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Surge Protection Device for Solar PV installation

Effects of Lightning Strike on Solar PV system: A solar PV installation is vulnerable to the effects of lightning stroke. The potenti...

Friday, 28 November 2014

Jamgodrani Wind Farms still with 98% machine availability

India is currently ranked at number five in Wind power production. The total installed wind power capacity at the end of January 2014 was 21,264 MW. At the top of the table is China, followed by US, Germany and Spain.
Although the state of Madhya Pradesh has not done much in the field of wind power production, but it has some successful wind farm projects to its credit. The wind power project (wind farm) at Jamgodrani Hills, near Dewas on Bhopal-Indore highway is one of the prominent wind power projects in Madhya Pradesh.
This 13 MW project was commissioned during 1995-1999 by MP Windfarms Ltd (MPWL) which is a joint sector company. The parties having stake in MP Windfarms Ltd are Consolidated Energy Consultants Ltd. (CECL), MP Urja Vikas Nigam (MPUVN) and Indian Renewable Energy Development Agency (IREDA). The project has total 58 numbers of Wind Electric Generators (WEG), each of 225 kW capacity. These wind turbines are in operation since 15-19 years and still performing nicely with 98% machine availability. The yearly operation and maintenance cost is only about 3% of the total project cost which is quiet remarkable considering that the wind farm is near the end of its life span. The life span of the project is assessed as 20 years.
These large numbers of turbines are connected to a metering point and the whole electrical energy produced is fed to the grid of the Western region Distribution Company of M.P. Figure shows the Wind turbines of Jamgodrani Wind farm, Dewas and was taken in 2010.

Jamgodrani Wind Farm, near Dewas on Bhopal-Indore highway

MP Windfarms Ltd is providing the engineering services and project coordination required for a wind farm project. It is actively engaged in the design and commissioning of wind farm projects which include preparation of detailed project report, detailed construction drawing, execution of civil and electrical works, supervision of erection and commissioning etc. They are also involved in operation and maintenance business of wind turbines.
Similarly, CECL which has 51% equity in MP Windfarms Ltd has handled many wind farm related assignment, ranging from conceptualization to commissioning of wind power projects, both in the country and abroad.  

Ref: www.cecl.in

Wednesday, 26 November 2014

Matlab Coding for the demonstration of Ferranti Effect in Transmission lines

"When a long transmission line is without electrical load or very lightly loaded, the voltage at the receiving end (Vr) is greater than the voltage at the sending end (Vs). This effect seen in transmission lines is called Ferranti effect."

"Ferranti effect" is because of the substantial amount of charging current drawn by the distributed shunt capacitance of the transmission line. This charging current is greater than the current drawn by the load at the receiving end (in case of light loads). This over-voltage at the receiving end can be nicely demonstrated with the help of MATLAB. The "Ferranti effect" can be well shown in a Lab using tube-light chokes and fan capacitors.

The typical values of series inductance and shunt capacitance of a 400 kV transmission line are:

Inductance, L = 1.044 mH per km of line length, and
Shunt capacitance, C = 12 nF per km of line length.   
Assuming a system frequency of 50 Hz and a line length of 800 km, the coding is as given below.

>> C = 12*10^-9;
% Length of the line (len) is taken as 800 km with an equal spacing of 25 km.
>> len = 0:25:800;
% Total capacitance of the 800 km long line,
>> C_tot = len*C;
% Series inductance, L = 1.044 mH per km of line length
>> L = 1.044*10^-3;  
>> L_tot = len*L;
>> f = 50;
% For a sending end voltage of 400 kV,
>> Vs = 400;
% phase shift,
>> beta = len.*(2*pi*f*sqrt(L*C));
% Characteristic Impedance is Zc,
>> Zc = sqrt(L/C);
% Sending end Voltage is Vr,
>> Vr = Vs./cos(beta);
>> plot(len, Vr);
>> axis([0 800 0 700]);
>> xlabel(‘line length in km’);
>> ylabel(‘Receiving end Voltage in kV’);

>> grid;

As can be seen from the MATLAB results and plot, the receiving end voltage of a 400 kV line of length 800 km at no-load may reach 635 kV which is quite harmful. 

This dangerously high over-voltage at the receiving end can be controlled by using shunt reactor. The lagging reactive current drawn by the shunt reactor compensates for the charging current of the line and hence the increase in voltage at the receiving end is controlled.

