Energy Basics

Direct Current (DC): A non –varying, unidirectional electric current (Example: current produced by batteries)
Characteristics:

  • Direction of the flow of positive and negative charges does not change with time.
  • Direction of current (direction of flow for positive charges) is constant with time.
  • Potential difference (Voltage) between two points of the circuit does not change polarity.

Alternating Current (AC): A current which reverses in regular recurring intervals of time and which has alternately positive and negative values, and occurring a specified number of times per second. ( Example: Household electricity produced by generators, Electricity supplied by utility).
Characteristics:

  • Direction of the current reverses periodically with time.
  • Voltage (tension) between two points of the circuit changes polarity with time.
  • In 50 cycles AC, current reverses direction 100 times a second (two times during one cycle).

Ampere (A): Current is the rate of flow of charge. The ampere is the basic unit of electric current. It is that current which produces a specified force between two parallel wires, which is 1 meter apart in a vacuum.

Voltage (V): The volt is the International System of Units (SI) measure in electric potential of electromotive force. A potential of one volt appears across a resistance of one ohm when a current of one ampere flows through the resistance.
                        1000 V = 1 Kilo Volts (KV)

Resistance:
Resistance = Voltage/Current
The unit of resistance is ohm (Ω)

Ohm’s Law
Ohm’s law states that the current through a conductor is directly proportional to the potential difference across it, provided the temperature and other external conditions remain constant.

Frequency
The supply frequency tells us the cycles at which alternating current changes. The unit of frequency is hertz (Hz: cycles per second)

Kilovolt Ampere (KVA)
It is the product of kilovolts and amperes. This measures the electrical load on a circuit or system. It is also called the apparent power.
For a single phase electrical circuit , Apparent power (KVA) = (Voltage X Amperes)/1000
For a 3 phase electrical circuit, Apparent power (KVA) = (√3 X Voltages X Amperes)/1000

Reactive Power (KVAr): KVAr is the reactive power; reactive power is the portion of apparent power that does no work. This type of power must be supplied to all types of magnetic equipment, such as motors, transformers etc. Larger the magnetizing requirement, larger the KVAr.

Kilowatt (KW) (Active Power): KW is the active power or the work –producing part of apparent power.
For single phase, Power (KW) = (Voltage X Ampere X Power factor)/1000
For 3 Phase, Power (KW) = (√3 X Voltage X Ampere x Power factor)/1000

Power factor: Power factor (PF) is the ratio between the active power (KW) and apparent power (KVA).
Power factor (cos ɸ)     = Active Power (KW) / Apparent Power ( KVA)
                = KW/{√(KW)2 + (KVAr)2}
                = 1.0 (when KVAr = 0)
When the current lags the voltage like in inductive loads, it is called the lagging power factor and when the current leads the voltage like in capacitive loads, it is called leading power factor.
    Inductive loads such as induction motors, transformers, discharge lamp, etc. absorb comparatively more lagging reactive power (KVAr) and hence, their power factor is poor. Lower the power factor; electrical network is loaded with more current. It would be advisable to have highest power factor (close to 1) so that network carries only active power which does real work. PF improvement is done by installing capacitors near the load centres, which improve power factor from the point of installation back to the generating station.
Kilowatt-hour (Kwh): Kilowatt-hour is the energy consumed by 1000 watts in one hour. If 1KW (1000 watts) of electrical equipment is operated for 1 hour, it would consume 1 Kwh of energy (1 unit of electricity). For a company, it is the amount of electrical units in Kwh recorded in a plant over a month for billing purpose. The company is charged/billed based on Kwh consumption.

Electricity Tariff:
Calculation of electric bill for a company
Electrical utility or power supplying companies charge industrial customers not only based on the amount of energy consumed (Kwh) but also on the peak demand (KVA) for each month.

Contract Demand
Contract demand is the amount of electrical power that a consumer demands from the utility in a specified interval. Unit used is KVA or KW. It is the amount of electric that the consumer agreed upon with the utility. This would mean that utility has to plan for the specified capacity.

Maximum Demand
Maximum demand is the highest average KVA recorded during any one-demand interval within the month. The demand interval is normally 30 minutes. The demand is measured using tri-vector meter/digital energy meter.

Prediction of Load
While considering the methods of load prediction, some of the terms used in connection with power supply must be appreciated.
Connected Load: It is the name plate rating (in KW or KVA) of the apparatus installed on the consumer’s premises.
Demand Factor: It is the ratio of maximum demand to the connected load.
Load Factor: The ratio of actual Load to maximum load
Load factor = Actual Load/Maximum Load

PF Measurement: A power analyzer can measure PF directly, or alternately KWh, KVAh or KVArh readings are recorded from the billing meter installed at the incoming point of supply. The relation KWh/KVAh gives the power factor.
Three phase AC Power Measurement: Most of the motive drives such as pumps, compressors, machines, etc. operate with 3 phase AC Induction motor. Power consumption can be determined by using the relation:
        Power = √3 X Voltage X Current X Cosɸ

 

BEE Manuals