The selection of a particular bus-bar arrangement is done depending upon the factors such as voltage level, simplicity, reliability, safety, cost of installation and maintenance, etc.

Bus-bar Arrangements

Different types of bus-bar arrangements available are,

1. Single bus-bar system
2. Single bus-bar with sectionalizer
3. Main and transfer bus-bar system.

Single Bus-bar System

Single bus-bar system is the simplest and cheapest arrangement of bus-bars. It consists of a single bus-bar to which all the electrical equipments viz., generators, transformers, isolators, etc., are connected. A circuit breaker is provided for every individual equipment for protection against fault currents. Similarly, isolators helps in the isolation of generators, transformers etc., during fault clearance and maintenance.

Single bus-bar system is used for voltages below 33 kV. Usually, it is employed for 11 kV indoor substations. Single line diagram of a single bus-bar system is shown in the following figure.  Further, it is to be noted that all the incoming and outgoing lines are connected to the single bus-bar only. The incoming lines at a voltage of 11 kv are connected to the bus-bar through isolators and circuit breakers. In the outgoing line, the voltage is stepped down to 400 V using 11 kV/400 V transformer, thereby making the voltage at outgoing lines to 400 V

Advantages of Single Bus-bar System

1. Due to the simplicity and low initial cost, single bus-bar systems are used.
2. It is easy to operate since, the connections of single bus-bar system are simple.
3. Single bus-bar system can be conveniently used where there is no future expansion of the substation is expected.

Disadvantages of Single Bus-bar System

1. In case of fault on the bus-bars, the supply to the whole system, including healthy feeders gets interrupted.
2. It is not possible to carry out repairs without interrupting the supply.

Single Bus-bar with Sectionalizer

Sectionalized single busbar means single busbar with 2 to 3 sections. Sections of busbar are separated by isolator with circuit breaker combination as shown in figure.  Sectionalization of busbar with only isolator is not preferable. We know that isolator is also called as no-load switch i.e., which works on no-load only. Hence, a circuit breaker is necessary to remove the load on bus-bar. It is clear that sectionalization of busbar prefers isolator with circuit breaker.

Sectionalized single bus-bar has following advantages (over single bus-bar arrangement),

Advantages of Single bus-bar with sectionalizer

1. Flexible operation can be achieved using single bus-bar scheme with sectionalization.
2. More reliable than simple bus-bar scheme.
3. The faulted section can be isolated without affecting the other section’s supply.
4. The maintenance or repair of one section is possible as it is shut downed, without affecting the other section supply.

Disadvantages of Single bus-bar with sectionalizer

1. If the circuit breakers are not used as the sectionalizing switch, then the coupling of the bus-bar may occur during the load transfer and the use of circuit breakers make the system expensive.
2. In order to maintain the continuity of supply to the system, during the maintenance of the circuit breaker, the circuit breaker must be provided with isolators on both sides, which further increases the cost of the system.
3. Single bus-bar system with sectionalization is uneconomical for small substations.

Main and transfer bus-bar system

Bus-bars are the copper rods, that are used to collect electrical energy at one place. The generators and feeders that are operating at same voltage (or) constant voltage are connected directly to these busbars. In order to avoid the interruption of power flow the study of bus-bar arrangement is very important.

The main and transfer busbar arrangement uses two buses, one as main bus and other as transfer (or) auxiliary (or) spare bus. The generators and feeders are connected to both main bus and as well as transfer (or) auxiliary bus.

Under normal conditions the generator and feeder, are connected to main bus. Suppose, assume that the fault has occurred in any breaker or main bus then the power flow gets interrupted. In order to avoid this, the entire equipment that are connected to main bus is shifted (or) transferred to a transfer bus (or) auxiliary bus without any interruption of power flow by using a bus- coupler, which uses double isolating switches. Thus the generator and feeders are transferred from main bus to auxiliary bus without any interruption of power.

Advantages of Main and Transfer Bus-bar System

1. There is no interruption of power supply i.e., the power supply is continuous even under fault condition.
2. The maintenance and repair of bus or circuit breaker is easy.
3. During fault conditions the circuit is transferred easily to an auxiliary bus.

