> <mo>(</mo> <mfrac> <mi>B</mi> <mi>A</mi> </mfrac> <mo>)</mo> </mrow> <mo>+</mo> <mi>α</mi> </mtd> <mtd> </mtd> <mtd> <mi>A</mi> <mo>></mo> <mn>0</mn> </mtd> </mtr> <mtr> <mtd> <mi>arctg</mi> <mrow> <mo>(</mo> <mfrac> <mi>B</mi> <mi>A</mi> </mfrac> <mo>)</mo> </mrow> <mo>+</mo> <mi>α</mi> <mo>+</mo> <mi>π</mi> </mtd> <mtd> <mi>A</mi> <mo>≤</mo> <mn>0</mn> </mtd> <mtd> <mi>B</mi> <mo>&GreaterEqual;</mo> <mn>0</mn> </mtd> </mtr> <mtr> <mtd> <mi>arctg</mi> <mrow> <mo>(</mo> <mfrac> <mi>B</mi> <mi>A</mi> </mfrac> <mo>)</mo> </mrow> <mo>+</mo> <mi>α</mi> <mo>-</mo> <mi>π</mi> </mtd> <mtd> <mi>A</mi> <mo>≤</mo> <mn>0</mn> </mtd> <mtd> <mi>B</mi> <mo><</mo> <mn>0</mn> </mtd> </mtr> </mtable> </mfenced> </mrow></math>

wherein,

B = \frac{- k_{2} - \sqrt{k_{2}^{2} - 4 k_{1} (P_{c} - D)}}{2 k_{1}},

<math><mrow> <mi>α</mi> <mo>=</mo> <mi>arctan</mi> <mfrac> <mi>R</mi> <mi>X</mi> </mfrac> <mo>,</mo> </mrow></math>

Y = \frac{1}{\sqrt{R^{2} + X^{2}}},

K₁＝Ysinα，K₂＝-U_sYcos2α，D＝U_sYAsin2α-A²Ysinα，X＝ωL，

in the formula of U_dIs a direct current voltage, U_SIs an effective value, Q, of fundamental phasor of side bus voltage of an AC system_sReactive power, P, output for AC systems_cFor the active power that VSC absorbed, R is VSC's equivalent loss resistance, and omega is AC system's angular frequency, and L is the equivalent inductance of converter reactor.

According to the invention, VSC-HVDC and HVDC are combined into the double-feed direct current transmission method, the electrical distance between the VSC-HVDC and the HVDC is very close or zero, and as the VSC-HVDC has the capability of quickly controlling reactive power, when the voltage of an alternating current system at the side of an inverter of an HVDC subsystem is suddenly reduced, the VSC-HVDC subsystem can be used for dynamically compensating the reactive power, so that alternating voltage support is quickly provided for a receiving end of the HVDC subsystem, and the inverter of the HVDC subsystem is prevented from being failed in. Compared with the existing HVDC multi-feed system (including double feed, and the existing double feed direct current or multi-feed system is an alternating current system only including two or more HVDC feed), the invention can effectively reduce the probability of commutation failure of the HVDC subsystem inverter and improve the operation reliability of the HVDC subsystem.

The present invention will be described in further detail with reference to the accompanying drawings.

Drawings

Fig. 1 is an electrical schematic diagram of HVDC and VSC-HVDC dual feed systems:

FIG. 2 is P_c、Q_sActive power and reactive power control algorithm schematic diagram:

FIG. 3 is a three-phase AC bus voltage simulation waveform of the inversion side of the HVDC system in a non-double-feed DC system during disturbance;

FIG. 4 is a simulation waveform of the HVDC subsystem inverter side three-phase AC bus voltage during disturbance;

FIG. 5 shows DC voltages of the double-fed system and the inversion side of the HVDC system during disturbance;

FIG. 6 is an effective value of alternating current voltage on the inversion side of the double-fed system and the HVDC system during disturbance;

FIG. 7 is a turn-off angle between a double-fed system and an inversion side of an HVDC system during disturbance;

fig. 8 shows the dc voltage across the VSC-HVDC system in case of disturbance:

FIG. 9 is a DC waveform across the VSC-HVDC system under disturbance;

fig. 10 is a simplified physical model of a VSC-HVDC system converter VSC.

