About Mahindra and Mahindra’s Farm Equipment Division-

Mahindra & Mahindra’s Farm Equipment division is the top-selling tractor company in the world, with annual sales over 150,000, a presence in more than 40 countries and more than 1,000 dealers worldwide.  They manufacture tractors at four state-of-the-art plants in India, two in China, three in the US, and one in Australia, grounding us in four major agricultural hubs to give us close on-the-ground understanding of what farmers need from their tractors.  Their strong R&D team of more than 600 engineers constantly push technology forward.

State Estimation:

It is a process where the state of the power system is estimated.All the telemetry values of power flow, voltage levels are considered.These values are used to calculate the phase angle of the voltage which helps in determination of the power flow between two nodes in the power system as the power flow is directly proportional to sine of that angle.

The power flow monitoring is important for the power system as any kind of overloading on the lines can lead to cascade tripping and can also lead to black out.

As a VNITian, you guys might have observed that VNIT stands out from Nagpur city when it comes to power cut. Power cuts are rare and even when there is a power failure due to some fault we have supply for our mess and in exam time in our hostel even. Wouldn’t it be interesting to know how the electrical distribution and substations our college are configured meets the demand of uninterrupted supply? Let’s find out!

SAIL AT A GLANCE

Steel Authority of India Limited is India’s largest steel producing company and ranks 24th in the World. It was founded in 19th January 1954 with headquarter at New Delhi. It was incorporated to central public sector on 24 January 1973.  With an annual turnover of Rs. 49350 crore, the company is among the seven Maharatnas of the country’s Central Public Sector Enterprises.

NTPC Introduction

National Thermal Power Corporation is one of Navratna of India. It is a coal based power plant. NTPC lights 1 out of every 3 bulbs in India. NTPC Mouda was setup in year 2009. Its 1st stage comprising of 2 units of 500MW was commissioned on March 2013. The 2nd stage comprising of 2 units of 660MW is under erection (scheduled to be commissioned in 2016). This 2320MW Power Plant has the parts and models provided by BHEL (Bharat Heavy Electronics Limited). Two necessary raw materials that a power plant needs for its operation are coal and water. The coal is grabbed from Mahanadi Coal Field or sometimes from the coal belt region comprising of Jharkhand, Chhattisgarh, and Orissa etc. while Goshikhurd Dam (Bhandara District) serves as the source of water.
 

India is a very large country with 173GW generating capacity. The most important point to ponder on is how such huge amount of energy is transferred to such a huge number and variety of customers. Surely the network needs to be too much efficient and rules and regulation laid down concrete.
The power is generated at 21kV in the power plants. These are stepped Up to 400kV and fed to the National Grid (transmission line). Substations of 400kV and 200kV are connected to the appropriate transmission lines and step down the voltage to a value required by the various customers. Also we have 765kV HV lines to which they can be stepped up to.

As an electrical engineer an important point to note is: The stability of the power system decreases with decrease in size. To explain it a bit further let me define a term Power Number as the Power output per unit frequency. A 500MW plant producing at 50Hz has a power number of 10MW/Hz. Consider an isolated power grid or system with 10 generating plants and capacity 1GW at 50 Hz. In a generating station of 200MW trips, a fall of 10 Hz would be observed. This number will go on getting small as we increase the total capacity ie. As we go on increasing the size of power system the stability will improve. Thus for a large country like India producing at more than 100GW, tripping of a 1000MW unit would produce a minute change in of .5Hz would be observed.

The need for widening the power system was recognized in way back in 1950s and by 1990s we were able to prepare a national grid synchronized at a common frequency and various regions combined together through hvdc and ac links. This has helped to increase the stability of our system to a very great extent.

Our power grid is divided into 5 regions:

1.    Northern Power Grid
2.    Eastern Power Grid
3.    Western Power Grid
4.    Southern Power Grid
5.    North-Eastern Power Grid
 

Interconnection between various regions:

International Interconnection:

Now the important question is why do we require load dispatchers in this system?  

