AN LCC AND MMC SERIES-CONNECTED HVDC SYSTEM WITH DC FAULT RIDE-THROUGH CAPABILITY

Abstract

The present invention discloses an LCC and MMC series-connected HVDC system with DC fault ride-through capacity, comprising rectifier and inverter linked by DC transmission line; Both the positive pole and the negative pole of the rectifier and the inverter consist of line-commutated converter and modular converter in series-connection; the modular converter adopts one MMC or several parallel-connected MMCs. The present invention has the advantage of low cost, low power loss and high reliability of the LCC, as well as flexible control, low harmonics and AC voltage support of the MMC. Further, the present invention is able to deal with DC fault by itself, hence additional DC fault clearing equipment is not needed. As a result, the present invention is suitable for the field of long-distance large-capacity power transmission and has broad development potential.

Claims

1. A series hybrid bipolar DC transmission system having a DC fault ride-through capability, comprising: a rectifier and an inverter, both of which are connected by a DC transmission line; characterized in that: the rectifier is used to transform three-phase AC current from sending system to DC current and further transmits the DC current to the inverter by DC transmission line; the inverter is used to transform the DC current to three-phase AC current and further transmits the three-phase AC current to receiving system; both the rectifier and the inverter adopt commutation unit of bipolar structure, namely the positive commutation unit and the negative commutation unit are series-connected and the connecting point is grounded; the commutation unit consists of line-commutated converter (LCC) and modular converter; wherein, one end of the LCC is connected to the DC transmission line, the other end of the LCC is connected to one end of the modular converter; the other end of the modular converter is connected to the ground.

2. The series hybrid bipolar DC transmission system according to claim 1, characterized in that: the modular converter adopts one modular multilevel converter (MMC) or several parallel-connected MMCs; if several parallel-connected MMCs are adopted, the modular converter equips current balance control.

3. The series hybrid bipolar DC transmission system according to claim 1, characterized in that: a passive filter is connected to the three-phase AC bus of the sending system and the receiving system.

4. The series hybrid bipolar DC transmission system according to claim 1, characterized in that: the positive and negative pole of the rectifier and the inverter connect the DC transmission line through smoothing reactor.

5. The series hybrid bipolar DC transmission system according to claim 1, characterized in that: the LCC is in parallel connection with DC filter.

6. The series hybrid bipolar DC transmission system according to claim 1, characterized in that: the LCC adopts twelve-pulse bridge converter; each bridge arm comprises several series-connected thyristor valves.

7. The series hybrid bipolar DC transmission system according to claim 1, characterized in that: the LCC is connected to the AC system through one three-winding transformer of Y.sub.0/Y/Δ connection, or two two-winding transformers of Y.sub.0/Δ and Y.sub.0/Y connection, respectively; the MMC is connected to the AC system through one two-winding transformer of Δ/Y.sub.0 connection or Y/Y.sub.0 connection.

8. The series hybrid bipolar DC transmission system according to claim 1, characterized in that: the MMC adopts the three-phase six-arm structure; an arm consists of several series-connected commutation modules; the commutation module adopts half-bridge submodule (HBSM); the HBSM consists of two switch tubes (T1 and T2) and one capacitor; wherein, one end of T1 and one end of T2 are connected and form the positive pole of HBSM; the other end of T1 connects one end of the capacitor; the other end of T2 and the other end of the capacitor are connected and form the negative pole of HBSM. Both control ends of T1 and T2 receive external switch signals.

9. The series hybrid bipolar DC transmission system according to claim 1, characterized in that: the system has three operation modes, namely bipolar current balance mode, monopolar mode with ground return path and monopolar mode with metallic return path.

10. The series hybrid bipolar DC transmission system according to claim 1, characterized in that: the system is able to deal with DC fault by the force retard of LCC at the rectifier side, the unidirectional continuity of LCC at the inverter side and the blocking of MMCs at both sides; for the force retard of LCC at the rectifier side, the firing angle is about 135°˜145° while applying a latching control to the modular converter in the rectifier station fault pole commutation unit so that the series hybrid bipolar DC; the transmission system outputs a negative voltage on the rectifier station fault pole commutation unit to eliminate the fault current provided by the fault pole commutation unit; and the inverter station pole commutation unit due to the unidirectional continuity of the thyristor converter The current will be cleared naturally.

