Electricity Funda

Electricity-Some Fundamentals: • • • • • • • • • • • • •

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Voltage is the energy difference between the positive and negative terminals of the battery. Voltage causes current to flow in the wire. Voltage=Current x Resistance (Ohm's law) The energy consumed per second is the power. Power=Voltage x Current Power=Current x Current x Resistance Units: Voltage-Volt, Current-Ampere, Resistance-Ohm, Power-Watt Energy=Power x Time Electricity Consumption Unit=Number of hours of operation x Power in KW DC Circuit: Current always flows in one direction AC (Alternating Current): The higher the voltage, the higher the current and vice-versa. The number of times the current changes its direction in a second is called frequency. Nearly all the power systems today operate on AC because AC Power can easily be 'transformed' or changed from one voltage to another. This is of great help in sending the power over long distances.Current will be less if voltage is increased. Power loss in the wires in the form of heat is proportional to the square of the current. Therefore, higher the voltage level, the greater the reduction in power loss. Furthermore, it is easier to generate AC power and motors operating on AC are cheaper and easier to maintain. High voltages are dangerous since there is a higher chance of electric shock. AC helps resolve this. Power can be generated at low voltage at generating stations. Then it can be transformed to high voltage and transmitted to consumer locations. At consumer's place voltage can be lowered. In AC circuit there are two components which resist the flow of current. They are resistance and reactance. Reactance is caused by coils(typically found in motors) or capacitors. Net effect of resistance and reactance is called impedence. If current wave and voltage wave coincide then power factor is one as in the case of a resistor like bulb.If current lags behind voltage then the load is said to be inductive, as in the case of a motor. If current leads voltage, the load is capacitive, caused by capacitors. In an AC System, active power and reactive power depend on voltage, current and power factor. The power required by electrical equipment to operate is called load. Load is made up of an active part(measured by watt) and a reactive part(measured by VAR(Volt-Ampere-Reactive)s) Depending on consumer behaviour, load keeps changing from second to second. Average Load=Sum of hourly loads/24 Load Factor=Average Load/Maximum Load The installed Capacity of a generating unit is its maximum MW capacity at the time of installation.

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The minimum capacity is the minimum MW at which the generating unit can operate in a stable way. Energy generated by a generating unit is measured by metering instruments at the station and can also be calculated if one knows the average hourly MW generation. It is typically measured in Millions of Units(MU) and is usually calculated for a period of one year,i.e 8760 hours (8784 hours for a leap year.) MU=Sum of hourly MW values for one year/1000 Average Capacity is the average of all the hourly MW generation values. Average Capacity=Annual Energy Generation/ Number of hours in a year. Firm Capacity of a unit is the MW power that can be assured from the unit at any point in time. Plant Load Factor = 100 x (Energy Generated in a Year) (Maximum energy generation possible in a Year)3 Base load stations have high PLF and peaking stations have low PLF. That's why power generation by base load stations is cheap and that by peaking stations is costly. High PLF implies economy of scale due to high capacity. Since base load is assured, high capacity stations can be set up and run continuously to meet this load. Availability of a generating unit is the per hour average of the declared generating capacity values over a period of time (typically a year) Availability = 100 x (Hours for which the unit is available for generating power)/ (Total Hours in the Year).

Hydropower • • • • •

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Penstocks are huge pipelines that carry water from the reservoir to the turbine. The Full Reservoir level is the maximum height of the water in the reservoir. The Minimum Draw Down Level is the minimum permissible level in the reservoir. Hydro stations located right on the water stream are called 'run of the river' stations. They don't have a reservoir to store and regulate water flow. Pumped Storage Stations: These are special type hydro stations with two reservoirs-one upstream, near the dam at high level and another downstream, after the tail race at low level. Water from the dam reservoir is guided to the turbine making it and the generator rotate to generate power. Water leaves the turbine to the tail race reservoir through a pipe called the tail race. This is the generating mode. In the pumping mode, water is pumped up from the tail race reservoir to the dam reservoir. In this mode, it draws power from the grid. The pumped storage station is run in pumping mode during off-peak hours so as to increase the storage in the dam reservoir. During peak hours, it is run as a generator to supply energy needs at that time. This arrangement can reduce the cost of peak period energy generation. The difference in levels of water at the storage reservoir and the turbine is called the 'Head'. Head is measured in metres. When water flows through the penstock and the valves, some pressure is lost due to friction. The friction head is around 5% and the remaining 'net head' contributes to power generation. Power (kw) = 8 x Net Head (metres) x Flow Rate(Litres/sec)/100