Disadvantages of Main and Transfer Bus-bar System

1. The main and transfer bus-bar arrangement is very expensive than other bus-bar arrangements.
2. The service may be interrupted during switch over from one bus to another bus.

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Objectives of Tariff :

Like other commodities, electrical energy is also sold at such a rate that it not only returns the cost but also earns reasonable profit. Therefore, a tariff should include the following terms:

1. Recovery of cost of producing electrical energy at the power station.
2. Recovery of cost on the capital investment in transmission and distribution systems.
3. Recovery of cost of operation and maintenance of supply of electrical energy e.g., metering equipment, billing etc.
4. A suitable profit on the capital investment.

Types of Tariff

The different types of tariff are as follows,

1. Simple tariff
2. Flat rate tariff
3. Block rate tariff
4. Three-part tariff
5. Two-part tariff
6. Maximum demand tariff
7. Power factor tariff.

1. Simple Tariff

In this type of tariff, the rate per unit of energy consumed is fixed and every consumer will be charged the same. The simple tariff is also called as uniform tariff and is the simplest form of tariff. The simple tariff remains constant and does not vary with the increase or decrease in number of units consumed by a consumer.

2. Flat Rate Tariff

In this type of tariff different types of consumers are charged at different rates. The rate for each type of consumer is arrived at by taking its load factor, diversity factors into account. The bill will be total units consumed × rate/unit.

Advantage :

1. It is easy to understand by different types of consumers and calculations at suppliers end are simple.

Disadvantages :

1. If the consumer has got two types of loads i.e., (i) fan and lighting load (ii) power load, then two meters are installed at his premises i.e., for different types of power supply, separate meters
   are required.
2. It is very difficult to derive at the load factor and diversity factor to be used in deciding the traffic.

3. Block Rate Tariff

Here, cost of units consumed will change as the consumer crosses particular number of units its. If the number of units generated increases, then the cost of generation per unit, automatically decreases. For example, for first 50 unit there may be one ratio, 51 to 200 there may be other rate. From 201 to 400 there may be some another rate. Such type of tariff is commonly used now a days for domestic and small industrial consumers.

Advantage :

If the consumer has large demand of  number of units, then he has to pay less amount only. Due to that reason, the consumers show more interest for consuming more electrical energy.

Disadvantage :

It lacks a measure of the consumers demand.

The overall annual cost of electrical energy generated by a power station can be expressed in two forms, three-part tariff forms and two-part tariff form.

4. Three-part Tariff

In this method, the overall annual cost of electrical energy generated is divided into three parts i.e., Fixed cost, Semi-fixed cost and Running cost.

Total annual cost of energy =Fixed cost + Semi-fixed cost + Running cost

= Constant + Proportional to maximum demand+ Proportional to kWh generated

= (a+ b kW + c kWh)

Where,

a – Annual fixed cost independent of maximum demand and energy.

b – Constant which when multiplied by maximum kW demand on the station gives the annual semi- fixed cost.

c – A constant which when multiplied by kwh output per annum gives the annual cost

It may be seen that by adding fixed charge or consumer ‘s charge to two-part tariff, becomes three-part tariff.

5. Two-part Tariff

The total charge under this kind of tariff is divided into two parts i.e., a fixed charge based on the maximum demand and variable charge (running charge) per unit of energy consumed. The expression for the annual cost of energy then becomes,

Total annual cost of energy = (A kW + B kWh)

Where,

A – A constant which when multiplied by maximum kW demand on the station gives the annual cost of the first part.

B – A constant which when multiplied by the annual kWh generated gives the annual running cost.

The fixed charges are dependent upon the maximum demand of the consumer, while the running charges are dependent upon the number of units consumed by the consumer. It is easy to understand and this type of tariff applies to consumers industrial consumers.

6. Maximum Demand Tariff

Maximum demand tariff is almost similar to the two-part tariff. The only difference is that, in a maximum demand tariff, the maximum demand is measured by an indicator known as maximum demand indicator.

