 |  | Village Electrification (SKAT, 1992, 128 p.) | ||
 |  | Part 10: Typical application examples | ||
|  | (introduction...) | ||
| | 1. Introduction | ||
| | 2. Namche bazar (category a) | ||
| | 3. Chame (category b) | ||
| | 4. Syangja (category c) | ||
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from Nepal
Hereunder some case studies of the electrical control system for three MHP schemes in Nepal will be presented.
The equipment of all three plants has been designed, supplied and set into operation by BYS. The electrical control systems have basically been designed in line with the concept outlined in chapter 1.2, though a few additional options regarding protection have been incorporated.
The Namche MHP scheme was executed by BYS on a turn key basis.
2.1 Operating Conditions:
net head: 75 m max turbine discharge: 651/s
plant capacity:
27 kWe
altitude: 3500 m a. s .1.
load type: lighting and heating (mostly
resistive)
Figure 1 Schematic transmission and
distribution system.
2.2 Generating Equipment:
turbine: BYS splitflow
generator: Markon (U.K.), synchronous
3F, brushless type, 37.5 kVA, 230/400 V, 50 Hz
speed control: manual flow
regulator
speed transmission: not required
penstock valve: gate
valve
control panel: BYS standard control, protection and instru mentation
cubicle category A, see below for details
2.3 Electrical Design Considerations,
Components and Wiring Details
As the site is located 3500 m a.s.l. where the air density is low, a aeration factor of approx. 0.85 needs to be considered to keep the generator from getting overheated. Required rated power of the generator kW =
As the loads are mostly lighting, heating and cooking with very low overall inductance, the power factor may be assumed close to unity. To be on the safer side, assuming a lagging pf = O.9, requires a kVA rating of the generator of
The nearest, higher-rated, standard size generator used at site is 37.5 kVA, which with 6% above the estimated value, is safe enough. The generator selected is of brushless-type with an electronical automatic voltage regulator, which has resulted in a constant generator voltage. Even at varying speeds (as the equipment is manually governed) down to 90 % of the rated speed the voltage is stable, below that the generatorvoltage falls proportionally to the speed. Both features also resultin low maintenance requirements and are best suited to the site's remoteness.
The total load actually exceeds the plant's installed capacity, therefore, manual load managementis practised to keep the generator from overloading and, at the same time, to maximize the load factor. The load centres are several 100 m away from the powerhouse and are connected through an underground cable network. As Namche Bazar is a touristic centre this solution was also chosen considering esthetics. Most of the lodges, requiring much heating and cooking power, are connected by two different lines to the distribution centre, one line for lighting only and the other for cooking and heating. Load management is done by switching these lines remotely from the power house by remote controlled air break contactors. They are housed in a sheet metal enclosure at the distribution centre.
The main control, protection and instrumentation features of the equipment in the power house include: heating/lighting line on/off selector switches, circuit breaker back-up fuses, over voltage relay, excitation/ de-excitation system, time totalizer, energy meter, kW meter, ampmeter, voltage meter, frequency meter, instrument current transformers and indicating lamps. A phase selector switch is added to connect the power house load (lighting and cooking for the operator) to the least loaded phase, to improve the phase balance.
BYS scope of work was limited to the design, supply and installation of the generating equipment only.
3.1 Operating Conditions:
net head: 65 m
max turbine discharge: 130 Vs
plant
capacity: 45 kKWe
altitude: 3000 m a.s.l.
load type: lighting, heating,
radio, tape recorders etc.
3.2 Generating Equipment:
turbine: BYS crossflow T6
generator: Leroy Somer
(France),230/400 V, 50 Hz
speed control: automatic oil hydraulic, mechanical
governor
speed transmission: not required
penstock valve: gate
valve
control panel: BYS standard control, protection and instru mentation
cubicle category B, see below for details.
3.3 Electrical Design Considerations,Components and Wiring Details
The site is located at approximately 3000 m a.s.l., therefore, a
aeration factor of 0.88 is to be applied. Although most of the loads are
resistive, a power factor of 0.8 is considered. The required generator kVA is
therefore:
generator rating kVA =
The generator used at this site is a l00kVA frame, which is actually much too big. The reason for using this generator was that it was originally selected for another site, which turned out to be not feasible. As there was no way to return the generator and no other, more adequate site, it was selected. The generator is a brush less type with electronic automatic voltage regulator.
The automatic flow controlling governor with the generator AVR keeps the voltage and frequency within internationally accepted limits at all operating conditions.
The main control, protection and instrumentation features of the control panel are excitation/de- excitation system, over and under voltage relays, solid state over current relay (inverse time characteristics), circuit breakers, earth fault relay (RCCB type), Voltmeter, Ampmeter, KW meter, energy meter, time totalizer, frequency meter, instrument current transformers, and status indicating lamps.
Photographs, circuit diagram and component list of the control cubicle are given in the next page.
BYS' scope of work was limited to the design, supply and installation of the generating equipment only.
4.1 Operating Conditions:
net head: 13.3 m max turbine discharge (unit) 4251/s plant capacity: 2 x 40 kWe altitude: below 1000 m load type: lighting, heating and motor loads
4.2 Generating Equipment:
turbines: BYS crossflow T7
generators: Markon (UK), Brushless
230/400 V, 50 Hz
speed controls: automatic water hydrau lic, mechanical
governor
speed transmission: V-belt drive
penstock valves: gate
valve
control & synch. panel: BYS standard control, protection,
instrumentation and synchronisation
cubicles category C, see below for
details.
4.3 Electrical Design Considerations, Components and Wiring Details
The site is located below 1000 m.s.l., therefore, no aeration
factor needs to be applied to the generator rating. Load types are such that the
worst case power factor at full load is better than 0.8 lagging. The required
generator kVA rating is therefore:
generator rating kVA =
The nearest standard size generator used at site is 62.5 kVA, which is on the safer side (+12%). The two generators used are both brushless with an electronic automatic voltage regulator. Droop kit for reactive load sharing and short circuit maintenance unit are installed in the generator terminal box itself. There are in total 3 control cubicles, namely generator control panel 1, generator control panel 2 and synchronisation panel. The main control, protection, synchronising and instrumentation features of the equipment comprise of excitation and de-excitation control system, air break contactor control system, overvoltage relay, under voltage relay, switch-fuses unit, solid state over current relays, earth fault relay (RCCB type), voltmeter ampmeter instrument, transformers, power relay, time totalizer, frequency meter, kW-meter, energy meter, auto/manual synchronising system, status indicating lamps and feeder circuit breaker.
The design of the synchronisation control circuit allows for both synchronising of the two units and paralleling with the grid.
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