CN105870927B - Unified power flow controller with multiple operation modes - Google Patents

Abstract

The present invention provides a unified power flow controller with multiple operation modes. The unified power flow controller comprises a transformer, a converter module (12) and a mode conversion switch module (11). The transformer comprises a series transformer (3) connected in series to a power transmission line, and a parallel transformer (8) connected in parallel to an AC system. One end of the converter module (12) is connected to the parallel transformer (8) via the mode conversion switch module (11), and the other end is connected to the series transformer (3) and the mode conversion switch module (11). When the present invention is used in a power transmission or distribution line, it can greatly improve the line power flow control capability and voltage stability, and has the advantages of flexible functions and high device reliability.

Description

A unified power flow controller with multiple operation modes

Technical Field

The invention relates to the technical field of power electronics, and in particular to a unified power flow controller with multiple operating modes.

Background technique

With the vigorous development of the power system, the scale access of new energy, the increasingly complex grid structure, the uneven distribution of power flow, and the insufficient voltage support capacity have put forward new requirements and challenges for the safe and stable operation of the power grid. The power supply bottlenecks in some areas cannot meet the needs of load development. The operation of the power grid shows that the uneven distribution of power flow is an important factor restricting the transmission capacity of the power grid. In view of the lack of effective power flow regulation means in traditional power grids, the use of a new Flexible Alternative Current Transmit System (FACTS) to improve the system operating conditions and ensure stable operation is a realistic and ideal choice to increase the transmission capacity of the power grid.

The Unified Power Flow Controller (UPFC), which is the representative of the third generation FACTS system, is the most comprehensive so far because it has multiple basic functions such as parallel compensation, series compensation, phase shifting and terminal voltage regulation, which can be realized separately or simultaneously. UPFC can not only realize power flow regulation while ensuring the stable operation of the power system, reasonably control active power and reactive power, improve the transmission capacity of the line, and achieve optimized operation; it can also dynamically support the voltage of the access point through fast reactive power throughput in dynamic conditions, and improve the voltage stability of the system; not only that, it can also improve the system damping and improve the power angle stability. The existing unified power flow controller is connected to the system in series and parallel by two voltage source converters, the series side realizes the power flow control function, and the parallel side realizes the reactive voltage regulation function. However, its functional configuration is not flexible enough. For example, when power flow control is not needed but a large amount of reactive power support is needed, it is not possible to flexibly configure the two converters of the system to operate in parallel for reactive power compensation.

In view of the above problems, it is necessary to propose a unified power flow controller that meets different system regulation requirements, reduces device losses, and improves equipment reliability.

Summary of the invention

In order to overcome the above-mentioned deficiencies of the prior art, the present invention provides a unified power flow controller with multiple operation modes, comprising the following steps:

The unified power flow controller comprises: a transformer, a converter module (12) and a mode conversion switch module (11); the transformer comprises a series transformer (3) connected in series to a power transmission line, and a parallel transformer (8) connected in parallel to an AC system; one end of the converter module (12) is connected to the parallel transformer (8) via the mode conversion switch module (11), and the other end is connected to the series transformer (3) and the mode conversion switch module (11).

The converter module (12) comprises: a first converter (13) and a second converter (14); the DC sides of the first converter (13) and the second converter (14) are connected to each other; the AC side of the first converter (13) is connected to one end of the mode conversion switch module (11); the AC side of the second converter (14) comprises a branch connected to the series transformer (3) via a first circuit breaker (5), and another branch connected to the mode conversion switch module (11).

The series transformer (3) is connected in parallel with the bypass switch (2); a second circuit breaker (7) is provided between the parallel transformer (8) and the AC system.

The mode conversion switch module (11) comprises: a first soft start circuit (9), a first mode conversion switch (6) and a second mode conversion switch (10);

The AC side of the second converter (14) is connected to one end of the first soft start circuit (9) via the first mode conversion switch (6), and the AC side of the first converter (13) is connected to one end of the first soft start circuit (9) via the second mode conversion switch (10), and the other end of the first soft start circuit (9) is connected to one end of the parallel transformer (8).

The mode conversion switch module (11) comprises: a first soft start circuit (9), a second soft start circuit (15), a first mode conversion switch (6) and a second mode conversion switch (10); one end of the first soft start circuit (9) and the second soft start circuit (15) are interconnected, the other end of the first soft start circuit (9) is connected to the AC side of the second converter (14) via the first mode conversion switch (6), and the other end of the second soft start circuit (15) is connected to the AC side of the first converter (13) via the second mode conversion switch (10). The parallel transformer (8) is a three-winding transformer.

