Energy Management for Water Supply Industry
Energy conservation is the element of utmost attention for every industry, looking into increasing energy demand and limited resources of fuel. While meeting the objective of making water available on 24 X 7 basis to growing consumers, it is also crucial to address the need of energy conservation.
Water distribution system of any township covers mainly Lifting stations, pumping stations, WTP, MBRs ESRs, GSRs, Main & trunk pipelines, ETP & STPs. Various process involve pumps driven mainly by motors, which consumes more than 80% of energy supply available for water distribution of any municipal authority. Power management system proves as solution to address the need of energy conservation. It can be implemented through various schemes as mentioned below.
Monitoring & recording electrical parameters like Voltage, Current, frequency, KWh, KVArh & KVA through special panel meters.
Recording the status of electrical control element like contactors, breakers, relays controlling Motors, transformers, load buses, substations equipment
Interfacing the panel meters & relays to automation system so as to measure and record energy usage to allocate energy cost process area wise.
Record Maximum demand so as to control consumption of energy and avoid additional charge by electricity board for irregular power consumption and reduce the operational cost.
Recording the health parameter like of winding temp, Oil level , Vibration level of motors & transformers for better maintenance and reduce downtime.
Reactive Power Control
Active & Reactive power are part of any electrical network. The active power is used by load and meet its output requirement. However reactive power is used to meet the magnetic field requirement. Majority of load of electrical network are inductive in nature like motors, transformers. Electrical network for water distribution is always under varying load condition so as to meet the varying water demand.
Reactive power lags behind active power. It is therefore necessary to control & manage the flow of reactive power to achieve higher efficiency of electrical system and reduction in cost of electricity consumed.
Effective method of reducing and managing reactive power is by power factor improvement through power capacitors. This can be achieved through Fixed compensation or Variable compensation depending on type of load. Automatic power factor correction systems helps in achieving consistent higher power factor through out varying load condition.
Power factor management reduces losses of transformers, switchgears and cables; avoid over sizing, electricity cost due to levy of penalties/ loss of incentives.
Variable Frequency Drives
As mentioned above pumps consume most of the energy supply for water distribution and transmission.
Normally it is observed that irrespective of load demand, motors driving pumps during operation in water distribution & transmission run at full speed utilizing 100% power when they are connected to direct on line.
Power consumption is product of head and flow. So reducing the flow during low demand period can help in achieving power saving considerably. Reducing the speed of the motor can reduce flow of the pump. Variable Frequency drive (VFD) which work on 3 Phase , 415 V supply is able to deliver the variable Voltage and Frequency output. VFD drives motor at reduced speed and varies the flow output of pump as per demand. This helps in achieving saving in energy.
Apart from energy saving, VFDs protect motors from high starting in rush current (600%) reducing it to (150%). This increases the life and ultimately efficiency of motor for longer period.
Motor Rating: 360 KW
Rated flow: 231 M3/HR, Discharge Pressure: 37.27kg/cm2, Suction Pressure: 2.87 kg/cm2,
Differential Pressure: 34.4 kg/cm2, Pump efficiency: 72%
Present Running Parameters: (as measured)
Flow: 280 MT/HR (Specific Gravity: 1.37) i.e. 204 M3/HR, Suction Pressure: 2.4 kg/cm2, Discharge Pressure: 42.5kg/cm2, Differential Pressure: 40.1kg/cm2, Valve Opening: 24.1%, Pump efficiency at 204 M3/HR: 68%, Motor efficiency (assumed): 90%
Input Power to the Motor P= (40.1*9.81*10)*(204/3600)/(0.68*0.90)= 364kW
(The effect of static head is not considered, as it will be same for the next case with variable speed drive)
Ratio (Actual Flow / Rated Flow) = 204/231= 0.8847
If the pump speed is reduced by the same proportion, using variable speed AC Drive, the differential pressure will reduce by square of the same ratio. Hence, new differential pressure will be =34.4* (204/231) 2 = 26.92 kg/cm2
Revised Input Power to the Motor P' = 26.92*9.81* 10)*(204/3600)/(0.68*0.90)= 244 kW
Net Savings: (364 - 244) = 120 kW
VSD Losses = 10 kW ( 96% efficiency)
Net Power (Saved) : 110 kW
With Unit Power rate of Rs 4.00, Total savings per year (est.): 110*8760*4.0 = Rs 38.5L
Note: It is assumed that the monitoring points of suction & discharge pressures are such that the difference in these two pressures eliminates the effect of static pressure. However, if actual static head is given, the calculations can be refined further. Similarly, resistive drop in the piping is neglected.
Looking at power scarcity in some areas, especially in urban & rural, where water lifting stations & pumping stations for water transmission are located, it has become imperative to implement alternate source of energy like diesel generator set. However to protect such high cost capital equipment, it is advised to use a means of reducing starting current of large AC induction motors. There are many types of methods like Wye-delta Starters, auto-transfomer & solid-state starters.
Out of above methods, Solid-state starters, or Soft Starters gives an edge over other in terms of ease of operation and maintainability.
Soft Starters control the voltage applied to motor by use of solid state AC switches (SCRs) in series with supply to the motors.
Being electronic controller, Soft Starters deliver much more advantages, like Soft stopping, its compact modular design occupying less space, a low voltage test mode that aids commissioning, a real time clock for event monitoring & logging of electrical parameters, simulation, storage of multiple motor characteristics pattern.
- Anand K Nigam