Transposition of conductors in Power Transmission Lines

Parameters of Transmission Line:

A transmission line has four parameters, namely resistance, inductance, capacitance and conductance. The resistance ‘R’ of a line is because of conductor resistance, series inductance ‘L’ is due to the magnetic field surrounding the conductors, shunt capacitance ‘C’ is due to the electric field between conductors, and shunt conductance, ‘G’ is because of the leakage current between phases and ground.

What is Transposition of Conductors?

The interchange of conductor positions of a transmission line at regular intervals along the route is known as Transposition of Conductors.

Why transposition is needed?

In the power transmission line when the line conductors are asymmetrically spaced i.e. not equally spaced, the inductance of each phase is different causing voltage drops of different magnitudes in the three phases even if the system is operating under balanced condition (load currents are balanced in the three phases). Also the magnetic field external to the conductors is not zero thereby inducing voltages in adjacent communication lines and causing what is known as “telecommunication interference”. This can be overcome by the interchange of conductor positions at regular intervals along the route and this practice is known as “transposition of conductors”.

How transposition is done?

In a transposed transmission line each of the three conductors occupies all the three positions relative to other conductors (position 1, position 2, and position 3) for one-third of the total length of the transmission line. Transposition also balances out the line capacitance so that electro-statically induced voltages are also balanced. Figure shows the transposition of conductors over a complete cycle.

A complete cycle of transposition of line conductors.

Complications of Conductor Transposition:

Frequent transposition usually leads to complication of support structures (as can be seen by the picture below), increase the cost because of increased number of insulator strings and total weight of supports. 

Transposition on 400 kV, double circuit transmission line, near Bhopal, M.P.  

MATLAB coding for Y Bus partition

Last Updated: Feb 26, 2017

Voltage Stability:

A power system is said to be voltage stable if it is able to maintain steady voltages at all its buses after a disturbance. In other words, one can say that voltage stability is the ability to maintain steady voltages at all the buses in the power system after being exposed to a disturbance. The disturbances may be:

1. Line or Generator outages,
2. Increase in loading,
3. Generators, synchronous condensers and other reactive power sources inching close to their reactive power limits.

A power system is voltage stable if the magnitude of  voltage at a bus increases as the reactive power injection (at the same bus) is increased. At a given operating condition, this is true for every bus in the system.

When the reactive power demand of the load is not fulfilled, voltage collapse occurs. Voltage stability of a power system is on the verge of collapse when a disturbance increases the reactive power demand beyond the available capacity of the system components. Voltage collapse is a usual phenomenon in a heavily loaded power system or a system having shortage of reactive power. The voltage drop in the line impedance during power flow is the main cause of voltage instability. This reduces the power transfer ability of the transmission system and also reduces voltage support ability.

A system is "voltage unstable" if the magnitude of voltage at one or more bus decreases when the reactive power injection at the very bus or buses is increased.

Thus, for a power system, if the V-Q sensitivity is positive for every bus, the system is voltage stable, otherwise for a negative V-Q sensitivity, the system is voltage unstable.

Voltage Stability Index:

Voltage stability analysis of a power system involves determination of an index called the “voltage stability index” which is used as a measure of inclination of Power system towards voltage collapse. These indices are helpful in determining the weak bus so that adequate reactive power allocation can be done.

Methods of determining the Voltage Stability Index:

There are few methods of determining the voltage stability index and “L-index method” is one such method. 

In L-index method, one has to partition the Y bus matrix as Y_GG, Y_GL, Y_LG, and Y_LL, where ‘G’ stands for generator and ‘L’ stands for Load. Matrix Y_LG, and Y_LL are required to calculate the matrix FLG needed for calculation of L-index. Detailed theory can be seen in many research papers.

MATLAB coding for Y bus partition:

The MATLAB coding for Y bus partition is as given below:

 clear; clc;

% File gives the partition of Y bus.

num=6;   % specify the bus system if you to work with many examples.

%  a function file “volt_ang” gives the admittance, magnitude and angle of bus voltage.

%  This file is a part of the NR load flow code  and not given here.

