Concept of excitation systems in modern alternator (Excitation System)

Definition: The system which is used for providing the necessary field current to the rotor winding of the synchronous machine, such type of system is called an excitation system. In other words, excitation system is defined as the system which is used for the production of the flux by passing current in the field winding. The main requirement of an excitation system is reliability under all conditions of service, a simplicity of control, ease of maintenance, stability and fast transient response.

The amount of excitation required depends on the load current, load power factor and speed of the machine. The more excitation is needed in the system when the load current is large, the speed is less, and the power factor of the system becomes lagging.

Types of Excitation System

The excitation system is mainly classified into three types. They are

1. DC Excitation System
2. AC Excitation System
• Rotor Excitation System
• Brushless Excitation System
3. Static Excitation System

1. DC Excitation System

The DC excitation system has two exciters – the main exciter and a pilot exciter. The exciter output is adjusted by an automatic voltage regulator (AVR) for controlling the output terminal voltage of the alternator. The current transformer input to the AVR ensures limiting of the alternator current during a fault.

When the field breaker is open, the field discharge resistor is connected across the field winding so as to dissipate the stored energy in the field winding which is highly inductive.

The main and the pilot exciters can be driven either by the main shaft or separately driven by the motor. Direct driven exciters are usually preferred as these preserve the unit system of operation, and the excitation is not excited by external disturbances.

The voltage rating of the main exciter is about 400 V, and its capacity is about 0.5% of the capacity of the alternator. Troubles in the exciters of turbo alternator are quite frequent because of their high speed and as such separate motor driven exciters are provided as standby exciter.

2. AC Excitation System

The AC excitation system consists of an alternator and thyristor rectifier bridge directly connected to the main alternator shaft. The main exciter may either be self-excited or separately excited. The AC excitation system may be broadly classified into two categories which are explained below in details.

a. Rotating Thyristor Excitation System

The rotor excitation system is shown in the figure below. The rotating portion is being enclosed by the dashed line. This system consists an AC exciter, stationary field and a rotating armature. The output of the exciter is rectified by a full wave thyristor bridge rectifier circuit and is supplied to the main alternator field winding.

The alternator field winding is also supplied through another rectifier circuit. The exciter voltage can be built up by using it residual flux. The power supply and rectifier control generate the controlled triggering signal. The alternator voltage signal is averaged and compare directly with the operator voltage adjustment in the auto mode of operation. In the manual mode of operation, the excitation current of the alternator is compared with a separate manual voltage adjustment.

b. Brushless Excitation System

This system is shown in the figure below. The rotating portion being enclosed by a dashed line rectangle. The brushless excitation system consists an alternator, rectifier, main exciter and a permanent magnet generator alternator. The main and the pilot exciter are driven by the main shaft. The main exciter has a stationary field and a rotating armature directly connected, through the silicon rectifiers to the field of the main alternators.

The pilot exciter is the shaft driven permanent magnet generator having rotating permanent magnets attached to the shaft and a three phase stationary armature, which feeds the main exciter field through silicon rectifiers, in the field of the main alternator. The pilot exciter is a shaft driven permanent magnetic generator having rotating permanent magnets attached to the shaft and a 3-phase stationary armature, which feeds the main’s exciter through 3-phase full wave phase controlled thyristor bridges.

The system eliminates the use of a commutator, collector and brushes have a short time constant and a response time of fewer than 0.1 seconds. The short time constant has the advantage in improved small signal dynamic performance and facilitates the application of supplementary power system stabilising signals.

3. Static Excitation System

In this system, the supply is taken from the alternator itself through a 3-phase star/delta connected step-down transformer. The primary of the transformer is connected to the alternator bus and their secondary supplies power to the rectifier and also feed power to the grid control circuit and other electrical equipment.

This system has a very small response time and provides excellent dynamic performance. This system reduced the operating cost by eliminating the exciter windage loss and winding maintenance.

 

Power factor correction/improvement

 

Power factor correction

Power factor basics:

Power quality is essential for efficient equipment operation, and power factor contributes to this.