11. The series hybrid bipolar DC transmission system according to claim 1, characterized in that: the rated DC voltage ratio of the LCC to the modular converter is not less than 0.8.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 is a structure diagram of the present invention;

[0035] FIG. 2 is a structure diagram of the twelve-pulse bridge converter;

[0036] FIG. 3 is a structure diagram of the MMC;

[0037] FIG. 4 is a structure diagram of the HBSM;

[0038] FIG. 5 is a schematic diagram of the present invention in bipolar current balance mode;

[0039] FIG. 6a is an operation diagram of the present invention when DC fault of positive pole occurs;

[0040] FIG. 6b is an operation diagram of the present invention when DC fault of negative pole occurs;

[0041] FIG. 6c is an operation diagram of the present invention when bipolar DC fault occurs;

[0042] FIG. 7a is a waveform diagram of rectifier DC voltage of the present invention when DC fault of positive pole occurs;

[0043] FIG. 7b is a waveform diagram of rectifier DC current of the present invention when DC fault of positive pole occurs;

[0044] FIG. 7c is a waveform diagram of inverter DC voltage of the present invention when DC fault of positive pole occurs;

[0045] FIG. 7b is a waveform diagram of inverter DC current of the present invention when DC fault of positive pole occurs.

PREFERRED EMBODIMENTS OF THE INVENTION

[0046] In order to more specifically describe the present invention, the technical solutions of the present invention and the related principles thereof will be described in detail with reference to the accompanying drawings and specific embodiments.

[0047] FIG. 1 is a structure diagram of the LCC and MMC series-connected HVDC system, which comprises rectifier and inverter linked by DC transmission line.

[0048] Wherein, the bipole of the rectifier and inverter consist of LCC and modular converter in series connection, the modular converter adopts one MMC or several parallel-connected MMCs.

[0049] The three-phase AC buses of both sending and receiving system connect passive filters; the type, capacity, number of banks and tuning point of the AC filters depend on engineering condition. Generally, double-tuned filter and shunt capacitor can be applied to filter out the characteristic harmonics; C type filter can also be applied to filter out the low-order harmonics if needed.

[0050] As shown in FIG. 2, the LCC adopts twelve-pulse bridge converter; wherein, each bridge consists of several series-connected thyristors; the LCC at rectifier is configured in rectification mode and is in control of constant DC current; the LCC at inverter is configured in inversion mode and is in control of constant DC voltage.

[0051] Each LCC is connected to the AC system through two two-winding transformers of Y.sub.0/Δ and Y.sub.0/Y connection, respectively. The transformers offer AC voltage transformation to match the rated DC voltage. The difference of the connection modes of the transformers offers two types of AC voltage with 30° phase deviation for the two series-connected six-pulse bridge converters in one twelve-pulse bridge converter.

[0052] As shown in FIG. 3, the MMC adopts three-phase six-arm structure; wherein, each arm consists of several series-connected commutation modules. The MMC is in control of constant DC voltage and constant reactive power. The MMC is connected to the AC system through a two-winding transformer of Δ/Y.sub.0 connection.

[0053] The commutation module adopts half-bridge submodule (HBSM), and the structure of HBSM is shown in FIG. 4. A HBSM consists of two switch tubes (T1 and T2) and one capacitor; a switch tube adopts insulated gate bipolar translator (IGBT) and diode in antiparallel connection. The emitter of IGBT in T1 connects the collector of IGBT in T2 and forms the positive pole of HBSM; the collector of IGBT in T1 connects one end of the capacitor; the emitter of IGBT in T2 connects the other end of the capacitor and forms the negative pole of HBSM. The gates of IGBT in T1 and T2 receive external switch signals.

[0054] The switching strategy of HBSM is based on the nearest level modulation and the voltage balance control of submodule capacitor.

[0055] In this preferred embodiment, the system is in bipolar current balance mode under normal condition, as shown in FIG. 5. The LCCs in the rectifier of both the positive pole and negative pole control DC current with same current order; the LCCs in the inverter of positive pole and negative pole are in constant DC voltage control. All the MMCs are in constant DC voltage control and constant reactive power control. In this control mode, the DC voltages of the positive pole and the negative pole are the same if their active power orders are the same; the positive pole or the negative pole will has larger DC voltage if the active power order of it is larger. Generally, the active power orders of both poles are set as the same to maintain the current of grounding electrodes to be zero.