[Y, Vm, Va]= volt_ang(num)

linedt= line_data(num);                     % calling the line data for the system

busdt= bus_data(num);                     % calling the bus data for the system

nb= max(busdt(:,1)) ;                        % gives the total number of buses in the system

type =busdt(:,2) ;                    % identify the type of bus i.e. ref., generator, and load      

pv = find(type==2 l type==1);           % identify the PV bus           

npv = length(pv);                               % gives the number of PV buses
pq = find(type==3);                          % identify the PQ bus 
npq = length(pq);                               % gives the number of PQ buses

for m=1:npq,
for n= 1:npq,
Y_LL (m,n) = Y (pq(m), pq(n));
end
end

for m=1:npq,
for n = 1:npv,
Y_LG(m,n)= Y(pq(m), pv(n));
end
end

FLG = (Y_LL)^-1*Y_LG

Special arrangements for transportation of Large Power Transformer

Large Power Transformers (LPT) are large in dimension, and heavy in weight. They can cost millions of dollars and weigh between 100 to 400 tons. For example a 765 kV, 750 MVA, three phase transformer with size 56 ft (W) x 40 ft (L) x 45 ft (H) can weigh 410 tons  They pose unique requirements to ensure safe and efficient transportation. Hence the weight and dimension of large power transformers need careful planning and the critical transportation aspect should be kept in mind.  

Power transformers can be transported by rail, road, air and sea route. Depending on the size of the transformer unit and on the route and transport conditions, a transformer may be transported completely or partially assembled. LPTs have to be transported with bushings, conservator, cooling arrangements and all other minor accessories removed. If the transformer tank has been drained for transportation, it is necessary that the oil should be replaced by dry air or nitrogen maintained at a slightly positive pressure above the atmosphere. This ensures the dryness of the winding during the entire transportation.

Large power transformers cannot be transported on normal rail cars. The heaviest load a rail-road normally carries is 100 tons whereas the LPT can be 4 times of that weight. A specialized rail-road car called Schnabel car, is used to transport extremely heavy loads. Some LPT are designed and made as an integral part of the Schnabel car. The transformer is designed so that it can be attached to rail car frames with the help of a pinning system. These cars may have 20 or more axles depending on the weight of the transformer to be transported.     

When LPTs are to be transported via road, special permits are also required from various government agencies. Before issuing these special permits, careful inspection of the entire route through which the transformer has to pass, is carried out. Inspection of bridges and their load bearing capacity are of prime importance while issuing such permissions. Hasty permits may lead to serious accidents as one happened in Madhya Pradesh in year 2011, in which a huge trailer carrying a 380 ton power plant equipment on Sagar-Bhopal road was washed away when the bridge through which the consignment was passing collapsed due to the heavy weight killing at least 3 persons and damaging the power plant equipment. 

    

Use of Vegetable oil as dielectric medium in Power Transformers

Electric transformers while in operation produce heat due to iron and copper losses (although stray losses are also there). The heat thus produced must be carried away swiftly to avoid excessive temperature rise in various parts of the transformer such as winding and insulation. The cooling medium used must prevent excessive rise in temperature in any portion of the transformer and should avoid formation of “hot spots” within the transformer.

Mineral oil is normally used as an insulating and cooling medium in power transformers. The oil covers the core and coil assembly completely and fills small voids in the insulation to enhance the transformer performance. 

Advantages of Vegetable Oil:

Over the years mineral or silicone oil has been used as insulating and cooling medium in the transformers. Vegetable oil such as rapeseed, sunflower oils etc. are bio-degradable and have a much higher flashover point, are environment friendly and less inflammable. Vegetable oil has higher flash and fire point when compared to mineral oil. Similarly the dielectric strength is also higher. 

Properties of  Vegetable Oil-based Envirotemp Insulating fluid:

The flash point and dielectric strength of “Envirotemp FR3” vegetable oil-based insulating fluid is 330^oC and 56 kV at 25 ^oC whereas for mineral oil the values are 147 ^oC and 45 kV respectively.  Transformers using vegetable oil will require lesser fire safety systems. Since transformers with vegetable oil are better in terms of fire hazard protection, hence can be used in environment sensitive and densely populated areas.

First EHV class Power Transformer with Vegetable oil:

Siemens has successfully produced and commissioned, in 2014, the world’s first EHV class power transformer that uses vegetable oil as the dielectric medium. The transformer, which is a 380/110 kV power transformer, uses nearly 100 tons of vegetable oil and is commissioned in Bruchsal-Kandelweg substation in Germany. 

Although using vegetable oil in power transformers is not new. Siemens have produced and commissioned more than 30 transformers that use vegetable oil as a dielectric medium up to 69 kV class transformers with individual capacity of 30 MVA.   

