MODERN POWER SYSTEM

- FUTURISTIC TRENDS

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Indian Power System Before & After Independence

The total generating capacity in India, in 1947: 1360 Mw, (all owned by private sector.)

Within 5 years the figure went up to 1950 Mw of thermal and 555 Mw of Hydel.

Generation and distribution was confined to limited areas such as metropolitan cities like Kolkata and Bombay, some industrial areas like Dishergarh(primarily to feed the mines) and some tourist spots like Darjeeling where hydro-power was harnessed. There was practically no transmission lines what to speak of an interconnected grid system.

Indian Power System Before & After Independence

Power system in India in the post-independence era is characterized by large scale expansion - in generation, transmission, distribution and utilization.

Manifold increase has occurred in all these fields and that too at an unthinkable high rate. Our first prime minister Pandit Jaharlal Nehru took the pioneering role.

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Five Year Plans The five year plans were projected with a view to rapid

industrialization which gave rise to urbanization (the city of Durgapur in W.B. is an example).

It was also projected to power agriculture and rural electrification.

It was the motto of the first few governments to feed electricity to power-starving millions of India as a mission. They also aimed at rapid increase in GNI and GNP.

Five Year Plans The progress was supported partly by developed

countries (particularly USSR & US), partly by financial institutions and partly by internal resources.

Power system expansion had a humble beginning with a number of power stations, substations and transmission lines but it grew up very fast during the first few decades after independence, leading to a grid system in India.

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Development of Techno-Administrative Structure

This development was followed by the establishment of a techno-administrative structure under the Central Govt.

Electricity supply act was enacted in 1948.

Barring a few licenses, the ownership was transferred to the state govts.

State electricity boards (SEB) were formed- the generation and supply of electricity was looked after by the boards.

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Development of Techno-Administrative Structure

In 1975, Central govt. directly entered into the field of generation and transmission forming PSUs like NTPC, NHPC, NLC etc.

In 1989, Power Grid corporation was formed to develop the transmission system in the country with an ultimate aim for formation of national grid.

The operating voltage went up to 765 Kv in 1998.

Development of techno-administrative structure The electricity act was revised in 2003 to allow open

access to transmission.

The national electricity plan was finalized in 2005-06 and a big step was taken in 2006 to form the national grid.

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Division of Indian power system The Indian power system has now been divided into five

regions viz. Northern, Western, Southern, Eastern, and North-Eastern.

The power stations within a region are synchronized with each other to form the regional grid.

Some of the regions are also now interconnected. we are gradually proceeding towards a national grid. The national grid may be further expanded to form a sub-

continental grid for making power export/import business. The regions are looked after by regional electricity boards

(REB). The central electricity authority (CEA) monitors the activities

of all the boards and makes supervisions. Power Grid Corporation (PGC), formed later on, looks after

transmission of electricity from one place to another-regional and inter-regional.

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The statutory members and the Load Dispatch Centers There are many statutory members under a regional

board.

For the Eastern Region, the statutory members are:-NTPC, NHPC, DVC, WBSEB (split up into WBPDCL, WBSETCL & WBSEDCL), Jharkhand SEB, Bihar SEB, Sikkim Electricity Board etc.

CESC is also deemed to be a member though it is a privately run utility having its own generating stations and distribution systems. It is connected to WBSEDCL and DVC at a no. of points but it has the provision of islanding on occurrence of faults

The statutory members and the Load Dispatch Centers The power flow is controlled by load dispatch centers

belonging to each utility. They also take part in dictating the generation of a plant.

The state load dispatch center (SLDC) is near Botanical Garden at Howrah. It is backed after by a smaller unit at Siliguri which looks after the power flow at the northern part of the state.

These two in conjunction controls the power generation and power flow in the state.

CESC has its load dispatch center at Victoria House in Kolkata and DVC in Maithon.

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Regional Load Dispatch Centers Future Possibilities Each region has a regional load dispatch center (RLDC). It is

supposed to control the generation and flow in an entire region.

They are hierarchically above the SLDCs.

The RLDC of the eastern zone is situated at Golf Green of Kolkata.

In foreseeable future there will be a load dispatch centre, hierarchically above the RLDCs and will be engaged in controlling power generation and flow for the whole country.

It will also look after scheduled exchange with neighboring countries for the projected sub-continental grid.

Regional Load Dispatch Centers Future Possibilities

Intercontinental grid systems exists in Europe, even the grid systems of UK and France are connected by cable transmission passing over the bed of English channel.

In a similar way, we may get connected to Bangladesh and Pakistan to form a sub-continental grid and lay a HVDC line through Pak-strait to get connected to Srilanka.

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Table-I, showing growth in generation after independence.Region/State

Hydel Thermal Nuclear Others Total

As on 31/07/2007

Northern 12671 21476 1180 1220 36547

Western 6743 29090 1840 2670 40344

Southern 10646 20498 1100 5899 38143

Eastern 2598 14557 - 228 17384

N-Eastern 1116 1244 - 146 2506

All India 33776 86936 4120 10175 135006

As on 31/12/2013

All India 39893 159794 4780 29463 233929

Maharastra 3331 23715 690 4768 32505

West Bengal 1248 7329 - 131 8708

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Growth- the picture That there has been a massive growth in generation is evident

from table-I. The growth has been unprecedented but not uniformly distributed over the country. After about 60 years from the day of independence the total generating capacity has increased from about 2500 Mw to 135000 MW- 154-times. It is a stupendous increase.

It is also clear from the last part of the table that the growth rate has increased in the recent years. The generating capacity has been augmented by 98923 Mw within 6½ years, from 135006 Mw to 233929 Mw, at about 8.82% per annum, as against 6.87% in the sixty years following the day of independence.

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Notable Points The power generation in our country is dominated by

thermal except for the N-eastern region, where hydro-resources are abundant.

The N-eastern zone has the lowest installed capacity, next to it is Eastern. The capacities of Northern, Western and Southern region are near to each other.

