A black start system for a biomass unit combined with a coal-fired unit

Abstract

The present invention discloses a black start system for a biomass unit combined with a coal-fired unit. This system realizes black start by using the biomass unit when the entire power grid loses power, and then starts the coal-fired unit through the biomass unit to gradually restore the power supply capacity of the power grid system. The present invention includes a coal-fired generator, a biomass generator, a diesel generator, a No. 1 main transformer, a No. 2 main transformer, a high-voltage plant transformer, and a start-up and standby transformer. The coal-fired generator is connected to the 220 kV bus through the No. 1 main transformer T1 and connected to the 6 kV plant power 1A section and 1B section through the high-voltage plant transformer T3; the biomass generator G2 and the diesel generator G3 are both connected to the 6 kV plant power 1C section; the high-voltage side of the main transformer T2 is connected to the 220 kV bus, and the low-voltage side is connected to the 6 kV plant power 1C section; the high-voltage side of the start-up and standby transformer T3 is connected to the 220 kV bus, and the low-voltage side is connected to the 6 kV plant power 1A section and 1B section.

Description

Technical Field

[0001] The invention belongs to the technical field of auxiliary power supply for power plants, and particularly relates to a black start system for a biomass unit combined with a coal-fired unit. Background Art

[0002] Power system black start is a process in which, in the case of a large-scale power grid outage caused by human factors, natural disasters or equipment failures, and without the help of external power sources, the units with self-starting capabilities within the system are started, and then the units without self-starting capabilities are driven to start, gradually expanding the scope of system restoration, and trying to restore the operation of the entire power grid in the shortest possible time. Since black start can quickly restore the disintegrated power grid and can minimize the losses caused by power grid disintegration, it is of great significance for the power grid to cope with serious accidents, and the power grid attaches great importance to it.

[0003] The basic requirements for the power grid to restore black start units include: high reliability, fast start, reasonable location, close to the load center, and conducive to the restoration of the power grid structure. However, it is not easy to select a black start power source point that can meet the above requirements. Traditional black start power sources are generally hydroelectric units and small gas units, and there are also some thermal power units with FCB functions. Hydroelectric generating units have become the optimal black start power source due to their fast self-starting speed and small auxiliary load, and their technology is quite mature. However, hydropower plants are located in remote areas, generally far from the load-intensive areas, and are not on the best black start path of the power grid. At present, using gas units as black start power sources is also a research hotspot in the industry, but the investment cost of gas generating units is relatively high, which is not suitable for large-scale popularization at the present stage, and there are also disadvantages such as small capacity. Thermal power units have large capacity, large quantity and wide distribution, but only a very small number of them have FCB functions, and the tests have both successes and failures. Once a thermal power unit cannot perform FCB, due to the large auxiliary load, it is relatively difficult for the unit to perform black start. If it is to be made to have black start capabilities, it is necessary to build large-capacity high-voltage diesel generating sets, with a huge one-time investment and extremely low utilization rate after completion. Therefore, the black start of thermal power units is difficult and uneconomical. Unless there are special needs, thermal power units are generally not selected as black start power source points.

[0004] Renewable energy biomass power generation is a new energy source that has emerged in recent years. Replacing small urban coal-fired heating boilers with biomass cogeneration for centralized heating can well solve problems such as energy security, social development, and environmental protection. At the same time, it also increases farmers' income and improves the rural environment, and has been widely developed in northern China. According to statistics, there are more than 200 domestic biomass power generation projects under construction and preparation. According to investigations, currently built biomass units do not have black start capabilities and cannot quickly help the system restore power supply in the event of a power grid power outage. Summary of the Invention

[0005] In view of the problems existing in the prior art, the present invention provides a black-start system for a biomass unit combined with a coal-fired unit, realizing the interconnection and mutual backup between the auxiliary power supplies of the coal-fired unit and the biomass unit. In an emergency, the biomass unit is used to achieve black start, and then the coal-fired unit is started through the biomass unit to gradually restore the power supply capacity of the power grid system.

