It is a well known fact that considerable amount of energy is consumed by the Street lighting systems in the major metropolis and other cities and towns of our country. The ON/OFF operations are carried out manually, with no precise control. Sometimes, the street lights are ON upto late in the mornings and they are switched ON prematurely much earlier in the evenings. Lot of energy can be saved by proper planning of the ON/OFF sequence by due consideration to the day’s twilight time availability of which is very well known for the entire year.
In many places, illumination sensor based sequence is followed, which is not very reliable, and requires maintenance. The timing based sequence can be highly reliable and has enough scope of very good and reliable automation.
For the latitude spread (8ºN to 32ºN) of our country, good twilight time exists for about 20-30 minutes before and after Sunrise and Sunset respectively. Based on this data an ON/OFF sequence has been worked out for about 6 major towns and cities of the country. Comparison of house-hold energy consumption with the street lighting energy has also been done.
An estimate has been made for the energy consumption and the saving worked out based on an average 12-hour ON/OFF sequence. A peak saving of about 25 percent for New Delhi and adjoining areas during summer months is shown.
Presently High Pressure Sodium Vapour (HPSV) lamps are in wide use in most of the cities and towns of our country. They are available in many versions. The 250W and 150W type are popularly used. The actual wattage used depends on the type of street. For a small town, Ulhasnagar in Maharashtra, the total connected street light load is close to 3 MW(1), covering an area of about 13 sq-Km. 
Although it is very difficult to estimate the lighting load for major cities and towns, a first cut estimate on the above value can be made. The load for Bangalore can roughly be taken as about 60 MW, which is about 20 times that of Ulhasnagar. For other Metros, the value can be even higher.
Generally a household with modern gadgets consumes about 300 Units of Electrical Energy per month. Comparing this with the street lighting load, the monthly energy consumption of five households is close to the electrical energy consumption of the street lighting load for just one minute for Bangalore city. So, it is very important to save maximum energy in the street lighting systems.
A switching sequence based on the sunrise and sunset timings for a particular place and the twilight time available after sunset and before sunrise has been worked out.
For Indian latitude spread of 9ºN to 34ºN, the twilight time is about 20-30 minutes. The ON time is taken as 10 minutes after Sunset, since the light intensity is decreasing fast. The local topography of high buildings or cloudy days may require an alteration of about say 5-10 minutes advancement of the ON timing.
The OFF time in the morning can be taken as in between “civil twilight” and “nautical twilight” time, explained later, before the Sunrise, since the light intensity is increasing very fast. There are three types of twilights (2) defined based on the angular position of the Sun below the local horizon, before sunrise and after sunset. They are defined as Civil (6º), Nautical (12º) and Astronomical (18º) twilights. Hence for switching OFF, a value of about 9º can be considered. This leads to a time of about 30-40 minutes before sunrise for the Indian latitude spread.
The table above shows the data of ON/OFF times for six major cities including Bangalore. The timings shown are for every 15 days. It shows a yearly average of close to 11 hrs for these major cities, which also cover a wide range of latitude. In terms of percentage computed on the basis of 12-hrs ON duration, New Delhi’s average saving over a year can be about 9.7 per cent, with a peak saving of about 25 per cent, whereas for Bangalore the figures are 6.3 per cent and 12.5 per cent respectively.
It will be very advantageous if a centralized control is made for different parts of a city through computer. It can be programmed initially for one year, with a provision to change the data on-line to meet any contingency ON/OFF operations. The system should also have over-riding provisions to take care of the local conditions, like high rise buildings, etc. The working of this system should be very meticulously followed with strict maintenance schedules.
In major cities at present there are many large companies with vast campus areas. With about 200 lamp posts in the campus, there is load of about 50 kW. For 50 such units, the connected load can be as much as 2.5 MW. One hour energy consumption of these lights will be about one month’s consumption of a house-hold. Here also, it becomes important to conserve energy and follow the above computerized sequence.
In our country the electrical distribution losses connected with the utility, involving the 11 kV lines and further lower voltage (440/240V) lines is quite high as compared to international standards. These are in the region of 15 per cent, as compared to internationally achieved figure of about 5 per cent-10 per cent.
Generally a bank of lamps, upto 25 kW is powered by a single control panel. There is finite amount of power loss that takes place from the control panel to the lamp posts. We should set a small target of reducing this loss by say 1 per cent and try to achieve it.
This will certainly pave the way for means to achieve an overall bigger target. For Bangalore, this 1 per cent is about 600 kW, which is almost equal to the installed capacity of about 150-200 house-holds. Energy saving by controlling the lamp voltage during non peak hours (0000-0300 hrs) by means of tap changers at the distribution end can also be considered (a small reduction in the luminance). 
Even though there is a need for extra power generation, a more urgent need is the cutting down of the requirement wherever possible. One such area is discussed in this article. The energy saving should be aimed with the idea of no basic compromise at the luminance requirement on the roads, during the peak hours. The above discussed ON/OFF sequence requires only a simple planning, but a meticulous execution to get a significant saving in the street-lighting energy requirements.
The author is with the electrical integration division, ISRO