The Electrical Power Grid

The  Electrical Power Grid

L V Nagarajan

Mr. Contractor and Mr. Engineer are great friends. They are running a reasonably successful construction business for the last twenty years. They decided to construct houses for their own use, but in a small village far away from maddening crowd. The village they chose, did not even have electricity supply as yet. They decided to start construction of their houses adjacent to each other. They had a small diesel generator for construction purposes. After completing the construction they promptly applied for electricity connection. Due to lack of infrastructure, the same was delayed for more than two years. However they wanted to live in their new houses, hence they commissioned two  diesel generators, one for each house and connected the same to their individual main switch boards. They both moved to their new houses

Mr. Contractor started the generator and switched on his lights, fans and heaters etc and as he switched them on, one by one, he observed the whine of the rotating generator getting louder and shriller. But there were other changes happening in the machine which he could not observe. As he put on one appliance after the other, the extra load slowed down the rotating speed of the machine. The speed governor sensed this speed drop and picked up the speed to normal by automatically increasing the fuel input. Just as the speed of the machine is an indicator of generation-load balance, speed of the machine also decides the frequency of AC-power output. The frequency should be maintained close to 50 Hz, as in all AC systems. As the load on the generator kept varying with the use of different appliances, the speed/frequency also followed the changes to maintain the generation-load balance.

There was a time once, when Mr. Contractor’s gen-set developed a problem and got shut down. He suffered a loss of power. He immediately called his neighbor Mr. Engineer, to look into the problem. Mr. Engineer’s gen-set was up and running. He suggested to connect his neighbor’s load also on to his machine, at least till the faulty machine is repaired. He installed a pair of cables to jumper the outputs of both the machines, but took care to put it through a circuit breaker. He requested Mr. Contractor to switch on only essential loads to keep the load on the single machine within its limits. Mr. Engineer checked the faulty machine and found it was a minor fault. He repaired the same and started the machine. He took the machine in service. But he forgot to remove the jumper cable. Then he decided to leave the same on, to increase the reliability of power supply to both of them. Thus was born a GRID, a Power Grid.

When both gen-sets operated as a GRID, they enjoyed the increased reliability and stability of power supply, but they also experienced a few problems:

i) Any load changes in one system affected both the gen-sets.

ii) The response of the two speed-governors being slightly different, there were oscillations in actual power sharing between the two gen-sets.

iii) The power flow through the jumper cable (normally called Tie-line in GRID terminology) was varying widely, some time very close to its full capacity.

Mr. Engineer being very smart, introduced some changes in the governor systems as below:

a) He made the speed governors less sensitive by introducing a speed-load droop response in their systems.

b) He introduced a secondary frequency-control equipment in both the gen-sets, which responds based on ‘Speed/Frequency plus Tie-line flow’, as seen from respective systems. This secondary control helped individual systems in keeping generation/load balance on their own systems. (This secondary control is called Tie-line Bias Control in GRID terminology).

Under normal conditions, the GRID was operating quite well. they derived following advantages:

– Any minor restrictions on the individual gen-sets could be easily managed

– Any gen-set could be released for routine maintenance easily, with only minor load restrictions

– Voltage and frequency were better regulated, even during load surges and drops

There were also a few new problems:

– lack of proper accounting of Tie-line power flows either way, which could enable proper sharing of energy/fuel expenses.

– Mr. Contactor’s gen-set was inherently more prone to frequent failures. This created problems for Mr. Engineer also. Can he be compensated in any way?

– Once, Mr. Contractor’s gen-set-1 started dropping load gradually due to some mechanical problem. But Mr. Contractor ignored to take action and did not switch of his non-essential loads. This lack of discipline resulted in tripping of both the gen-sets giving a total shut down to the GRID.

– Of course there were other times when Gen-set-1 one failed instantly with the same result. Mr. Contractor could not do anything to prevent this and Mr. Engineer had to accept it.

They together discussed these problems and decided on the following solutions:

1. Tie line power flow will be metered either way and the net flow of energy from either system to the other will be billed to the receiver at an agreed rate.

2. The above is fine for inadvertent exchanges of power, happening due to system dynamics. For scheduled assistance from one system to the other, the charges could be agreed at a higher rate.

3. For unscheduled short-term emergency assistance of power, the charges could be agreed at a still higher rate.

4. Proper discipline should be followed during abnormal situations, to keep the loads to be within the limits of  ‘own generation plus agreed assistance’.

5. They also agreed to commission automatic load shedding (on under frequency and reverse power) and system separation (i.e. tripping the tie-line) in cases where such load-generation balance could not be achieved by self-discipline.

Both Mr. Engineer and Mr. Contractor (somewhat reluctantly) agreed and implemented these ideas. Surely, there were frequent bickering between the two, but the GRID basically worked well. Before the GRID could be affected by this bickering, fortunately they received the electrical connection from the power utility. Yes, now they are connected to a common grid managed by the utility. Mr. Contractor’s gen-set was disposed off. Mr. Engineer’s gen-set was retained as a stand-by supply till their connections were fully stabilized.

Though the above GRID was a small one, the principles of grid operations are the same even when two bigger systems are connected as a super GRID. To ensure that these principles are always upheld, usually a separate GRID operation department, (or a Load Dispatch Centre) is established, common and neutral to all the systems interconnected. It monitors the grid operation on a 24×7 basis and instructs the component systems to adhere to the principles of grid operation.  Failure by one or many of these connected systems to adhere to these principles will lead to major system black-outs, as occurred in the Northern Grid of India on 31 July 2012, which affected 500 million people. But out of these large population, even 0.01% would not have understood what exactly led to this shut down. With this note, I sincerely hope, at least 0.02% may now understand how any lack of discipline as described above could lead to failure of GRID operation and eventually lead to a black-out. (which means at least 50,000 people should read this blog !). I have kept this write-up intentionally simple to help even non engineers to understand.

(Refer: http://en.wikipedia.org/wiki/2012_northern_India_power_grid_failure).

25 Dec 2012

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2 Responses to “The Electrical Power Grid”

  1. Krishna Nandan Sharma Says:

    I am in power and Utilities operation field since a long time with small medium and large power station in various industries and countries. This write up gives me a real picture of electrical generation vs electrical distribution to voltage and frequency relation and its importance for all . One acn take this note in their social and industrial life.
    Rgds
    K.N.Sharma
    NSRP, VN
    Email: skrishnan58@gmail.com

  2. Nagarajan Says:

    Thanks for your comment. I wish more people who manage utilities in India and other developing countries will read this and adopt these principles in their grid operations.

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