[0006] The present invention is realized through the following technical solutions:

[0007] A black-start system for a biomass unit combined with a coal-fired unit includes a coal-fired generator, a biomass generator, a diesel generator, a No. 1 main transformer, a No. 2 main transformer, a high-voltage auxiliary transformer, a start-up and standby transformer, a No. 1 high-voltage switch, a No. 2 high-voltage switch, a No. 3 high-voltage switch, a No. 1 auxiliary switch, a No. 2 auxiliary switch, a No. 3 auxiliary switch, a No. 4 auxiliary switch, a No. 5 auxiliary switch, a No. 6 auxiliary switch, a No. 7 auxiliary switch, a No. 1 interconnection switch, a No. 2 interconnection switch, and a No. 3 interconnection switch;

[0008] The coal-fired generator is connected to the low-voltage side of the No. 1 main transformer and the high-voltage side of the high-voltage auxiliary transformer;

[0009] The high-voltage side of the No. 1 main transformer is connected to the 220 kV bus through the No. 1 high-voltage switch;

[0010] The low-voltage side of the high-voltage auxiliary transformer is connected to the 6 kV auxiliary power supply section 1A and the 6 kV auxiliary power supply section 1B through the No. 1 auxiliary switch and the No. 2 auxiliary switch respectively;

[0011] The biomass generator is connected to the 6 kV auxiliary power supply section 1C through the No. 6 auxiliary switch;

[0012] The diesel generator is connected to the 6 kV auxiliary power supply section 1C through the No. 7 auxiliary switch;

[0013] The low-voltage side of the No. 2 main transformer is connected to the 6 kV auxiliary power supply section 1C, and the high-voltage side is connected to the 220 kV bus through the No. 3 high-voltage switch;

[0014] The high-voltage side of the start-up and standby transformer is connected to the 220 kV bus through the No. 2 high-voltage switch, and the low-voltage side is connected to the 6 kV auxiliary power supply 1A and the 6 kV auxiliary power supply 1B through the No. 3 auxiliary switch and the No. 4 auxiliary switch respectively.

[0015] A further improvement of the present invention is that the auxiliary load of the coal-fired generator is connected to the 6 kV auxiliary power supply section 1A and the 6 kV auxiliary power supply section 1A1B, and the auxiliary load of the biomass generator is connected to the 6 kV auxiliary power supply section 1C. During normal operation, the coal-fired generator and the biomass generator supply power to their respective auxiliary loads, and the No. 1 interconnection switch, the No. 2 interconnection switch, and the No. 3 interconnection switch are disconnected.

[0016] A further improvement of the present invention is that the biomass generator has an FCB function.

[0017] A further improvement of the present invention is that when the power grid loses power and the FCB of the biomass unit is successful, the biomass unit immediately provides plant electricity for the coal-fired unit and starts the coal-fired generator; if the FCB of the biomass unit fails, the diesel generator set is used to provide plant electricity for the biomass unit, first realizing the black start of the biomass unit, and then starting the coal-fired unit through the biomass unit.

[0018] A further improvement of the present invention is that when the coal-fired generator is under maintenance, its plant load is supplied by the biomass generator through the plant interconnection bus, and the No. 1 plant switch, No. 2 plant switch on the low-voltage side of the high-voltage plant transformer, No. 3 plant switch, and No. 4 plant switch on the low-voltage side of the start-up and standby transformer are disconnected; the No. 1 interconnection switch, No. 2 interconnection switch, and No. 3 interconnection switch on the plant interconnection bus are closed.

[0019] A further improvement of the present invention is that when the biomass generator is under maintenance, its plant load is supplied by the coal-fired generator through the plant interconnection bus, and the No. 3 plant switch and No. 4 plant switch on the low-voltage side of the start-up and standby transformer are disconnected; the No. 1 plant switch, No. 2 plant switch on the low-voltage side of the high-voltage plant transformer, the No. 1 interconnection switch, No. 2 interconnection switch, and No. 3 interconnection switch on the plant interconnection bus are closed.

[0020] A further improvement of the present invention is that when the coal-fired generator and the biomass generator are under maintenance, the power grid supplies power to the plant loads of the coal-fired generator and the biomass generator through the start-up and standby transformer, and the No. 1 plant switch, No. 2 plant switch on the low-voltage side of the high-voltage plant transformer, and the No. 2 interconnection switch on the plant interconnection bus are disconnected; the No. 3 plant switch, No. 4 plant switch on the low-voltage side of the start-up and standby transformer, the No. 1 interconnection switch, and No. 3 interconnection switch on the plant interconnection bus are closed.

