SEM Writeup for NPTI

Energy Metering, Meter data collection, Processing and Accounting

What is a Meter * : Meter means a device suitable for measuring, indicating and recording consumption of electricity or any other quantity related with electrical system and shall include, wherever applicable, other equipment such as Current Transformer (CT), Voltage Transformer (VT) or Capacitor Voltage Transformer (CVT) necessary for such purpose.

What is interface meter * (inter-utility meter): Interface meter means a meter used for accounting and billing of electricity, connected at the point of interconnection between electrical systems of generating company, licensee and consumers, directly connected to the Inter-State transmission system or Intra-State transmission system who have to be covered under ABT and have been permitted open access by the appropriate commission.

* Definitions from CEA Installation and Operation of Meters Regulation, 2006.

Requirements for inter-utility metering

Energy Meters to be used for billing on account of power supply have basically to be designed for the tariff scheme that they have to cater to, and must be capable of precisely measuring and recording the electrical parameters which appear in the applicable tariff structure. For example, domestic supply meters need only to measure the energy going through and display the integrated energy quantum up to that time, i.e. just one figure. Meters for power supply to industry, however have to additionally measure and record the maximum demand, for the conventional two part tariff. And time of the day tariff would require meters with different registers for the daily time blocks having different paise/kWH rates.

For inter-utility exchanges in Availability Based Tariff (ABT) regime , the most important parameters from Commercial point of view are not the Energy and maximum demand, but the deviations from the agreed MW drawal /injection schedules. Accordingly, the metering requirements of inter-utility exchanges are also totally different from those of metering for domestic and industrial consumers.

Measurement for 15-minute time block

To determine the deviations from agreed MW schedule, one first has to determine the actual MW flow over the whole recording period. This can be done by measuring and recording the MWh for each 15-minute time block. This would be first and foremost requirement of the Meters. It would actually provide the average MW for each consecutive 15 minute time block, which is just the right parameter for this purpose. Minute by minute MW variations are not necessary in Availability Based Tariff (ABT) scheme.

The 15-minute MWh data would normally suffice for all inter-utility tariff requirements for active energy and MW. The total cumulative energy register is not really necessary, but

Metering & Settlement 1 4/8/2023

may be specified for cross-checking and record purpose. Logging of this register’s reading at every midnight would therefore be desirable.

Accuracy class

Accuracy of the energy measurement is crucial, particularly on EHV interconnections carrying hundreds of MW. The best accuracy class for which energy meters are available, and for which international standards are also available, is 0.2 S according to IEC-687. Meters with this accuracy level would be fully justified considering that even 0.1% error in energy measurement could mean a monetary error of many lacs of Rupees per year per meter location.

Time synchronization of meters

A corollary of the above is that the metering system must have a high-accuracy internal clock and calendar. For correct summation of net MW drawal, all such meters throughout the grid should always have the same time in their internal clocks. However, keeping them in synchronism would require very elaborate arrangements. Periodic clock correction may be unavoidable (to prevent any serious drift), but is not very much desirable. Any periodic correction would mean that the MW/MWh relationship would get disturbed in the time block in which the time correction is effected. The periodic corrections can be minimized by adopting high clock accuracy, such as one minute per year. It is to be noted further that the shift of time blocks by a few minutes in a particular meter may not really make any material difference in overall accounting.

Frequency measurement

Inter-utility tariff in India is linked to system frequency and deviations from agreed schedule are priced higher when frequency is below normal (which represents a deficit situation) and are priced lower when frequency is high (which represents a surplus situation). Accordingly, it is necessary to measure and record the average frequency for each successive 15-minute time block as well, so that deviations from MW schedule could be correlated with frequency during the relevant period.

Since frequency is same all over in an integrated AC network, it should not be necessary to measure frequency separately at each inter-utility metering point. However, a network could split at times, and therefore, it is desirable to measure and record average 15-minute frequency at all metering locations. The frequency record that would then be available can be usefully employed for monitoring meter functioning and their common synchronism with the standard time. A very precise measurement of average frequency is essential since the paise/kWH rate would change at specified frequency levels.

Reactive energy measurement

The Regional Grids in India also suffer from serious voltage profile problems due to unchecked reactive (VAR) drawals. These call for a voltage related VARh recording such


that VARh drawals are charged while voltage is low, and a credit is given for VARh drawal when voltage is high. Accordingly, the inter-utility metering have two cumulative VARh registers, one for voltage high condition, and one for voltage low condition. These registers operate on a continuous basis and there is no need to have any 15-minute time block wise data. Actual voltage thresholds 103% and 97% are decided from practical consideration. Logging of readings of two VARh registers are to be done at every midnight.