Power factor is the measure of how efficiently incoming power is used in an electrical installation. It is the ratio of active to apparent power, when:

Active Power (P) = the power needed for useful work such as turning a lathe, providing light or pumping water, expressed in Watt or KiloWatt (kW)

Reactive Power (Q) = a measure of the stored energy reflected to the source which does not do any useful work, expressed in var or Kilovar (kVAR)

Apparent Power (S) = the vector sum of active and reactive power, expressed in Volt Amperes or in KiloVolt Amperes (kVA)

The power triangle:

Poor power factor (for example, less than 95%) results in more current being required for the same amount of work.

Power factor correction

Power factor correction is obtained via the connection of capacitors which produce reactive energy in opposition to the energy absorbed by loads such as motors, locally close to the load. This improves the power factor from the point where the reactive power source is connected, preventing the unnecessary circulation of current in the network.

Determining the power factor correction required

• Calculation of the required reactive power

1st step we have to determine the required reactive power (Qc (kvar)) to be installed to  improve power factor (cos φ) and reduces the apparent power (S).

Qc can be determined from the formula Qc = P (tan φ – tan φ‘), which is deduced from the diagram.

Qc = power of the capacitor bank in kVAr

P = active power of the load in kW

tan φ = tangent of phase shift angle before compensation

tan φ’ = tangent of phase shift angle after compensation

The parameters φ and tan φ can be obtained from billing data, or from direct measurement in the installation.

Step 2: Selection of the compensation mode

The location of low-voltage capacitors in an installation can either be central (one location for the entire installation), by sector (section-by-section), at load level, or a combination of the latter two.

In principle, the ideal compensation is applied at a point of consumption and at the level required at any moment in time. In practice, technical and economic factors govern the choice.

The location is determined by:

the overall objective (avoiding penalties on reactive energy, relieving transformers or cables, avoiding voltage drops and sags)

the operating mode (stable or fluctuating loads)

the foreseeable influence of capacitors on the network characteristics

the installation cost

Step 3: Selection of the compensation type

Different types of compensation should be adopted depending on the performance requirements and complexity of control:

Fixed, by connection of a fixed-value capacitor bank

Automatic, by connection of a different number of steps, allowing adjustment of the reactive energy to the required value

Dynamic, for compensation of highly fluctuating loads

Step 4: Allowance for operating conditions and harmonics

Operating conditions have a great impact on the life expectancy of capacitors, so the following parameters should be taken into account:

Ambient temperature (°C)

Expected over-current related to voltage disturbances, including maximum sustained overvoltage

Maximum number of switching operations per year

Required life expectancy

Some loads (variable speed motors, static converters, welding machines, arc furnaces, fluorescent lamps, etc.) pollute the electrical network by reinjecting harmonics. It is therefore also necessary to consider the effects of these harmonics on the capacitors.

The benefits of power factor correction

Savings on the electricity bill

Power factor correction eliminates penalties on reactive energy, decreases demand on kVA, and reduces power losses generated in the transformers and conductors of the installation.

Increased available power

Fitting PFC equipment on the low voltage side increases the power available at the secondary of a MV/LV transformer. A high power factor optimises an electrical installation by allowing better use of the components.

Reduced installation size

Installing PFC equipment allows conductor cross-section to be reduced, as less current is absorbed by the compensated installation for the same active power.

Reduced voltage drops

Installing capacitors allows voltage drops to be reduced upstream of the point where the PFC device is connected, therefore preventing overloading of the network and reducing harmonics.

Why we connect Capacitor in parallel, not in series?

Reason 1

We know that in series connection Current is constant and voltage is varying but in parallel connection, voltage is constant and current is varying. 

So we need to keep constant the voltage across the load. So if we connect a capacitor in parallel it will be drawn leading current according to its rated value. But if we connect a capacitor in parallel then the flow of current through the capacitor will depend on the load.

Reason 2

As in the case of series connection of capacitor current fully depends upon the load so we need a capacitor of high value which can deliver the full load current.

Reason 3

If we connect a capacitor in series with the load for power factor improvement then a voltage will be dropped by the capacitor.

Reason 4

If we connect the capacitor in series with the load then if short circuit fault occurs in the load then the total voltage will be applied to the capacitor which may blow them.

Reason 5

In case of series connection, if we want to connect additional capacitor then we need to open the whole circuit. But in case of parallel connection, we can easily connect an additional capacitor in parallel with the existing capacitor.

Reason 6

If we connect the capacitor in series with the load for power factor improvement then the recovery voltage across the contacts of the switchgear shall be high.

