Special Types of Energy Meters

Special Categories of Energy Meters:

In the previous article, we discussed basics types of energy meters. In this article I will continue the discussion on other special types of energy meters.

Types of Energy Meters
Figure 1: Types of Energy Meters

Let’s take this hierarchy diagram again. We already understood about the types of energy meters with respect to technology. Now, let’s talk about category of energy meter with respect to load level.

Types of Energy Meters with respect to load level:

By load level I refer to the current bearing capacity of Energy Meters or you can say how much Electric Power, an Energy Meter can endure. Yes, Energy Meters have their current capacity as well. As Energy Meters and Load are always connect in series, so you must take care about the current capacity of Energy Meter before applying load on it.

There are three types of Energy Meters with respect to load level:

  1. WC (Whole Current) Meter
  2. LT (Low Tension) Meter
  3. HT (High Tension) Meter

Whole Current Meter:

Obviously, Whole Current Meters are those meters in which whole current passes through the Meter. Normally these meters come in the range from 0~100 Amps. If you ever worked on CTs (Current Transformers), you will notice that in the range of 0~100 Amps, CTs comes in small sizes that can fit anywhere. So, CT of these energy meters are present inside the meter. In other words, whole current must pass “through the Energy Meter” to calculate the Energy units, this is the reason why these energy meters named as WC (Whole Current) meters.

 Whole Current Energy Meter
Figure 2: 1 Phase Whole Current Energy Meter

LT (Low Tension) Energy Meters:

LT (Low Tension) Meters are those which can handle large load currents through externally present CT (current transformers). They are called as LT meters because they operate on not more than 440 volts. These meters are exactly same as WC meters but the only difference is their CT is not present inside the meter. A bulky high capacity CT installed in the series of this meter. This CT first step down the high current to the feasible level then the other CT present inside the meter step down the current even more, so that it can be measured easily.

LT Energy Meter
Figure 3: LT Energy Meter

Side Note: Energy meters only measure value of current that is given to it after the 1st CT. A multiplying factor is used to get the value of original current.

HT (High Tension) Energy Meters:

HT meters are those meters in CT (Current Transformer) as well as PT (Potential Transformer) present outside the energy meter. The reason behind calling them HT (High Tension) Meters because they normally installed on the feeder (11 kV) line. They are same Energy Meters as other meters but they have a combination of bulky CT and PT present outside the meters. PT is present there to step down the voltage from 11kV to 110 volts and then this voltage is given inside the meter to the PT or voltage divider depending upon the type of Energy meter.

Side Note: Mostly Energy meters uses voltage divider to step down the voltage due to their cheap cost and less bulkiness.

The function of CT is same as LT meter which is not necessary to explain here again.

Multiplying factor:

In HT Meters both multiplying factor of CT and PT are used. For example, if 500:5 (A) CT is present and 11000:1100 (V) PT is present then the MF will become like this.

MF = 100 X 10 = 1000

After multiplication we will get the original value of Energy Consumed.

Difference between WC, LT and HT meters
Whole Current Low Tension High Tension
Size Small Medium Big
TOD/TOU Functionality Yes Yes Yes
Voltage Level 3 Phase 440 V 3 Phase 440 V 3 Phase 11kV
Current Measuring Capacity 0~100 A 0 ~ 500 A 0 ~ 500 A
External Equipment Required Nothing Required CT CT & PT
Multiplying Factor Required No Yes (CT MF Required) Yes (CT and PT MF Required)

Table 1: Whole Current vs LT vs HT Energy Meters

Types of Energy Meters with respect to current direction:

Unidirectional Energy Meter:

These Energy meters are simple types of energy meters that you see every day. They just measure the current which is flowing inwards side or towards the consumer side. If the current is flowing towards outside then it does not count or negate that units. Therefor these meters name of these meters are unidirectional meters.

Bi-directional Energy Meters:

These are the special types of Energy Meters which measure currents in both directions. It means if current is flowing inward then it will add the units in its accumulated register and if current is flowing outwards then it will negate units from the register. With the advancement in solar technology, Electric consumers are shifting their loads towards the solar side. If someone has more solar panels than its load requirements, then they can sell additional Energy units (which are going to waste anyway) to the utility companies. This process is called as Net Metering. Utility companies can fulfill their energy deficiencies through this method. In Net Metering, Bi directional Energy Meters are the only solution to add or negate the energy units with respect to the current direction.

Types of Energy meters with respect to Class Accuracy:

Class Accuracies is another way to divide Energy Meters into different categories. In all energy meters, you will find written class 1, class 2 or class 0.5 etc. These class types is present on their bodies to define their accuracy level . Let’s take an example of 100-watt load to explain these class accuracies in an easy way.

Acceptable Ranges for different Meter on 100 Watts load:

  • Class-2 meters: 98 ~ 102 Watts
  • Class-1 Meters: 99 ~ 101 Watts
  • Class-0.5 Meters: 99.5 ~ 100.5 Watts

These acceptable ranges showing the meaning of class accuracies in a good manner. It clearly shows that classes give acceptable error percentage range of energy meter deviating from original value. If class 1 written on the body of meter, then it means the error percentage of this meter will lies in ± 1%.

Side Note: Modern Energy Meters are also coming in class accuracy of 0.1 or even more less.

Ismail Sarwar

Electrical engineering is my profession, my hobby and my passion. I completed bachelors of electrical engineering in 2015. Currently I am working with an Electric Utility Company. Power Electronics, Embedded Systems and Energy Metering are my fields of specialization. In free time I listen music and watch movies.

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