Expansion along nuclear line has not been promoted as our country is rich in fossil fuel resources. However, India is self-sufficient in nuclear technology.

Extraction of power from renewable sources has recently been given much importance- southern region is taking the lead.

Notable Points Installed capacity of hydel power is highest in the

Northern region, as it has enough of hydro-resources. The Hydel : Thermal ratio is 0.59 which is near-optimal (the optimal is 0.6). At other places the ratio is much lower which poses difficulty in peak management.

Maharastra is the state having highest installed capacity. West Bengal stands 10th in the list.

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Growth in Transmission

Powertransfer,MW

Distance,Km

Economicvoltage,KV

Powertransfer, MW

Distance,Km

Economicvoltage,KV

3500 500 765/800 120 150 220500 400 400 80 50 132

Table-II

Keeping pace with increased generation, transmission and distribution facilities have also increased. The increased quantum of power is to be evacuated through transmission lines and tie-lines for which Long HVAC transmission lines carrying bulk power have been installed to connect the generating stations with the load centers. In the earlier regime after independence 66 Kv lines and 132 Kv lines were installed. Later on, with increase in quantum of power, we installed 220 Kv lines.

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Present Status For about 15 years there was no further increase in voltage.

Then India installed 400 Kv lines (1991, after a debate on 400 Kv vs. 500 Kv). Later on 765/800 Kv power lines have been installed in many parts of India. 800 Kv HVDC lines are also operative in the country.

The choice of operating voltage depends on the bulk of power and the distance through which power is to be carried. The economic voltages are shown in table-II.

Now there exist a large no of 400 Kv long lines in our state e.g. Farakka - Jeerut, Jeerut -New Howrah etc., but as yet no 800 Kvline. In our state (also in other parts of India) 132 Kv, 220 Kvand 400 Kv power lines co-exist. The subsystems operating at different voltages are connected by auto-transformers.

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Tie-lines & Central Generation In addition to transmission lines, there are tie-lines with

tap-changers and phase-shifters for scheduled exchange of power from one region to another.

The central generating stations like NTPC, NHPC, NLC and NEEPCO etc. were established in mid-seventies. They produce bulk power. FSTPS in our state belongs to NTPC, our state gets a share of it. The Chukha HPS belongs to NHPC- its excess power is transmitted by 220 Kv long line to Birpara of Coochbehar, from where it is catered to consumers through the power grid of WB. A company was formed to deal exclusively with transmission. It was named National Power Transmission Corporation (NPTC). Later on, it was renamed as Power Grid Corporation.

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Problems with HVAC and solution High voltage has given rise to problems associated

with insulation coordination, corona and high voltage swings. Special designs are being used to address the problems e.g. bundle conductors to reduce corona, avoidance of sharp edges in substations for insulation coordination etc. Voltage instability is another problem arising out of reactive power mismatch which on occasions has given rise to voltage collapse and cascaded failure leading to complete black out.

FACT-devices like TCSCs (Thyristor-Controlled Series Capacitor) are also finding application in recent times. TCSC makes series compensation and enhances the power transfer capability of a line. It also reduces the voltage drop.

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High Voltage DC Transmission Extensive synchronous coupling may give rise to poor damping and

promote oscillatory behavior leading eventually to dynamic instability. Also it may not be practically feasible to couple two areas synchronously provided they operate at different frequencies. These problems can be overcome by using HVDC transmission lines. Power can be transferred from one area to another through asynchronous link, thus avoiding oscillatory problems of synchronously coupled systems.

HVDC links are particularly suitable for long cable lines for which a.c. transmission gives rise to Ferranti effect due to large capacitance of cable lines. For the same voltage, corona losses and radio interference are much less pronounced for HVDC, also the insulation requirement as its peak voltage is same as its average. There is no charging current or skin effect. The voltage regulation problem is much less serious. There is only one objection to its use- that is its higher cost arising out of terminal equipment for rectification and inversion.

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Growth in Distribution 6.6/11 Kv was used as feeder voltage in earlier days.

Now we use up to 33 Kv. Many more distribution transformers have been installed. The unit size of distribution transformers has also gone up to 5-10 MVA as compared to KVA-ratings of earlier days. Old meters are being replaced by Smart Electronic Meters.

Older 1-phase lines are being replaced by 3-phase lines and the old transformers are being replaced by higher unit sizes. Tough handling is made against hooking and tapping in rural areas to reduce the non-technical losses. Tariff concession is only being allowed for the agricultural sector for over-all benefits of the country. Private sector participation is also being encouraged e.g. IPCL, CESC, Tata Power etc.

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Modernization of Distribution Systems

Ring-main systems are being introduced for higher reliability in place of radial systems.

Cable distribution is being used for safety and cleanliness in densely populated cities in lieu of overhead lines.

Modernization of Distribution Systems The earlier protection schemes are also being

modernized. We are going to introduce field programmable gate array (FPGA) for detection and protection of distribution feeders. Steps are being taken to improve the voltage regulation and security of supply.

Networks or grids are appearing in distribution.

Researches are going on for optimizing the distribution system and for minimizing load-shedding.

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Modern practices following the growth The growth in power system has prompted the

technologists to assimilate and apply the techniquespertinent to modern power systems of large capacity.

This includes load forecasting, unit commitment,load scheduling, automatic generation control,automatic voltage regulation and excitation control,load flow study and its optimization, security andcontingency evaluation, reliability and expansionplanning. Also awareness has come towards stabilityproblems in a large interconnected system- powerangle stability and voltage stability, need for powersystem stabilisers and compensators and applicationof FACT-devices.

Modern practices following the growth

The protective schemes have also gone though revolution e.g. vacuum and SF6 CBs are replacing the older ones, solid state circuit-breakers, digital relaying and telemetry are coming up to replace the wire-connected analog systems.

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Introduction of SCADA/SASVoice communication was used in the earlier days to direct to control the power flow. But with the increase in grid size, it becomes almost impossible to manage the complicated system.

In course of time, SCADA systems has taken over the role over and above the voice communication in the load dispatch centers.