The drawbacks of two-part tariff are eliminated in maximum demand tariff. This type of tariff is applicable for bulk supplier and for large industrial consumers, who have control over their maximum demand

7.Power Factor Tariff

when the power factor of the consumer’s load are taken into consideration, then that tariff is known as power factor In A.C. system, the efficiency of a plant and equipment not only depends on kW, but also on the power factor. The power factor plays a major role in A. C systems. So, to increase the utility of the plant to a maximum extent, the plant must be operated at most economical power factor.

The three main classes are,

1. kVA Maximum Demand Tariff: The maximum demand of the consumer is measured in kVA, not in kW. The maximum kVA demand is charged in addition to the charge corresponding to the energy.
2. kWh and kVARh Tariff: In this type of tariff both kWh and kVARh of a consumer are charged separately.
3. Sliding Scale Method: Under this kind of tariff, average power factor say 0.8 lagging is assumed as a reference. If the p.f of a consumer falls by even 1 % then an extra amount will be charged for it. On the other hand, if the p.f of any consumer rises by even 1%, a discount will be given to him.

Requirements of Tariff Method

1. Tariff should be simple.
2. It should give lesser rates for more consumption.
3. It must be same for large population.
4. It should consider both the maximum demand charges and the energy charges.
5. It must encourage the consumers with high load factors.
6. It should motivate consumers for using power during off-peak hours.
7. It should charge more for lighting than power connection.
8. The penalty should be provided for low power factor.

Q1. Why is tariff for power load less than the lighting load?

Ans. Although tariff should include the total cost of producing and supplying electrical energy plus the profit, it cannot be the same for all types of consumers. This is because the cost of producing electrical energy depends to a considerable extent upon the magnitude of electrical energy consumed by the user and his load conditions. Therefore, in all fairness, due consideration has to be given to different types of consumers (e.g., industrial, domestic and commercial) while fixing the tariff. This makes the problem of suitable rate making highly complicated. The power load improves the load factor of a system to greater extent rather than a lighting load. Hence, the reason, why the tariff for power load is less than the lighting load.

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Figure 1: Electrical Insulator.

An electrical insulator in an overhead line is to hold the live conductor to prevent leakage of current from the conductor to the pole. These are made of porcelain clay and are thoroughly glazed to avoid the absorption of moisture from the atmosphere.

Properties of an Electrical insulator

1. It should have high electrical resistance in order to prevent the leakage of current.
2. It should have high mechanical strength in order to withstand the weight of conductor and the forces acting due to wind pressure, ice etc.
3. It should have high resistance to temperature variation in order to prevent it from any damage.
4. It should have high permittivity  in order to withstand the electrical stresses due to over voltages, switching surges, lightning
5. It should be non-porous and free from impurities like cavity, void, crack etc.
6. It should have high ratio of puncture strength to flashover.
7. It should not be brittle.
8. It should be non-hygroscopic.

Types of Electrical Insulator

The various types of insulators are as follows,

1. Pin-type insulators
2. Suspension type insulators
3. Strain insulators
4. Shackle insulators and
5. Stay insulators.

Puncture in an Electrical Insulator

Puncture is an electric breakdown in an insulator and it is most severe but least frequent type of electrical breakdown. Whenever, the voltage across the insulator exceeds the puncture voltage, an arc will strike between the conductor and passes through the body of the insulator. Since, the arc produced during puncture passes through the whole body of the insulator, the complete insulator gets damaged and has to be replaced with a new one. In order to avoid puncture of insulators, the thickness of the porcelain disc has to be increased.

Whenever flash over occurs, there are chances that the insulator will continue to work. But, whenever a puncture occurs,  there is no chance, that it will continue to work and the insulator has to be replaced. Hence for safety operation of the insulator, the insulator will be designed in such a way that flash over occurs before puncture. This can be ensured by keeping the value of safety factor as high as possible. The value of safety factor for a pin type insulator is about 10.