The mode conversion switch module (11) comprises: a first soft start circuit (9), a second soft start circuit (15), a first mode conversion switch (6) and a second mode conversion switch (10); the AC side of the converter (14) is connected to one end of the first soft start circuit (9) via the first mode conversion switch (6), and the AC side of the converter (13) is connected to one end of the second soft start circuit (15) via the second mode switch (10); the other end of the first soft start circuit (9) and the other end of the second soft start circuit (15) are respectively connected to two ends of a three-winding transformer.

The mode conversion switch module (11) comprises: a first soft start circuit (9), a second soft start circuit (15), a first mode conversion switch (6) and a second mode conversion switch (10); the AC side of the second converter (14) is connected to one end of the first soft start circuit (9) via the first mode conversion switch (6), and the AC side of the first converter (13) is connected to one end of the second soft start circuit (15) via the second mode switch (10); the other end of the first soft start circuit (9) is connected to one end of the first parallel transformer (16), and the other end of the second soft start circuit (15) is connected to one end of the parallel transformer (8), and the other ends of the parallel transformer (8) and the first parallel transformer (16) are connected to each other. The first soft start circuit (9) and the second soft start circuit (15) comprise a resistor and a circuit breaker connected in parallel therewith.

The first converter (13) and the second converter (14) include: a two-level converter, a three-level converter, a modular multi-level converter, an H-bridge cascade multi-level converter, a diode clamped converter and/or a flying capacitor converter.

The bypass switch (2) comprises: a switch composed of a circuit breaker, an isolating switch and/or a power electronic device; the switch is a single switch or a dual bypass structure composed of two switches.

The first mode conversion switch (6) and the second mode conversion switch (10) include: a circuit breaker, an isolating switch and/or a switch consisting of a power electronic device.

The operation modes include: UPFC operation mode, single STATCOM operation mode, dual STATCOM operation mode and SSSC operation mode.

Compared with the closest prior art, the technical solution provided by the present invention has the following beneficial effects:

1. The present invention realizes four different operation modes, including: UPFC operation mode, single STATCOM mode, dual STATCOM mode and SSSC mode.

2. The four modes of the present invention can be flexibly configured, and different operation modes can be switched according to the needs of power grid operation. In particular, the dual STATCOM mode can provide a wide range of dynamic reactive power compensation, meet the requirements of different operation modes, and improve the voltage stability of the system.

3. When the present invention is used for power transmission or distribution lines, it can greatly improve the line flow control capability and voltage stability, and has the advantages of flexible functions and high device reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the structure of a unified power flow controller according to an embodiment of the present invention;

FIG2 is a schematic diagram of the structure of another unified power flow controller according to an embodiment of the present invention;

FIG3 is a schematic diagram of the structure of another unified power flow controller according to an embodiment of the present invention;

FIG4 is a schematic diagram of the structure of another unified power flow controller according to an embodiment of the present invention;

1-AC bus; 2-bypass switch; 3-series transformer; 4-transmission line resistance; 5-first circuit breaker; 6-first mode conversion switch; 7-second circuit breaker; 8-parallel transformer; 9-first soft start circuit; 10-second mode conversion switch; 11-mode conversion switch module; 12-converter module; 13-first converter; 14-second converter; 15-second soft start circuit; 16-first parallel transformer.

Detailed ways

The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings.

As shown in FIG1 , a unified power flow controller with multiple operation modes includes at least one series transformer 3, one parallel transformer 8, two converters 13 and 14, mode conversion switches 6 and 10, and other necessary power equipment. The DC sides of the two converters are connected via switches, and the AC sides of the converters are connected to the series transformer 3 and the parallel transformer 8 respectively via the mode conversion switch. The series transformer 3 is connected to the transmission line in series, and the parallel transformer 8 is connected to the AC system in parallel.

The converter can be any one or more combinations of two-level converters, three-level converters, modular multi-level converters, H-bridge cascade multi-level converters, diode clamped converters, and flying capacitor converters. The converter protection needs to be equipped with a fast bypass switch. When the system or device fails, the converter is placed in bypass to avoid the adverse effects of the system on the device or the device on the system. The fast bypass switch can be a switch composed of a circuit breaker, an isolating switch, or a power electronic device. The switch can be a single switch or a dual bypass structure composed of two switches. Preferably, a thyristor bidirectional switch is used.

The converter on the side of the protection series connection needs to be equipped with a fast bypass switch 2. When the system or device fails, the converter is bypassed to avoid the adverse effects of the system on the device or the device on the system. The fast bypass switch can be a switch composed of a circuit breaker, a disconnector or a power electronic device. The switch can be a single switch or a dual bypass structure composed of two switches. A thyristor bidirectional switch is preferably used.

The mode conversion switch may be any one or more combinations of a circuit breaker, an isolating switch, or a switch composed of power electronic devices.