Sunday, 23 November 2014

Indian Power System Marching ahead with One Synchronized National Grid

Nearly 70 years ago, the power system in India consisted of small isolated generating plants catering the local electrical needs. The post independence era witnessed a significant growth in the power sector. To enhance the reliability of power supply and for achieving economical operation, interconnection of individual systems was planned which led to the formation of state electricity grid in 1950s.
By the sixties, the management of power grid started on regional basis. The state owned power grids were interconnected to form regional grid. With the goal to rapidly develop India at the power sector front, the country was divided into 5 power regions viz. Northern, Western, Southern, Eastern and the North-Eastern power region. Also by mid 60s, Regional Electricity Boards came into existence in the above mentioned five power regions. The move has facilitated interconnected operation of the power system within the regions. The basic role of these regional electricity grids was planning and operation of electric power system within their region.
Now the job was to integrate these regional grids. The basic theme was the formation of a synchronously connected national grid. Things started in 1990 with the asynchronous interconnection between regional power grids made with the help of back-to-back High Voltage Direct Current (HVDC) links such as the Vindhyachal link. Later on other HVDC links also came up such as the Chandrapur and Vishakapatnam-Gazuwaka HVDC links. These HVDC links had limited capacity for exchange of power. 
The first move towards achieving the “one nation, one grid, one frequency” status, was the interconnection of North-Eastern and the Eastern grid in 1991. Next was the interconnection of Western grid with the Eastern and North-Eastern grid in 2003. Similarly in 2006 the Northern grid was interconnected with the above three grids. The four regional grids i.e. Eastern, North-Eastern, Western and Northern collectively formed the Central Grid operating synchronously at one frequency. Initially these inter-regional transmission links carried the operational surplus energy from the energy sufficient region to the energy deficient region. Later on the inter-regional transmission links were planned considering the generation plants having beneficiaries spread over the nation.
On December 2013, months ahead of the schedule, the Southern grid too was synchronously connected to the rest or the Central grid through the 765 kV Raichur-Solapur transmission network. With this the mission “one nation, one grid and one frequency” was accomplished.
Interconnected power system and formation of one national grid will reduce the investments in generation reserves, and helps to utilize the benefits of generation mixes and load pattern to a greater extent. This will also facilitate the smooth and efficient operation of the Indian electricity market. For example, till now there was a large inconsistency in the short-term electricity prices in the southern region and the other region because of the inadequate transmission capacity. 

Now all the regional grids were synchronized but still the power transfer capability of the national grid was low considering the giant size of the Indian power system. At the end of the 11th Five Year Plan (FYP) the total inter-regional power transfer capacity through inter-regional transmission link was about 28 GW. This inter-regional capacity is expected to reach 65 GW by the end of 2017. So we have completed the “one nation, one grid, one frequency” challenge, but to make this unified grid of significant size having a matching transfer capability and its efficient operation is a bigger one.

Thursday, 20 November 2014

Growth of Transmission System in India

At the time of Independence, the power system in India consisted of small isolated power generating plants catering the electrical needs of major cities and towns. The total installed capacity at that time was merely 1300 MW and the highest transmission voltage was 132 kV AC. The post independence era witnessed an appreciable growth in the power sector.
With the goal to rapidly develop India at the power sector front, the country was divided into 5 power regions viz. Northern, Western, Southern, Eastern and the North-Eastern power region. Also by mid 60s, Regional Electricity Boards came into existence in the above mentioned five power regions. The move has facilitated interconnected operation of the power system within the regions. Interconnected power system reduces the investments in generation reserves, and helps to utilize the benefits of generation mixes and load pattern to a greater extent. The transmission voltage had increased to 400 kV AC by the 70s. Significant transmission networks were developed by Uttar Pradesh, Maharashtra, Madhya Pradesh, Gujarat, Orissa, Andhra Pradesh and Karnataka as these states were having the bulk of the electrical load.
In the year 1975, to enhance the generation capacity, which till the time was carried out at the state level, Central sector generation utilities i.e. National Hydroelectric Power Corporation (NHPC) and National Thermal Power Corporation (NTPC) were formed. These corporations established generating power plants of large capacity, say of 1000 MW capacity, and developed the much needed transmission systems. To speed up the transmission infrastructure development, Power Grid Corporation of India (POWERGRID) was created in 1989.
Till the time all the 5 power regions were operated independently and that too at different operating frequency. Therefore in 1990 asynchronous interconnection between regional power grids were made with the help of back-to-back HVDC links. This was the introduction of HVDC system in the country. In the year 2007, India touched the 765 kV AC transmission voltage and by 2009 we had a couple of ± 500 kV bipolar HVDC lines. 
Fig: Under construction 765 kV lines in Madhya Pradesh

Shortly (by 2015) we are going to have the first multi-terminal UHV DC system. The ±800 kV, 1728 km long Biswanath- Agra UHV DC transmission system with a  8 GW converter capacity, including a 2 GW redundancy, will transmit hydroelectric power from the country’s northeast region to Agra in Uttar Pradesh.
In the next decade 1200 kV UHV AC system is expected to emerge as the main transmission level in India along with the 800 kV UHV DC system. The power transfer capacity of 1200 kV UHV AC transmission system is expected to be between 6000 to 8000 MW. To develop 1200 kV AC transmission system in India, a joint venture for a test sub station and test line by Power Grid Corp. of India and CPRI is under progress at Bina in Madhya Pradesh.