 

ELECTRICAL LOAD ASSESSMENT

Ø  Connected Load :- The sum of continues rating of all  electrical equipment connected to the supply system is known as connected load. 

Ø  Maximum Demand :- Maximum demand is the Maximum load which a consumer uses at any time which consume for min  30 mints). It is the maximum demand which determines the size and cost of the installation.

Ø  Demand Factors:- The ratio of maximum demand to the connected load is called the demand factor. It is always less than unity.

Ø  Load Factor:- The ratio of the average load to maximum load during 24 hours is called as load factor.

                                     

The load factor can  also be defined as the ratio of energy consume during a period to the energy which would have been used if the maximum demand had been maintain throughout that period.

Ø  Diversity Factor: - The ratio of the sum of  individual maximum demands to the simultaneous maximum demand on the power station is known as diversity factor.  It is always greater than unity.

                                      

Ø  Utilization Factor :- It is the measure of utility of power plant capacity and is the ratio of maximum demand to the rated capacity of power plant

Ø  Power Factor:- The POWER FACTOR is a number (represented as a decimal or a percentage) that represents the portion of the apparent power dissipated in a circuit.

 Note:

                               I.            Diversity Factor is higher, the energy is cheaper

                            II.            Load Factor, Diversity factor higher the energy per unit cost is less.

                         III.            Diversity Factor has direct effect on the fixed cost of unit generated.

Different Types of Load:-

The different types of loads are as follows

1.      Residential or domestics load

2.      Commercial load

3.      Industrial load

4.      Government load

5.      Municipal load

6.      Irrigation load

 

1.      Residential or domestics load:- Domestics load mainly consists of lights,fans, domestics appliances such as heaters,refrigerators,air-conditioners for rooms, radio receivers,television,LCD,LED,electric cookers, electric water-heater, single phase I.M. for pumping.

      The diversity factor of such load is between 1.2 to 1.3. The load factor of such load is very poor and it is between 10% - 20% .  

2.      Commercial load:- This type of load mainly consist of lighting for shops, advertisement ,fans and electrical appliances use for commercial purpose likes shops ,restaurants, market places etc.

      The demand factor for such load is 100% & the diversity factor is between 1.1-1.2 .The load factor of such load is between 25%- 30%.

3.      Industrial  load:- This type of load can be subdivided into sections depending on the power range required

                                           I.            Cottage industries which requires about 5kw

                                        II.            Small scale industries about 25 kw

                                     III.            Medium scale industries which requires about 25-100 kw

                                     IV.            Large scale industries which requires about 100-500 kw

                                        V.            Heavy scale industries which requires more  than 500 kw

      For large scale industries the demand factor may be taken as 70-80% and the load factor between 60-65%.

      For heavy industries the demand factor is 85- 90% and the load factor is about 70-80%

4.      Government load:- This may be classified as separate type of load when it has separate feeders & special working condition for example Defense load.

5.      Municipal Load:- Street light is one of the municipal load. This type of load is practically constant throughout the hours of darkness so the demand factor is 100%, the diversity factor can be taken as 1 when considered for the duration of street lighting load and load factor is 25-30%.

6.      Irrigation load:- This type of load is required for supplying water for fields for crops in different seasons . The demand factor 90-100%,Diversity Factor 1-1.5 and load factor 20-25% for irrigation load.

TARIFF:-

The electrical energy produced at the generating station is delivered to a large number of consumers. The rate at which energy is sold to the consumers (called tariff) is fixed by the supplying company .While fixing the tariff, the supply companies are to ensure that they should not only recover the total cost of producing the energy but also earn some profit. However, the profit should be minimum possible so that electrical energy can be sold at reasonable rates and the consumers insured to use more electricity. 

The rate of electrical energy at which it is sold to the consumers is called tariff .The supply companies invest money to generate, transmit and distribution of electrical energy,a tariff is fixed. The cost of generation depends upon the magnitude of energy consumed by the consumers and his load conditions. Therefore, due consideration is given to different types of consumers (e.g. domestic, commercial and industrial) while fixing a tariff

The main objective of the tariff is to ensure the recovery of the total cost of generation and distribution .Tariff should include the following items:

(1) Recovery of cost of electrical energy generated at the generating system.

(2) Recovery of cost on the capital investment in transmission and distribution system.

(3) Recovery of cost of operation, supplies and maintenance of equipment.

