Energy Loss Measurement and Monitoring

Load curve

A load curve, also known as load profile, is a chart illustrating the variation of electrical load over a period of time. Generating companies or electrical utilities use this chart to study the pattern of load variation and to obtain information regarding the amount of power to be generated at a specific time.

An example of a load curve is given in the figure below.

Load factor

Load factor is defined as the ratio of average load to the peak load. It can also be expressed as the ratio of a number of units supplied during a certain period of the maximum energy demand during that period. Load factor refers to the energy load on a system as compared to its peak load for a given period of time and is most typically calculated on a monthly or annual basis.

Load factor is given as,

Load factor (LF) = Average load/Peak load

= No. of energy units supplied during a particular period/Maximum power demand during that period × Period

Loss factor

It is defined as a factor which when multiplied with energy loss at peak time and the number of loads periods gives the overall average energy loss. It is expressed as the ratio of average power loss to the peak power loss or power loss at peak load. If the load to a power plant is constant, throughout the day, the loss factor of the plant is 1. But in practice, the load to a plant varies throughout the day which is given by a load curve.

An approximate value of loss factor can be found by the following equation:

Loss factor = (0.3 × Load factor) + (0.7) × (Load factor)²

Numerical example:

The load curve of a power plant for a day is shown in the figure below. Calculate the load factor, loss factor, and energy loss in a day if the installed capacity of the plant is 150 MW and energy loss at peak load is 10 MW.

Solution:

According to the load curve,

Total units supplied in a day = (50 × 8) + (80 × 4) + (130 × 5) + (60 × 7) = 1790 units

Therefore, average load = 1790/24 = 74.58 MW

Load factor = Average load/Peak load = 74.58/130 = 0.5736

Loss factor = (0.3 × Load factor) + (0.7) × (Load factor)² = (0.3 × 0.5736) + (0.7) × (0.5736)² = 0.4023

Actual power loss = Power loss at peak load × Loss factor = 10 × 0.4023 = 4.023

Thus, energy loss in a day (24 Hours) = 24 × 4.023 = 96.552 KWh

Metering and measurement

Metering, or meter reading, is defined as the act of measuring physical quantities (energy in case of electric power system) with the aid of certain meters or instruments. It can also refer to the datum about some physical state which is obtained by using a meter and some other instruments. Meter reading, in the case of electric power system, is done to determine and record the amount of actual energy consumption by an individual customer over a period of specific time. Generally, meter readings are conducted on a monthly basis or once in two months or once in three months. They may be done either by the utility itself by appointing meter readers or on some contract basis through some contractors.

The amount of electric energy consumed by a user is determined by the utilities with electric meters installed at the consumers' homes. Electric meters are of two types. They are described briefly below.

• Electromagnetic type

Electromagnetic meters are the conventional type of meters which are commonly used by consumers. They consist of a non-magnetic but electrically conductive disc which rotates proportionally to the power passing through the meter.

An electromagnetic meter constantly consumes a small amount of power proportional to the voltage (which is not registered on the meter) and proportional to the amount of current flowing through the meter (which is registered on the meter). The rotating disc is therefore acted upon by two sets of coils which result in the formation of a small two-phase induction motor. The voltage coil produces a magnetic flux in proportion to the voltage and the current coil produces a magnetic flux in proportion to the current. The disc is therefore subjected to a flow of eddy currents and a force is exerted on the disc which is proportional to the instantaneous value of current, voltage and the existing power factor. A permanent magnet serves to act as a brake which exerts a braking force proportional to the speed of rotation of the disc. Therefore, the disc rotates at a constant speed which is proportional to the power flowing through the meter or in other words, the power consumed by the consumer.

Electromagnetic meters are cheap but have more moving parts which cause more friction and breakdown of the moving parts. They are also less accurate than the electronic meters. They are used on a single phase AC supply.

• Electronic type

Electronic or digital type meters are superior to the electromagnetic type in terms of functionality, features, and accuracy. However, they are costlier than the electromagnetic type meters. The meter usually consists of a power supply, a metering engine, and a processing system. Voltage and current are given as input to the electronic meters along with reference voltage. Some form of sampling and quantizing circuits are also used along with ADCs (Analog to Digital Converters) in order to obtain a digital output. The processed parameters are finally displayed on the LCD or LED screen. These meters also have the feature of data logging in order to store the data for future use.

Metering types

1. Low tension metering

Low tension metering is employed for low supply voltages. The voltage level, in this case, is either 230 V (single phase supply) or 400 V (three phase supply). The low tension metering can again be classified into two types which are listed below.

Whole current metering

This method of metering is employed when the flow of current is small. This is usually the case in households of the consumers and some small business ventures. The KWh meter or the energy meter is directly connected to the system voltage and the current passes through the meter directly. The schematic representation is shown below.

• CT metering

This method is employed when the flow of current is large. This is usually the case in small industries. As shown in the figure below, the energy meter is directly connected to the system voltage while the system current is fed to the energy meter after reducing it with a current transformer.

2. High tension metering

This type of metering method is employed when the system voltage is greater than 400 V and the system current is also substantially large. This is usually the case in commercial consumers such as shopping malls, entertainment centers, etc. and industries. The system voltage and current should be reduced before being fed to the energy meter. Therefore, current transformers and potential transformers should be used to reduce the current and voltage respectively. The schematic diagram of the metering method is shown below.

1. 1. Wikipedia. (2016)Electricity Meter.Retrieved from:https://en.wikipedia.org/wiki/Electricity_meter