Criticality of Power Distribution System in a Steel Plant

Reliability of the power source is very important for running a large scale steel plant. Coal-based captive power plant is a good option from the OPEX angle.

Criticality Of Power Distribution System In A Steel Plant

Power distribution system in a steel plant is different from any other industrial plants due the nature of load and its magnitude. An integrated steel plant of large scale consists of various process plants, non process plants, services and utilities. For a typical BF-BOF (blast furnace-basic oxygen furnace) route manufacturing process of steel, sinter plant, calcining plant, coke oven, blast furnace blowers, basic oxygen furnace, oxygen plant, slab casters, rolling mills constitute major process loads, whereas various services such as raw material handling, water, compressed air system and miscellaneous auxiliary system constitute non process loads. Steel plant being a continuous process plant with high capital expenditure,  design of power network should be realised from criticality point of view so that any eventuality such as grid power failure, power equipment failure  and internal power system disturbances due to poor power quality, constraint in the selection and sizing of equipment and accessories can’t drive the processes into jeopardy resulting in huge capital damage associated with financial loss due to loss of production. As an example, interruption of power supply to BF blowers or failure of BF blowers may cause capital damage to the blast furnace putting the steel plant operation into complete halt. Recovery time also is quite high as manufacturing and refractory work of capital equipment demand long delivery time. Hence if electrical power driven blowers are envisaged, 3 x 50% configuration is more beneficial from reliability point of view. Critical aspects of reliability and flexibility of power distribution network along with power sourcing necessitate formation of robust power system that will enhance safety of capital equipment. From the feasibility of CAPEX, optimisation of power network needs to be looked into as well. In a large scale integrated steel plant generally the philosophy of construction is phase wise in which plant loads come up in multiple phases along with power generation to ease out cash burden. Scaling of plant power distribution system capacity is also applicable to match with the enhancement of plant capacity along with all other services and utilities. Along with the load growth power source capacity and reliability should be enhanced. Options are available to select the power sources from utility grid, co-generation plant and diesel generator plant. To provide stability and reliability of the in-plant power system, power sources should be reliable along with the power distribution system. Sourcing of power should be looked into critically and possibility of sourcing from multiple sources should be explored. Power sourcing from utility grid is mandatory as high fluctuating loads particularly in the rolling mill area can be absorbed by grid without causing voltage fluctuation at rolling mill bus and other plant load buses beyond permissible limit. The irritation level of flicker and voltage variations at upstream high voltage buses as well as at the point of common coupling also remain well within permissible limit. In plant power generation is another reliable source of power which can be utilised for production of steel. In a large scale integrated steel plant there is availability of by product gases from blast furnace, coke oven and LD plant. Although calorific value of by product gases is low, but these gases can be utilised to generate electrical power and process steam which can be used by different process plants such as reheating furnace in mill. Co-generation plant run by by-product gas and waste gas form captive power plant which provide reliable power source at cheaper rate than grid power. Steam and power generation from available by product gases from steel plant is routed through proven and established HRSG (heat recovery steam generator) and STG (steam turbine generator) route instead of gas turbine route. The reasons are – (1) low calorific value of by product gas will result in higher power consumption in the fuel gas compressor, (2) the available by product gas is not clean which requires elaborate gas cleaning plant, (3) available gas is fluctuating in nature, and (4) adequacy of steam generation to meet the demand of steel plant process steam is uncertain.  From the availability of waste gas power can be generated through top recovery turbine (TRT) and coke dry quenching (CDQ) plant. Capacity of power and steam generation depends on the volume of gases produced in the steel production process. When steel plant is in operation with full capacity power import from grid will be low due to full power generation by captive power plants with higher amount of gas availability. Dependency on grid power import can be reduced with installation of coal based power plant with added advantage of reliability, accessibility for control of power generation and availability of start-up power in case of major grid failure. Coal based captive power plant is better option than centralised diesel generator power plant from OPEX point of view. The consideration as stated above needs study and in-depth techno-commercial analysis of different alternative configurations of power system with different connectivity of power sources in each alternative. This analysis will be the stepping stone for selection of most feasible alternative of power network in terms of reliability, flexibility and cost optimisation.