(4) Recovery of cost of metering equipment, billing and miscellaneous services .

(5) A marginal return (Profit) on the capital investment

TYPES OF TARIFF

There are various types of consumers ( domestic, commercial and industrial etc.) and their energy requirements are also different. Accordingly, several types of tariffs have been designed so far, out of which the most commonly applied are described below:

1.      SIMPLE TARIFF

2.      FLAT RATE TARRIF

3.      BLOCK RATE TARIFF

4.      TWO-PART TARIFF

5.      MAXIMUM DEMAND TARIFF

6.      POWER FACTOR TARIFF

1.Simple Tariff: The tariff in which the rate per unit of energy is fixed, is called simple tariff.

This is a simplest possible tariff. The rate per unit of energy consumed by the consumer is fixed irrespective to the quantity of energy consumed by a consumer. This energy consumed is measured by installing an energy meter

The following are the advantages :

            1.         It is in simplest form and easily understood by the consumers.

            2.         Consumer is to pay as per his consumption.

Disadvantages

1.                  Consumer is to pay the same rate per unit of energy consumed irrespective of the number of units consumed by him. Hence, consumers are not encourage to consume more energy.

2.                  The cost of energy per unit delivered is high.

3.                  The supplier does not get any return for the connection given to the consumer if consumer does not consume any energy in a particular month.

Application :

Since it is very simple form of tariff, it is generally applied to tube wells which are operated for irrigation purposes

2. Flat rate tariff.  The tariff in which different types of consumers are charged at different per unit rates is called flat rate tariff. This type of tariff is similar to simple tariff. Only difference is that consumers are grouped into different classes and each class of consumer is charged at a different per unit rate. For example flat rate for fan and light loads is slightly higher than that for power loads.

Advantages

(1) It is fairer to different types of consumers.

( 2) It is quite simple in calculations.

Disadvantages

(1) Consumers are not encouraged to consume more energy because same rate per unit of energy consumed is charged irrespective of the quantity of energy consumed.

(2) Separate meters are required to measured energy consumed for light loads and power loads.

(3) The suppliers does not get any return for the connection given to the consumer if he does not consume any energy in a particular period or month. 

Application :Generally applied to domestic consumers. Since it is simple and easy for explanation  to consumers, therefore this tariff is  

3. block rate tariff:

The tariff in which first block of energy is charged at a given rate and the succeeding blocks of energy are charged at progressively reduced rates is called block rate tariff

In this type of tariff, the energy units are divided into numbers of blocks and the rate per unit of energy is fixed for each block. The rate per unit of energy for the first block is the  highest and reduces progressively with the succeeding blocks. For example, the first 100 units may be charged at the rate of Rs. 3.00 per unit; the next 100 units may  be charged at the rate of Rs.2.50 per unit and the remaining additional units may charged at the rate of Rs. 2.00 per unit. 

Advantages:

(1)   By giving an incentive, the consumers are encouraged to consume more energy. This increases the load factor of the power system and hence reduces per unit cost of generation.

(2)    Only one energy meter is required to measure the energy .

Disadvantages:

(1)   The supplier does not get any return for the connection given to the consumer if consumer does not consume any energy in a particular period.

Application

This type of tariff is mostly applied to domestic and small commercial consumers.

 

 

4. Two – Part Tariff

The tariff in which electrical energy is charged  on the basis of maximum demand of the consumer and the units consumed by him is called two- part tariff.

In this tariff, the total charges to be made from the consumer are split into two components namely fixed charges and running charges. The fixed charges are independent of energy consumed by the consumer but depend upon the maximum demand, whereas the running charges depend upon the energy consumed by the consumer. The maximum demand of the consumer is assessed on the basis of the kW capacity of all the electrical devices owned by a particular consumer or on the connected load

Thus, the consumer is charged at a certain amount per kW of energy is consumed i.e.

        Total charges= Rs. (a X kW + b X kWh )

where, Rs. a= charges per kW of maximum demand

              Rs. b= charges per kWh of energy consumed

In this tariff basically, the charges made on maximum demand recovers the fixed charges of generation such as interest and depreciation on the capital cost of building and equipment, taxes and a part of operating cost which is independent of energy generated. Whereas, the charges made on energy consumed, recovers operating cost which varies with variation in generated (or supplied) energy.

Advantages

(1)   It is easily understood by the consumers.

