SPECIAL ENERGY METER: 101 COMPREHENSIVE GUIDE

ENERGY METER

The meter used for the measurement of electrical energy consumed by loads is called an energy meter. Generation, Transmission, or distribution stations from where energy is transmitted in bulk need to keep a measurement of the energy transmitted, and hence special energy meters are often found installed at the substations.

SEM or special energy meter is an application-specific microprocessor-based solid-state energy meter. This coupled with data collecting device DCD and a local computer forms a powerful system for metering and data logging. This type of data logging gives a unique method for the measurement of all required electrical parameters.

These meters are manufactured in 0.2s class as per IEC 62053 standard. Its working voltage is 63.5V. it is a composite meter capable of measuring active and reactive energy in all four quadrants.

Direct measurement is shown at the display as per CT/PT secondary quantities. It can filter the harmonics and measure energy at fundamental frequency. It can operate on power from a connected PT supply which is 110V Phase to Phase or 63.5V Phase to neutral.

VA burden of these meters is not recommended to be more than 10 on any of the phases. It works on a real-time clock and hence data remains time-stamped to an accuracy of 60sec/month. These types of meters require no calibration due to the absence of any moving parts.

DATA RECORDING FORMAT OF ENERGY METER:

Data recorded by these meters are normally not a readable text file. Therefore data downloaded from these meters have to be converted to readable text format for computation and analysis. All data from the last 10 days and the most recent complete 15-minute block data is available up to the time of collection in non-volatile memory.

DATA RECORDED IN EVERY BLOCK:

Net active energy in 15 min time block watt-hour.

Average frequency in 15 min. Time block records frequency in the time band of 49 to 51 Hz in steps of 0.02 Hz in a coded form as an integer from 00 to 99. Here FC means frequency code and f means frequency.

Voltage failure flag: A * is recorded if the average voltage in the three phases falls below 70%

Time correction flag: ‘aa’ is recorded if the time is advanced and ‘rr’ is recorded if time is retarded.

Data recorded once a day at 00:00 hrs:

• SL no of meter
• Cumulative active energy register- C
• High voltage reactive energy register- H
• Low voltage energy register- L
• Date

MEASUREMENT PRINCIPLE OF ENERGY METER:

Wh Measurement

The active energy Wh measurements are carried out on a 3-phase, 4-wire principle with accuracy as per 0.2s class of IEC-62053-22. The meter computes the net active energy Wh in each successive 15-minute block and stores it in non-volatile memory along with a plus sign for net export or a minus sign when there is net import.

VARh Measurement:

The meter also computes the reactive power (VAR) on a 3-phase 4-wire principle with accuracy as per class 0.2S of IEC-62053-23. There are two reactive energy registers, one for the period when the average RMS voltage is above 103% and the other for the period when the voltage is below 97%. The registers will move forward when there is a VARh export and will move backward when there is a VARh import.

Average Frequency

The meter also counts the number of cycles in VT output during each successive 15-minute block and divides the same by 900 (15*60=900) to arrive at the average frequency. The average frequency is stored in the meter’s memory in 2-digit code neglecting decimals. In case the average frequency is less than 49.0Hz, it is recorded as 00 and in case it is 51.0Hz or higher it is recorded as 99. The actual frequency corresponding to two consecutive codes differs by 0.02Hz.

Operating Section of the Meter

Analog Section:

Precision Current transformer and voltage transformer step down input currents and voltages which are fed to an analog-to-digital converter.

Digital Section:

A powerful microprocessor controls the Analog and Digital sections. Analog to Digital converters are fed from high-precision instrument transformers. The Digital samples from Analog to Digital converter are used to process the metering data. The sampling rate is 3000 samples per second which gives the meter exceptional accuracy and results.

Power supply Section:

A switch mode power supply (SMPS) unit supplies power to the meter’s internal circuit.

Sign convention of Active and Reactive energy recorded

For SEMs on all locations except GTs at the Generating station. When energy (active/reactive) flows out of the bus to which the SEM is connected, the energy is recorded with a ‘+’ sign and vice versa.

For SEMs on GTs at Generating Stations: When energy (active/reactive) flows into the bus to which the SEM is connected, the energy is recorded with a ‘+’ sign and vice versa.

Accordingly, the values in the cumulative registers for active energy and reactive energy keep incrementing when the net energy for a particular time period recorded is ‘+ve’ and decrementing when it is ‘-ve’. For example, if the initial value of these registers is 00000.0 (say) and the net energy in a block is 25.0 (say), the cumulative value of the register increments to 00025.0. Once the value reaches 99999.9, the same will get reset to 00000.0

It may be noted that no multiplication factor is incorporated in the meter and hence, the data recorded is of secondary side Wh. To get the data on the primary side in MWh the recorded data has to be multiplied with the multiplication factor.

WIRING IN ENERGY METERS

connection from the current transformer’s metering core secondary is given to the SEM for current measurement via the test terminal block and for voltage measurement connections of potential transformers are extended to the special energy meter.

FAQ’s