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LAB 3 1. 2. Sketch a Single Line Diagram (SLD) of the power plant on the basis of data provided in Lab 2. Show all the basic structure of the power plants: Generators, Generator Neutral Grounding Method, Generator Leads, Medium Voltage Bus Bar, Generator Circuit Breakers, High Voltage Bus Bar, Transformer Connection, Station Supply Connection, Head Works Supply, Low Voltage Distribution System, HV Switchyard etc. 3. Prepare an AUTOCAD drawing and submit the drawing in A3 paper size The following attachment consists of the single line diagram of the power plant showing all the basic structure of it like generator, Generator Neutral Grounding Method, Generator Leads, Medium Voltage Bus Bar, Generator Circuit Breakers, High Voltage Bus Bar, Transformer Connection, Station Supply Connection, HV Switchyard. Almost all the hydraulic turbine-driven generators used are synchronous alternating current machines, which produce electrical energy by the transformation of the rotational mechanical energy. The electrical and mechanical design of each generator must confirm to the electrical requirements of the power distribution system to which it will be connected, and also to the hydraulic requirements of its specific plant. The electrical characteristics of the generator depend on the following factors: a) Capacity and the power factor b) Generator power output rating c) Generator voltage d) Insulation e) Short-circuit ratio f) Line charging and condensing capacities g) Power factor h) Reactance i) Amortisseur windings j) Efficiencies The Specification of a Generator used in our Power Plant : Salient Pole Synchronous Generator . Size/capacity - 27.390 MVA. Efficiency - 90%. Power Factor - 0.85. Generation Voltage -66 KV. Duty -Continuous. Alignment -Vertical. Insulation type -Class F. Cooling type -Forced Air. No. of poles Synchronous speed -8 -690.71322RPM Our hydropower project consists of 3 units each of 23.281MW. So we have selected salient pole synchronous generator of 66 KV, MVA with unit transformer scheme. That is the power is generated at 66 KV level and stepped up to 132 KV and then the two units are synchronized at 132 KV bus and the generated power is evacuated. Drawing baki cha three unit bhako EARTHING SELECTION: The various grounding system in practice used for the earthing of generator is as follows: 1. Solid Neutral grounding. 2. Reactor Neutral grounding. 3. Resistor Neutral grounding. 4. Distribution Transformer Secondary Resistor Neutral grounding. Among these methods we choose the distribution transformer secondary resistor neutral grounding method for our power plant generator. It is high resistance neutral earthing. It consists of a low ohm resistor shunted across the secondary of a step down transformer whose primary is connected in the generator neutral circuit. In this method of grounding the generator neutral current, during a phase to earth fault is limited to a very low value between 5 to 15 A. The primary advantage of this method is it limits the fault current during the short circuit condition. With solid grounding the fault current is very high which cause various problems hence to limit the current we use this method. This method is widely use in many hydropower plant such as Kaligandaki, Marsyangdi, Khimti etc. GENERATOR GROUNDING METHOD: The main reasons for grounding the neutrals of synchronous generators are to limit over voltages on the generators and connected equipment under phase to ground fault conditions, and to permit the application of suitable ground fault relaying. We choose Distribution transformer-secondary resistor neutral grounding, which is in effect high-resistance neutral grounding. With this method of grounding, the generator neutral current, during a phase-to-ground fault, is limited to a very low value, usually between 5A and 15A, by the use of a relatively low-ohm resistor shunted across the secondary of a conventional step-down transformer whose primary is connected in the generator neutral circuit. The possible damage at the fault is therefore least of any of the various grounding methods. Fig :-Distribution transformer-secondary resistor neutral grounding We preferred to select the resistance grounding method. Its value is selected such that the value of Xcg >> Rn or Xcg/Rn>>1. The Phase Capacitance to Ground Co=0.13µF. Surge Capacitance Cs=.25µF. The total capacitance to ground Ccg=3(.13+.25) µF=1.14 µF Xcg=1/(2pf Ccg)=2792.190 Ω Resistance in secondary=2792.19/(11/.41)2=3.90 Ω. The cost of grounding devices and switch gear for other grounding methods with compared to this grounding method is excessive due to large values of ground fault current. Hence this method of earthing is suitable for our plant. EXCITATION SYSTEM: The excitation system used in this hydropower plant is brushless DC excitation system since to avoid the cost of regular maintenance of brush replacement due to wear and tear of the brushes during operation and since our unit is small there is no stability concern. It consists of dry type transformer. A solid state rectifier rotates with the shaft converting the AC output from the generator into the DC which is the supplied to the rotating field coils of the main generator without the need of brushes. GENERATOR LEADS The term “generator leads” applies to the circuits between the generator terminals and the low voltage terminals of the GSU (generator step-up) transformers. The equipment selected depends upon the distance between the generator and transformer, the capacity of the generator, types of generator breaker employed,the economics of installation. There are three types of generator leads: Non segregated Segregated Isolated We have, Generator output = 27.390 MW Generated Voltage =66KV So current in each line is I = (27.390)/(√3*66*.85) = 0.28188 KA. = 281.88A Types of bus bar to be used baki cha Selection of scheme We use unit generator-transformer scheme • • • • • This is a conventional scheme preferred for medium sized power plants in a small power system where the plant size is significant for the system. With the use of individual transformer for each generator unit, unit protection of generator and transformer becomes easy. Overall protection system becomes simple and easy to locate the faults. The fault level at the generator bus is less as compared to other schemes. ( refer to relevant section) This scheme makes maintenance outage simpler to arrange. In case of fault in a unit, other units can run independent of it and the plant’s availability is higher. It requires higher number of switchgear. Drawing baki cha Voltage bus bar pani baki cha