Watch out for development of national grid in the coming blog.

Monday, 10 November 2014

Optical Ground Wire for Transmission System Protection

An Optical Ground Wire (OPGW) or an Optical Fiber Composite Overhead Ground Wire, as known in the IEEE standards, is a type of cable mainly used by electric utilities to facilitate the function of grounding and communications in a transmission system. These cables are run at the top of a transmission line parallel to the power conductors. The conducting part of the cable shields the power conductors from direct lightning strokes whereas the inner fiber optics are used for high speed data transmission for the purpose of protection and control of the transmission system, communication etc. These cables look the same as an ACSR conductor normally used as ground wire and have the same dimension and weight. The OPGW are suitable for high loads and longer spans. From 1985 onwards OPGW has been extensively used as ground wires in transmission lines particularly in China.  
The OPGW contains one or more optical fibers surrounded by a layer of steel and aluminum wire. A typical cable may have 8 and 48 optical fibers placed in a plastic tube which is again inserted in a tube made up of stainless steel or aluminum. Buffer tubes filled with jelly are provided to protect the fibers and steel tubes from water and corrosion respectively. The number of fibers may go up to 144 fibers in a cable. Several other types of cable construction are also available in the market which may employ aluminum rods with spiral grooves for fibers. These cables are also custom made to suit the requirement of customers. figure shows a optical fiber ground wire.
Optical Fiber Ground Wire

Optical fiber ground wire is seen as an alternative to power line carrier system used for data transfer and communications. Optical fiber cables when used as ground wire on transmission lines has lesser installation cost as compared to buried optical fiber cables. As they are provided on towers hence are unlikely to get damaged by excavation and other repair works. Since the optical fiber is an insulator it prevents the induction effect of power line and the lightning stroke, external noise and cross talks. Vibration dampers are also provided on OPGW cables to reduce Aeolian vibrations.

Optical fiber ground wires have been so successful that power utilities are replacing the existing steel or ACSR ground wires of their transmission system. In India, Sterlite Industries and several other manufacturers have the technical expertise to manufacture optical fiber composite ground wire and its related hardware. These OPGW technologies have been implemented in many Indian transmission projects commissioned by Adani Power, UPPTCL, Areva & Vedanta Aluminum, etc.

Saturday, 8 November 2014

Carrier-current protection schemes for Long Transmission lines

Different protective schemes and the choice
The overhead transmission and distribution lines are more prone to faults because of their length and exposure to atmospheric conditions. There are several protective schemes for the protection of these lines and feeders viz. over-current protection, distance protection and pilot protection. The choice of a particular protection scheme depends upon the cost of the scheme, type of the feeder, length of the feeder, method of operations etc.
Pilot relaying protection
Pilot relaying protection is a form of unit protection used for the protection of transmission line sections. In these protection schemes some electrical quantities such as the phase angle of current, direction of power flow etc. at the two ends of the transmission lines are compared. Some form of interconnecting channel, called pilot, is required to transmit information from one end to the other. The three different types of interconnecting channel or pilots used are wire pilot, carrier-current pilot, and microwave pilot.
Carrier-current protection
For long overhead lines the power line itself may be used as the interconnecting channel between the terminal equipments. Carrier-current protection is the most widely used scheme for the protection of Extra High Voltage (EHV) and Ultra High Voltage (UHV) power lines. The carrier signal is directly coupled to the power line itself which is to be protected. Carrier-current protection is faster and superior to distance protection schemes and is more reliable when used for long transmission lines, although the terminal equipments are more expensive and complicated. In addition to protection the carrier signals can also be used for communication, supervisory control and telemetering.
In carrier-current protection or any other unit protection, the circuit breakers at both the ends of the line trip simultaneously when a fault occurs at one of the ends of the protected line sections. This helps in improving the stability. The carrier signals can be used either to initiate or to prevent the tripping of a protective relay according to which they are classified. When a carrier signal is used to initiate tripping of relay, the scheme is known as carrier inter-tripping, or transfer tripping or permissive tripping scheme. The scheme is known as carrier-blocking scheme when the carrier signals are used to prevent the operation of a relay.
Different operating techniques used in carrier-current protection
The two operating techniques mainly used in carrier-current protection are:
1.      Phase comparison technique, and
2.      Directional comparison technique.
In phase comparison technique, the phase angle of the current entering at one end is compared with the phase angle of the current leaving the other end of the protected section. During normal operating conditions or in case of an external fault, the currents at both the ends of the protected line are in phase. In case of an internal fault i.e. fault in the protected section, the currents at the two ends will be 180o out of phase.
The direction of power flow at the two ends of the protected sections is compared in the directional comparison technique. During normal conditions or external faults, the power flows into the protected section at one end and leaves at the other end. During internal faults, the direction of power flow is inwards at both the ends.