SCADA/SASSCADA stands for Supervisory Control And Data Acquisition. It has

the following subsystems:

A Human-Machine Interface to monitor and control the process. A supervisory (computer) system, gathering (acquiring) data on

the process and sending commands (control) to the process. Remote Terminal Units (RTUs) connecting to sensors in the

process, converting sensor signals to digital data and sending them to the supervisory system.

Programmable Logic Controllers (PLCs) used as field devices because they are more economical, versatile, flexible, and configurable than special-purpose RTUs.

Communication infrastructure connecting the supervisory system to the RTUs.

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Load-Forecasting The primary task of a power system engineer is

to forecast the load. The forecasting may be shortterm- on daily basis, weekly basis or monthlybasis or it may be long term. The base load andthe peak load both are to be estimated fromstatistical data and forecast. Short termforecasting is required for unit commitment andscheduling the generators. It is particularlyrequired for making the utility compatible withABT.

Load-ForecastingHowever, there is always an uncertainty in

load-forecasting due to errors in estimating the weather or festival dependent loads or errors in statistical formulation.

Long term forecasting is required for preparing maintenance schedule and for future planning of expansion or scheduled exchange with neighboring utilities.

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Unit Commitment Unit commitment follows load-forecasting. It is the

selection of units that will supply the forecast load, keeping adequate spinning reserve. It is made primarily on economic basis, but there are other points to be considered.

Thermal and nuclear units cannot be started and loaded quickly. It takes several hours to synchronize a set from cold start. So they are used for supplying the base load.

Hydel units are best-suited for peak management as they can be started and fully loaded within a few minutes and their operating cost is low. In places or in utilities where there is no or little hydro-resources, the peak management has to be made by gas-turbine plants or diesel engines.

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Pumped storage plants as alternatives The cost of a BOT unit produced by GTR or diesel engine is

much higher compared to that by traditional methods. To reduce the cost of running GTRs, we may opt for pumped storage plants, if possible. In our state, The hydro: thermal ratio was very poor, creating problem of peak management. Recently this problem has been partly overcome by installing a pumped storage plant at Ajodhya Pahar of Purulia district.

Now we have concentrated more on non-conventional sources of energy e.g. solar, wind, bio-mass, fuel-cell etc. Maximum power is extracted from the non-conventional power plants with a look to preserve the fossil fuels for future use and also to reduce the pollution, though the unit cost is higher.

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Load-Scheduling Load scheduling has to be made after unit commitment. For the

steam units it is made on the basis of fuel cost. While makingthis commitment, the cost of starting and shut-down should alsobe taken into consideration. The operating point for economicscheduling is found out by using Lagrangian functions. It showsthat the best possible economy is obtained for equal incrementalfuel cost for the sets housed in a plant.

If the sets are housed at different places and connected to eachother by transmission lines, then the line losses are also to beconsidered. This is made by multiplying the individual fuel costsby penalty factors. The penalty factors are found out bycalculating the so-called B-coefficients.

If the fuel costs are same for a no of units (as found for singlevalve turbines) then merit order approach should be made i.e.the one with least fuel cost should be fully loaded at first,followed by those in the ascending order of fuel costs.

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Scheduling Hydro-Thermal Mix Scheduling for a hydro-thermal mix is a much more complicated

problem. For steam units it has been assumed that the fuel is readily available- the operation is not restricted by availability of fuel (though it is not strictly true in the present time). So the optimization is independent of time.

But this is not true for hydro-units. Their operation depends on availability of water- the energy output is fixed for a particular year depending on the rainfall and the catchment area. Factors like flood, irrigation, pressure exerted by the stored water on the dam-walls etc. are also to be accounted for in addition to the usual economic considerations.

Scheduling becomes more complicated in presence of nuclear

units and smaller units running on non-conventional sources.

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Need for automationThis growth has given rise to additional complexity. It is not possible to monitor and control a modern power system manually. Automatic systems are required for control and supervisory systems for monitoring. At present, in a Turbo-Generator set, there are three feedback loops viz. the boiler-firing control, the turbine-governor control and the AVR-excitation control (fig. 1, given below).

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Coordination & Tuning These three feedback loops are again coordinated with each

other by a major loop. In hydro-generators, the first one is absent. In nuclear power plants, the boiler-firing control is replaced by reactor control.

In a power system, control loops are being used for automatic generation control and for automatic switching of tap-changers and phase-shifters with a look to optimal power flow and keeping a good voltage profile.

This is an age of automation. Automatic control systems make the performance better, but their parameters must be properly tuned with the system so that they may not malfunction and give rise to instability.

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Automatic Generation ControlAutomatic Generation Control (AGC, also known as load-frequencycontrol) in a power system fulfills the following objectives:It holds the frequency at or very close to a specified nominal value. Asper ABT, it must be within (+1%) and (-2%) of the nominal frequencyi.e. between 49 Hz. and 50.5 Hz.

It has to maintain each unit’s generation at the most economicalvalue.

It has to maintain the correct value of interchange power betweencontrol areas.

The characteristic of turbine-governor must have a static droop, R.This is required for load-sharing during parallel operation. A changein active power gives rise to a frequency deviation. It depends on thedamping, D.

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Area Frequency Response-single and multi-area It can be shown that the frequency deviation under steady state for a

single area equals:

where is the change in load. is called the area frequency response characteristics (AFRC), expressed as a % of spinning capacity.

The static frequency error can be reduced (or eliminated) by injecting supplementary signals. Generally P-I control is used- it tends to reduce the frequency-deviation to zero. It was once tried at Bandel TPS but it could not be tuned with our system.

The behavior of a multi-area system is similar to that of a single area system. In an uncontrolled multi-area system, there will be a static frequency error combined with a static interchange error if there be a step load change in one area. This is the only difference. In this case also supplementary signals may be used to eliminate the static errors.

/ ; 1/Df P D R

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Automatic Voltage Regulator and Excitation Control The basic requirement and

function of an excitation system is to provide direct current to the synchronous machine field winding. The excitation system

performs control and protective functions essential to the satisfactory performance of the power system by controlling the field voltage and thereby the field current (fig. 2).