Parameters to be measured by inter-utility meters

To recapitulate, the meters deployed for inter-utility exchanges would have to measure and record

(i) Wh transmittal for each successive 15-minute block.(ii) Average frequency codified for each 15-minute block(iii) Cumulative Wh register reading at each midnight(iv) Cumulative VARh register reading for voltage high condition at each midnight(v) Cumulative VARh register reading for voltage low condition at each midnight

These readings should be properly dated and time tagged, and also tagged with the unique meter identification number and model (1A or 5A). In addition, all 15-minute blocks in which a VT supply abnormality occurs should also be identified.

Display parameters

Since the Static meter do not have rotating disc, and one would not know otherwise whether they are functioning, the meters should have a visual pulse output for energy. The meters must also display, on demand, all parameters being measured and recorded such as

(i) Cumulative Wh reading(ii) Average frequency for the previous 15-minute block(iii) Net Wh transmittal during the previous 15-minute block(iv) Average voltage(v) Reactive Power(vi) Voltage high VARh register reading(vii) Voltage low VARh register reading

In addition, the meter’s unique serial no., date and time should also be displayed.

Weekly data collection and transmittal to central location

Determination of the net MW drawal / injection by an utility requires time block-wise summation of energy transmittals recorded by meters on all interconnections of the utility. This summation has to be done centrally, and requires transmission of the data recorded in the meters to the central location who would be doing computations. This can be carried out once a week, and accordingly data collection and transmittal from meters also can be once in a week say every Monday.


Static Meters : need and requirements

If the foregoing measurements are to be made using induction type meters, at least four meters would be required for each circuit – Wh export, Wh import, VARh export and VARh import – with reverse stops and pulse transmitting devices. In addition, frequency transducers, voltage relays / transducers and a microprocessor would be required for the full metering scheme. All this can be replaced by just one compact meter. Another major advantage of static meters is the achievable accuracy 0.2% against 0.5% of induction meters.

The meters should also be totally tamper-proof. This requires that the meters should not have any possibility of any adjustment at site, which in turn means that CT and VT ratios should either be permanently pre-set at factory, or should be accounted for separately. The latter is preferable, since it allows total interchangeability with just two standard models-one for 110 V, 1A and the other for 110 V, 5A. The meters should also be sealed for life, and any repair should be possible only at the manufacturer’s works. This removes any requirement of stocking spare parts.

The static meters are particularly prone to electrical spikes and surges. It is therefore preferable to avoid any electrical connections except for CT and VT leads (which are unavoidable). In particular, data collection should be through optical coupling, and no auxiliary supply should be required. Meters should accordingly work from the VT supply backed-up by an internal, long life battery.

The EHV system invariably has solid neutral grounding, and is therefore 4-wire systems. Energy measurement should accordingly be on 3-element, 4-wire basis, for the required high accuracy. Since the static meters have no moving parts, they are not prone to accuracy drifts due to friction, transport stresses etc. As such, it should not be necessary to test and calibrate them too frequently. The trend is towards testing them only at 5 year interval.

Special Energy Meters in India (The inter-utility meters)

With the implementation of Availability Based Tariff (ABT) in India, Special Energy Meters (SEM) with Data Collecting Device (DCD) has been adopted for inter-utility metering. In inter-State level, CTU (POWERGRID) is responsible for installation as well as maintenance/testing of SEMs and RLDCs validate & process the meter readings received from various locations. STUs and SLDCs are mandated to carry out similar roles in intra-State level.


SEM is Microprocessor base solid state static energy meter and works on 3-phase 4-wire principle. Picture of a typical SEM is as below:

Data Collecting Device (DCD) is a hand held terminal, capable of communicating with SEM as well as PC. Both communications requires suitable software at DCD and SEM.


LCD Display

Optical Port

Scroll Buttons

R Phase PT Lead

Y Phase PT Lead

B Phase PT Lead

Neutral Lead

RS-485 TB

B Phase CT Lead

R Phase CT Lead Y Phase

CT Lead

Measurement Principle

a) Wh Measurement The active energy (Wh) measurements are carried out on 3-phase, 4-wire

principle with an accuracy as per class 0.2S of IEC-687/IEC-62053-22. The meter computes the net active energy (Wh) in each successive 15-minutes block and stores it in its non-volatile memory along with plus/ minus sign. The 15-minute Wh shall have a +ve sign when there is a net Wh export and a –ve sign when there is a net Wh import. Cumulative Wh reading at each midnight shall be stored in the meters memory along with plus/ minus sign. The meter shall store all data in its memories for a period of ten (10) days. The data older than ten (10) days shall get erased automatically. Memory storage in SEMs follow FIFO rule.

