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The hydraulic turbines should determine the turbine speed for maximum

December 28, 2020

arangwal SHP with Induction Generators and capacitor Bank

 

Speed (rpm)

The speed of a generator is established by the turbine speed. The hydraulic turbines should determine the turbine speed for maximum efficiency corresponding to an even number of generator poles. Generator dimensions and weights vary inversely with the speed. For a fixed value of power a decrease in speed will increase the physical size and cost of generators. Low head turbine can be connected either directly to the generator or through a speed increaser. The speed increaser would allow the use of a higher speed generator, typically 600, 750 or 1000 (1500) r/min, instead of a generator operating at turbine speed. The choice to utilize a speed increaser is an economic decision. Speed increasers lower the overall plant efficiency by about 1% for a single gear increaser and about 2% for double gear increaser. (The manufacturer can supply exact data regarding the efficiency of speed increasers). This loss of efficiency and the cost of the speed increaser must be compared to the reduction in cost for the smaller generator. It is recommended that speed increaser option should not be used for unit sizes above 5 MW capacity.

 

 

Dimension

Three factors affect the size of generator. These are orientation, kVA requirements and speed. The turbine choice dictates all three of these factors for the generator. The size of the generator for a fixed kVA varies inversely with unit speed. This is due to the requirements for more rotor field1 kW Rating: The kilowatt rating of the generator should be compatible with the kW rating of the turbine. The most common turbine types are Francis, fixed blade propeller, and adjustable blade propeller (Kaplan).

 

Each turbine type has different operating characteristics and imposes a different set of generator design criteria to correctly match the generator to the turbine. For any turbine type, however, the generator should have sufficient continuous capacity to handle the maximum kW available from the turbine at 100-percent gate without the generator exceeding its rated nameplate temperature rise. In determining generator capacity, any possible future changes to the project, such as raising the forebay (draw down) level and increasing turbine output capability, should be considered.

 

In a variable head power plant the turbine output may vary depending upon available head. In general the generator is rated for turbine output at rated head. kVA Rating and power factor: kVA and power factor is fixed by consideration of location of the power plant with respect to load centre. These requirements include a consideration of the anticipated load, thelication No. 280 (Economical) AHEC/MNRE Guidelines Up to 750 kVA 415 volts Up to 400 kW (or kVA) 415 volts

 

751 – 2500 kVA 3.3 kV 401 – 2500 kW (or kVA) 3.3 kV

2501 – 5000 kVA 6.6 kV 2501 – 5000 kW (or kVA) 6.6 kV

Above 5000 kVA 11 kV Above 5000 kW (or kVA) 11 kV

Preferred voltage rating of generator as per IEC 60034-1 is as follows:

3.3 kV - Above 150 kW (or kVA)

6.6 kV - Above 800 kW (or kVA)

11 kV - Above 2500 kW (or kVA)

 

Stator Winding Connection: Star, stator winding connection are providing for both grounded or ungrounded operation and six terminal (3 on line side and 3 on neutral side) are brought out, except forsmall generators when only one neutral is brought for ground connections.

 

 

Excitation Voltage: Rated generator rotor voltage is specified by the manufacturer, based on the rotor winding resistance and the excitation current required for full loSynchronous Generators

a)Stator:Class F insulation level and Class B temperature rises are recommended. The American practice is to provide Class H insulation with a temperature of not more than 80o C.

 

b)Rotor :

 

The insulation level should normally be Class-F and temperature rises Class-B. Asynchronous (Induction) Generator

 

a)Stator

 

Class F insulation level and Class B temperature rises are recommended.

b)Rotor

 

Squirrel cage construction, Class F insulation and Class B temperature rises are recommended. These units should be designed to withstand continuous runaway conditions.

 

 

Typical Characteristics

SHP Projects Remarks

Awapan(2 x 250 kW)

Sebari SHP(2 x 500 kW)

Kitpi SHP (2 x 1500 KW)

 

Type Salient pole Cylindrical pole Cylindrical rotor Operation Isolated/grid connected Grid connected Grid connected Grid connected Configuration Horizontal Vertical Horizontal Sebari SHP has siphon intake Rating (kW) 250 500 1500 Voltage (volts) 415 415 3.3 kV Rated power factor 0.8 Lag 0.85 0.8 Speed rpm 1500 750 600 Runaway Speed/withstand in minutes 2700/15 min 2175/30 min 1080/15 min

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Micro Hydro

Synchronous generators are generally used. Generators may be selected in accordance with quality standard issued by AHEC extracts enclosed as Annexure 2. These generators are self excited and factory assembled and classified as category-1 generator in American Practice. They are shipped to site completely assembled depending on the rpm selected, unit speed/weights and method of transportation to site. In case of isolated units, small capacity Induction generators with variable capacitor bank may be used up to a capacity of about 50 kW especially if there is no or insignificant Induction motor load i.e. less than about 20%.enerator Efficiencies

 

The efficiency of an electrical generator is defined as the ratio of output power to input power. Typical Efficiency values for some generators are given in table 9.4. There are five major losses associated with an electrical generator. Various test procedures are used to determine the magnitude of each loss. Two classes of losses are fixed and therefore independent of load. These losses are (1) windage and friction (2) core loss. The variable losses are (3) field copper loss, (4) armature copper loss and (5) stray loss or load loss. Windage and friction loss is affected by the size and shape of rotating parts, fan design, bearing design and the nature of the enclosure. Core loss is associated with power needed to magnetize the steel core parts of

 

the rotor and stator. Field copper loss represents the power losses through the dc resistance of the field.

Similarly, the armature copper loss is calculated from the dc resistance of the armature winding. Stray loss for load loss is related to armature current and its associated flux. Typical values for efficiency range from 91 to 98%. This efficiency value is representing throughout the whole loading range of a particular machine; i.e., the efficiency is approximately the same at ¼ load or at ¾ load. Typical efficiencies of some SHP generators as installed is given in table