The techniques of metering can be classified depending upon the voltage at which consumers are fed as:
• LT (Low Tension) Metering, and
• HT (High Tension) Metering.
We describe these techniques in the following subsections.
LT Metering
The following types of meters are commonly used for LT consumers:
Whole Current Meters
i)Single phase meters
ii) 3-phase 4 wire meters
iii) 3-phase 3 wire meters
CT Operated Meters
iv) 3-phase 4 wire meters with CT and MD
v) 3-phase 3 wire meters with CT and MD
We now briefly describe these meters.
i)Single phase meters
Single phase meters are generally rated for 240 V AC supply; the current ratings are 5/10, 10/20, 2.5/10 or 5/20 A and are for direct connection to the mains. They start registration with small load of about 6 W and record up to 4800 W.
The main feature of a single-phase meter (electromechanical induction type) is its simplicity, compactness and robust design. It comprises a potential coil and a current coil. The potential coil is fitted on the middle limb of an E shaped electro-magnet and connected across the supply mains.
Similarly, the current coil consisting of a few turns of heavy gauge copper wire is wound on two limbs of a U shaped electro-magnet. The two fluxes,produced by the voltage (pressure or potential) coil and current coil, set up a mechanical torque on the non-magnetic aluminium disc (which is located between the 2 coils) causing it to rotate.A brake magnet of C shaped alloy steel is provided to control the movement or rotation of the disc set up by two fluxes. The disc rotates through the narrow air gap of the C shaped magnet and eddy currents are set up, which interact with the field and exert braking effect.
ii) 3-phase 4 wire meters
3-phase 4 wire meters (10/20, 20/50 A) are used for agricultural and industrial consumers. For loads up to 50 A, the meters are directly connected to the supply. For greater loads, it is preferable to provide CTs.They are rated for 415 V.In a 3-phase 4 wire meter, 3 pairs of coils of each type, i.e., potential coil and current coil are provided for each of the three phases. Each of the 3 elements (coil pairs) of the meter produces equal torque, i.e., one-third of the total torque under all conditions of varying power factor.
iii) 3-phase 3 wire meter
A 3-phase 3 wire meter has 2 elements of each type instead of 3 elements as in the case of 3-phase 4 wire meter. The basic principle of operation is the same as that for 4 wire meter. The torque produced by 2 elements is equal to each other when power factor (P.F.) is unity. At other power factors, the torques produced by the 2 individual elements are not equal and are of varying proportion. B phase element produces more torque at lagging power factors, while the R phase element produces more torque at leading power factor. This particular feature can be made use of to find out the P.F. of the load current of the consumer for the purpose of checking at site. Power factors less than 0.5 are indicated by the reverse running of the meter with only the R element in operation.
Usually, 3-phase 3 wire meters are not used in recording consumption of LT power consumers for the simple reason that the lighting load has to be properly connected on one of the phases R or B. Otherwise, if it is connected on the Y phase, where no current coil is provided, the energy consumption would not get recorded.iv) 3-phase 4 wire meters with CT and MD If load is generally more than 50 A, CT operated meters should be used. It is to be noted that CTs should be properly selected for accurate recording. Usually rating of CTs should fall within 50 to 80 percent of the maximum load current of the consumer.
For two part tariff, meters having recording arrangement for maximum demand in kVA are to be provided. The maximum demand (MD) indicators are additional mechanism attached to the meters to record the rate of consumption over a fixed period (usually half an hour) each time and then get reset with the help of a time switch. A pointer indicating the highest ever rate of consumption thus recorded by the MD indicator is left behind,which has to be manually reset while taking down the reading every month.
v) 3-phase 3 wire meter with CT and MD
It is exactly like 3-phase 4 wire meter and works on the same principle.It is also used in the same way and under the same circumstances as a 3-phase 4 wire meter – the only difference being that it is connected through CTs rather than being directly connected to consumer.
HT Metering
The following types of meters are commonly used for HT consumers:
• Trivector Meter,
• Bivector Meter, and
• Summation Meters.
• Trivector Meter
A well planned tariff covering an electricity service takes into account both the energy consumed, maximum demand and recording of power factor.
Tariff of this kind comprises:
− charge on energy consumed in kWh;
− charge on maximum demand in kVA; and
− charge on account of power factor, if it is below the specific limit (these charges are not required where energy charge is on kVAh basis)
A trivector meter is designed to record active, reactive and apparent energy along with MD indicators on all. The trivector meter is a compact unit. It replaces the set of instruments, which must otherwise be installed and has the advantage of yielding full and more accurate data.The trivector meter consists of three different recording elements, namely, kWh, kVAh and kVARh. The recording principle of kWh and kVARh element is the same as described in 3-phase meters whereas kVAh element records mechanically with the differential gearing arrangement.The meter is very reliable, robust and practically free of any adjustment even for a long period of service.