b) VARh MeasurementThe meter shall also compute the reactive power (VAR) on a 3-phase 4-wire

principle with an accuracy as per class 0.2S of IEC-687/IEC-62053-23. There shall be two reactive energy registers, one for the period when average RMS voltage is above 103% and the other for the period the voltage is below 97%. The registers will move forward when there is VARh export and will move backward when there is VARh import.

c) Average FrequencyThe meter shall count the number of cycles in VT output during each

successive 15-minute block and divide the same by 900 (15*60=900) to arrive at the average frequency. The average frequency is stored in 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 differ by 0.02Hz.

Display ParametersSpecial Energy Meters have the facility to display the following parameters on

pressing a push buttion:

Display Parameter: Indication Display format 1. Meter identification code A NP1234A2. Date (day, date, month, year) d dd-mm-yy 3. Time (hour, min, sec) t hh:mm:ss 4. Cumulative Wh reading c xxxx.x Wh5. Reactive Power Pr xxx:x VAr 6. Average freq. of previous block F xx:xx 7. Net Wh transmittal-previous block E xx:xx 8. Average % voltage U xx:xx 9. Voltage high VArh register reading H xxxx:x VArh

10. Voltage low VArh register reading L xxxx:x VArh 11. Real time indication rtC Fit 12. Low battery indication Low Bat


Location of Meters

Special Energy Meters are to be installed at different locations in the following manner as per CEA Installation and Operation of Meters Regulation, 2006 dated 17.03.06 :

Location Main Meter Check Meter Standby Meter

GeneratingStation

On all outgoing feedersOn all outgoing

feeders

HV side of GTHV side of StationAux. Transformers

TransmissionSystem

At one end of the line between the S/Ss of the same licensee and at the both ends of the line between S/Ss of

two different licensees. Meters at both ends shall be considered as main meter.

----

There shall be no separate standby

meter.Meter installed at other ends of the

lines in case two different licensees shall work as standby meter.

ICT HV side of ICT -----LV side of ICT

Main meter is a meter which would primarily be used for accounting and billing of electricity.

Check meter is a meter which shall be connected to the same core of CT and VT to which main meter is connected and shall be used for accounting and billing of electricity in case of failure of main meter.

Standby meter is a meter connected to CT and VT, other than those used for main meter and check meter and shall be used for accounting and billing of electricity in case of failure of both main meter and check meter.

Meter Data Collection

1. Data collection through DCD:

Data Collecting Device (DCD) is used for data collection from SEM and transfer of the same to a local PC.

Each DCD generally have :

1. A lead with optical head for coupling it to the SEM (Optical Cable)2. A lead for plugging it to PC (Data Cable).3. An internal battery for powering the device.4. A battery charger.


Data Cable: RJ-11 (telephone jack) or round 9-pin male connector (D-cable) at one end for connecting with DCD and 9-pin female connector at other end for connecting either with optical cable or PC.

Optical Cable: A cable with optical head at one end for coupling with SEM optical port and 9-pin male connector at other end for connection with data cable.

Suitable software is provided with DCD to enable data collection from SEMs and a software is used for dumping the same data to local PC in a pre-decided format.

Optical port of DCD: Each meter shall have an optical port on its front for tapping all data stored in its memory. Dimensions of optical port and optical head as per IEC-62056-21 standard are as below:


LCD Display

9-Pin Connector

Power ON / OFF switch

Status LEDs

RJ-11 Port

Battery Charging Socket

Data CableD-Cable Port

Optical Port Optical Head

2. Data collection through RS-485 Scheme (automatic data downloading):

RS-485 scheme facilitates automatic data downloading from all the meters in a location to a local PC. RS-485 standard is a balanced data transmission scheme with two wire connectivity (having ground). SEMs are RS-485 compatible having DATA (+), DATA (-) & GROUND terminals in its RS-485 terminal block. However, as RS-232 (serial binary data communication) is used in Computer serial ports, RS-485 to RS-232 converter is required for communication between a meter and PC. RS-232/9 pin serial port of PC is connected with RS-232 port of converter and RS-485 port of converter is connected with the RS-485 terminals of SEM.

The following connection diagram shows four meters connected in parallel to a PC using RS-485 scheme of data communication.


20 mm

30 mm

34 mm max.

6.5 mm +/- 0.5mm symmetrical

13 +/-1

28 mm

With the help of Scheduler option of meter software, data is automatically collected from SEM to local PC.

After successful collection of data from SEM, same is transmitted to RLDC through e-mail.

