Study material for SSE/JE examination by RRB on Electrical Engineering Part II

AC Transmission and Distribution

AC power is generated at Power Generating Stations and the size of the machine goes up to 1000 MW. The voltage selection is based on the current levels and the dielectric strength of the insulating material for the winding and generally ranges between 6 kV to 30 kV. For a 660 MW machine operating at 23.5 kV voltage at 0.85 power factor will draw a current of 19000 A in each phase. This power is transmitted to load center which may be located at a distance of about 100-500 kms. Transmitting power at generating voltage  will result heavy energy losses and voltage drop in the transmission line. We know P=√3*V*I*PF and higher voltage and unity power factor will reduce current, and for this reason, the transmission voltage is increased to 132, 220, 400 kV and also 760 kV (HVDC) depending on the economics of capital cost and transmission losses. There exists grid networking for different voltages for utilisation of power generating at any station for any consumer.

Cost of Electrical Energy: The cost of electrical energy depends on the capital cost of generating station, variable cost of fuel, transmission and distribution, operation and maintenance cost. There are two parts of any costing namely Fixed and Variable cost as follows

Fixed Cost: This includes the cost of land, building, equipment, machinery and complete related infrastructure in generation, transmission and distribution to the consumer end, interest, depreciation etc.  There is generating station using clean fuel namely wind, solar, hydro and bio-mass whereas others using non-renewable sources of energy namely thermal, nuclear etc. The fixed cost per MW also varies considerably with present order of minimum for thermal power generation and going up for  hydro, nuclear, wind and solar and variable cost in same but  reverse order . The present trend is showing a downward trend for capital cost of solar and the day is not far when solar will dominate the energy scenario. There is associated advantage of water management with hydro but with issues of environmental clearances for hydro as well as nuclear. Three nuclear accidents have influenced the discontinuation of nuclear power: the 1979, Three Mile Island partial nuclear meltdown in the United States, the 1986  Chernobyl disaster in the USSR, and the 2011 Fukushima nuclear disaster in Japan. This prompted many countries to phase out nuclear plants namely Sweden (1980),Italy(1987), Belgium(1999), and Germany (2000). Austria and Spain have enacted laws to stop construction on new nuclear power stations. Several other European countries have debated phase-outs.

Variable Cost: The cost of fuel and operation & maintenance results variable cost.

Tariff: On lines of the cost of power generating station, the energy cost also consists of fixed cost, variable cost (depending on the energy consumed), penalty for

1. Low power factor: Low power factor at the consumer end cost the generating and transmission company for reactive power and not consumed and therefore penalty is imposed. Such penalty is generally for HT consumer. In order to charge the LT consumer, some utilities bills for kVA and not kW. (one unit is kWh but charged for kVAh which is about 5-10% more)
2. Poor Load factor: Load factor is defined as Average Load/Maximum load in a given time period. There are certain utility which imposes penalty if this value goes below 0.25 (DVC), 0.35 (BSEB) etc.
3. Fuel surcharge and other taxes etc.

Factors: Power plant don’t run to the maximum capacity all the time due to varying demand during the 24 hours time period. The maximum demand is during night time between 18-22 hours and very low between 00-05 hours. Therefore while working out the economics various demands are to be considered like

Connecting load: It is the total sum of all the loads connected on the system. All the loads will not be ‘ON’ at any instant of time and it is the diversity factor resulting into the actual demand on the system.

Maximum Demand: It is the maximum load which a consumer uses at any time during the 24 hours period.

Average load: The consumer does not consume power depending on the maximum demand but average consumption in kWh may be only 40-45% of the maximum demand and is called load factor.

Based on these aspects, power generating company monitors the following factors.

1. Availability factor: The total time it is available to produce electricity over a month (say) divided by the total time during the period
2. Capacity factor: Actual output over a period of time to its potential period of delivering the output
3. Utilization factor: The time period for which the equipment is in use to the total time period it could have been use.
4. Demand factor: It is average demand divided by the maximum demand possible
5. Load factor: It is the average load on the system divided by the maximum load in given period of time
6. Diversity factor: It is the percent of time for which a machine or any equipment is on full or rated load.
7. Coincidence factor: It is the reciprocal of the diversify factor and as per IEC coincidence and simultaneity factors are same.

Power Transmission: Transmission voltages are very high as compared to generation and distribution voltage to reduce I²R joule losses.

Distribution System: Distribution system starts by stepping down the transmission voltage of 132 kV and above to 33 kV at 33 kV substation and than at 11 kV distribution network before being transformed to 415/230 V network at consumer end. Distribution at consumer end is of three types namely

1. AC single phase where there only single phase loads and limited to few kW
2. 3 Phase, 3 wire where there three phase loads only
3. 3 phase, 4 wire for all types of load and where the load  is high.

The distribution network is either radial having advantage of simplicity and low cost but during failure there is no alternative feed arrangement or ring main  having the advantage of reliability and reduced losses but with complex switchgear arrangement.

Material Selection for transmission line: The material selection is based on economics and technical suitability. High electrical conductivity, low specific gravity (weight), low cost  and high tensile strength are the key requirements. The most commonly material used are Cu, Al, Al conductor reinforced with steel (ACSR), galvanised steel and Cd-Cu. Kelvin law is relevant to assess the most economical size of conductor. It states that the variable part of the annual charges is equal to the cost of energy losses per year.

Visit for Study material on Electrical Engineering Part I 

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