Part Power Network Showing Connectivity Of Utility Grid And Cpp Generators
Figure-1: Part power network showing connectivity of utility grid and CPP generators

Power System Configuration     

The criteria for selection of power distribution configuration is based on reliability, adequacy of redundancy, optimum loading of equipment, system fault current, optimum utilization of layout, reliable and instantaneous islanding operation of captive power plant generators from grid in case of grid disturbances, provision for expansion of power system for allowing future capacity enhancement and cost optimisation. System voltage variation should not cause the tripping of critical motors or/and drives, or of generators due to the operation of under voltage and over voltage relays. Similarly frequency variation should not trip the generator due to the operation of under frequency and over frequency protections. While sourcing grid power, quality of power is also to be explored with respect to frequency and duration of grid interruptions, current and voltage harmonics among other things. Power network should be configured considering 2×100% redundancy such that outage of one transformer or feeder does not cause power interruption and jeopardise the operation of process plant connected to the distribution network. Steel plant is a continuous process plant where loads are categorised as critical, non critical and emergency based on the criticality of power supply. Quantum of critical and non critical loads has to be assessed after discussion with process and operation group. For a typical BF-BOF route of steel making process, critical loads are comprised of electrical power driven BF blowers along with its auxiliaries, part load of BF process, coke plant, oxygen plant, steel melt shop, services and utilities. Under any contingency condition continuity of power supply has to be maintained for critical loads. With reduction in cost gas insulated switchgears (GIS) are better option than air insulated switchgears (AIS) as it can save 60-70% of foot print space with added advantage of higher reliability, safety, least maintenance and interference caused by external factors such as environmental pollution. GIS can be used at different voltage level up to 33 kV with double bus bar configuration and bus sectionalizers. The primary power distribution system should be arranged such that critical and non critical loads can be operated in different islands with captive power generation and utility grid supply. In other words, power supply to critical loads are made from a bus section where grid and captive power generation are connected. Furthermore, utility grid and captive power generation sources are connected at different voltage level to ensure less voltage variation in case of any grid disturbances. A typical part power distribution scheme showing the connectivity of utility grid and CPP (captive power plant) generators is shown in Figure-1. From the figure it is observed that grid power and captive power are connected at 220 kV and 132 kV bus respectively. Under normal operation of power system grid and captive power generators will be running in parallel at 132 kV bus with bus coupler breaker (B2_IB-1) and bus section breaker (B2_IB-2) closed condition. Captive power generation is operated to match the demand of steel plant loads such that import from grid is minimised. In case of grid disturbance, grid system is isolated from the plant power system with tripping of both the breakers (B2_IB-1 and B2_IB-2) so that total power failure to the plant loads particularly critical loads can be avoided with availability of generation from captive power plant.

It is desirable to deploy automatic load shedding system for a critical process plant. The operation of the islanding scheme triggers the dynamic load shedding scheme whose operation will restore balance between availability of power and load demand within frequency stability limit of the in-plant generators. When one or more in-plant generation facilities are shut down and power demand is expected to exceed power generation, load management system (LMS) is activated to initiate dynamic load shedding of non-critical loads to ensure balancing of power demand and supply of in-plant generation. In the event of failure of the dynamic load shedding scheme, the under frequency-based back-up scheme (with time delay) comes into play and sheds sufficient load to arrest the fall in frequency to sustainable level. Emergency power generators are installed at certain manufacturing processes to ensure protection of equipment and emergency evacuation under total power failure condition. Priority loads are identified according to the nature of the criticality to ensure that LMS carries out load shedding with least priority loads in stages to maintain balance between power supply and demand. Entire load shedding process has to be completed within certain time as obtained from power system analysis to ensure that captive power plant generators do not cross stability limit.

Conclusion

After various alternative studies based on the criteria as discussed in previous sections the selected power distribution scheme has to undergo power system analysis comprising of load flow, short circuit, motor starting, transient stability, islanding & load shedding, insulation coordination and flicker studies to revalidate the scheme, finalise equipment rating, critical fault clearing time, time to shed non critical loads within stability limit and flicker limit. Critical and non critical loads and its magnitude have to be assessed correctly and taken into account while developing power distribution scheme. Sourcing of power from multiple sources including captive power generation, connectivity with the power network, islanded operation of the power system to ensure continuity of power supply under any contingency are significant requirement to bring reliability in the power distribution system for integrated steel plant. Deployment of LMS also is a prime requirement to match between power supply and demand in case of outage of grid and planned shutdown or outage of 1 or 2 generating units of captive power plant. As the extent of damage of capital equipment, cost, loss of revenue and recovery time of a steel plant is quite high, extreme care has to be taken while finalising power distribution system such that in the event of failure of grid power supply and plant equipment, operation of steel plant can continue or safe shutdown can take place depending on the availability of power and plant equipment. Reliability, redundancy and cost optimisation shall be blended and balanced during the planning of power distribution system for a large scale integrated steel plant project.


Anupam Roy

Anupam Roy
Discipline Head- Electrical,
Tata Consulting Engineers Limited

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