If the P.F. of the installation is leading, then kVARh element of the trivector has a tendency to record in reverse direction. But this is checked by providing a back stopper in kVARh element and if the power factor is leading, it records at unity power factor.
•Bivector Meter
As it is clear from the name given ‘Bivector’, it records kWh and kVAh consumption along with maximum demand on both or on any one as
required. The working principle of kWh element is as usually described.But the kVAh element is an independent unit and the recording of kVAh is made proportional to the arithmetic sum of kVAh in the three phases. The principle of kVAh meter is that line currents are rectified to DC current and this DC current is fed to the control winding of a transducer. The kVAh element always records in forward direction irrespective of PF lagging or leading. Thus, it gives the average PF of the installation.
In the case of bivector, if the PF is a leading one, kVAh element records in forward direction and the average PF is always shown as lagging one. Over compensation does not help as it would record more kVARh, thereby the power factor if calculated would be very low. The consumers’ PF so arrived is less than the prescribed limit, and he is liable for penalty for low P.F. In this respect bivector meters may be of importance but results in
penalizing the consumer, if compensation is not varied with varying loads in 24 hours of the day.
•Summation Meters
Summation meters are another special type of meters, which are used for recording the total consumption of a consumer fed at more than one point.The main purpose for connecting a summation meter is to facilitate the correct recording of the Simultaneous Maximum Demand, since adding up of the MDs recorded by the individual meters, arithmetically may not be correct because of the possible diversity in their periods, even though the total unit consumption can be worked out arithmetically. There are a number of methods for summing up the consumptions through one meter,some of which are as indicated below:
− Paralleling of the CT leads on the secondary side before connecting them to the meter current coil.
− Providing for each phase, one intermediate CT, known as summation CT, with a number of primary windings for connections to the
secondaries of individual CTs and only one secondary winding to feed the meter current coil.− Use of multiple current coils in the same meter for direct connections to the individual CTs, the measuring elements being common to all for the same phase.
− Having independent elements for all CTs mounted on a common shaft of the same meter so that the shaft gets the total torque and records total consumption thereby.
− Separate elements to measure the consumption independently as in the preceding method but having separate driving shafts connected to
a common one through differential gears to record the total consumption
− Separate meters, each fitted with an electrical impulse transmitting device, to record the consumption at various points independently and pass on the impulses simultaneously to a common meter which then records the total consumption.
• LT (Low Tension) Metering, and
• HT (High Tension) Metering.
We describe these techniques in the following subsections.
LT Metering
The following types of meters are commonly used for LT consumers:
Whole Current Meters
i)Single phase meters
ii) 3-phase 4 wire meters
iii) 3-phase 3 wire meters
CT Operated Meters
iv) 3-phase 4 wire meters with CT and MD
v) 3-phase 3 wire meters with CT and MD
We now briefly describe these meters.
i)Single phase meters
Single phase meters are generally rated for 240 V AC supply; the current ratings are 5/10, 10/20, 2.5/10 or 5/20 A and are for direct connection to the mains. They start registration with small load of about 6 W and record up to 4800 W.
The main feature of a single-phase meter (electromechanical induction type) is its simplicity, compactness and robust design. It comprises a potential coil and a current coil. The potential coil is fitted on the middle limb of an E shaped electro-magnet and connected across the supply mains.
Similarly, the current coil consisting of a few turns of heavy gauge copper wire is wound on two limbs of a U shaped electro-magnet. The two fluxes,produced by the voltage (pressure or potential) coil and current coil, set up a mechanical torque on the non-magnetic aluminium disc (which is located between the 2 coils) causing it to rotate.A brake magnet of C shaped alloy steel is provided to control the movement or rotation of the disc set up by two fluxes. The disc rotates through the narrow air gap of the C shaped magnet and eddy currents are set up, which interact with the field and exert braking effect.
ii) 3-phase 4 wire meters
3-phase 4 wire meters (10/20, 20/50 A) are used for agricultural and industrial consumers. For loads up to 50 A, the meters are directly connected to the supply. For greater loads, it is preferable to provide CTs.They are rated for 415 V.In a 3-phase 4 wire meter, 3 pairs of coils of each type, i.e., potential coil and current coil are provided for each of the three phases. Each of the 3 elements (coil pairs) of the meter produces equal torque, i.e., one-third of the total torque under all conditions of varying power factor.
iii) 3-phase 3 wire meter
A 3-phase 3 wire meter has 2 elements of each type instead of 3 elements as in the case of 3-phase 4 wire meter. The basic principle of operation is the same as that for 4 wire meter. The torque produced by 2 elements is equal to each other when power factor (P.F.) is unity. At other power factors, the torques produced by the 2 individual elements are not equal and are of varying proportion. B phase element produces more torque at lagging power factors, while the R phase element produces more torque at leading power factor. This particular feature can be made use of to find out the P.F. of the load current of the consumer for the purpose of checking at site. Power factors less than 0.5 are indicated by the reverse running of the meter with only the R element in operation.