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A graph obtained between the load consumption and time is called as load curve. Load curve is the graph between varying load on power station and time by taking load on y-axis and time on x-axis. It is also defined as the curve representing the load demand with respect to time.

The curve is referred as daily, monthly and annually load curve, when the time is represented in hours, days and months represented. However, a general daily load curve is shown in figure 1.

Load on power station is not constant, it varies from time to time. If the variations in load considered hourly then it is called daily load curve. Monthly load curve can be drawn from daily load curves of specified month by calculating average value of power at a particular time of the day. In similar manner we can draw annual load curves from monthly load curves of particular year.

Significance of Load Curve

Load curves provides following information.

1. Load variations during different hours and a day.
2. Maximum and minimum load on the station.
3. Area under load curves represents total number of units generated for considered period.
4. Average load on the station obtained by dividing the area under the curve by number of hours.
5. Station load factor can be determined.

Q1. How load curves help in the selection of size and number of generating units.

Ans: In power systems, the load is a function of time (varies from time to time). Hence, it is not worth while to consider a single unit as economical proposition to meet the load changes. However, the above problems can be eliminated by selecting the number and size of generating units, which can be done by using load curves.

Apart from the above: the following points should be kept in view while choosing size and number of generating units.

1. The installation of units should be of different capacities rather than that of identical one’s which reduces the capital cost of the unit
2. The selection of size and number of unit should be such that it should closely fit the load curve in order to obtain continuous and reliable service.
3. To meet future requirements the capacity of unit should be considered 20-25% more than the maximum demand.
4. A spare generating unit should be considered in order to avoid breakdowns during repairs and over hauling’s.

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Figure 1: Load Duration Curve.

Load Duration Curve is drawn with time on x-axis and variation of load on y-axis, but the loads are an-angled in particular order usually descending Order Of magnitude. This curve can be obtained from ordinary load curve that the greatest load is on left and least load on extreme right of the graph. Utility Of Load and Load Duration Curves in the

Significance of Load Duration Curve

1. Maximum and minimum demands can be determined by using load duration curve.
2. It is useful for financial analysis of a power plant
3. Load duration curve gives the amount of energy which can be generated by taking load factor of the system as 100%.
4. Total energy consumption can be found out by considering the area under load duration curve.
5. Capacity of base load plant and peak load plant can be decided by studying the load duration curve carefully.
6. It helps to predict the capacity of generator.
7. It helps in working out the operational economics of the plant.
8.  It helps in evaluating the average demand on the station.

The above information is useful in deciding the size of units to be installed and also in preparing operation schedule of generating units.

Integrated Load Duration Curve

Integrated Load Duration Curve represents the total number of units (kWh) generated for the given demand in kW The ordinates represent the demand in kW and the abscissa represents kWh. Such a curve can be obtained from the load duration curve. The number of units generated (h₁) corresponding to this load demand are represented by the area 0abc. It corresponds to point P₁ on the integrated duration curve of figure 2. The integrated load duration curves are useful, where the given number of kWh units are available. Also the load in kW that could be carried at the base or peak from a river flow can be easily determined by the integrated load duration curve. The total number of units generated for a given demand can be obtained from this curve.

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Figure 1: Thyristor Controlled Reactor (TCR).

Working Principle and Waveforms of Thyristor Controlled Reactor (TCR)

A basic TCR is shown in figure (1). It consists of two main components, thyristor switch (T_y) and linear reactor ‘L’. Thyristor switch comprises of two back to back thyristors which conduct on alternate half cycles of the supply frequency. If a gate pulse is applied to the thyristors, it results in conduction of thyristor valve or switch. The current in the reactor can be controlled from maximum to zero value by varying the firing delay angle α.

Figure 2: Waveforms of Thyristor Controlled Reactor (TCR)

The duration of current conduction intervals is controlled by delaying the closure of the thyristor switch with respect to the peak applied voltage once in each half cycle as shown in figure (2).