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Hydro plants can also generate some reactive power. Hydro plants in the condenser mode can generate only reactive power. In this mode of operation, a minimum water flow is to be maintained and the unit will consume some active power from the grid. A hydro station can be started up or put off in a few minutes. It can be easily operated over a wide range of power output with high efficiency. Ideal for peak load. Low auxiliary power consumption-2-3% of the power generated Simple to operate, high overall efficiency Hydro-power is clean Hydro-power is cheap-no fuel cost (though construction costs are high) If there is a natural high head, initial costs are lower. Environmental costs and R&R costs. Run of the river stations without adequate storage cannot be used as peaking stations. Hydro-power is renewable-long life Execution of hydro projects requires thorough survey and investigation, preparation of DPR, development of infrastructure, Environment Impact Assessment and other preparatory works which are time consuming and take 2-5 years.

Coal-based Station : • •

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Main elements-Coal Yard, furnace, boiler, cooling tower, Condenser Coal based thermal stations are typically run as base load, generating at the same level most of the time. This is because of the fact that the generation level of these stations is changed by controlling steam flow and pressure. But since there is a limit to the permissible changes, generation levels cannot be changed fast. Large number of devices-higher maintenance cost Power is generated typically at 10-15 kv and the voltage is stepped up at the substation which links the generating station to the grid. The fixed cost of a coal based station is less than that of a hydro station. Variable cost and auxiliary consumption values are high. The overall efficiency of power generation is quite low compared to hydro stations. Auxiliary consumption-8-10% of the power generated The coal based station is expected to run at full steam all the time. But even the coal plants may be asked to reduce generation at night, when the demand falls. Such instructions are given by the load dispatch centre.

Gas-based Station: • •

Fuel(LNG,Oil or Naphtha) arrives at the generating station through pipeline from a refinery. For safety reasons, very little fuel is stored at the station.

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Open Cycle Gas Turbine Mode: Air is compressed and fuel burnt in a combustion chamber. This releases high-pressure hot gases which drive the gas turbine. The turbine turns the generator, producing power. Closed Cycle Gas Turbine Mode: the exhaust gas from gas turbine is sent back to the compressor, not to the atmosphere. Combined Cycle Gas Turbine has a gas turbine followed by a steam turbine. Output gases from the gas turbine flows into the boiler/steam generator. Steam produced here turns a steam turbine. Gas based stations can be started, stopped and the generation level changed quite easily, making it a convenient choice to meet peak loads. These stations are usually required to change generation levels at short notice by the Load Dispatch Centre. Less polluting. Can be constructed quite fast. Average life-10-15 years High fuel cost Auxiliary Consumption-3-4% of the power generated

Diesel-based Station: • • • • • •

Similar to gas based station Ideally suited to handle peak load conditions and emergency power requirements. Low capital cost and requires little space. Average life-5 years Fuel cost and O&M cost is high. Highly polluting.

Nuclear Station: • • • • • • • • • • •

Similar to coal stations. Instead of burning coal, the process of nuclear fission produces heat High Capital Cost Need for stringent safety measures Problem of Radioactive Waste Disposal De-commissioning problems Takes 6-10 or more years to build High auxiliary consumption-11-12% of the power generated Take 1-2 days to start up and shut down-used as base load plant Overall Efficiency-30-35% Typical life-30-40 years

Generation Scheduling: •

Planning the level of generation of the available generating units to meet the load is called generation scheduling.