Consider a case in which a power plant trips, or some auxiliaries or components in the system needs to be repaired or replaces. If every such unit was free to take related to these issues on their own, there would be lot of instability in the system. Power cuts are very disastrous for industries, hospitals, and various emergency services. It also brings down the economy. Apart from maintaining the stability of the system, the load dispatchers act as the link between the generation side and the customer side. They ensure that all the rules regulations are being followed. They maintain the operating frequency within desired limits thus making our national grid more reliable.
Hierarchy followed among Load Dispatch Centre:

 

• National Load Dispatch Center
• Regional Load Dispatch Center
• State Load Dispatch Center

NLDC is the apex body in nation which looks after enhancing and optimizing national grid. It is responsible for keeping the grid synchronized and grid security. It has the authority to schedule and dispatch electricity over inter regional links. The RLDC’s have to comply with the orders of NLDC failing which they are heavily fined.

RLDC looks after the scheduling and dispatching of electricity to the states within its region. They have the authority to give directions to the SLDC’s. SLDC’s are heavily fined if they don’t comply with it without a proper reason. They keep an account of electricity transmitted between various states in their region.

SLDC is the apex body in a state. They keep account of electricity transmitted from licensees, the generating companies, substations and the consumers. It is responsible for optimum scheduling and dispatch of electricity within state.

Details of regular schedule of load dispatch centre:

Pre Dispatch function

Work in LDC start with a day ahead scheduling. Load forecasting is done based on the previous year’s data available for the same date. Also national holidays, festivals and other important events are taken into account. Load dispatchers collect the information from various generating stations about how much they will be providing the next day. Then a schedule is prepared with 96 time blocks of 15minutes each by 3PM. Schedule preparation takes into outage planning, electricity from central pool, cost of electricity from various generating companies and the forecasted demand. The schedule is sent to the generating companies at 10PM and given time for any revision up to 11PM and then freeze. This schedule becomes applicable from 12AM. The various licensees, withdrawing companies are given the schedule how much they will be withdrawing and at what time.

Few lines above I used the term outage planning. Let me explain what outage planning is. There are various auxiliaries and components in substation, or generating plant etc. which would require maintenance. They cannot be disconnected without prior permission of the area LDC. This would seriously affect the scheduling. The substation or the responsible authority have to apply for undertaking maintenance to the LDC. After looking at various aspects the LDC give them a time slot to undertake the repairing. This is called Outage Planning.

Various appliances that are used in homes or factories are designed to operate at 50Hz ideally. Large deviating from operating frequency would cause large damage to the appliance. The losses will be huge in case of large machines and equipment. The turbines at the generating plants are set to trip at around 5percent deviation (<47.5 and >52.4 or 51.5) after generating a series of alarms. It is the task of LDC to keep the frequency in the operating range. Because if the a power plant trips, frequency will fall leading to tripping of other plants and finally the grid would collapse.

Another important thing is the active and reactive voltage availability. A minimum amount of reactive voltage is required by induction motors and few other appliances, but large amount of reactive power lead to lowering of voltage which is undesirable. To cope with all these capacitors banks are installed at various power stations and substations to absorb reactive power. Also the LDC can direct the power plants to absorb reactive power which is very rare. It causes loss to the power plant so they are very reluctant to do so.

As for the frequency, the LDC can ask the power plants to boost their generation in case of low frequency or reduce their generation in case of high frequency. If all these do not help to make the scene better, they go for load shedding option.

Load shedding is done in a much planned manner. First of all the areas are classified into various groups depending upon the total cost the distribution company incurs and the revenue they collect.
The load shedding is maximum in those area where this ratio is high in other words the region where there is too much of electricity theft.

A lot of relief came in the year 2005 with the introduction of self-correcting tariff system named ABT or Availability based tariff. The ABT system divides the total cost into 3 parts:

1. Fixed Charges
2. Energy Charges
3. UI Charges

Fixed charge is levied if the OLC declared is greater than 85% availability. Energy charge is the charge per unit. Most interesting is the UI or the unscheduled charges. These charges vary from 0 to Rs 16 per unit. If we overdraw power at frequency higher than ideal frequency, the UI charges are null. However if we overdraw the power at lower frequency UI charges are levied depending upon how large is the deviation. This helps to discourage the consumers to overdraw the power at lower frequency thus helping to maintain the optimum frequency. An important question that might arise is who will be paying for the overdrawn power at high frequency if we are not paying. Well this is paid as a penalty by those who are under drawing from what they have been scheduled to draw.
One more case can arise is that the generating plant fails to deliver what it had been scheduled to. The customer is drawing power according to the schedule. So extra charges can’t be levied on him. But the frequency has fallen down. Who will be paying for this? Here penalty would be imposed on the generating plant. One way to escape so much loss due to all this is to ask for revision of schedule to the LDC which help them to reduce the losses incurred.
Also it can happen that the power generating companies in a lure of earning more give the LDC lower capacity, and when the frequency goes down, they would inject more power to the grid to become wealthier. To prevent this sort of exploitation, rules have been laid that they can’t inject more than 2 to 3% to the scheduled amount. If they do so, they won’t be given the extra UI charges they are expecting.

Hydro Power plant are a great relieve to the LDC. One important property of the hydro power plant is that they can boost or reduce their generation in very less time. So the SLDC use them to optimize and enhance the grid in case of emergencies. Area LDC of Nagpur uses Koyna Project frequently for this purpose.

Now let us come to the most disastrous part that can happen ie. Failure of a grid. How do the LDC cope and bring the things to normal. They ask for power from the neighboring regional grids and power its own generating stations first and high importance areas like hospitals. As the generating power stations start they ease the load from the neighboring link. Thus after all the generating stations have started, the normal power is restored.

Located at a distance of 30km from Nagpur (main city), Butibori is one of the many substations in Nagpur. This 220kV substation has twenty four 33kV feeder, four 132kv feeders and fourteen 220kV feeder of which 2 belongs to VIPL category.

Before going further let me first clear why do we require a substation?
The main purpose the substation is to make the power generated at the power plant available to the people. The high voltage in which they transmit the power can’t be directly put to use. These need to be stepped down to convenient level before it is send to the customers. Here comes the role of substation. The substation steps down the voltage according to the requirement of the utility.
The other term which may trouble you a bit is Feeder. What do we mean by feeder?
In simple terms, all the connection which draw and give power to the grid are called feeder.

Components of power plant substation:

1.    Transformer
2.    Bus bars
3.    Circuit Breakers
4.    Current Transformers(for auxiliary)
5.    Potential Transformers(for auxiliary)
6.    Isolators
7.    Lightening Arresters
8.    Insulators
9.    Overhead Ground Wire
10.    Control Room Containing various auxiliaries
11.    DC Battery

Transformer:

 

 
Butibori has three 500MVA 3phase power transformer of 220kv/33kv. These transformers are provided with ONAN and ONAF cooling mechanism. Tapings are provided to alter the voltage available to the utilities in case they complain against less terminal voltage appearing at their end. Complete detail of the transformer is as under:

Accessories of transformers:

1.    Buchholz relay
2.    Conservator
3.    Silica jell breather

Action of Buchholz Relay:

Buchholz relay is a mechanical actuator, which trips the transformer in case of internal faults like insulation breakdown, short circuit of winding etc. Due to these faults the temperature rises and there is decomposition of insulating oil and gases are produced which get accumulated in the upper part of the Buchholz relay. This leads to tilting of the mercury switch. Based upon the level, the alarm is produced and relay comes in action.
The operator in control room receives an alarm, the transformer is turned off. The mouth of the gas valve on Buchholz relay is opened and a match is lighted. If the gas escaping the transformer catches fire, then this is an indication that the winding have burned and it requires immediate maintenance. On the other hand if it does not catch fire, then it is an indication that air was collected. The air is allowed to escape and the normal function of the transformer is resumed.

Conservator

 

Now it happens that the oil which is circulated through the transformer for cooling may expand or contrac
t depending upon the varying temperature and pressure condition to which it is exposed. In summer there is an expansion of the oil. If suitable arrangement is not made for the storage of excess oil, then the oil tube may bust. On the other hand, in winter season, due to contraction, the availability of oil necessary for cooling the transformer will be less leading to shortening of the life of transformer. Here comes the action of conservator. The conservator stores the oil in case of expansion and provides the deficit amount in winter.