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Functions of Excitation Control

The excitation control of synchronous machines is the most important approach in improving the quality, reliability and stability of electrical service. It is the heart of the generator control. A static droop is required in the voltage-load current characteristic for equitable sharing of the reactive power. However, the control may be made a static (i.e. no static error) by auxiliary signals (fig. 2).

Functions of Excitation Control

The excitation system should fulfill the followingrequirements:

Ensure good damping of free and forced oscillation ofsmall and large amplitude.

It should have high operational reliability. It should ensure high quality of voltage under steady

state. It should ensure steady state stability of the electrical

system under all possible operating conditions. It should ensure a desired limit of transient stability.

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Evolution of the excitation control A lot of changes and evolutionary processes have occurred

in the area of excitation control through ages. In the earlierregimes there was a main DC exciter with automatedRheostatic control. Then a pilot exciter was added. ThenDC exciters were replaced by HF AC exciters and rectifiers.Then rotating exciters with rectifiers were used. Now-a-days the excitation is realized mostly by transformationand controlled rectification of the alternator terminalvoltage no separate exciter is used. There are a no ofversions of solid state excitation.

Intelligent devices like Micro-controller, PLC anddedicated computer are being used for more precisecontrol and better voltage response ratio.

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Load Flow & its optimalityIn a grid system, the problem of proper power flow and maintenance of voltage profile arises. Load flow study is conducted for assessment of the power system under existing conditions and for future extensions. It is important for planning, control and operation of present and future systems. It throws light upon the effects of interconnections, new loads, new generating stations, new transmission lines etc. It also throws light upon the requirement of VAR-compensators at specific locations.

Load Flow & its optimalityThe following information is obtained from a

load flow study:

Magnitude and phase angle of voltages at eachbus

Active and reactive power flow in each line-whether any line is overloaded or not.

Power generation in the slack bus.

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Optimization of load flow In load flow, a set of non-linear algebraic equations in

complex variable have to be solved, in presence of inequality constraints. As no solution in compact is available, recourse is made to computer.

People felt happy with a feasible solution in the earlier days as there was less concern about economics of power flow. But now-a-days people are aiming at optimal load flow. The objective function may be minimum active loss or a weighted combination of active and reactive loss/generation or minimum load-shedding or minimum fuel cost.

Optimization of load flow The formulation of optimal load flow study starts from

sensitivity analysis. The operating limits give rise to inequality constraints.

The optimal solution is found out generally by using gradient search technique applied to the control variables. The constraints are accounted for by penalizing the objective function. Active generation and the voltage magnitudes of generating buses are control variables. The tap-setting positions of on-load tap-changers and phase-shifters are also varied to reach the optimality conditions.

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The stability problems arising out of expansion There are three kinds of vulnerability in a power system.

There may be a failure due to sudden impact e.g. a sudden short-circuit or a sudden loading, a sudden loss of load or a sudden loss of generation. This impact in a synchronized system gives rise to electromechanical oscillation. If the oscillations gradually die down, the system is stable. If the amplitude of the oscillation grows with time eventually leading to pole slips and decoupling of synchronous machines, the system is unstable. This is called transient instability. Adequate angle margin has to be kept to combat transient instability. Also the excitation control-AVR combination has to be equipped with field-forcing feature to counteract transient instability.

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Dynamic Stability behavior under small impacts For economic reasons, now-a-days the

generators are run with reduced anglemargin- there is more reliance on the actionof the fast-acting excitation-controllers. Sothe turbo-sets may become unstable if theexcitation system malfunctions.

The steady state stability may be static ordynamic. If the load changes are very slowsuch that automatic control systems do notbecome operative, the stability is static. Forsmall impacts such as 1% change in load or 1%change in voltage, the stability is dynamic.

Dynamic Stability behavior under small impactsThe other kind of vulnerability is due to these

small impacts.

They may not immediately give rise to poleslip and instability but the resultingoscillations may grow gradually with time ifthe system damping is negative. Large scaleinterconnection, particularly in presence offast-acting AVR, may give rise to negativedamping and make the system prone todynamic instability.

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Power system stabilizerThe over-all inertia of a system increases with

more coupling. This is beneficial for transientstability. However, it reduces the over-alldamping and makes the system oscillatory. Inthe worst case the damping may be negativeleading to dynamic instability. In order tocounteract it, modern power systems areequipped with power system stabilizer (PSS)

Power system stabilizerTo counteract the tendency to dynamic

instability due to negative damping, supplementary stabilizing signals are added. The network used to generate these supplementary signals is known as Power System Stabilizer or PSS. Stabilizing signals are added in the summing junctions of the excitation controllers, as shown in fig. This is working in conjunction with a solid state excitation-controller represented by type 1s of IEEE.

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PSS and Excitation Control Combined

PSS generates the supplementary signal Vs and adds in the summing point. It is a feedback element from the shaft speed which tends to make the damping positive (fig. 3).

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Reactive Power Management-Compensation Another problem faced by the power system engineer in

recent times is voltage instability which arises out of poor management of reactive power. In a long high voltage transmission line the shunt capacitance is appreciably high. They generate VAR all the time the line remains charged. The consumption of VAR in the series inductance depends on the line current. In the peak hours the line charging VAR is beneficial as it partly offsets the inductive VAR consumed in the line and in the load. But during off-peak hours, the VAR consumption is much less compared to that at the peak hours. Therefore, the capacitive VAR dominates and gives rise

to Ferranti effect.

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A look into Voltage Instability At present for proper management of reactive power, both

capacitive and inductive compensation are used. Shuntinductive compensation is provided to long EHV lines toavoid VAR-IN and Ferranti effect. For short lines andfeeders, capacitive compensation is used to improve thepower factor. Series compensation is used for longtransmission lines to increase the power transfer capabilityand reduce the voltage drops. Currently FACT devices arefinding widespread use for compensation. For example,static synchronous compensators (STATCOM) are beingused in AC transmission networks. These are fed fromvoltage source inverters and can act as either a source or

sink of reactive power.