[0021] Compared with the prior art, the present invention has the following beneficial technical effects:

[0022] The present invention realizes the interconnection of the plant electricity of the coal-fired unit and the biomass unit through the provided plant interconnection bus. A switch is installed at each plant electricity section to connect with the plant interconnection bus. Through the operation of the interconnection switch, the interconnection of the plant electricity of the coal-fired unit and the biomass unit can be realized, so that the plant bus of the operating unit can supply power to the plant bus of the maintenance unit. In the case of power grid power failure, if the FCB of the biomass unit is successful, the biomass unit immediately provides plant electricity for the coal-fired unit and starts the coal-fired generator. If the FCB of the biomass unit fails, the diesel generator set is used to provide plant electricity for the biomass unit, first realizing the black start of the biomass unit, and then starting the coal-fired unit through the biomass unit to realize the black start of the coal-fired unit and gradually restore the power supply capacity of the power grid.

[0023] Furthermore, when the coal-fired unit or the biomass unit is under maintenance, the present invention can supply power to the plant load of the maintenance unit through the interconnection of the plant electricity. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the schematic diagram of the black start system for the combined biomass unit and coal-fired unit in the present invention.

[0025] Figure 2 This is the schematic diagram of the normal operation of the system in the embodiment of the present invention.

[0026] Figure 3 This is the schematic diagram of the first method for the FCB black start of the biomass unit in the system in the embodiment of the present invention.

[0027] Figure 4 This is the schematic diagram of the second method for the FCB black start of the biomass unit in the system in the embodiment of the present invention.

[0028] Figure 5 This is the schematic diagram of the diesel generator black start of the biomass unit in the system in the embodiment of the present invention.

[0029] Figure 6 This is the schematic diagram of the biomass unit providing maintenance power for the coal-fired unit in the system in the embodiment of the present invention.

[0030] Figure 7 This is the schematic diagram of the coal-fired unit providing maintenance power for the biomass unit in the system in the embodiment of the present invention.

[0031] Figure 8 This is the schematic diagram of the start-up and standby transformer providing maintenance power for the coal-fired unit and the biomass unit in the system in the embodiment of the present invention.

[0032] In the figure: G1 is the coal-fired generator; G2 is the biomass generator; G3 is the fuel generator; T1 is the No. 1 main transformer; T2 is the No. 2 main transformer; T3 is the high-voltage auxiliary transformer; T4 is the start-up and standby transformer; 11 is the No. 1 high-voltage switch, 12 is the No. 2 high-voltage switch, 13 is the No. 3 high-voltage switch, 21 is the No. 1 auxiliary switch, 22 is the No. 2 auxiliary switch, 23 is the No. 3 auxiliary switch, 24 is the No. 4 auxiliary switch, 25 is the No. 5 auxiliary switch, 26 is the No. 6 auxiliary switch, 27 is the No. 7 auxiliary switch, 31 is the No. 1 interconnection switch, 32 is the No. 2 interconnection switch, 33 is the No. 3 interconnection switch. Detailed implementation manners

[0033] The following further elaborates on the present invention in conjunction with the attached drawings, which is an explanation rather than a limitation of the present invention.