 Silica Jell

 

One of the biggest enemy of transformer is the moisture. To deal with these, the transformer are provided with silica jell box. It is blue in colour. As it absorbs moisture it becomes pinkish. Then it is required to be replaced. They can be reused after placing them in light for some time. They lose their moisture content and become blue again and ready to be reused.

Bus Bars:

 

The bus bars are aluminium or copper conductors which carry electric current in open air to which various connection (feeders, equipment) can be made. There arrangements are such that work on any equipment can be done without causing any trouble to incoming and outgoing feeders.
The 220kV butiburi substation has Double bus bar with auxiliary bus arrangement. It has two main and 1 auxiliary line. This ensures that the supply is not affected if one line goes faulty.

Circuit Breakers:

 

Circuit Breakers used for making and breaking of electrical contacts. This task can’t be done in open air as this high voltage will ionize the air and produce spark. Circuit breaker contains inert medium in which the making and breaking of contact is done. Their outer cover is made of porcelain. Also they ensure that the operation is done very fast so that arc is quenched faster. In butibori, the circuit breaker are inert gas type which contain SF6. Before the circuit breaker is operated, it is necessary to ensure that correct pressure is maintained in the circuit breaker and the gas cylinder.

Isolators:

 

Isolators are disconnection switch. They can be controlled from switch yard or from the control room. These isolators are in open air, so they are used only in no load condition ie. they are operated only after the circuit breaker is brought to action. By looking at the position of the isolators we can tell whether it is connected or disconnected.
Current Transformer and Potential Transformer:
For the safety of the instruments and economic reasons, we need to keep a record of the current and voltage at various points in the circuit. Such high voltage and current is highly unsafe to directly give to the measuring equipment. Their values have to be lowered down. For this we require CT and PT. These can be seen at various points in the switch yard. These are step down transformers. Both of them appears almost same but PT is a bit conical compared to cylindrical structure of CT. They have two input wires and two outgoing wires.

 

Current Transformer and Potential Transformer:

 

For the safety of the instruments and economic reasons, we need to keep a record of the current and voltage at various points in the circuit. Such high voltage and current is highly unsafe to directly give to the measuring equipment. Their values have to be lowered down. For this we require CT and PT. These can be seen at various points in the switch yard. These are step down transformers. Both of them appears almost same but CT is a bit conical compared to cylindrical structure of PT. They have two input wires and two outgoing wires.

Insulators:

 

The heavy current carrying conductors if directly mounted on the poles or support will directly ground them to earth. So we requires some insulator through which we can hang them on poles and propagate them. The insulators should have high resistivity to prevent the leakage of current to earth. Along with this it should be able to withstand the weight of the conductor (wire). Depending upon the rated kV the line is carrying the size of conductor changes so does the insulator required to support them. For 33kV and below we use pin type insulator and for higher rating we use suspension type insulators. These insulators have porcelain covering. These have discs whose number depend upon the kV rating of the line. Eg. If the discs are of 11kV rating, then we would require 20 of them to hang the conductor through the support. Butibori substation has suspension type insulators in switch yard.

Lightening Arrester:

 

One of the biggest thread to the Substation is Lightening. 100s of kV smashing on the equipment all of a sudden renders them useless. Lightening arrester protect the substation (most probably the switchyard) from the effect of lightening. These are connected between line and ground. They have a spark gap in between them which remains non-conducting as long as normal power is flowing through it. But when the lightning strikes, it becomes conducting and all power in the lightening goes to ground safely and the spark gap becomes non-conducting again.
Overhead ground wire:
These are steel wires which run over the top of each transmission line tower. These are solidly grounded at each tower. They also help to minimize the effect of lightening surges by grounding them safely.

 

Overhead Ground Wire

 

These are steel wires which run over the top of each transmission line tower. These are solidly grounded at each tower. They also help to minimize the effect of lightening surges by grounding them safely.

Control room:

It has arrangement for the remote monitoring of each of the auxiliaries, feeders etc. and controlling them. Various components in control room are as shown below:
Slides

DC  Battery:

 For the proper functioning of the relay and other protective gears we requires uninterrupted dc supply. They are crucial even when the whole grid terminates. Butibori substation has 110 cells of 2V each connected in series to provide 220V for the proper functioning of the auxiliaries.