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The base load generating units (big coal based, nuclear, irrigation dependent hydro etc.) are scheduled first to meet the base load. Then the peaking stations (open cycle gas, small hydro) are scheduled during the morning and evening peak hours. The intermediate loaded stations (CCGT, small coal based and hydro) are used to meet the remaining load. While scheduling the units of a particular type (say peak load), units with low operating cost are scheduled first and the costlier ones last. Ordering of units on the basis of operating costs is called the merit order, which is prepared based on the Variable Cost values. If there is sufficient generation capacity, some units may not be scheduled at all. On the other hand, if available generating units are not sufficient to meet the load, then power may have to be imported from another utility. If all the generating units and the imported power cannot meet the demand of the state, and then load shedding has to be resorted to. Broad plans for load shedding should be prepared as part of the annual plan. Generation Capacity has to be planned to meet the load forecast. The total available generation capacity in the state and power imports should be sufficient to meet the peak load at all times of the year. Energy from these should meet the annual energy requirement of consumers. Step 1: Prepare a load duration curve for the 10 year horizon. (Load Duration Curve captures the load behaviour and the energy requirements) Step 2: After finding the energy requirement at the consumer location this is converted to the requirement at generating stations by adding the estimated T&D losses over the planning horizon. Step 3: Then account for the auxiliary consumption, spinning reserve and expected outage of generating units. (Spinning reserve means generation capacity which is already spinning and can be used at short notice.This is managed by ensuring that some generators, which are 'on' and connected to the system, are not fully loaded so that they have some spare capacity.) Expected Outage includes planned outage and forced outage. Planned Outage is owing to routine maintenance planned for all generators. On the other hand unforeseen failure of a generator is called forced outage. Based on the type of the generator, it is possible to reasonably predict the percentage of time in a year during which such failures occur. When generation is not sufficient to meet the load it is called loss of load. Loss of load probability (LOLP) is the probability that available installed capacity falls short of load. If high excess capacity is planned and the spinning reserve is high, LOLP will be low and vice-versa. The transmission system consists of transmission lines, substation and support services. Transmission voltages are very high and can be AC or DC. Therefore transmission lines have to be very tall. Lines are connected to towers by a chain of insulators. Lines run from one tower to another which are 100-200m apart. A set of three lines is called a circuit. Normally a tower carries one circuit but some carry two. Tower also carries one or two more lines which are lightning arrestors.

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The utility purchases the land around the route of the line which is called 'right of way'. A transmission line starts and ends at 'substations'. Substation consists of a transformer, bus bar, circuit breaker, protective relays and isolator. A transformer is used to convert one AC voltage to another. Power generated at 10-15 Kv at the generating station is 'stepped up' to high transmission voltage at the substation in the generating stations. Local substations 'step down' the voltage to distribution level voltage. These step up and step downs are done by the transformers. Another function of the transformer is to correct for small variations in voltage. A substation that handles 500 MVA power and has 400 and 220 kv voltage levels, may be called a 500 MVA-400/220 kv substation. Transformers are identified by the upper and lower voltages and the power they handle. At a 220 kv substation, there are two 220/132 kv transformers, each capable of handling 100 MVA power. Bus bar is a line at one voltage level to which many connections are made. In a 220/132 kv substation there will be one 220 kv and one 132 kv bus bar. All 220 kv lines and the 220 kv terminals of the transformers terminate on the 220 kv bus bar. In big substations, there can be more than one bus bar at the same voltage level. A Circuit Breaker is an elaborate switch used at high voltages and currents.A typical CB used in a 220 kv substation is designed to withstand 220 kv, 600 Amperes and weighs hundreds of kilograms.A CB is designed to automatically turn off when there is very high current.It can be operated remotely from a control room. Protective Relays in the substation are designed to sense when there are abnormal voltages and currents, high/low frequency, mismatch in frequency etc. An Isolator is also a switch but it cannot be operated when there is current flowing in the circuit. To isolate a piece of equipment, first CB is opened, then the isolator. To bring the equipment back to circuit, first isolator is closed, then CB. Most of the substation's equipments are located in open air, the switchyard. The protective relays, meters, switches for remote operation, communication equipments etc are located in the control room. The transmission system consists of many substations inter-connected by transmission lines. It is like a mesh and is called the transmission grid. Power is fed into grid at the generating stations; power flows on the lines towards load centres; substations near load centres step the voltages down and supply power to the loads. Grid collapse occurs when many transmission lines open(i.e., blocked), loads lose supply and many generators trip. A fault occurs when two transmission lines touch each other, or if one or many of the lines touch the ground. Then very high currents may flow through equipments thereby damaging them. The relays detect the fault and give a signal to open the CB. The cycle of open/close operation to test the fault is called 'auto reclosing'.