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Collapse and Black out Load flow studies on large systems at peak and lean hours

reveal the possibility of voltage collapse for specifiednetwork configurations and loading conditions. At least,the voltage profile of the system is found to beunacceptably poor. To avoid such unhappy situation, VARcompensators are interposed at vulnerable points.

Now-a-days, the power systems are being operated closerto their stability limits for economic reasons andenvironmental constraints. Maintenance of stable andsecure operation of a power system under such stringentconditions is a very important and challenging issue.

Collapse and Black out Voltage instability is a major source of power system

insecurity. It is characterized by fall in receiving end voltage much below its normal value, or continuous oscillation against sudden disturbances due to poor damping. Voltage collapse is the process by which the voltage profile falls to a low, unacceptable value due to cumulative actions. Once upon a time, this phenomenon was associated with weak systems and with long lines. But now this problem exists even in highly developed networks due to heavier loading.

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Security concept and contingencies A power system under normal operating condition, may

face a contingency e.g. a loss of generation due to outage ofa unit, a large change in demand from the forecasted value,a sudden loss of load due to short-circuit of a largetransformer, sudden tripping of a transmission line. Thecontingencies that are highly probable are called credible.Others are incredible.

Conditions under credible contingency must be analyzedand steps to be taken under such contingencies must bedecided upon beforehand. This will increase the security ofoperation. It is very much needed for planning acontinuously expanding system as like ours.

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Operating States There are three states of a power system- normal, emergency

and restorative. Under normal conditions, the demand canbe met satisfying all the network constraints. Emergenciesarise out of short-circuit, trippage, loss of load orgeneration. If the system remains stable but some of theoperating constraints are violated, it may be tolerated forsome time. If the system becomes unstable, then restorativeactions have to be taken. In restoration, corrective actionsare taken, like operation of CB to isolate a faulty section orload-shedding, so that the system settles at a new normalstate. It may be necessary to start new rapid-start units likeHydel sets, pumped storage units or GTRs during this timeto feed the load.

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Deregulation Deregulation is a concept and practice of new origin. In the

earlier days, generation, T&D were entrusted on a singleorganization, mostly under govt. or federal bodies. There waslittle privatization. The organizations grew in size withincrease in demand and supply and eventually they lost theireconomic efficiency. In deregulated environment,generation, transmission and distribution are treated asindependent activities. There is a competition amonggenerators of electricity for customers.

Main benefits from the deregulation are:

Cheaper electricity, Efficient capacity expansion planning,Cost minimization, more choice and better service.

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Historical Background Since the mid-1980s the electrical power supply industry

around the world has experienced a period of rapid andirreversible change. The need for more efficiency in powerproduction and delivery has led to a restructuring of thepower sectors in several countries like UK, Spain, NewZeeland, Argentina, Chile, Norway and Sweden. Even incountries with privately own utilities, e.g. US, there has beena strong drive toward deregulation and a more intenseparticipation of third party generation. Other countries arealso considering the restructuring of their electricity powersector so as to introduce more competition among

producers and to offer more choices for customers.

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Secured System A secured system is one which has the ability to undergo a set of

disturbances without getting into an emergency condition. This has some similarity with robustness of a control system.

A secured system must satisfy the loading and operating constraints and in addition, the security constraints for each of the credible contingencies. Contingency evaluation is required to ensure that the system is secured for all of them.

Security monitoring is the on-line identification of the actual operating conditions. It requires large scale instrumentation spanning the whole system. A large part of it is telemetric. It can be done by SCADA.

The operator should take preventive action if he finds the system unsecured. The best preventive action can be determined by running a security-oriented optimisation program.

The optimal load-flow should also be security-constrained-otherwise it is not meaningful and practical.

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Deregulation in India In India, the deregulation process has been started. Some private

power generating systems are under constructions. Rules and regulation in this regard has already been formulated. The changes are concerned with ownership and management of the industry. The basic objectives of restructuring Electricity Supply Industries are as follows:

To introduce competition into the monopolistic industry. This is to be achieved by vertical unbundling i.e. separating power generation, transmission, and distribution and by horizontal unbundling i.e. creating several competing generation companies and utlities.

To make special regulatory provisions in this respect, recognizing that the power transmission system is a natural monopoly.

To allow consumers to exercise choice between suppliers (generation companies) who will be using the existing transmission facilities to cater power to the consumers.

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Revision of Tariff Systems Tariff is the principle of charging electricity from the

consumers. It has also undergone many changes with increase in generating capacity and interconnections. In earlier days, the general tariff form for industry was: A=cx+dy+f; where x=max. demand (KVA/KW), y= total energy units consumed ; c,d,f: constants. This is 3-part tariff. Hopkinson advanced 2-part tariff in which there is no fixed charge. In addition there were flat rate, straight meter rate, and block rate for domestic consumers. Many of these systems of tariff are still in use e.g. block rates are being used by CESC.

Recently changes have been brought about in the tariff systems for bulk consumers, based on economic considerations. The examples are ABT, ToD, RPT etc.

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ABT :: Availability Based Tariff Availability-based Tariff (ABT) is a rational tariff structure for

power supply from generating stations on contractual basis.The power plants have fixed and variable costs. The fixed costelements are interest on loan, return on equity, depreciation, O& M expenses, insurance, taxes and interest on working capital.The variable cost comprises of the fuel cost, i.e., coal and oil incase of thermal plants and nuclear fuel in case of nuclearplants.

In the Availability-based Tariff, the fixed and variable costs aretreated separately. The payment of fixed cost to the generatingcompany is linked to availability of the plant i.e. its capability to

deliver MWs on a day-by-day basis.

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Capacity Charge & Energy Charge The amount payable to the generating company over a year

towards fixed cost depends on the average availability (MW-capability) of the plant. In case the average actually achievedis higher than the specified norm for plant availability, thegenerating company gets a higher payment. In case theaverage availability achieved is lower, the payment is alsolower. Hence the name, ‘Availability Tariff ’. This is the firstcomponent of Availability Tariff, and is termed ‘capacitycharge’.

