MDI based electricity bill


#1

AOA

I need help in calculating the MDI (Max Demand Indicator??) electricity bill for a commercial LESCO connection. Also want to know what effect does the devices locally called "power factor" have on the bill. Any help is appreciated.

Regards

Khawar


#2

I know only about your second part of the question, sorry about first part.

As long as you are in DC voltage domain, the simple Ohm's law applies. When you come to AC domain, things start getting complex. Since the AC is alternating (a sine wave), the current drawn by various kinds of load on AC is not in-phase with the voltage, causing a phase shift between Voltage and Current. This phase shift, in turn causes many problems. Power factor is basically COSINE value of this angle (phase shift)

Ideally the power factor should remain at 1.0 at which the voltage and current are in phase, this happens only in case of resistive loads like old incandescent lamps, heaters etc. If power factor is 1.0, the simple Ohm's Law still applies and W = V x I (watts = volts x amperes)

Now if power factor is lower than 1.0, for example a 23W energy saver lamp has a PF = 0.55 typically, the electricity company has to supply you 23W/0.55 = 41.8 VA. In commercial or industrial usage, this loss due to low power factor is a financial loss to electricity company and they charge the customers.

In old days, the energy meters were Watt-Hour meters and for commercial / industrial consumers, WAPDA used to install MDI meters to check how much low the power factor becomes and this is still in practice.

Now with digital energy meters, this MDI (Maximum Demand Indicator) thing is built into the meter and consumers are automatically billed according to their power factor.


#3

Thank you Ijaz for your input. That cleared up some things for me.

I wanted to know more about the device locally known as the "power factor".

It is connected to the main line after the WAPDA meter and is supposed to somehow enhance / maintain the power factor , which in turn reduces the electricity bill.

[quote=", post:2, topic:14815"]

I know only about your second part of the question, sorry about first part.

As long as you are in DC voltage domain, the simple Ohm's law applies. When you come to AC domain, things start getting complex. Since the AC is alternating (a sine wave), the current drawn by various kinds of load on AC is not in-phase with the voltage, causing a phase shift between Voltage and Current. This phase shift, in turn causes many problems. Power factor is basically COSINE value of this angle (phase shift)

Ideally the power factor should remain at 1.0 at which the voltage and current are in phase, this happens only in case of resistive loads like old incandescent lamps, heaters etc. If power factor is 1.0, the simple Ohm's Law still applies and W = V x I (watts = volts x amperes)

Now if power factor is lower than 1.0, for example a 23W energy saver lamp has a PF = 0.55 typically, the electricity company has to supply you 23W/0.55 = 41.8 VA. In commercial or industrial usage, this loss due to low power factor is a financial loss to electricity company and they charge the customers.

In old days, the energy meters were Watt-Hour meters and for commercial / industrial consumers, WAPDA used to install MDI meters to check how much low the power factor becomes and this is still in practice.

Now with digital energy meters, this MDI (Maximum Demand Indicator) thing is built into the meter and consumers are automatically billed according to their power factor.

[/quote]

#4

[quote=", post:3, topic:14815"]

Thank you Ijaz for your input. That cleared up some things for me.

I wanted to know more about the device locally known as the “power factor”.

It is connected to the main line after the WAPDA meter and is supposed to somehow enhance / maintain the power factor , which in turn reduces the electricity bill.

[/quote]

IIRC, these power factor devices are primarily for inductive loads. Usually, there aren’t enough inductive devices in a residence to make any significant difference in the bill compared to the cost of such devices on each such load.


#5

[quote=“Asad, post:4, topic:14815”]

IIRC, these power factor devices are primarily for inductive loads.

[/quote]

True

[quote=“Asad, post:4, topic:14815”]

Usually, there aren’t enough inductive devices in a residence to make any significant difference in the bill compared to the cost of such devices on each such load.

[/quote]

You will be surprized to know that most of the loads in household appliances are inductive with few exceptions like electric iron, electric heater and old incandescent bulbs. Even the PC, monitors, chargers, energy savers, TV, fridge are all inductive.

Correcting power factor is just a matter of attaching a certain amount of capacitors to your electric mains in your home at any place. This total value of capacitance required to correct the power factor depends upon how much inductive laod is being connected. But if you add more capacitors than required, there would be loss instead of benifit. So there should be some intelligent control which measures how much low the power factor is and how much capacitors are required to correct it to optimum value. This is commonly known as capacitor bank switching.


#6

^ Thanks for correcting.