Energy conservation in Residential and Commercial buildings

"Residential and Commercial buildings account for a significant portion of the total energy consumption in India." 

These buildings use electricity and other energy sources such as natural gas etc. Electricity is used for lighting and operating other useful appliances. The potential for energy conservation in buildings remains large despite of the improvements in energy efficiency and house keeping. For any building, the envelope that is walls, roofs, floors, windows and doors has a significant impact on the energy consumption.

Recommended Energy Conservation Measures:
The commonly recommended energy conservation measures are:

• The energy use of a building is dominated by weather, especially in extreme weather conditions. Heat gain and loss from direct conduction of heat or from air passage are significant. Addition of thermal insulation can be cost effective. CO₂ based ventilation controller can be implemented in various commercial buildings including cinemas, classrooms, retail stores and establishments to reduce the energy requirement. Recently several materials such as selective glasses or chromogenic glazing have been used to improve energy efficiency of buildings.
•  Simple and inexpensive measures to improve the efficiency of a lighting system include the use of energy efficient lamps and ballasts, reflective devices, de-lamping and maximum use of day light. Sensors and controllers can be used to reduce electrical lighting consumption. Energy efficient equipments of standard make should be used to comply with the efficiency standards and to save money.
• The energy use due to Heating Ventilation and Air-conditioning (HVAC) may be significant particularly in the light of increasing living standards.  30 to 40% of the total energy consumption in any commercial building is because of HVAC. Measures to improve energy performance include appropriate setting of thermostat, retrofitting of central heating and cooling plants, installation of heat recovery system etc.
• An automated energy management and control system (EMCS) can control the building energy use by continuously monitoring the energy consumption of various equipments and making necessary adjustments.   
• Regular energy audits should be conducted to identify opportunities of energy saving. The short term and cost effective measures should be implemented immediately. 

Optimize the electrical loads for a successful solar PV system

As per a recent survey, India is second among the countries in terms of growing electrical demand. To meet this ever increasing electrical demand in an economically feasible and environmentally friendly way, Renewable Energy and Energy Conservation are the best options. In this regard, the Indian government is planning at a fast pace and the result is schemes and programs like Jawahar Lal Nehru National Solar Mission (JNNSM) and many more. Many states in the country have implemented solar schemes such as roof top net metering arrangements and so on.

The current scenario in India is that the roof top solar PV system along with major solar PV plants is coming up at a rapid pace. A good population is keen to know and eager to install a roof top solar PV module. Here are some of the vital basics needed as a priori to install a roof top solar PV system for the residential sector in India.

What should be the ideal capacity of my solar PV system? (a FAQ)

I hope, the following paragraphs will answer most of your queries.  

   

First and foremost thing is to know the key elements of any solar PV system. Any solar PV system, I am talking of stand-alone solar PV system, consists of the below given equipments.

1.  Solar PV module,

2.  Charge controller,

3.  Battery,

4.  Inverter,

5. Miscellaneous items such as supporting frame, wires,   switches, change-over etc.

Of the above listed electrical equipments and elements each has its own rated capacity, and to get a proper matching between them is very important. Prior to working out the optimum capacity of the solar PV system, one has to determine the right electrical loads which can be connected to make this solar PV system viable with a low pay back period. The steps in planning for a solar PV system are:

Step 1: Determination of proper electrical load

Make a list of electrical appliances that you are planning to run from the solar PV system. Prior to this one has to find out how much power each piece of equipment draws for its operation. The electrical power needed by the equipment is given on its name plate from where one can note down and prepare a list. The electrical wattage of some of the commonly used appliances is:

Sr no	Equipment	Electrical load (in Watts)
1	Microwave oven	1200- 2000 W
2	Electric Geyser	2000-3000 W
3	Washing Machine	300-500 W
4	Electric Iron	600-800 W
5	Water pump (domestic, 1-phase)	375-750 W
6	Ceiling fan	80 W
7	PC	100-150 W
8	Color TV	150 W
9	Tubelight (including choke)	45 W
10	Freezer	150-250 W
11	Room Cooler	150-250 W
12	AC	1000-2000 W

The electrical load given above is for reference only. Actual rating may be obtained from the equipment name plate or manufacturers specification only, as the rating or electrical load of any equipment may vary according to its capacity, features, etc.