The parallel transformer can be a double-winding or three-winding transformer, and at least one soft start circuit is included between the parallel transformer and the converter. The specific connection forms include but are not limited to the following four forms. Figure 1 shows two converters connected to a double-winding transformer 8 through a soft start circuit 9; Figure 2 shows two converters connected to a double-winding transformer 8 through two soft start circuits 9 and 15; Figure 3 shows two converters connected to a three-winding transformer 8 through two soft start circuits 9 and 15; Figure 4 shows two converters connected to two double-winding transformers 8 and 16 through two soft start circuits 9 and 15.

It should be noted that the present invention does not mark the arresters and gaps used to protect the coupling transformer, the inter-converter switch, the switch device, etc., which does not mean that these devices do not exist during the design and manufacture of the device and the actual implementation of the project. At the same time, there will be many isolation switches, circuit breakers, current measuring devices, and voltage measuring devices during the actual project implementation. The fact that these devices are not drawn here does not mean that these devices do not exist during the actual implementation of the project.

The unified power flow controller with multiple operation modes in the present invention can realize different operation modes through the switching operation of the mode conversion switch, and the achievable operation modes include but are not limited to: UPFC operation mode, single STATCOM mode, dual STATCOM mode and SSSC mode. The embodiment of Figure 1 is used for illustration, but is not limited to the operation mode and switching mode of this embodiment.

①UPFC operation mode:

The first circuit breaker 5, the second circuit breaker 7, the circuit breaker in the first soft start circuit 9 and the second mode conversion switch 10 are in the closed state, the bypass switch 2 and the first mode conversion switch 6 are in the open state, and the parallel side converter 8 is connected to the system through the parallel transformer to realize dynamic reactive power compensation on the parallel side. The series side converter 3 is connected to the system through the series transformer to realize independent control of active and reactive power flows.

②Single STATCOM mode:

Taking the parallel side converter as a single STATCOM as an example, the bypass switch 2, the second circuit breaker 7, the circuit breaker in the first soft start circuit 9 and the second mode conversion switch 10 are in the closed state, and the first circuit breaker 5 and the first mode conversion switch 6 are in the open state. The series side converter does not operate and there is no power loss. The parallel side converter is connected to the system through the parallel transformer to achieve dynamic reactive power compensation. Dynamic reactive power support can be achieved, the bus voltage regulation capability can be improved, and the different requirements of the grid for absorbing excess cable charging power and peak reactive power shortages can be met. Both the series side and the parallel side converters can operate in STATCOM mode separately, with mutual redundancy, to improve the reliability of the device operation.

③Dual STATCOM mode:

The bypass switch 2, the first mode conversion switch 6, the second circuit breaker 7, the circuit breaker in the first soft start circuit 9 and the second mode conversion switch 10 are in the closed state, and the first circuit breaker 5 is in the open state. After the two converters are connected in parallel, they are connected to the system through the parallel transformer to achieve dynamic reactive power compensation. The dual STATCOM mode can provide greater dynamic reactive power support, improve the bus voltage regulation capability, and meet the different needs of the power grid for absorbing the excess charging power of the cable and the reactive power shortage at peak times.

④SSSC mode:

The first circuit breaker 5 is in a closed state, and the bypass switch 2, the first mode conversion switch 6, the second circuit breaker 7, the circuit breaker in the first soft start circuit 9 and the second mode conversion switch 10 are in an open state. The parallel side converter does not operate and there is no power loss. The series side converter is connected to the system through a series transformer to realize the SSSC operation mode and perform power flow control, which can balance the line power flow and improve the cross-section transmission power limit.

In addition, it should be noted that the main connection of multiple operating modes is not limited to the structure shown in the embodiment, and any unified power flow controller involving the use of a conversion switch to increase different operating modes falls within the scope of the present invention.

The embodiments of the present invention are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described in the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Ordinary technicians in the relevant field can still modify or make equivalent substitutions to the specific implementation methods of the present invention with reference to the above embodiments. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention are within the scope of protection of the claims of the present invention to be approved.

Claims (9)

1. A multi-mode unified power flow controller comprising a transformer, a converter module (12) and a mode changeover switch module (11), characterized in that,

The transformer comprises a series transformer (3) connected in series with the power transmission line and a parallel transformer (8) connected in parallel with the alternating current system;

one end of the converter module (12) is connected with the parallel transformer (8) through the mode change-over switch module (11), and the other end of the converter module is connected with the series transformer (3) and the mode change-over switch module (11);

The converter module (12) comprises: a first converter (13) and a second converter (14);

The direct current sides of the first converter (13) and the second converter (14) are connected to each other;

the alternating-current side of the first converter (13) is connected with one end of the mode change-over switch module (11);

The alternating current side of the second converter (14) comprises a branch connected with the series transformer (3) through a first circuit breaker (5) and another branch connected with the mode change-over switch module (11);