The second component of Availability Tariff is the ‘energycharge’, which comprises of the variable cost (i.e., fuel cost) ofthe power plant for generating energy as per the givenschedule for the day.

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Deviation Charge Energy charge is based on Scheduled Generation (SG), not

Actual Generation (AG). If excess drawal is made it has tobe paid at a rate dependent on the system conditionsprevailing at the time. This is Deviation Charge.

If the grid has surplus power at the time and frequency isabove 50.0 Hz, the rate would be lower. If the grid hasshortfall at the time and the frequency is below 50.0 Hz.the rate would be higher.

The Central generating stations have various states of theregion as their beneficiaries or bulk consumers. The latterhave shares in these plants calculated according to Gadgilformula. The beneficiaries have to pay the capacity chargefor these plants in proportion to their share.

Implementation of CERC (Deviation SettlementMechanism and related matters)Regulations,2014

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DSM Regulation DSM Effective from 17.02.2014 DSM is Replacement of existing UI Regulations which is

effective since 17.09.12

Objective:

• Discourage deviation from Schedule• Operate within the freq band• Ensure grid security• Operation of network elements within acceptable limits

Important Changes vis-à-vis Existing UI regulation:

• Buyer/Seller• Generating Station

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RDC for Thermal Stations regulated by CERC and using coal as fuel

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Main points under DSM RegulationOver injection:

Charges for Deviation receivable for over injection in a time block within the volume capping of 12% of Schedule or 150MW whichever is less in normal Deviation Charge subject to a capping of 303.04p/u depending on freq.

For any over injection beyond the volume capping limit in a time block, the charges receivable is zero.

Additional Charges for Deviation for over injection in any time block when f>=50.10Hz at 178 p/u

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Main points under DSM regulationUnderinjection:A. Charges for Deviation payable for underinjection in a time block in

normal Deviation Charge subject to a capping of 303.04p/u depending on freq.

B. Additional charges for Deviation levied for underinjection beyond the volume capping limit as follows:

• Freq ---- 49.7 Hz and abovea. 12% of Sch in a time block < = 150 MW

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Main points under DSM regulation

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Implementation of RGMO(Restricted Governor Mode of Operation) through CERC Regulations

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RGMOThe restricted governor mode of operation shall essentially have the following

features:a) There should not be any reduction in generation in case of improvement in

grid frequency below 50.05 Hz (for example, if grid frequency changes from 49.9 to 49.95 Hz, there shall not be any reduction in generation). For any fall in grid frequency, generation from the unit should increase by 5% limited to 105 % of the MCR of the unit subject to machine capability.

b) Ripple filter of +/- 0.03 Hz. shall be provided so that small changes in frequency are ignored for load correction, in order to prevent governor hunting.

c) If any of these generating units is required to be operated without its governor in operation as specified above, the RLDC shall be immediately advised about the reason and duration of such operation. All governors shall have a droop setting of between 3% and 6%.

d) After stablisation of frequency around 50 Hz, the CERC may review the above provision regarding the restricted governor mode of operation and free governor mode of operation may be introduced.

e) All Users, SEB,, SLDCs , RLDCs, and NLDC shall take all possible measures to ensure that the grid frequency always remains within the 49.90-50.05 Hz band.

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Pattern of Daily Load curve in our area The daily load curve in our country is not flat. It has

two peaks- morning peak and evening peak, theevening peak being higher (fig. 4). There are also twooff-peaks

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Grid indiscipline prior to ABT This payment is dependent on the declared output capability of

the plant for the day and the beneficiary's percentage share in that plant, and not on power / energy actually drawn by the beneficiary from the Central station.

The mechanism works by collaborative effort of the central generating stations and the RLDCs. The schedules are fixed up beforehand but in case of contingencies, revisions can be made.

Before ABT-age, regional grids were very undisciplined, everyone trying to generate as much as possible, thus forcing the system to higher frequency. There was no incentive for backing down. Grid indiscipline was rather encouraged. ABT has solved all these

problems.

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Time of the Day (ToD) Tariff Another type of tariff used is Time of the Day (ToD) tariff. It is a

pricing policy to flatten the load curve.

The peak to lean ratio is quite high in our state due to poor or no demand side management. This was involving high cost for meeting the evening peak by running GTRs- it has reduced a little due to the pumped storage plant.

Time of the day (ToD) tariff is an incentive scheme which encourages the industrialists to shift their load to off-peak hours. They are charged at a considerably lower rate during lean hours, which is the incentive and they are to pay more for consuming power at peak hours which is a disincentive. The tariff structure aims at flattening the load curve, as far as possible, and reducing the peak demand.

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Reactive Power Tariff Another tariff may be thrust upon the industries- this is

reactive power tariff. Till date only active power is beingmetered and billed, not the reactive power. But reactive poweris required to maintain the power factor in the grid. It causesadditional loss in the transmission lines and should also becharged. The regional power sectors have reported to theElectricity Regulatory Commission (ERC) regarding thisadditional loss and have asked for permission to collect areactive power tariff.

It has been proposed to be fixed at Re. 0.25 per unit lag energyfor p.f. in the range of 0.95 to 1.0 and Re. 0.50 per unit for p.f.less than 0.95. This additional burden of tariff will give anincentive to the industries to improve the p.f.

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Smart Grid-Concept and Realization The concept of smart grid originated in the last decade of

20th. Century in the context of inefficiency and unreliability of complex interconnected power grids. A smart grid uses modern communication technology to gather and act on information about the behaviors of suppliers and consumers, automatically to improve the efficiency, reliability, economics, and sustainability of production and distribution of electricity. Recently, improvements in communication technology have opened up avenues to resolve the limitations and costs of the electrical grid. Technological limitations on metering no longer force peak power prices to be averaged out and passed on to all consumers equally. Smart meters are now available.