The signals generated in a carrier-current protection scheme are at a frequency between 50 and 500 kHz. Below 50 kHz the size and cost of the coupling equipments would be too high and above 500 kHz the line losses and therefore the signal attenuation would be too high on long lines. Carrier-current protection can be used only on overhead lines and cannot be used for underground cables as the capacitance of a cable would attenuate the carrier signals appreciably.   

Tuesday, 4 November 2014

Shielding method Protection of Transmission Lines against Lightning

Last updated: January 20, 2017

Surges due to lightning are mostly injected into the power system through the long transmission lines. Substation apparatus is always well protected against direct lightning strokes. The commonly adopted and effective method of protecting transmission lines against direct strokes is by the use of overhead ground wires. This method of protection of transmission lines is known as shielding method which does not allow an arc path to form between the line conductor and the ground.

Ground wire is a conductor run parallel to the main conductors of the line. It is supported on the same towers, is placed higher than the main conductors and is adequately grounded at every tower. For horizontal arrangement of conductors, there are two ground wires to provide effective shielding to power conductors from direct lightning strokes whereas in vertical configuration of conductors there is only one ground wire.

Fig 1: A 765 kV transmission line with ground wire

The ground wire is made up of galvanized steel or ACSR conductors. Modern Extra High Voltage (EHV) transmission lines have ground wires of ACSR conductors of the same size as the power conductors. In case of a direct lightning stroke, the ground wire intercepts the stroke and by providing multiple paths for conducting the stroke to the ground they reduce the induced voltage. It also helps to increase the effective capacitance between the conductor and the ground which in turn reduces the voltage induced on the conductors from nearby strokes.

The ratio of the induced voltage on a conductor of a line provided with ground wire protection to the induced voltage which would exist on the conductor in the absence of ground wire is known as protective ratio. Each ground wire has a protective angle which is defined as the-
" Angle between the vertical line passing through the ground wire and the line passing through the outermost power conductor is called the protective angle."
 The protective angle is in the region of 20o to 45o.

The voltage to which a transmission line tower is raised when a lightning strikes the tower is independent of the operating voltage of the system. 

Basic design requirements for protection against direct lightning strokes:

The basic requirements for the design of a line to safeguard it against direct lightning strokes are-

1.      The ground wire used should be mechanically strong and should be so located that they provide sufficient shielding.
2.      There should be sufficient clearance between the power conductors and the tower structure.
3.      There should be an adequate clearance between the line conductors and the ground wires, particularly at the mid-span, so as to avoid flashover to the power conductor upto the protective voltage level used for the line design.
4.      The tower footing resistance should be as low as permissible.  

Sunday, 2 November 2014

Right of Way Requirement for Transmission System

Last Updated: January 20, 2017

A Right of Way (ROW) is an integral part of a transmission system (transmission lines, towers, sub-station etc.) that carry electricity. In other words, 
ROW is the strip of land immediately below and adjacent to a transmission line or tower. This is the strip of land used by Electrical Utilities to construct, operate and maintain the transmission line facilities."
 The obstacles like tall trees etc. have to be removed in the width of the ROW so as to prevent electric power outages.

As per the sub section 10 (b) of The Indian Telegraph Act, 1885, ROW is not purchased. The power utility in India only acquire the users right in the property under, over, along, across, in or upon which any transmission line or tower is placed. The ROW is used by the utility to construct, operate and maintain the transmission system. The owner of the land retains the ownership and use of land. The Act also states that the construction agency and the power utility will have to prevent un-necessary damage to the property during the construction as well as operation of the transmission system. They have to pay compensation for any damage to standing crops or fruit bearing trees as approved by the competent local revenue officers. In fact this is way leaves; means the right to use the property of another without possessing it.

The width of the ROW depends on the voltage of the transmission system and the height of the tower used. As per the rule, the ROW includes the area extending for a distance of 26 meter on each side from the centre of the tower for 400 kV transmission system and hence the ROW is 52 meter. The ROW requirements for transmission facilities of other voltages are:   

Transmission Voltage
Recommended Right of Way (ROW)
132 kV
27 m
220 kV
35 m
400 kV
52 m
800 kV
85 m
For technical and safety reasons, the following vertical clearances shall be maintained within the transmission line ROW.

Transmission Voltage
Minimum clearance between conductor and trees
132 kV
4.0 m
220 kV
4.6 m
400 kV
5.5 m