The series transformer (3) is connected with the bypass switch (2) in parallel;

a second circuit breaker (7) is arranged between the parallel transformer (8) and the alternating current system;

the mode changeover switch module (11) includes: a first soft start circuit (9), a first mode changeover switch (6) and a second mode changeover switch (10);

The alternating current side of the second converter (14) is connected with one end of the first soft start circuit (9) through a first mode change-over switch (6), the alternating current side of the first converter (13) is connected with one end of the parallel transformer (8) through a second mode change-over switch (10), and the other end of the first soft start circuit (9) is connected with one end of the parallel transformer (8);

The operation modes include: a UPFC mode of operation, a single STATCOM mode of operation, a dual STATCOM mode of operation, and an SSSC mode of operation;

wherein, single STATCOM mode:

The bypass switch (2), the second circuit breaker (7), the first soft start circuit (9) and the second mode change-over switch (10) are in a closed state, and the first circuit breaker (5) and the first mode change-over switch (6) are in an open state; the series-side converter does not operate, and no power is lost; the parallel side converter is connected into the system through a parallel transformer to realize dynamic reactive compensation; dynamic reactive power support can be realized, the voltage regulating capability of a bus is improved, and different requirements of a grid valley on the absorption of redundant charging power of a cable and peak reactive power shortage are met; the converters at the serial side and the parallel side can be independently operated in a STATCOM mode and are redundant for standby, so that the reliability of the operation of the device is improved;

wherein, dual STATCOM mode:

The bypass switch (2), the first mode change-over switch (6), the second circuit breaker (7), the first soft start circuit (9) and the second mode change-over switch (10) are in a closed state, and the first circuit breaker (5) is in an open state; the two converters are connected in parallel and then connected into a system through a parallel transformer, so that dynamic reactive compensation is realized; the double STATCOM mode can provide larger dynamic reactive power support, improves the bus voltage regulation capability, and meets different requirements of the grid valley on the absorption of the redundant charging power of the cable and the peak reactive power deficiency.

2. The unified power flow controller according to claim 1, characterized in that the mode transfer switch module (11) comprises: a first soft start circuit (9), a second soft start circuit (15), a first mode changeover switch (6) and a second mode changeover switch (10);

The first soft start circuit (9) is connected with one end of the second soft start circuit (15), the other end of the first soft start circuit (9) is connected with the alternating current side of the second converter (14) through the first mode change-over switch (6), and the other end of the second soft start circuit (15) is connected with the alternating current side of the first converter (13) through the second mode change-over switch (10).

3. The unified power flow controller according to claim 1, characterized in that the shunt transformer (8) is a three-winding transformer.

4. A unified power flow controller according to claim 3, characterized in that the mode transfer switch module (11) comprises: a first soft start circuit (9), a second soft start circuit (15), a first mode changeover switch (6) and a second mode changeover switch (10);

the alternating current side of the second converter (14) is connected with one end of the first soft start circuit (9) through the first mode change-over switch (6), and the alternating current side of the first converter (13) is connected with one end of the second soft start circuit (15) through the second mode switch (10);

The other end of the first soft start circuit (9) and the other end of the second soft start circuit (15) are respectively connected with two ends of the three-winding transformer.

5. The unified power flow controller according to claim 1, characterized in that the mode transfer switch module (11) comprises: a first soft start circuit (9), a second soft start circuit (15), a first mode changeover switch (6) and a second mode changeover switch (10);

the alternating current side of the second converter (14) is connected with one end of the first soft start circuit (9) through the first mode change-over switch (6), and the alternating current side of the first converter (13) is connected with one end of the second soft start circuit (15) through the second mode switch (10);

The other end of the first soft start circuit (9) is connected with one end of a first parallel transformer (16), the other end of the second soft start circuit (15) is connected with one end of the parallel transformer (8), and the parallel transformer (8) and the other end of the first parallel transformer (16) are connected with each other.

6. The unified power flow controller according to claim 5 wherein the unified power flow controller comprises,

The first soft start circuit (9) and the second soft start circuit (15) comprise a resistor and a circuit breaker connected in parallel thereto.

7. A unified power flow controller according to claim 1, characterized in that the first converter (13) and the second converter (14) comprise: two-level converters, three-level converters, modular multilevel converters, H-bridge cascaded multilevel converters, diode clamped converters, and/or flying capacitor converters.

8. The unified power flow controller according to claim 1, characterized in that the bypass switch (2) comprises: a circuit breaker, a disconnector and/or a switch of power electronics;

The switch is a double bypass structure formed by a single switch or two switches.

9. The unified power flow controller according to claim 1, characterized in that the first mode switch (6) and the second mode switch (10) comprise: circuit breakers, disconnectors and/or switches of power electronics.