(2)   The supplier gets the return in the form of fixed charges for the connection given to the consumer even if he does not consume any energy in a particular period.

Disadvantages

(1)   If a consumer does not consume any energy in a month even then he has to pay the fixed charges .

(2)   Since the maximum demand of consumer is not measured, therefore, there is always conflict between consumer and the supplier to assess the maximum demand.

Q.1      A power supply is having following loads:   

Types of load	Maximum demand(kW)	Diversity of Group	Demand Factor
Domestic	1500	1.2	0.8
Commercial	2000	1.1	0.9
Industrial	10000	1.25	1

            If the overall system diversity factor is 1.35, Determine The maximum demand and Connected load of each type .

 

 Q.2      An industrial consumer has connected load (mechanical) of 100 HP,Driven by electric motors. The overall system effieciecyis 90% with power factor of 0.86 (lag).Calculate the maximum demand if the demand factor is 0.6. If the industries operate for 500 Hrs per month,with a load factor of 0.75. Calculate the monthly energy consumption. Also Calculate monthly energy bill if energy charges is Rs 4/kWh and demand charge is Rs 150/kVa/month.Assume two part tariff.                                                   

 

Load Forecasting:- There are mainly four method of load forecasting

                                                    I.         Section method or load survey method

                                                 II.         Method of extrapolation

                                              III.         Mathematical method

                                              IV.         Mathematical method using economics parameters 

I.                   Section method or load survey method: In this method the area under consideration is visited and the existing and future load requirement  are forecast, Here the consumer group are classified into residential or domestics consumer, commercial consumers, industrial consumers etc. are made for each consumer categories and their respective work time also noted. This  method may cover a periods 8-10 years for planning of equipment and erection. 

II.                 Method of extrapolation: This method involves study of data collected from various records and used in the comparison of trend of increase in demand and energy consumption during past period comparable with period of forecast. 

III.             Mathematical method: The curve are plotted with the data of energy consumption for past years and trends is observed by the direct extrapolation of rate of change of energy requirement. 

IV.             Mathematical method using economics parameters: This method using economics parameters are based on the assumption that per capita consumption of electrical energy in a country depends upon economics factor like cost, efficiency ,production ,consumption ,transport etc. Indices for these parameters are found and suitably used for extrapolation of linear or exponential approximations.

Regression analysis:

            It is the study of the behaviour  of a time series or a process (e.g. power system load) in the past and its mathematical modeling  so that the future behaviour can be extrapolated from it, in time, in time varying events such a power system load can be broken down into  the following major component.

•  Basic trend :
•  Seasonal variation
•  Cyclic variation :It includes periods larger than seasonal  variation and cause the load pattern to depleted load from two or three years.
• Random variation: It occurs on account of day to day changes and depends upon type of week, weekend.

Regression analysis depends upon the following basic trends (i) linear trend (ii) exponential trend

(i)                 Linear trend: A past trend where the increase in consumption from year to year is more or less constant. Tabulate  the past consumption data and plot it on an graph which give a straight line.The projection of this line  will give a forecast of future demands. But in real life, such growth trend is unlikely in the power supply industries. The such growth trend in the power industries can be mathematically expressed as follows 

                                                                                           

                                                        C_t  = a +  b t

Where C_t  is electrical consumption at any year t

a is consumption for base year t= 0

b is constant annual increase in energy consumption

t = T-1+ n

Where T is the no. of year for which the statistical trend is studied

n is the no. of year for which the forecast is desired.

Q)    Find the electrical consumption in 15 years it consumption of base year is 2,annual increase in consumption is 0.18 mv/year number of year for which the forecast is required is 5 and number of years for which the statistical trend is studied is 11 years.

(ii)          Exponential trend: In this case past data are drawn on a logarithmetic graph to give straight line projection for forecasting. Its mathematical expression is

                     C_t = C₀ (1+ m )^t

Where  m is mean annual growth of consumption during T years

       &   C₀ is consumption in reference year .

By applying log at both side of the equation we get

              log C_t =  log C₀ + t log(1+ m)

                        =  a + bt

 Where  a = log C₀  & b = log(1+ m)

Q)        Find the consumption at the end of 25 years for which the statistical trend is studied is 21 years, consumption in reference year is C₀ = 5000 mV mean annual growth is 10 % and  n = 5 years.