When α = 0°. the thyristor switch (T_y) gets closed at the peak of the applied voltage, the amplitude is maximum and hence the resulting current in the reactor is equal to the steady state current. When α = 90°. the amplitude is zero and hence there is no current flow during the corresponding half cycle. The TCR current as a function of angle, (a) can be expressed as,

\[{{I}_{LF}}(\alpha )=\frac{V}{\omega L}\left( 1-\frac{2\alpha }{\pi }-\frac{\sin 2\alpha }{\pi } \right)\]

Figure 3.

Where.

V – Amplitude of applied voltage

L – Inductance of TCR

ω – Angular frequency of applied voltage.

The amplitude variation of the fundamental  TCR current with the delay angle a is shown in figure (3). From figure (3), it can be observed that the TCR can control the current continuously from zero to a maximum value.

Characteristics of Thyristor Controlled Reactor (TCR)

Figure 4: V-I Characteristics of Thyristor Controlled Reactor (TCR)..

The TCR ratings are decided as per the operational requirement. It can be operated in specified V-I region. The boundaries of this region is determined by maximum voltage, current and admittance which are shown in figure (4).

Where,

V_L(max) – Maximum voltage

I_L(max) – Maximum current

Y_L(max) – Maximum admittance

If TCR operates at fixed delay angle, α = 0° (say), then it act as Thyristor Switched Reactor(TSR). When TSR is fed from A.C supply it gives fixed inductive admittance and its reactive current is directly proportional to the supply voltage.

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Figure 1.

Design of SVC possesses thyristors without gate tum-off capability. Separate apparatus for leading and lagging VAR are incorporated in SVC. To absorb reactive power, thyristor-controlled or thyristor-switched reactors are used and to supply reactive power thyristor-switched capacitor is used. Figure 1 shows a basic model of SVC.

Fixed Capacitor-Thyristor Controlled Reactor (FC-TCR)

Figure 2.

As shown in figure (2), an FC-TCR consists of a fixed (permanently connected) capacitor with a thyristor controlled reactor. In this the method of firing delay angle control is employed to vary the current in the reactor. A filter network with required capacitive impedance usually replaces the fixed capacitor fully or partially at the fundamental frequency. This is done so as to generate the required reactive power. But at certain frequencies it provides a low impedance in order to avoid the dominating harmonics generated by TCR.

Characteristics:

The V-I characteristic of Fixed Capacitor-Thyristor Controlled Reactor is defined by maximum admittance of inductor and capacitor and by their voltage and current rating. The VI characteristic of FC-TCR is shown in below figure (2),

Where,

I_CM = Maximum capacitive current

I_LM = Maximum inductive current

V_CM = Maximum capacitor voltage

V_LM = Maximum TCR voltage

Y_C = Capacitor admittance

Y_LM = Maximum Inductor admittance.

Advantages of Static VAR Compensator (SVC)

1. SVC is simple in operation.
2. It improves the steady state and transient stability
3. It has higher voltage capacity
4. It gives faster and reliable response.
5. It is less expensive.

Applications of Static VAR Compensator (SVC)

1. SVCs are employed in long transmission lines to increase the power transfer capability.
2. These VAR compensators are also employed at sub transmission and distribution system levels for balancing the three individual phases of the system supplying unbalanced loads.
3. Fluctuations in the supply voltage can be minimized by employing static var compensators.
4. Control of dynamic over voltage is also possible.

Difference between SVC and STATCOM

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Figure 1: Voltage Source Converter.

Symbolic representation of a voltage sourced convener is as shown in figure 1. The symbol has a box with a gate tum-off device paralleled by a reverse diode, and a D.C capacitor as its voltage source.

In a voltage source converter, devices undergo sequential switching to present unidirectional D.C voltage of a D.C capacitor, as A.C voltage to the A.C side. The A.C output voltage can be varied in magnitude and also in any phase relationship to the A.C system voltage, using suitable converter topology. The power reversal here involves only the reversals of current but not the voltage. In case the storage capacity of the D.C capacitor reduces and if the D.C capacitor has no other source of power, the converter can no more impart or consume the real power for more than a cycle. A converter imparts or absorbs the reactive power alone, as long as the A.C voltage and A.C current (leading or lagging) are at 90º phase angle, where the A.C current is taken as reference.

