| 6 ELECTRICAL SYSTEM
In according to the requirements of the Owner, 1x200MW units will be constructed in the power plant. The unit will be connected to the 110kV outdoor switchgear and will be transmit by one 110kV outgoing line.
As lack of information and data from the site, the supposed criterion for design will be as follows:
Ø The maximum short-circuit current of 110kV system will be no more than 40kA (r.m.s), and there will be one outgoing line.
The design and equipment specification will comply with the following codes and standards:
Ø IEEE-Institute of Electrical and Electronics Engineers;
Ø IEC-International Electromechanical Commission;
Ø GB-The national standards of The People Republic of China.
The design scope of electrical system of the project will include following items:
A. The related electrical design in the main block.
B. Design of generator as well as their enclosed bus bars and power transformers.
C. Electrical design of 110kV switchgear (including 110kV line protections, RTU).
D. Electrical auxiliary system outside the main block.
E. Lightning protection and grounding.
F. Cable facilities of the whole station.
G. Design of A.C uninterrupted power supply and DC system.
H. Fire-fighting system.
I. Lighting of the whole station.
6.2 Key Electrical Connection
In this project, the 110kV switchgear will adopt generator transformer line connection and outdoor type equipment. The units will be connected with one double-winding step-up transformer in unit form, and then linked with the 110kV switchgear through the overhead line.
The generator and LV terminals of the step-up transformer will be connected through the IPB (isolated-phase busbar).
One HV auxiliary transformer will be brunched from main circuit of IPB between the generator and low voltage side of the step-up transformer; it will provide power for the unit auxiliary load.
The HV start-up/standby auxiliary transformer will be installed in this project, and it will supply start-up and standby power to each 6.6kV auxiliary unit board.
The neutral point of HV winding of the step-up transformer and HV start-up/standby transformer will be directly grounded.
The neutral point of the LV side of the HV auxiliary transformer will be grounded through resistance.
The neutral point of the LV side of each LV auxiliary transformer will be directly grounded.
The neutral point of the generator will be grounded through a single phase grounding transformer with a resistance on the second circuit.
6.3 Main Equipment Specification
The equipment will be selected based on the short-circuit current level as follows:
Ø 110kV bus: 40kA (r.m.s);
Ø 6.9kV busbar: 40 kA (r.m.s)(preliminary).
The main electrical equipment specification will be as follows:
Ø Generator
Rated capacity 200MW
Maximum continuous output power match with turbine MW
Rated power factor 0.85(lagging)
Rated voltage 15.75kV
Rated frequency 60Hz
Ø HV Step-up transformer
Rated capacity 240MVA
Rated voltage and tap range 121±2x2.5�?/SPAN>/15.75kV
Vector group YN,d11
Impedance Ud%=18
Cooling mode ONAN/ONAF
Off load tap changer
Ø HV auxiliary transformer
Rated capacity 25MVA
Rated voltage and tap range 15.75±2x2.5�?/SPAN>/6.9kV
Vector group D,yn11
Impedance Ud%=10.5
Cooling mode ONAN/ONAF
Off load tap changer
Ø HV start-up/standby transformer
Rated capacity 25MVA
Rated voltage and tap range 15.75±8x1.5�?/SPAN>/6.9kV
Vector group YN,d11
Impedance Ud%=10.5
Cooling mode ONAN/ONAF
On load tap changer
Ø The 110kV circuit breaker will be single-breaking pole, SF6 type, rated breaking current will be 40kA.
6.4 Auxiliary Power System
Auxiliary power system will adopt 6.9kV and 480/240V power distribution systems.
6.4.1 6.9kv Distribution System Connection
Two 6.9kV unit buses will be installed , which will be fed by the HV auxiliary transformer via 6.6kV non segregated phase busduct, supplying power to 6.6kV motors and LV auxiliary transformers . Meanwhile, the HV auxiliary start-up/standby transformer provided start-up and standby power source for each 6.9kV unit bus.
The 6.9kV system will be grounded via neutral grounding resistances.
6.4.2 480/240V Distribution System Connection
480/240V distribution system will consists of power center (PC) and Motor control center (MCC).
The low voltage auxiliary electrical system will supply power to 460V loads through indoor dry-type transformers connected to 6.9kV high voltage system.
Some LV dry-type transformers will be installed, supplying power to LV auxiliary load in the power plant.
One emergency diesel generator set and one emergency bus (PC) will be installed for unit to supply AC emergency power for unit shut down as system fault. The emergency load will be supplied by unit auxiliary buses (PC) in normal operation, while it will be supplied by diesel generator if emergency happened.
The neutral point of LV auxiliary electrical system will be directly grounded.
6.5 Arrangement of Electrical Equipment
6.5.1 Outdoor Oil-Immersed Transformer
The step-up transformers, HV start-up/standby transformer and the HV unit auxiliary transformers will be arranged in front of the column A out of the main block building. The HV terminals of step-up transformers and HV start-up/standby transformer will be connected to 110kV switchgear through the overhead line. The HV terminals side of HV unit auxiliary transformer will be connected to the low voltage side of step-up transformer by isolated phase busduct.
6.6kV non-segregated phase bus will be adopted to connect the LV terminals of the HV auxiliary transformer, HV start-up/standby transformer the 6.6kV auxiliary switchgears.
6.5.2 110kv Switchgear
110kV outdoor switchyard will be placed in the front of the main block, which will be connected to 110kV grid with overhead line.
6.5.3 6.9kV and 480/240V Auxiliary Switchgears
Based on arrangement in main block, the arrangement of auxiliary switchgear is as follow:
6.6kV auxiliary switchgear will be arranged in turbine hall.
480/240V unit auxiliary switchgear will be arranged in main block,
Other 480/240V auxiliary switchgears out of the main block will be arranged near the center of the electrical loads.
6.5.4 Selection Of Auxiliary Equipment
A assemble configure proposal of vacuum circuit break & (F-C) will be designed for 6.6kV auxiliary power system.
LV transformers will be dry-type.
480/240V PC & MCC switchgear cubicle will be draw-out type.
6.6 Overvoltage Protection and Grounding
6.6.1 Overvoltage Protection
6.6.1.1 Overvoltage Protection against Invading Lightning Wave
Zinc-oxide lightning arrester will be installed on the high-voltage side of step-up transformer and HV start-up/standby auxiliary transformer, at generator outlet and bus of 110kV switchgear.
6.6.1.2 Protection Against Lightning Stroke
Lightning Rod will be installed on top of steam turbine hall and 110kV switchgear to protect outdoor transformers and 110kV equipments against direct lightning stroke.
For protection against direct lightning stroke, lightning belt will be installed on the structure such as coal handling facilities as necessary.
6.6.2 Grounding
Grounding installation will be provided to ensure the security of the operational staff and the electrical equipments and to maintain normal function of the electrical equipments.
The material to be used for the grounding installation will be copper. |