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Management of Grid-connected renewable energy sources In parallel, growing concerns over environmental

damage from fossil-fired power stations has prompted the use large amounts of renewable energy e.g. solar power, wind power etc. These are variable sources of power- if these are to be grid-connected, more sophisticated control is required. Clean Power (or green power) from non-conventional sources is a viable alternative to large centralised power stations. The falling energy costs of PV-cells is an incentive towards change from centralised to dispersed and distributed grid.

Management of grid-connected renewable energy sources Amin and Wollenberg in 2005 coined the word smart grid.

It is in essence the application of digital processing and communications to the power grid, making data flow and information management for giving improved services to the consumers.

Smart meters having continuous communications are parts of the smart grid- they can monitor the demand in real time and bring forth changes as and when required within the limits of system capability.

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Centralized Generation- the environmental aspects Industrialized countries generate most of their electricity

in large centralized power stations, in thermal (using coal or gas), nuclear (using fissionable materials) or hydropower plants. These plants are economically good, but the power produced by them is to be transmitted over long distance, particularly for hydel plants, giving rise to heavy power loss, poor voltage profile and creating pollution.

Centralized Generation- the environmental aspects

The site for most of the power plants are chosen on the basis of factors like economic, health & safety, logistical, environmental, geographical and geological.

Coal-based thermal power plants are built away from the cities to reduce pollution. Ideally such plants should be built near to the collieries.

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Hydroelectric Power Plants. Hydroelectric plants are sited at places with sufficient

water flow, and having favourable terrain wherecatchment areas can be built up at relatively low cost.These are mostly in hilly places, far off from the loadcenter. They are environmentally less harmful but theybreed virus and insects, slow down the water flow andcause silting. There by they affect the eco-system.

Nuclear Power Plants Nuclear power plants eject pollutants in less quantity

compared to thermal, but the pollutants are far moredangerous. It affects the animals, flora and fauna, thehuman beings and to a lesser extent, the plants andvegetation. Therefore expansion along the nuclear lineis not advised by reasonable persons.

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Distributed and Dispersed Generation Considering environmental impacts and keeping a look at our fast-

depleting fossil fuel resources, many power system engineers areadvocating distributed and dispersed generation in lieu of centralized.Distributed generation is, in essence, generation of electricity from manysmall energy sources. It is also known as on-site, dispersed, embedded ordecentralized generation.

In distributed compared to centralized generation, power is generated inmultiple stations which are all near to the points of consumption. It maybe generated in the same building where it is consumed as in the case ofroof-top solar cells. This approach also reduces the size and number ofpower lines to be constructed.

These are used in a Feed-in-Tariff (FIT) scheme. They have lowmaintenance, low pollution and high efficiencies. In the past, there wasneed for dedicated operating engineers. But the modern embeddedsystems are automated in operation.

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Typical Distributed Power sources The energy sources are renewable, such as sunlight or wind, tidal power or

ocean power. This approach gives greater economy and increases the profit of the utilities employed in distributed generation.

The sources of energy fall under the category of non-conventional, characterized by low pollution. This is true for solar, wind, biomass and biogas etc. Low pollution is also a crucial advantage of combined cycle plants that burn natural gas. The low pollution permits the plants to be near to populated cities and their waste heat can be used for district heating and cooling.

Distributed energy resources (DER) are small-scale units, in the range of 3 kW to 10,000 kW. They are alternatives to or enhancement of traditional power systems. The main problem is their high costs.

The examples are solar cells, wind turbine-generators, bio-gas plants, micro-hydel plants, tidal plants, fuel cells etc. Cogeneration sources using combined cycle also come under the category of DER.

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Decentralized Generation Decentralized Generation is the production of

electricity at or near the point of use, irrespective of size, fuel or technology. It reduces capital investment, lower the cost of electricity, reduce pollution and greenhouse gases, and decrease vulnerability of the system to extreme weather and terrorist attacks. Decentralized Generation, powered by a wide variety of fossil fuels, may be distributed or dispersed.

Decentralized Generation Distributed power generation is any small-scale power

generation technology that provides electric power at a site closer to customers than central station generation.

Dispersed generation is a decentralized power plant, feeding the distribution level power-grid (10 and 150 MW).

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Distributed Generation Distributed generation is used mainly for onsite

power generation. Dispersed generation isstrategically located on the transmission grid toovercome bottlenecks in the transmission anddistribution system and to improve the stability ofthe system.

Distributed GenerationAdvantages Low cost of electricity - the consumer is benefited by the low

cost of generation.

Geographical factors- it reduces transmission congestion andassociated high price in major metropolitan areas.

Saving on outage cost - The risk of power outages is less.

Increasing demand in intermediate sector – It has flexibilityto meet accelerated demand.

Low payback period- dispersed generation calls for lesserinvestment and lower payback period.

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Restraints and Industry ChallengesRestraints

Utility attitude- Utility owners are worried about recoveryof their stranded assets - they offer resistance toimplementation of dispersed generation

Consumer perception –The consumers are apprehensiveabout the future of dispersed generation.

Industry Challenges

Government regulations (state and federal) – Futuredevelopment of dispersed generation markets largelydepends on the regulator’s policy and framework.

Grid interconnection issues - various issues like safety, lackof uniform standards, impact on grid snag dispersedgeneration.

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The future of Decentralized Generation Capitalizing on the potential available in North

America for decentralized generation, WärtsiläCorporation has set up many dispersed generation plants. It is also coming up in our country.

The decentralized generation is unlikely to replace centralized generation entirely but its share in power production will increase dramatically in the coming years, with important benefits to all segments of population and significant environmental benefits.

The future of Decentralized Generation Dispersed generators are unlikely to compete with central

power stations, but its reliability and quality of service may be driving factors to attract the consumers.

The quality of centralized power system and its ability to transmit power to the load continuously have become questionable. In this perspective, dispersed and distributed generation may grow in dimension and be able to supplement the centralized generation to increase the overall reliability.