[0034] As Figure 1As shown in the figure, a black start system for a biomass unit combined with a coal-fired unit provided by the present invention includes a coal-fired generator G1, a biomass generator G2, a diesel generator G3, a No. 1 main transformer T1, a No. 2 main transformer T2, a high-voltage auxiliary transformer T3, a start-up and standby transformer T4, a No. 1 high-voltage switch 11, a No. 2 high-voltage switch 12, a No. 3 high-voltage switch 13, a No. 1 auxiliary switch 21, a No. 2 auxiliary switch 22, a No. 3 auxiliary switch 23, a No. 4 auxiliary switch 24, a No. 5 auxiliary switch 25, a No. 6 auxiliary switch 26, a No. 7 auxiliary switch 27, a No. 1 interconnection switch 31, a No. 2 interconnection switch 32 and a No. 3 interconnection switch 33; the coal-fired generator G1 is connected to the main transformer T1 and the high-voltage auxiliary transformer T3; the main transformer T1 is connected to the 220 kV bus through the No. 1 high-voltage switch 11, and the high-voltage auxiliary transformer T3 is connected to the 6 kV auxiliary power supply 1A section and the 6 kV auxiliary power supply 1B section through the No. 1 auxiliary switch 21 and the No. 2 auxiliary switch 22 respectively; the biomass generator G2 is connected to the 6 kV auxiliary power supply 1C section through the No. 6 auxiliary switch 26; the diesel generator G3 is connected to the 6 kV auxiliary power supply 1C section through the No. 7 auxiliary switch 27; the high-voltage side of the main transformer T2 is connected to the 220 kV bus through the No. 3 high-voltage switch 13, and the low-voltage side is connected to the 6 kV auxiliary power supply 1C section through the No. 5 auxiliary switch 25; the high-voltage side of the start-up and standby transformer T3 is connected to the 220 kV bus through the No. 1 auxiliary switch 21, and the low-voltage side is connected to the 6 kV auxiliary power supply 1A section and the 6 kV auxiliary power supply 1B section through the No. 3 auxiliary switch 23 and the No. 4 auxiliary switch 24 respectively. The auxiliary interconnection bus BUS is connected to the 6 kV auxiliary power supply 1A section, the 6 kV auxiliary power supply 1B and the 6 kV auxiliary power supply 1C section through the No. 1 interconnection switch 31, the No. 2 interconnection switch 32 and the No. 3 interconnection switch 33 respectively.

[0035] The biomass generator G2 has the function of FCB (Fast Cut Back, quickly shedding the load of the unit to operate with auxiliary power).

[0036] The working principle of the present invention is as follows:

[0037] When the coal-fired generator and the biomass generator are operating normally, as Figure 2 shown, the auxiliary loads of the coal-fired generator and the biomass generator are supplied by their respective units. The No. 1 high-voltage switch 11, the No. 3 high-voltage switch 13, the No. 1 auxiliary switch 21, the No. 2 auxiliary switch 22, the No. 5 auxiliary switch 25, and the No. 6 auxiliary switch 26 are in the closed position; the No. 2 high-voltage switch 12, the No. 3 auxiliary switch 23, the No. 4 auxiliary switch 24, the No. 7 auxiliary switch 27, the No. 1 interconnection switch 31, the No. 2 interconnection switch 32, and the No. 3 interconnection switch 33 are in the open position. The 6 kV auxiliary power supply 1A section and the 1B section are supplied by the coal-fired generator G1 through the high-voltage auxiliary transformer T2, and the 6 kV auxiliary power supply 1C section is directly supplied by the biomass generator G2.

[0038] When the power grid loses power and the FCB of the biomass generator is successful, as Figure 3As shown in the figure, the method one is used to provide auxiliary power for the coal-fired generator G1 by the biomass generator G2 for black start. The No. 6 auxiliary switch 26, the No. 1 interconnection switch 31, the No. 2 interconnection switch 32, and the No. 3 interconnection switch 33 are in the closed position. The No. 1 high-voltage switch 11, the No. 2 high-voltage switch 12, the No. 3 high-voltage switch 13, the No. 1 auxiliary switch 21, the No. 2 auxiliary switch 22, the No. 3 auxiliary switch 23, the No. 4 auxiliary switch 24, the No. 5 auxiliary switch 25, and the No. 7 auxiliary switch 27 are in the open position. The 6kV auxiliary power supply 1A section, 1B section, and 1C section are all powered by the biomass generator G2.

[0039] The power grid loses power, and the biomass generator FCB is successful. As Figure 4 shown in the figure, the method two is used to provide auxiliary power for the coal-fired generator G1 by the biomass generator G2 for black start. The No. 2 high-voltage switch 12, the No. 3 high-voltage switch 13, the No. 3 auxiliary switch 23, the No. 4 auxiliary switch 24, the No. 5 auxiliary switch 25, and the No. 6 auxiliary switch 26 are in the closed position; the No. 1 high-voltage switch 11, the No. 1 auxiliary switch 21, the No. 2 auxiliary switch 22, the No. 7 auxiliary switch 27, the No. 1 interconnection switch 31, the No. 2 interconnection switch 32, and the No. 3 interconnection switch 33 are in the open position. The 6kV auxiliary power supply 1A section, 1B section, and 1C section are all powered by the biomass generator G2.

