Emergency lighting

Emergency lighting

The name Emergency lighting itself implies that it is required in an emergency. In the present day conditions of technological advance this not only includes lighting but also supply to communication and other essential equipment.

Today’s emergencies

Today communication is becoming a part of life even in households in all countries. Therefore the wording Emergency supply for these could be more appropriate. Even in residential homes in all countries where electrical supply is not stable, the computer system is supplied through what is called an (UPS) Uninterruptible Power Supply to avoid interruption as well as damage to the same. This concept also normally applies to any industry or a public place catering to any type of humanitarian work, such as hospitals, nursing homes, testing laboratories etc. where also the use of electronic gadgets with information technology are very common.

In other industries such as food processing etc. this emergency condition creates havoc for the process needs spoiling the end product. This results in a great loss not only industry but also to general public.

Since reliability of the Electrical power supply in the Power generating stations is very critical because it is the starting point which generate and supply to other consumers, this case has been discussed in detail here.

Power generating station

These emergencies when applied to a power generating station are of a very complicated nature. This generally has the effect that all the lights go off. This not only includes the general lighting but also the indicating lighting on the operators’ console including localized lighting to monitoring indications such as boiler and deaerator gauge glasses etc. Further the essential equipment like turbine lube oil pumps, turbine (thermal Power Station#Barring gear) barring gear etc. will also lose the power supply causing damage to them.

The normal supply for these in a well designed system in a utility generating station fails only when there is a station shut down, and also when getting isolated from Grid, creating favorable condition for equipment damages.

Hence emergency and normal system designs require detailed study. These are analyzed here under different categories.

Location of power station

Generally the locations of new power stations are selected primarily on the basis of reliability of power supply to the consumers. Local environmental conditions, the requirement demanded by the power station requirement itself, such as sufficient land, condenser cooling water facility, connection to existing grid (electric power transmission) lines etc. have also to be taken into account. In advanced countries the power supply system as a whole for the country is studied to day by use of net work analyzers only. It is interesting to note that even in 1962 highly advanced utilities were using mechanical type net work analyzer occupying huge space and requiring all the parameters to be set manually on the board. This was a time consuming job considering the complicated nature of the power supply system of the whole country or of a particular zone. In other small countries it may be only local conditions. After careful study only the location of the power station is decided. Therefore in a well designed system the possibility of station shutdown and also isolation from grid are far remote. However such occurrences do happen once in a way. Under such conditions it is essential to protect the equipment from damages. The emergency supply therefore has to be very reliable and do its share of work reliably under the designed conditions.

Reliability achievement

The power supply to all the units and common station auxiliaries in a well designed system is generally as shown in the typical schematic diagram and explained hereunder.

Normally large units of a utility power station now days are designed on a unit system basis, i.e. the boiler, the turbine generator unit and its power (step up) and unit (auxiliary) transformers are solidly connected as one unit. Some times two units are grouped together with one common station auxiliaries. As each turbine generator unit has its own unit auxiliary transformer solidly connected this comes into circuit automatically as soon as the unit goes on line. This transformer supplies its own unit auxiliaries. For starting the unit or before going on line, the unit auxiliaries are supplied by another unit (auxiliary) transformer or station auxiliary transformer. The change over from unit transformer to other unit transformer or to station auxiliary transformer is designed for automatic operation in such a way that at no time these unit auxiliaries will be without power supply. Therefore the power to unit auxiliaries cannot fail during station shut downs (black-out) (i.e. all units are off bars) as long as the station is connected to grid and not got isolated from grid. The system of relays coordinating these auto operations are generally reliable.

Grid system operation

During station shut downs (black-outs) the grid supply is expected to remain as a back up. However due to reasons beyond control the grid system may get isolated once in a way. The possible causes for grid isolation are the operation of reverse power relays and frequency operated relays on grid lines due to severe grid disturbances created. Under these circumstances the emergency station supply has to come into operation automatically to avoid damage to any equipment and also to avoid creation of hazardous situation like release of hydrogen gas from generators to atmosphere. Manual operation in that emergency is generally not effective on all operations called for simultaneously as this action cannot be faster than the auto change over incorporated.

Emergency requirement

This normally indicates power supply requirement to the emergency circuits. It is essential that these emergency circuits should get switched on automatically without any external aid and without any time delay. These emergency circuits are therefore clubbed together, different than the normal lighting circuits etc. and taken out as a separate circuit from an AC main power supply system. The local communication system also being essential, this system also is clubbed together on this emergency circuit.

Emergency supply system

The backup and most reliable 230V AC emergency supply in a generating station therefore is obtained from a Central battery system with controls, located in the Power station building itself, to avoid long electric supply leads. This Central Battery System consists of lead-acid battery cell units to make up 240 V DC and few more standby cells, with its own battery charging unit, inverter unit to get 230 V AC, and auto switch-less change over (to the Emergency supply circuit) in case of failure of AC 230 V station supply. This also supplies essential equipment, such as turbine lube oil pumps and rotating air preheaters during station shut downs with isolated grid operations. Some times this emergency battery supply with controls is fully duplicated.

Installation of emergency supply unit

The battery stack is installed on acid protected stands in a separate room with all round protection from acid fumes with acid proof painting and with exhaust fans etc. taking into consideration the design factors stipulated in battery room#Design Issues. The control circuits for battery charging, including quick and trickle charging, and necessary emergency change over circuit panels are installed in a separate room with monitoring instruments locally and on operators’ console.

Conclusion

These requirements such as battery stack with all controls etc. installed with all care are essential as a last reliable emergency power supply for smooth and damage less shut down of the units and bringing them back on line, safely. This emergency supply system caters for both, the steam generator units and TG units and other essential station auxiliaries. It is also designed to cater for a group of two units if required.

Similar power system design is applicable for all other industries as well with modifications to suit their specific needs.