Basic Operating Principle of a Voltage Source Converter

Figure 2: Voltage Source Converter working principle.

The basic operating principle of a voltage source converter generating reactive power is comparable to that of a conventional rotating synchronous machine (see Figure 2). Single line diagram for a basic voltage source convener scheme for reactive power generation is as shown in figure. The convener gives a set of controllable three phase output voltage with system frequency by the charged capacitor C_S when a D.C input voltage is given. Each output voltage is in phase with and coupled to the respective A.C system voltage through a small tie reactance. The tie reactance is of the order 0.1 to 0.15 p.u and it is provided by the per phase leakage inductance of the coupling transformer. The reactive power exchange between the converter and the A.C system is controlled by changing the amplitude of output voltage produced. This means that when the output voltage is increased than that of the A.C system voltage, current flows via the tie reactance from the converter to the A.C system. Thus reactive (capacitive) power for the A.C system is generated by the convener. Now if the output voltage is decreased then the reactive current flows to the converter from the A.C system. Thus reactive (inductive) power is absorbed by the converter. In case if the output voltage and the A.C system voltage are equal in amplitude, the reactive power exchange is zero.

Circuit Diagram of a Voltage Source Converter

Figure 3: Circuit Diagram of a Voltage Source Converter

The circuit diagram of single-phase full wave bridge converter is shown in figure (3).

Working of a Voltage Source Converter

Voltage source converter generates A.C voltage from D.C voltage. A single phase full wave bridge converter consists of 4 valves, namely valve-1, valve-2, valve-3 and valve-4 and each valve consists of a turn-off device T, and diode D, connected in series with each other. On the D.C side, as the voltage is unipolar, it is supported by a capacitor. The capacitor is used to handle the current that accompanies the switching sequence of the converter valve and shifts in phase angle of the switching valves without change in D.C voltage and two ac connection points A and B.

Conversion of AC voltage to D.C voltage is possible by changing the turn-ON and turn-OFF sequence of valves. In one cycle the single phase full wave bridge operates in four different operating modes as given below,

1. T₁ and T₄ ON, T₂ and T₃ OFF (Inverter)
2. T₁ and T₄ ON, T₂ and T₃ OFF (Rectifier)
3. T₁ and T₄ OFF, T₂ and T₃ ON (Inverter)
4. T₁ and T₄ OFF, T₂ and T₃ ON (Rectifier)

When the turn-off devices T₁ and T₄ are turned ON, voltage becomes positive i.e., +V_d for one half cycle and with T₂ and T₃ turned ON, V_AB becomes negative. The interaction of the converter generated A.C voltage with the A.C system voltage and impedance results in A.C current, which is generally a sinusoidal wave form.

From the 1^st operating mode time t_a to t_b with the turn-off devices T₁ and T₄ ON and T₂ and T₃ off, V_AB is positive and current i_AB is negative. Here power flow is from D.C to A.C. Hence inversion action takes place. In 2^nd operating mode time t_b to t_c, the current i_ab is positive and flows through diodes D₁ and with power flow A.C to D.C. Hence, it acts as a rectifier. Similarly 3^rd and 4^th operating modes are conducted. The operations of all the modes are shown below in comparison table.

The output waveforms of single phase converter is shown in figure (4).

Figure 4.

Figure 5.

Figure 6.

Figure 4 gives the output waveforms of voltage and current as per the given table. Figure 5 is the voltage across the valve 1 i.e., V_T1-D1. It is also known as lost waveform. Figure 6 shows the power flow from A.C to D.C with a power factor lagging. It gives the relationship between A.C voltage and current phasors.

Disadvantages of a Voltage Source Converter

The following are the disadvantages of voltage-source converters,

1. The output current limitation is low and it cannot control the capability of the semi-conduction device system faults.
2. In voltage-source converter, it is very difficult to protect converter against internal faults.
3. They have high-short circuit current and the transformer connections are complex compared with CSC.
4. Rapid increase in rise of capacitor discharge current results in the damage of valves.

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