The symbols used in the figures and in the text are: s₁An equivalent transmitting end alternating current system of the HVDC subsystem; z₁、S₁The system equivalent impedance of (1); t is_s1A converter transformer of the HVDC subsystem; DC1, HVDC converters and transmission lines; the filter and the AC filter of the HVDC subsystem; t is_r1A converter transformer of the HVDC subsystem; l is_rAn alternating current transmission line; z_rReceiving end alternating current system S_rThe system equivalent impedance of (1); s_rThe public equivalent receiving end alternating current system of the double-fed system; s₂A VSC-HVDC transmission end alternating current system; z_S2、S₂Its equivalent system impedance; t is_S2A converter transformer for VSC-HVDC; DC2, a VSC-HVDC converter and a transmission line; t is_r2A converter transformer for VSC-HVDC; l is₁₂An alternating current transmission line;

fundamental wave phasor of bus voltage at the side of the alternating current system;

voltage fundamental phasor of a bus at the VSC alternating side; delta, delta,

Hysteresis

The angle of (d); p_sActive power output by the alternating current system; q_sReactive power output by the alternating current system; p_cActive power absorbed by the VSC; q_cReactive power absorbed by the VSC; p_cref、Q_srefThe reference input value of the controlled quantity is a set value; l, equivalent inductance of the converter reactor; r, VSC equivalent loss resistance; u shape_dA direct current voltage; i is_dDirect current; m, VSC degree of modulation of the PWM technique employed; A. an intermediate variable; B. an intermediate variable; uabc, system inversion side three-phase alternating current bus voltage; ω, angular frequency of the ac system.

Detailed Description

Referring to fig. 1, a double-fed dc transmission method comprises a VSC-HVDC subsystem and an HVDC subsystem, which are coupled in parallel to the same bus of an ac system or two substations which are very closely electrically connected. The HVDC subsystem comprises: feeding deviceEnd communication system (S)₁) System impedance (Z)_s1) Converter transformer (T)_s1) HVDC converter, transmission line (DC1), converter transformer (T)_r1) (ii) a The VSC-HVDC subsystem comprises: sending end alternating current system (S)₂) System impedance (Z)_s2) Converter transformer (T)_s2) VSC-HVDC converter and transmission line (DC2), converter transformer (T)_r2) (ii) a Wherein the VSC-HVDC subsystem is connected with the same bus of the alternating current system at the receiving end of the HVDC subsystem inverter or is connected with the alternating current transmission line (L)₁₂) And the HVDC subsystem inverter receiving end alternating current system bus is connected.

When a VSC-HVDC system supplies power to an active network, a control strategy is usually adopted in which one end of a converter station controls the voltage of a direct-current line and the other end controls the current of the direct-current line. In order to improve the stability of the output voltage of the VSC AC side, the direct current voltage is required to be controlled, so a constant direct current voltage control mode is adopted in a VSC-HVDC rectifier. Because the connected active alternating current network needs to control direct current, a constant direct current control mode is adopted in the VSC-HVDC inverter. The phase delta and the modulation degree M of the PWM can be adjusted simultaneously and independently, so that the VSC-HVDC can realize the simultaneous and independent control of active power and reactive power. The physical model of the VSC is shown in fig. 10, and the active and reactive control system relationship of the VSC is shown in fig. 2. In fig. 2, except that the PWM trigger generation circuit and the phase-locked loop refer to specific circuits or circuit modules, the rest are schematic diagrams of active and reactive control algorithms, and the units 1 to 4 are control function relationships, which are described in detail below, and the main variables and the relational expressions referred to herein are as follows.