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At the State level, the minute to minute operation of the power system is coordinated from the Load Dispatch Centre, typically located in the State capital. The LDC is connected to three or four Sub-LDCs, which in turn are connected to major substations and generating stations. LDC gets information about: 1. on/off status of the generating unit 2. power generation level 3. frequency 4. voltage 5. reservoir level/ coal availability 6. on/off status of transmission lines 7. power flows, voltage on transmission lines 8. voltage, frequency in major substations 9. substation equipments 10. weather conditions from different locations

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The State LDC is connected to the Regional LDC. In India there are five RLDCs(North, South, West, East and North-East) These would be connected to a National LDC. Typically the LDC prepares an hourly generation schedule for each generating station in advance. The maintenance schedule, relative power generating cost, contractual agreements, water/fuel availability and the load requirement forecast are used by the LDC to prepare this. The schedule is followed until some unforeseen event occurs. This could be a sudden increase/decrease of load as against the forecast, failure of a generating station or opening up of a transmission line. If the generation is not sufficient to meet the load (which is indicated by low frequency) or if some lines are overloaded or if voltages are abnormally low, load-shedding instructions are issued by the LDC to substations. In cases where the frequency drops very quickly to a very low value the underfrequency relays located at substations are expected to act to shed loads automatically. Most often, the load shedding requirements are known in advance. This is based on the load forecast and generation schedule. The LDC issues these scheduled load shedding instructions. The LDC also coordinates sudden repair work as well as planned maintenance of transmission lines and generating stations. Whenever such repair work is to be undertaken, the LDC issues a 'Line Clear' instruction, which indicates that the line/generator is no to be put on service during the repair time. Sometimes disturbances happen in the power system. These are in the form of sudden opening of transmission lines, tripping of generation units, sudden changes in load, etc. Very often, the power system manages to regain its balance after such events. But sometimes, one event leads to another(could be another generator/transmission line tripping) and a set of cascading events causes the grid to collapse. Most of the generators trip and many transmission lines open

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resulting in a whole state or many states losing power for long periods. This is typically called a black-out. It may take only a few minutes from the first trigger to the collapse. Sudden swings in frequency or voltage are indications the LDC gets of a possible grid collapse. Immediate actions like load shedding, changing generation levels, reducing voltage levels to reduce the load, etc help to save the situation. A 'Brown Out' is the reduction of voltage in some areas, which helps to reduce the system load. Lights will dim and motors will turn slower when this happens. It takes a long time to restore the collapsed power system. In such a system, all generators would be 'off' and most transmission lines 'open'. Generators need some external power to start up. Only a few stations have their own source of this start up power, in the form of small generator sets. This capability is called black-start capability. After a collapse, these stations are started first. After a few generators start up, transmission lines are closed one by one, connecting load to the system.It must be ensured that voltage, phase and frequency are equal before the final instruction to close the circuit breaker is given. This procedure is called 'synchronisation'. One by one, generators and transmission lines have to be synchronised to the grid. All the time, the LDC has to ensure that LoadGeneration balance is maintained. The change in load from one hour to the next is much less during off peak hours(afternoon, middle of night). During peak hours the change can be quite high(a few thousand MW). The load dispatcher prepares an hourly generation dispatch schedule to meet the load forecast and issues instructions to the generating stations. There are variations in the value of load during the hour and also the actual load conditions may not exactly follow the forecast. Control mechanisms are put in place to handle this. Every generating unit has a piece of equipment called speed governor. When the load is more than generation, it acts like a brake on the rotating shaft of the generator and the generator slows down, bringing the frequency down. Speed governors installed on the generator sense the slowing down of speed and increase the input to the generator turbine so that additional power is generated and the speed is not further reduced. There are many generators in a power system. Since they are all connected, they all need to operate at the same frequency. Load>Generation => Speed Governors at work => Lower system frequency (The whole process is called 'free governor operation') System Frequency is brought to its nominal value while generators producing the same power through another corrective action called Automatic Generation Control. AGC commands the speed governors of each generator in the system. Increase the speed of rotation => generation=load => System frequency is brought back to nominal value. LDC is expected to calculate the AGC commands of the different generating stations and issue them. All these steps happen in reverse when the load is less than generation. Operation of the system at very high or low frequency and frequent variation of frequency is harmful for the generating units and motors. Generator life reduces,

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motor speeds change and low frequency operation reduces the efficiency of the system. Whenever current flows through a line, which has resistance, voltage will reduce. Therefore, voltages of substations far away from generating stations are low. In a DC System, the only solution to this problem is to locate generating stations all over the state. However this is not practically viable. In an AC System the following steps can be taken: 1. Locate Extra High Voltage substations close to load centres throughout the state. Generating stations may be located far away from load centres. 2. Managing reactive power by making a minor adjustment at the transformers located at the substations. By changing the ratio of turns in the coils in the input side and output side the output voltage can be maintained at 220 kv despite the reduction in input voltage. ('tap changers' help to change the ratio of the number of turns)