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Cover Story | August 2016

Interruption of power supply has huge impact on steel plant

Amrendra Ranjan | Chief of Electrical Maintenance, Shared Services Division, Tata Steel Limited

What are the power quality (QoP) issues you face in your day-to-day operations and how do you handle them?
For a continuous process plant like an integrated steel plant, reliability and quality of power is very important. The quality of power is generally defined by the voltage variation, frequency variation, and number and duration of interruption. The frequency of the supply is not in control of the supplier, is same throughout India and monitored and controlled by the state, regional and national load despatch centres. The frequency which used to vary from 47 Hz to 53 Hz about 15 years ago, has improved significantly over the years and now varies between 49.5 Hz to 50.3 Hz, and is not a concern for steel plant operation.

The supply voltage is normally maintained by the supplier within the band specified in the standard of performance regulation. It is +10% to -12.5% for supply voltage above 33000 Volt, +6% to -9% for supply voltage between 650 V to 33000 V and +6% to -6% for supply voltage of 250 V and 415 V. The voltage is controlled within the specified range by changing the tap of the power/ distribution transformer. It is also controlled by the customer at their end by changing transformer tap or by installation of capacitor banks and Static VAR Compensator (SVC).

The interruption of power supply from utility is very critical and has large impact on the steel plant process. To ensure continuous supply of power to important loads, Tata Steel has generating units installed within its premises, which are, connected with the utility. So, even during outage of the supply from utility, these generating units keep on supplying power to all important plants. The power system is designed such that outage of single equipment-/ line does not have any impact on the power supply. In 2015-16, there were two cases of outage of utility supply at 132 kV grid to which Tata Steel Jamshedpur is connected.

Both the time, power supply to Tata Steel plants continued from generating units within its area.
For a continuous process plant like an integrated steel plant, voltage dip (momentary sag of supply voltage, of millisecond duration) due to fault in the power system network is an important quality parameter of the power supply. This is normally not well appreciated by the utility because it generally does not lead to power failure anywhere. However, severe voltage dip may lead to complete stoppage of the continuous process and normalisation may take a few hours. Since the power system networks are interconnected and vast and it passes through rough terrains and forests, it is not possible to eliminate fault in the network. However, the numbers can be controlled by proper inspection and action. The duration of the fault can be controlled by proper designing of protection and relaying system. In FY16 there were 18 cases of voltage dip due to fault in 132 kV grid and 10 cases of voltage dip due to fault in 400 kV grid to which Tata Steel is connected. Out of these in 13 occasions some of the plants of Tata Steel got affected.

What are the power quality equipments you use to monitor and mitigate the problems and please narrate their uses?
Capacitor banks and SVC are installed at various plants to control the reactive power and hence the voltage. To control the harmonics generated by the solid state switching devices, suitable harmonic filters are installed in a number of plants at Tata Steel. All power transformers are provided with OLTC (on-load tap changer) to control the load end supply voltage. The whole power system distribution is designed for (n-1) contingency, i.e, outage of one transformer or feeder will not result in any power interruption. Numerical relays are installed to disconnect the faulty part of power system in minimum time. All the HT switch room and control rooms are air-conditioned to keep them dust proof and at controlled temperature.

Apart from having the generating units installed within the premises of Tata Steel which are connected with the utility, the company is also harnessing the renewable energy for power generation. The company has recently installed a 3 MW Solar PV Power Plant at its iron ore mine site at Noamundi, Jharkhand. This is the first Solar PV Power Plant of this size in a mining location in India and is expected to reduce CO2 emissions by 3,000 tons every year. Noamundi is a mining location in the West Singhbhum district of Jharkhand, around 130 km south west of Jamshedpur. The iron ore deposits were discovered in 1917 and today the mines cover an area of 1,160 hectares with mine production capacity of 10 million tonnes per annum. The Jharkhand State Electricity Board (JSEB) is the only source of power supply to run the mine and the township. The combined daily requirement of power is around 13 MVA per day - 11MVA for mines and 2MVA for the town. The emergency power requirement is currently being met through multiple diesel generator sets. This project ushers Tata Steel into a new era of global co-operation on climate change. Furthermore, the solar power plant is believed to improve biodiversity in the area.

In an ideal situation what kind of QoP supplies do you expect from the distribution companies and what are the inadequacies you are facing now?
The number of voltage dips due to fault in the distribution company network should be controlled. Presently, this is not a measure of performance for a distribution company.

There are restrictions of power imposed by the distribution company, due to shortage of power at their end. This needs to get eliminated with the power market opening up and short-term purchase and sell of power from power exchange getting established.

What are the measures that may be taken to improve the power supply scenario?
At the national level, stricter rule and regulations are being framed to further narrow down the frequency variation bands. The transmission network is being strengthened to enable inter-state transfer of power.

However, the burden of huge regulatory asset on the distribution companies is making them sick. Measures need to be taken to make regulatory assets competitive and reliable.

How do you think the power supply scenario compares with advanced countries in terms of QoP?
In advanced countries, frequency remains almost fixed at 50 Hz or 60 Hz as per the system design. This requires sufficient spinning reserves and fast response generating units like gas turbine-/ large hydro generating stations.

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