Step 2: Optimize your electrical load

As per the given list one can very well judge that appliances such as microwave ovens, geysers, AC, hotplates etc are equipments capable of drawing heavy current and hence power. To run these equipments or appliances on solar you have to go for a much higher values of installed solar capacity, which is going to increase the capital cost and the pay-back period (period necessary to get back the investment). So it is advised not to connect or run these equipments on solar PV system.

(If you an inverter technology based refrigerator or AC, then the possibility of running them on solar PV system is there.)

 In fact equipments such as Tubelight, CFL, fan, room cooler, TV, laptop etc, and in some emergency situations, electric iron, should be connected and run on solar PV system.

To separate the equipments requiring heavy power, moderate and low power one has to re-wire the distribution system or separate the circuits from the distribution mains (MCB). One can use a change-over switch also.

Step 3: Battery size

Appropriate battery size is the key element in making your PV system a success. The life of a PV module is around 25 years as claimed by the manufacturer, whereas the expected battery life is 3 to 5 years. Also the battery cost is significant. As one goes for a higher Ampere-hour (Ah; the rating of battery is given in Ah) battery, its cost increases. 

For example; recently I have installed a 400 W solar PV system costing around 52 thousand INR in Bhopal, India, the battery of 150 Ah capacity, with 5 years warranty, alone was of 14 thousand INR.     

So the summary is:

Battery is a vital element of your solar PV system whose life is less as compared to other equipments of the system and is comparatively costlier. The equipments which you have selected to get connected to the solar PV system, if works mostly during day time, is the optimum load as per the solar PV system design. This in turn will permit you to have a battery with lower Ah capacity which in turn will reduce the overall system cost.

If your load is ‘switched off’ mostly during the day time, then you have to keep a large battery to store the whole energy produced by the PV module during the day. Thus, to optimize your electrical requirements you have to look into your usage pattern and the criticality of your application.

For a ready reference, the back-up time for a particular capacity battery is given below

Load	Inverter capacity	Battery capacity/ Back-up time
		100 Ah		150 Ah		200 Ah
		Full Load	Half Load	Full Load	Half Load	Full Load	Half Load
2 TL + 2 F+ 1 PC+ 3 CFL	650 VA	1 hr.	2 Hr, 40 m	1 Hr, 50 m	4 Hr, 20 m	2 Hr, 40 m	6 Hr, 10 m
2 TL + 4 F+ 1 PC+ 3 CFL	850 VA	40 m	1 Hr, 50 m	1 Hr, 10 m	3 Hr	1 Hr, 50 m	4 hr, 20 m

TL and F stands for Tube-light and fan respectively.     

The charge controller is an equipment that controls the charging of the battery and thus helps in improving the battery health and life. The capacity of charge controller is in ampere.  The charging current ( CC ) of a battery is given in manufacturer’s specification, but for ready reference the CC  of a 100 Ah battery is 10 A,  150 Ah battery is 15 A and so on. So for a 150 Ah battery a charge controller of 20 or 30 A is sufficient.

Step 4: Inverter size

Inverter is the equipment which converts the DC voltage of the battery into AC 230 V, so that your normal AC appliances can be connected to the solar PV system. 

Selecting a proper inverter size is very important. Use only the inverter which provides a pure sine wave otherwise your equipments are going to suffer. The output capacity of inverter is given in Volt-Ampere (VA), whereas the appliances are rated in watts. We know that VA multiplied by power factor is watts. So you have to know the power factor of commonly used electrical gadgets for a precise calculation. For simplicity you can assume that the power factor is 0.8, which is the value for most commonly used inductive household equipments.

So a 850 VA inverter is of 850 x 0.8 = 680 W only. So the total electrical loads which can be connected to a 850 VA inverter can be about 600 W. This does not mean that for better utilization of resources one has to keep an inverter of higher capacity. The answer is:

The efficiency of an inverter is about 80% to 90% i.e. 10% to 20% of the energy given to an inverter is consumed by the inverter itself. So higher the inverter capacity higher the losses. Also a higher capacity inverter is useless unless the battery is also appropriately sized.

For a better understanding, have a look into my roof top solar PV system (photo below).

I have installed a 400 W solar PV module (4 panels of 100 W each, make Topsun) along with a 40 A charge controller. The battery used is of Luminous make, 150 Ah, 5 year warranty and the inverter is of 850 VA sine wave of Su-Kam make. 