Meter data processing

1. Converting binary file to text file: Meter software is installed at RLDCs and with this software, binary meter data file is converted to text file in “*.npc” format. The structure of *.npc file is as below:

Average Frequency is recorded for every 15 min block as a 2-digit code ( 0 to 99) and is computed based on the no. of voltage cycles in one 15 min block divided by (15*60=900). Computation of average frequency is done only for the period during which the VT supply was available in the 15 min block under consideration. The frequency data is available for each successive 15 min block in encoded form as (Average frequency – 49)*50 and neglecting decimals. When frequency is <=49 it is recorded as 0 and when the frequency is >=51 it is recorded as 99. The actual frequency corresponding to two consecutive codes differ by 0.02Hz.

2. Appending text files : Multiple npc files are combined (appended) into one single large file (for an entire Region) for better file management and for further processing.


SEM No.

Blockwise Avg. Freq. code

Blockwise active energy

MidnightNet Wh

Reactive High Energy

Reactive Low Energy

The appended file contains complete meter data of the region for the relevant period in text form and is used in all subsequent processing. The appended text file is used for calculating the actual figures pertaining to Drawal, Injection and inter-Regional flow.

3. Master Frequency Meter : One SEM is chosen as Master frequency meter and frequency code recorded by that meter is used for all calculations. At present one SEM at Korba in WR is used as master frequency meter for NR-ER-NER-WR (NEW) Grid and data of this meter is sent to all RLDCs by WRLDC.

If in any particular week/day, the Korba meter reading is erroneous or missing or incorrect then one SEM at Durgapur in ER is used as Master Frequency meter.

4. Computation of injection / drawal through data processing software

i. Output files indicating drawal / injection : Output MWh files pertaining to Drawal, Injection and inter-regional exchanges are created.

Output file for the State Manipur in North Eastern Region is as below:

First column : TimeSecond column : Coded FrequencyColumn 3 to 6 : Inlet point-wise drawal by Manipur. Here + indicates drawal as provided in the software for convenience of data processing. YURAM, NINGTH, KARONG and JIRIBAM are the inlet points (Sub-Stations) of Manipur where Meters are installed.Column 7 : Indicates total drawal by Manipur as metered by SEMs.

Similar files are created for each State for everyday on 15-minute time block basis.


Total Manipur drawal

Output file for the Central Sector Generating Station Doyang in NER is as below:

First column : TimeSecond column : Coded FrequencyColumn 3 to 6 : Feeder-wise injection by Doyang generating station. Here + indicates injection and – indicated drawal as provided in the software for convenience of data processing. DIMR-I, DIMR-II, DOY-KOH and DOY-MOK are the outgoing lines from Doyang where Meters are installed.Column 7 : Indicates total injection / drawal by Doyang as metered by SEMs.

Similar files are created for each Generator for everyday on 15-minute time block basis.

Data checking and validation

Before proceeding with computation of Energy Accounting, it is very important to carry out data checking and validation. Through this process it can be established whether the SEMs are recording data correctly. In case of any discrepancies observed, corrective action has to be taken, like say, intimation to concerned agency for replacement of meters, use of alternate meters for computation of energy in case main meter is showing incorrect data, etc.


Total DoyangInjection

Identification of any metering error can be done through the three broad categories of checks :

(a) Net Bus of Sub-Stations(b) Net bus of Generating Stations and(c) Pair check (End1 – End2 of lines)

(a ) Net bus check of substations-

As an example, consider net bus of Kumarghat (PG) sub-station in NER

First column : TimeSecond column : Coded FrequencyColumn 3 to 7 : Metered Energy flow through different lines /ICTs connected to the Sub-StationColumn 8 : Summation of all incoming / outgoing energy for the sub-station (should be ideally close to zero).


( b ) Net bus check of generators-

For an example, consider net bus of Doyang :

First column : TimeSecond column : Coded FrequencyColumn 3 to 6 : Metered Energy flow through different lines /ICTs connected to the generating station.Column 7 : Total export/import (Sum of 3 to 6)Column 8 to 11 : Injection by all GTs and consumption by Station transformer.Column : 12 : Total injection (Summation of 8 to 11).Column 13 : Difference between 7 and 12 i.e. difference between total injection and total export through all feeders/ICTs. Should be ideally close to zero.


( c ) End1-End2 (pair) check-

These file are created to facilitate pair check of back to back meters connected at two ends of transmission lines.

Check for time drift and voltage failure condition

Checks are also carried out to ascertain time drift in the meters with reference to a Master meter. Validation process also includes check to find out voltage failure condition in any meter during any time-block.


SEM Writeup for NPTI

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