[0056] The mechanism of DC fault clearance in this preferred embodiment is as below. When monopolar DC fault occurs (as shown in FIG. 6(a) and FIG. 6(b), respectively), the MMCs of the fault pole are blocked; the rectifier LCC of the fault pole is forcedly retarded by increasing its firing angle to output negative DC voltage, hence making the total rectifier DC voltage of fault pole below zero and the DC fault current from rectifier damped. Due to the unidirectional continuity of LCC, the energy path from the inverter AC system to the fault point is blocked; hence the inverter AC system will not contributes to the fault current. The monopolar DC fault is cleared if the DC fault current drops to zero; after that the fault pole stands by for recovering while the other pole keeps operating.

[0057] When bipolar DC fault occurs, all the MMCs are blocked, as shown in FIG. 6(c) , the rectifier LCCs of bipole are forcedly retarded by increasing their firing angle to output negative DC voltage, hence making the total DC voltage of rectifier below zero and the DC fault current from rectifier damped. Due to the unidirectional continuity of LCC, the energy path from the inverter AC system to the fault point is blocked; hence the inverter AC system will not contributes to the fault current. The bipolar DC fault is cleared if the DC fault current drops to zero; after that the system stands by for recovering.

[0058] In order to further demonstrate the effectiveness of the preferred embodiment, a time-domain simulation model is built up in power system transient simulation software PSCAD/EMTDC, the simulation parameters are listed in TABLE 1.

TABLE-US-00001 TABLE 1 Rated DC voltage ±800 kV   Rated DC current  4 kA Rated active power .sup.  6400 MVA Rated DC voltage of LCC at 400 kV rectifier side Rated DC voltage of MMC at 400 kV rectifier side Rated DC voltage of LCC at 380 kV inverter side Rated DC voltage of MMC at 380 kV inverter side AC RMS voltage at rectifier side 500 kV AC RMS voltage at inverter side 500 kV LCC commutation transformer Group number 1 2 at rectifier side Wiring mode Y.sub.0/Δ Y.sub.0/Y Ratio of 500 kV:165 kV 500 kV:165 kV transformation Capacity .sup.  1000 MVA 1000 MVA MMC commutation transformer Wiring mode Y.sub.0/Δ at rectifier side Ratio of transformation 500 kV:200 kV Capacity 480 MVA LCC commutation transformer Group number 1 2 at inverter side Wiring mode Y.sub.0/Δ Y.sub.0/Y Ratio of 500 kV:150 kV 500 kV:150 kV transformation Capacity .sup.  1000 MVA 1000 MVA MMC commutation transformer Wiring mode Y.sub.0/Δ at inverter side Ratio of transformation 500 kV:200 kV Capacity 480 MVA Smoothing reactor 0.3 H.sup.  DC transmission line 1000 km  Number of MMCs in parallel 4 connection Number of HBSMs in one arm 50 (300 in one MMC) Value of HBSM capacitor 1665 uF.sup.   Value of arm reactor 0.055 H  

[0059] Monopolar DC transmission line fault is the most likely to occur; in this preferred embodiment, the monopolar DC transmission line fault is applied at the middle point of the positive DC transmission line at 1.0 s with ground resistance 1.0Ω. The DC fault clearing method is as below. The DC fault is detected by the system if DC current is larger than 1.5 pu. After the fault is detected, the MMCs at both sides of the fault pole are blocked; α.sub.R is set to 135°; α.sub.I is set to 90° and kept until the system restarts. After the fault current cleared, the action of LCCs and MMCs is kept for another 0.2 s so that insulation of the fault point can be recovered. After deionisation, the MMCs at both sides are deblocked; for LCCs, α.sub.R is set to 45° and then linearly decreased to 15°, α.sub.I is set to 120° and then linearly increased to 140°; the restart process lasts 0.2 s. After these actions completed, the system is shifted to the normal operation mode.

[0060] The fault response is shown in FIG. 7. FIGS. 7a and 7b are the waveform of rectifier DC voltage and DC current, respectively. The two diagrams show that, after the positive DC fault occurs, the force retard of rectifier LCC makes the rectifier DC voltage negative and hence the rectifier DC current drops to zero; the rectifier DC voltage and DC current of negative pole return to normal after slight fluctuation.

[0061] FIGS. 7c and 7d are the waveform of inverter DC voltage and DC current, respectively. The two diagrams show that, after the positive DC fault occurs, the inverter DC current drops to zero due to the unidirectional continuity of LCC; the inverter DC voltage and DC current of negative pole return to normal after slight fluctuation.

[0062] After the fault cleared, the system returns to normal smoothly.