The whole system is working quite satisfactorily since May 2015. On this system I have used a 150 W room cooler and one 80 W fan the whole day during the summer with occasional load of a 150 W  color TV. I have tried to operate a 375 W water pump and a 600 W automatic iron also. Both the equipments worked nicely, but one at a time. The motor during starting draws a higher current (which is natural for motors), thanks to the in-built feature of the inverter which permits a 300% plus over-current for a few ms to cater such loads. 

Recently I have connected the entire Light & Fan load of 2 bedrooms and a hall. The total connected load on the solar inverter system is now 850 W (4 x 40 tubelights, 4 x 80 W ceiling fan, 1 x 150 TV, 2 x 150 room cooler) but the maximum load at a time is restricted to 400 W.             

SF6 CIRCUIT BREAKERS FOR MODERN POWER SYSTEM PROTECTION

Electrical faults give rise to abnormal operating conditions and can damage or disrupt the power system in many ways. It is necessary that the faulty section should be immediately disconnected so that the normal operation of the rest of the system is maintained. The protective relay should immediately detect the fault and initiate the operation of circuit breaker or breakers.

What is Circuit Breaker?

A circuit breaker is an automatic switch designed to protect an electrical circuit from damage caused by heavy currents, mainly caused due to overloading, short circuit, or earth fault. The basic function of a circuit breaker is to interrupt the current flow and isolate the faulty section from the rest of the network. Circuit breakers are made in different sizes ranging from the low voltage and low current circuit breaker used in low voltage residential, commercial and industrial premises to very high voltage and high current breakers used in electrical power stations and generating stations.

When a heavy current is interrupted, an electric arc is initiated. This arc must be quenched in a safe and reliable manner so that the breaker contacts are safe and the gap between the contacts can regain the dielectric strength in the desired time span. Different circuit breakers use different arc quenching medium and are accordingly classified as oil circuit breakers, air blast circuit breakers, vacuum circuit breakers and SF₆ circuit breakers.    

Properties of Sulphur hexafluoride (SF₆) gas:

Sulphur hexafluoride (SF₆) is a chemically stable, odourless, inert, non-inflammable and non-toxic gas. This gas has a high dielectric strength and outstanding arc quenching properties. At atmospheric pressure, the dielectric strength of SF₆ gas is about 2.5 times of air and may increase up to 5 times. 

SF₆ and its decomposition products are electro-negative. This property permits electron capture at relatively higher temperature. The ability of an atom to attract and hold electrons is called “electro-negativity”. Thus, the dielectric strength rises rapidly which enables the breaker to withstand the recovery voltage even under extreme switching conditions.

Construction of SF₆ circuit breaker:

Double pressure breaker is the early design of SF₆ circuit breaker and its operating principle is the same as that of air blast circuit breaker. Because of its complicated construction and the need for various auxiliary equipments such as compressors, control device etc., this type of SF₆ circuit breaker has become obsolete.

The puffer type or single pressure type SF₆ circuit breaker is the most popular and is available in the voltage range 3.6 kV to 765 kV. In such SF₆ breakers, the SF₆ gas is compressed by a moving cylinder and is released through a nozzle to quench the arc. Figure shows the working principle of single pressure type SF₆ circuit breaker. 

The operating mechanism, may be pneumatic or hydraulic, and is installed on the base. This operating mechanism is connected to the movable contacts located in the interrupter with the help of insulated rod of fibre glass. The interrupter and support insulator are filled with SF₆ gas at a pressure of about 5 kg/cm².

Figure shows the interrupter of a puffer type SF₆ circuit breaker in fully closed and a position in which contacts are separating. The moving cylinder or puffer cylinder and the moving contacts are coupled together. As a result, when the contacts are separated, the trapped SF₆ gas is compressed. This compressed gas is released axially through a nozzle. The gas removes the heat of the arc by axial convection and radial dissipation. The arc diameter reduces with the decrease of current and becomes very small during current zero and thus the arc is extinguished. Due to the electro-negativity and low arc time constant of SF₆ gas, it rapidly regains the dielectric strength after final current zero.     

Application of SF₆ circuit breaker

SF₆ circuit breaker has the ability to interrupt high fault currents, magnetizing and capacitive currents without too much over-voltages. Thus, it can perform duties like clearing line faults, and switching of capacitors, transformers and reactors. Because of the various advantages mentioned above, SF₆ circuit breakers are preferred for voltages above 132 kV.