Is the fundamental wave phasor of the bus voltage at the side of the AC system;

is VSC alternating-current side bus voltage fundamental phasor; delta is

Hysteresis

The angle of (d); p_s、Q_sActive power and reactive power output for the alternating current system; p_c、Q_cActive power and reactive power absorbed by the VSC; p_cref、Q_srefThe reference input of the controlled quantity is a set value;

(wherein:

Y = \frac{1}{\sqrt{R^{2} + X^{2}}},

x ═ ω L, L is the equivalent inductance of the converter reactor, and R is the equivalent loss resistance of the VSC); u shape_dIs a direct current voltage; and M is the modulation degree of the PWM technology adopted by the VSC.

In fig. 2, the control relationship of the unit (1) is:

the control relationship of the unit (2) is as follows:

B = \frac{- k_{2} - \sqrt{k_{2}^{2} - 4 k_{1} (P_{c} - D)}}{2 k_{1}},

(wherein: K)₁＝Ysinα，K₂＝-U_sYcos2α，D＝U_sYAsin2α-A²Ysin α); the control relationship of the unit (3) is as follows:

the control relationship of the unit (4) is as follows:

usually the active power of the VSC-HVDC is controlled by a direct voltage or a direct current, i.e. byOne side of the DC voltage and DC current is P_cref(ii) a The reactive power is obtained by directly setting the reactive power Q output by the alternating current system_sIs controlled by the constant value or the alternating voltage. The rectification side of the VSC-HVDC subsystem adopts a constant direct-current voltage and constant reactive power control mode; the inverter side adopts a control mode of fixing direct current and fixing alternating voltage so as to achieve the purpose of stabilizing alternating bus voltage.

A simulation analysis was performed on the dual feed system of fig. 1. The system comprises 2 direct current transmission lines, wherein direct current lines DC1 and DC2 are direct current lines of HVDC and VSC-HVDC subsystems respectively. The HVDC subsystem adopts a CIGRE direct current transmission standard test system; VSC-HVDC subsystem parameters are: the rated line voltage of a transmitting-end VSC alternating-current side system is 62.5kV, the equivalent loss resistance R is 0.1 omega, the equivalent inductance L of a converter reactor is 37mH, the direct-current side capacitance C is 250 mu F, a direct-current line is a 100km cable line, and the direct-current voltage setting value U is_d120 kV. AC overhead line L connecting HVDC and VSC-HVDC₁₂The length is 1 km. AC transmission line L_r10km overhead line. And the receiving end alternating current system is the same as that of the CIGRE direct current transmission standard test system.

For comparison with the above-mentioned double feed system, a single feed HVDC system was used for the study, except that there was no VSC-HVDC subsystem, and the wiring and parameters of the other parts were the same as in fig. 1.

The control mode of the HVDC subsystem is as follows: the rectifying side adopts constant current control and minimum trigger angle limitation; the inversion side adopts constant extinction angle control and constant current control. All control modes adopt PI regulators. In addition, both the rectifying side and the inverting side are provided with voltage dependent current rating limit (VDCOL) control. The VDCOL control can avoid long-time commutation failure of the inverter and improve the recovery characteristic of a direct current system after a fault.

The VSC-HVDC subsystem control mode is as follows: the rectification side adopts a constant direct-current voltage and constant reactive power control mode; the inverter side adopts a control mode of fixing direct current and fixing alternating voltage so as to achieve the purpose of stabilizing alternating bus voltage.

Supposing that the double-fed system is disturbed in a distant place of the receiving end alternating current system for 1.6s, the voltage of the receiving end system is reduced by about 0.1, interference is eliminated after 0.1s, and the system recovers to normal operation. In contrast, the inversion side ac voltage waveform of a HVDC system without VSC-HVDC feed under the same interference condition is as shown in fig. 3, while the inversion side ac bus voltage of a double feed system HVDC subsystem is as shown in fig. 4. The simulation experiment results of the dc voltage, the ac bus voltage (effective value), and the arc-extinguishing angle are shown in fig. 5, 6, and 7, respectively.