[0040] The power grid loses power, and the biomass generator FCB fails. As Figure 5 shown in the figure, the diesel generator G3 is used to provide auxiliary power for the biomass generator G2 for black start. The No. 7 auxiliary switch 27 is in the closed position; the No. 1 high-voltage switch 11, the No. 2 high-voltage switch 12, the No. 3 high-voltage switch 13, the No. 1 auxiliary switch 21, the No. 2 auxiliary switch 22, the No. 3 auxiliary switch 23, the No. 4 auxiliary switch 24, the No. 5 auxiliary switch 25, the No. 6 auxiliary switch 26, the No. 1 interconnection switch 31, the No. 2 interconnection switch 32, and the No. 3 interconnection switch 33 are in the open position. The 6kV auxiliary power supply 1C section is powered by the diesel generator G3. After the biomass generator starts, it serves as the auxiliary power supply for the black start of the coal-fired generator.

[0041] The gas turbine generator is under maintenance. As Figure 6 shown in the figure, the biomass generator G2 is used to provide maintenance power for the coal-fired generator G1. The No. 3 high-voltage switch 13, the No. 5 auxiliary switch 25, the No. 6 auxiliary switch 26, the No. 1 interconnection switch 31, the No. 2 interconnection switch 32, and the No. 3 interconnection switch 33 are in the closed position; the No. 1 high-voltage switch 11, the No. 2 high-voltage switch 12, the No. 1 auxiliary switch 21, the No. 2 auxiliary switch 22, the No. 3 auxiliary switch 23, the No. 4 auxiliary switch 24, and the No. 7 auxiliary switch 27 are in the open position. The 6kV auxiliary power supply 1A section, 1B section, and 1C section are all powered by the biomass generator G2.

[0042] The biomass generator is under maintenance. As Figure 7As shown in the figure, the coal-fired generator G1 provides maintenance power for the biomass generator G2. The No. 1 high-voltage switch 11, the No. 1 auxiliary power switch 21, the No. 2 auxiliary power switch 22, the No. 1 interconnection switch 31, and the No. 3 interconnection switch 33 are in the closed position; the No. 2 high-voltage switch 12, the No. 3 high-voltage switch 13, the No. 3 auxiliary power switch 23, the No. 4 auxiliary power switch 24, the No. 5 auxiliary power switch 25, the No. 6 auxiliary power switch 26, the No. 7 auxiliary power switch 27, and the No. 2 interconnection switch 32 are in the open position. The 6kV auxiliary power 1A section, 1B section, and 1C section are all powered by the biomass generator G2.

[0043] For the maintenance of the biomass generator and the coal-fired generator, as Figure 8 shown in the figure, the maintenance power for the auxiliary power loads of the coal-fired generator G1 and the biomass generator G2 is provided by the start-up and standby transformer T3. The No. 2 high-voltage switch 12, the No. 3 auxiliary power switch 23, the No. 4 auxiliary power switch 24, the No. 1 interconnection switch 31, and the No. 3 interconnection switch 33 are in the closed position; the No. 1 high-voltage switch 11, the No. 3 high-voltage switch 13, the No. 1 auxiliary power switch 21, the No. 2 auxiliary power switch 22, the No. 5 auxiliary power switch 25, the No. 6 auxiliary power switch 26, the No. 7 auxiliary power switch 27, and the No. 2 interconnection switch 32 are in the open position. The 6kV auxiliary power 1A section, 1B section, and 1C section are all powered by the start-up and standby transformer T3.