Usually, 3-phase 3 wire meters are not used in recording consumption of LT power consumers for the simple reason that the lighting load has to be properly connected on one of the phases R or B. Otherwise, if it is connected on the Y phase, where no current coil is provided, the energy consumption would not get recorded.iv) 3-phase 4 wire meters with CT and MD If load is generally more than 50 A, CT operated meters should be used. It is to be noted that CTs should be properly selected for accurate recording. Usually rating of CTs should fall within 50 to 80 percent of the maximum load current of the consumer.
For two part tariff, meters having recording arrangement for maximum demand in kVA are to be provided. The maximum demand (MD) indicators are additional mechanism attached to the meters to record the rate of consumption over a fixed period (usually half an hour) each time and then get reset with the help of a time switch. A pointer indicating the highest ever rate of consumption thus recorded by the MD indicator is left behind,which has to be manually reset while taking down the reading every month.
v) 3-phase 3 wire meter with CT and MD
It is exactly like 3-phase 4 wire meter and works on the same principle.It is also used in the same way and under the same circumstances as a 3-phase 4 wire meter – the only difference being that it is connected through CTs rather than being directly connected to consumer.
HT Metering
The following types of meters are commonly used for HT consumers:
• Trivector Meter,
• Bivector Meter, and
• Summation Meters.
• Trivector Meter
A well planned tariff covering an electricity service takes into account both the energy consumed, maximum demand and recording of power factor.
Tariff of this kind comprises:
− charge on energy consumed in kWh;
− charge on maximum demand in kVA; and
− charge on account of power factor, if it is below the specific limit (these charges are not required where energy charge is on kVAh basis)
A trivector meter is designed to record active, reactive and apparent energy along with MD indicators on all. The trivector meter is a compact unit. It replaces the set of instruments, which must otherwise be installed and has the advantage of yielding full and more accurate data.The trivector meter consists of three different recording elements, namely, kWh, kVAh and kVARh. The recording principle of kWh and kVARh element is the same as described in 3-phase meters whereas kVAh element records mechanically with the differential gearing arrangement.The meter is very reliable, robust and practically free of any adjustment even for a long period of service.
If the P.F. of the installation is leading, then kVARh element of the trivector has a tendency to record in reverse direction. But this is checked by providing a back stopper in kVARh element and if the power factor is leading, it records at unity power factor.
•Bivector Meter
As it is clear from the name given ‘Bivector’, it records kWh and kVAh consumption along with maximum demand on both or on any one as
required. The working principle of kWh element is as usually described.But the kVAh element is an independent unit and the recording of kVAh is made proportional to the arithmetic sum of kVAh in the three phases. The principle of kVAh meter is that line currents are rectified to DC current and this DC current is fed to the control winding of a transducer. The kVAh element always records in forward direction irrespective of PF lagging or leading. Thus, it gives the average PF of the installation.
In the case of bivector, if the PF is a leading one, kVAh element records in forward direction and the average PF is always shown as lagging one. Over compensation does not help as it would record more kVARh, thereby the power factor if calculated would be very low. The consumers’ PF so arrived is less than the prescribed limit, and he is liable for penalty for low P.F. In this respect bivector meters may be of importance but results in
penalizing the consumer, if compensation is not varied with varying loads in 24 hours of the day.
•Summation Meters
Summation meters are another special type of meters, which are used for recording the total consumption of a consumer fed at more than one point.The main purpose for connecting a summation meter is to facilitate the correct recording of the Simultaneous Maximum Demand, since adding up of the MDs recorded by the individual meters, arithmetically may not be correct because of the possible diversity in their periods, even though the total unit consumption can be worked out arithmetically. There are a number of methods for summing up the consumptions through one meter,some of which are as indicated below:
− Paralleling of the CT leads on the secondary side before connecting them to the meter current coil.
− Providing for each phase, one intermediate CT, known as summation CT, with a number of primary windings for connections to the
secondaries of individual CTs and only one secondary winding to feed the meter current coil.− Use of multiple current coils in the same meter for direct connections to the individual CTs, the measuring elements being common to all for the same phase.
− Having independent elements for all CTs mounted on a common shaft of the same meter so that the shaft gets the total torque and records total consumption thereby.
− Separate elements to measure the consumption independently as in the preceding method but having separate driving shafts connected to
a common one through differential gears to record the total consumption
− Separate meters, each fitted with an electrical impulse transmitting device, to record the consumption at various points independently and pass on the impulses simultaneously to a common meter which then records the total consumption.
No comments:
Post a Comment