As can be seen from fig. 3 and 4, under the same interference, the ac bus voltage of the HVDC system is reduced and the distortion is large, while the waveform of the ac voltage of the double-fed system containing VSC-HVDC is relatively stable, and the waveform is substantially undistorted and is similar to a sine wave except that the amplitude is reduced. In fig. 5, the dc voltage of the HVDC system without VSC-HVDC feed drops about directly to 0.2; in fig. 6, the fluctuation of the alternating voltage on the inverting side is large, in fig. 7, the turn-off angle of the inverter is immediately reduced to 0 °, the inverter has a phase commutation failure, the fluctuation range of the direct voltage and the alternating bus voltage of the double-fed system containing VSC-HVDC is obviously reduced, the turn-off angle is reduced to 9.39 ° during the fault, the reduction of the turn-off angle is effectively suppressed, no phase commutation failure occurs, and the recovery process after the interference elimination is relatively stable and rapid. The VSC-HVDC can dynamically compensate the reactive power of a receiving end alternating current system through constant alternating current voltage control, and the alternating current bus voltage is stabilized.

The waveforms of the direct voltage on both sides of the VSC-HVDC system in case of disturbance are shown in fig. 8; the dc current waveforms across the VSC-HVDC system in case of disturbances are shown in fig. 9.

As can be seen from fig. 9, the dc voltage and current fluctuations of the VSC-HVDC system under disturbance are small, which indicates that the designed control system can effectively improve the dynamic characteristics of the VSC-HVDC system.

The simulation experiment shows that in the double-feed system, the probability of phase commutation failure of the HVDC inverter station when the voltage of the alternating current system is suddenly reduced is reduced through the coordination control of the double-feed system and the double-feed system, and the operation reliability of the HVDC system is improved.

Claims

1. A double-fed direct current transmission method is characterized in that an HVDC subsystem and a VSC-HVDC subsystem are connected to the same bus of a transmission alternating current system in parallel or are respectively connected to two substations which are electrically connected tightly; the HVDC subsystem is composed of a transmitting end alternating current system (S)₁) System impedance (Z)_s1) Converter transformer (T)_s1) HVDC converter, transmission line (DC1), converter transformer (T)_r1) Composition is carried out; the VSC-HVDC subsystem is composed of a transmitting end alternating current system (S)₂) System impedance (Z)_s2) Converter transformer (T)_s2)、VSC-HVDC converter, power transmission line (DC2) and converter transformer (T)_r2) Composition is carried out; the VSC-HVDC subsystem is connected with the same bus of the receiving end alternating current system of the HVDC subsystem inverter or is connected with the receiving end alternating current system of the HVDC subsystem inverter through an alternating current transmission line (L)₁₂) And the HVDC subsystem inverter receiving end alternating current system bus is connected.

2. The double-feed direct-current transmission method according to claim 1, wherein a constant direct-current control mode is adopted on a rectification side of the HVDC subsystem, and a constant arc-quenching angle control mode is adopted on an inversion side; the VSC-HVDC subsystem adopts constant direct current voltage and constant reactive power control on the rectification side, and adopts a constant direct current and constant reactive power control or constant alternating current voltage control mode on the inversion side.

3. A double feed-in dc transmission method according to claim 1 or 2, characterized in that the inverter side ac system voltage of the HVDC subsystem is detected in real time, and if the inverter side ac system voltage of the HVDC subsystem suddenly drops, the control circuit causes the VSC-HVDC subsystem inverter side to adopt a constant ac voltage control.

4. A double feed direct current transmission method according to claim 1, 2 or 3, characterized in that the modulation degree (m) of the VSC-HVDC subsystem inverter is determined by:

Fundamental phasor of side bus voltage of lagging AC system

Is determined by:

wherein,

B = \frac{- k_{2} - \sqrt{k_{2}^{2} - 4 k_{1} (P_{c} - D)}}{2 k_{1}},

Y = \frac{1}{\sqrt{R^{2} + X^{2}}},

k₁＝Ysinα，K₂＝-U_sYcos2α，D＝U_sYAsin2α-A²Ysinα，X＝ωL，