[0044] The above are only the preferred embodiments of the present invention patent, and do not impose any limitations on the present invention patent. Any simple modifications, changes, and equivalent structural changes made to the above embodiments according to the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A black start system for a biomass unit combined with a coal-fired unit, characterized in that, it includes a coal-fired generator (G1), a biomass generator (G2), a diesel generator (G3), a No. 1 main transformer (T1), a No. 2 main transformer (T2), a high-voltage plant transformer (T3), a startup and standby transformer (T4), a No. 1 high-voltage switch (11), a No. 2 high-voltage switch (12), a No. 3 high-voltage switch (13), a No. 1 plant switch (21), a No. 2 plant switch (22), a No. 3 plant switch (23), a No. 4 plant switch (24), a No. 5 plant switch (25), a No. 6 plant switch (26), a No. 7 plant switch (27), a No. 1 interconnection switch (31), a No. 2 interconnection switch (32) and a No. 3 interconnection switch (33); The coal-fired generator (G1) is connected to the low-voltage side of the No. 1 main transformer (T1) and the high-voltage side of the high-voltage plant transformer (T3); The high-voltage side of the No. 1 main transformer (T1) is connected to the 220 kV bus through the No. 1 high-voltage switch (11); The low-voltage side of the high-voltage plant transformer (T3) is connected to the 6 kV plant power supply section 1A and the 6 kV plant power supply section 1B through the No. 1 plant switch (21) and the No. 2 plant switch (22) respectively; The biomass generator (G2) is connected to the 6 kV plant power supply section 1C through the No. 6 plant switch (26); The diesel generator (G3) is connected to the 6 kV plant power supply section 1C through the No. 7 plant switch (27); The low-voltage side of the No. 2 main transformer (T2) is connected to the 6 kV plant power supply section 1C, and the high-voltage side is connected to the 220 kV bus through the No. 3 high-voltage switch (13); The high-voltage side of the startup and standby transformer (T4) is connected to the 220 kV bus through the No. 2 high-voltage switch (12), and the low-voltage side is connected to the 6 kV plant power supply 1A and the 6 kV plant power supply section 1B through the No. 3 plant switch (23) and the No. 4 plant switch (24) respectively.

2. The black start system for a biomass unit combined with a coal-fired unit according to claim 1, characterized in that, The plant load of the coal-fired generator is connected to the 6 kV plant power supply section 1A and the 6 kV plant power supply section 1A 1B, and the plant load of the biomass generator is connected to the 6 kV plant power supply section 1C. During normal operation, the coal-fired generator and the biomass generator supply power to their own plant loads respectively, and the No. 1 interconnection switch (31), the No. 2 interconnection switch (32) and the No. 3 interconnection switch (33) are disconnected.

3. The black start system for a biomass unit combined with a coal-fired unit according to claim 1, characterized in that, The biomass generator (G2) has the FCB function.

4. The black start system for a biomass unit combined with a coal-fired unit according to claim 3, characterized in that, When the power grid loses power, if the FCB of the biomass unit is successful, the biomass unit immediately provides plant power for the coal-fired unit to start the coal-fired generator; if the FCB of the biomass unit fails, the diesel generator set is used to provide plant power for the biomass unit, first realizing the black start of the biomass unit, and then starting the coal-fired unit through the biomass unit.

5. The black start system for a biomass unit combined with a coal-fired unit according to claim 1, characterized in that, For the maintenance of the coal-fired generator, the auxiliary load of the plant is powered by the biomass generator via the auxiliary interconnection busbar. The No. 1 auxiliary switch (21), No. 2 auxiliary switch (22) on the low-voltage side of the high-voltage auxiliary transformer, the No. 3 auxiliary switch (23), and No. 4 auxiliary switch (24) on the low-voltage side of the start-up and standby transformer are disconnected; the No. 1 interconnection switch (31), No. 2 interconnection switch (32), and No. 3 interconnection switch (33) of the auxiliary interconnection busbar are closed.

6. A black start system for a biomass unit combined with a coal-fired unit according to claim 1, characterized in that when the biomass generator is under maintenance, its auxiliary load is powered by the coal-fired generator via the auxiliary interconnection busbar. The No. 3 auxiliary switch (23) and No. 4 auxiliary switch (24) on the low-voltage side of the start-up and standby transformer are disconnected; the No. 1 auxiliary switch (21), No. 2 auxiliary switch (22) on the low-voltage side of the high-voltage auxiliary transformer, the No. 1 interconnection switch (31), No. 2 interconnection switch (32), and No. 3 interconnection switch (33) of the auxiliary interconnection busbar are closed.

7. A black start system for a biomass unit combined with a coal-fired unit according to claim 1, characterized in that when the coal-fired generator and the biomass generator are under maintenance, the grid supplies power to the auxiliary loads of the coal-fired generator and the biomass generator via the start-up and standby transformer. The No. 1 auxiliary switch (21), No. 2 auxiliary switch (22) on the low-voltage side of the high-voltage auxiliary transformer, and the No. 2 interconnection switch (32) of the auxiliary interconnection busbar are disconnected; the No. 3 auxiliary switch (23), No. 4 auxiliary switch (24) on the low-voltage side of the start-up and standby transformer, the No. 1 interconnection switch (31), and No. 3 interconnection switch (33) of the auxiliary interconnection busbar are closed.

Images