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Compact Transmission Line Design and implementation of compact

transmission line is another modern trend. Compact transmission lines are not fundamentally different from traditional transmission lines, but they are designed to take up less space. Traditional transmission lines are designed very conservatively with wide spaces between phase conductors.

Compact Transmission Line Compact line design is the result of this space-saving

strategy. They take up far less lateral space by utilizing modern materials and altering tower geometries. The structures in these modern designs are simpler and require less space, reducing their visual impact. These designs reduce phase-to-phase and phase-to-structure distances, which in turn increase voltage gradients on conductors and reduce flashover voltage thresholds. Methods used in design of EHV transmission are utilized here in order to keep audible noise (AN), radio noise (RN), and EM fields within acceptable levels. The towers are replaced by tubular poles. The idea is equally appreciated by the utilities and the land-owners.

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Power system quality It is a big concern now-a-days. The power quality has

deteriorated particularly due to injection of harmonics into the system by power electronic devices. Power-electronic devices are being used extensively for drive control, induction and dielectric heating, traction, illumination, flexible AC transmission and various other fields of utilization. So the harmonic levels are going up. Present day designs are being made with a look to improving the power quality by reducing the harmonic level.

Other important features of power system quality are the variations in voltage and frequency, power and voltage pulsations, voltage dip, and load-shedding. The aim is to reduce these unwanted features to minimum.

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Quality of supply to be ensured There is practically no variation in voltage at the

generator terminals due to the presence of AVR. Also the turbine-governor keeps the frequency close to nominal. Voltage at the load buses are to be kept within statutory limits. It may be achieved by optimizing the generation and the load flow. Starting of large motors give rise to voltage dip- it can be avoided by soft-starting. Pulsations are produced by swings and asynchronous operations. Their magnitude can be reduced by increasing the damping.

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Reliability The over all performance of the system in a desired

manner is called reliability. It is in essence theprobability of the system or any of its subsystems orcomponents to perform adequately and to oursatisfaction.

Reliability engineering is thus concerned with theprobability of the system to remain in the operativestate. Reliability is the availability of the systemexpressed quantitatively. If there be no failure, thereliability is 100%.

Reliability There are 3 types of failures- early failure, wear out

failure, chance failure. Early failures are due tosubstandard components or poor design. It can beeliminated by checking components and improvingdesign.

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The modes of failures The wear out failure can be avoided by periodic

overhauling and preventive replacement of components.Chance failures are unexpected and unpredictable.Reliability engineering deals with the probability ofoccurrence of these events.

The 1st phase is burn-in period, while a trial run is madeand substandard components replaced, the 2nd phase isthe useful period when chance failures may occur, the 3rd

phase is the wear out period. Reliable operation ispossible only during the useful period. The predictions ofreliability are made by using the probability theory.

The modes of failures The installed capacity should be always above

the peak load. A reserve capacity is required forscheduled maintenance.

There are two types of outages- planned outagewhich is made for maintenance and forcedoutage due to some fault. A reserve capacity isrequired for both to ensure reliable operation.

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Up-time, Down-time and reliability Also there must be adequate spinning reserve to take

care of sudden increment in load or a forced outage.

A generating unit may be either up or down. It may enter into the down state from up due to occurrence of fault. The mean time between failures (MTBF) should be large.

The reciprocal of MTBF= (no. of failures/ total operating period) is the mean failure rate. The mean down time is: r = (total down time/no. of down states).

Up-time, Down-time and reliability Since repair is taken up as soon as a failure occurs,

the reciprocal of r is the mean repair rate.

The reliability=R=[m/(m+r)] where m is the mean up-time.

Hence, the unreliability=Q=[r/(m+r)]

The task of power system planner and operator is to increase R as far as possible.

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References “Electrical transmission and distribution reference book”, 4th. Ed.,

Westinghouse Electric Corporation, East Pittsburg, 1964.

V. Venikov, “Transient processes in electric power system”, MIR Publishers, Moscow, 1977.

R.N. Dhar, “Computer-aided power system operation and analysis”, TMH, New Delhi, 1984.

B.R. gupta, “Generation of electrical energy”, s. Chand and Co. Ltd., ISBN: 81-219-0102-2

C. W. Taylor, “Power system voltage stability”, McGraw-Hill, 1994.

T. V. Cutsem, C. Vournas, “Voltage stability of electric power systems”, Kluwer Academic Publishers, 1998

J. Machowski, J. W. Bialek, J. R. Bumby, “Power system dynamics and stability”, John Wiley & Sons, 1997.

D.V. Razevig (translated by M.P. Chowrasia, “High voltage engineering”, Khanna Publishers, 6th. Reprint, 1998

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References J.J. Grainger & W.D. Stevenson, Jr., “Power system analysis”, Tata-McGraw-

Hill, 2003; ISBN 0-07-058515-6

A. Chakraborty, S. Halder, “Power system analysis, operation and control”,2nd. Ed., PHI.

P.M. Anderson and A.A. Fouad, “Power system control and stability”, 2nd.Ed., IEEE, Wiley-interscience, 2011.

P. Kundur, “Power system stability and control”, McGraw-Hill Inc., theEPRI Power System Engineering Series.

P. Kundur, et. Al, “Definition and classification of power system stability,”IEEE trans. on Power Systems, Vol. 19, no. 2, May, 2004.

M.D. Singh, K.B. Kanchandani, “Power electronics”, TMH, ISBN: 978-0-07-058389-4

J. Arilagga, & N.R. Watson, “Power system harmonics”, Wiley: ISBN 978-0-470-85129-6.

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References

R.C. Dugan, M.F. Mc Granaghan, S. Santoso & H.W.Beaty, “Electrical power systems quality”, TMH, 2nd.Ed.

Y.H. Song, A.T. Johns, “Flexible AC transmissionsystems (FACTS)”, IEE; ISBN 0-85296-771-3.

Bharat Heavy Electricals Ltd., Operating manuals

Documentation library of CEO, ERLDC, SLDC, etc.,Wikipedia.com

Thank you all,for patient audience.

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