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NSMP Business Requirements Work Stream Current Transformers and

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					National Smart Metering Program




                         NSMP Business
                         Requirements Work Stream
                         Current Transformers and
                         Meter Resolution
                          Version number:   Version 0.2

                          Status:           Draft

                          Author:           Dr Martin Gill

                          Date published:   1st October 2009

                          File name:        NSMP Current Transformers and
                                            Meter Resolution v02.doc
NSMP Business Requirements Work Stream
Current Transformers and Meter Resolution




Table of Contents
1       DOCUMENT CONTROL .......................................................................................................................... 3 
     1.1      VERSION CONTROL ................................................................................................................................ 3 
     1.2      APPROVAL .............................................................................................................................................. 3 
     1.3      REFERENCES ........................................................................................................................................... 3 
2       INTRODUCTION ....................................................................................................................................... 4 
     2.1      BACKGROUND ........................................................................................................................................ 4 
     2.2      PURPOSE OF DOCUMENT ......................................................................................................................... 4 
3       THREE PHASE CT METERS................................................................................................................... 5 
     3.1      CURRENT TRANSFORMERS ..................................................................................................................... 5 
     3.2      RELEVANCE TO THE FUNCTIONAL SPECIFICATION.................................................................................. 7 
4       RECOMMENDATION............................................................................................................................... 8 
5       APPENDIX A – GLOSSARY..................................................................................................................... 9 




File Name: NSMP Current Transformers and Meter Resolution v02.doc                                                                                     Page 2 of 10
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Current Transformers and Meter Resolution




1            Document Control
       1.1        Version Control

 Version       Date                     Description                 Amended by

 0.1           6/09/2009                First Draft                 Dr Martin Gill
 0.2           1/09/2009                Updated after BRWG #7       Dr Martin Gill




       1.2        Approval

 Authorised by                            Signature                           Date

 NSSC Program Director



       1.3        References
The following documents are referred to in this document.

 Document Name                                                          Version

 NSMP Smart Metering Infrastructure Minimum Functional                  Version 0.14
 Specification
 National Electricity Rules Chapter 7 Metering                          Version 26




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2           Introduction
      2.1         Background
The Metrology Reference Group (MRG) has raised some questions about meter resolution for CT
connected meters.

      2.2         Purpose of document
This paper (briefly) attempts to discuss CT metering to explain the concepts involved before suggesting
modifications to the National Functional Specification.




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Current Transformers and Meter Resolution




3           Three Phase CT Meters
The supported meter configurations define a three phase Current Transformer (CT) connected electricity
meter. CT connected meters are required when the customer load is too large to pass directly through the
meter.

      3.1         Current Transformers
Voltage transformers are clearly visible throughout the distribution network. They are used to transform
one voltage to another. This is depicted in the following figure:




                                          Figure 1: Voltage Transformer
One side is designated the primary side and the transformed voltage appears on the secondary side. The
transformer ratio will determine the ratio between the two primary and secondary voltages.
In exactly the same manner Current Transformers can be installed to transform one current to another:




                                        Figure 2: 200:1 Current Transformer
The above figure shows a CT with a primary to secondary ratio of 200. As depicted in the figure a current
of 1000Amps on the Primary side of the transformer will appear on the Secondary side of the transformer
as 1000/200 = 5Amps. The lower current can be directly measured by an electricity meter.
The customer load is connected to the primary side of the current transformer and the meter is connected
to the secondary side of the transformer. For a three phase installation three separate current
transformers are required, one for each phase. This is depicted in the following figure:




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Current Transformers and Meter Resolution




                                        Figure 3: Three Phase CT connected meter


If each of the CT’s in Figure 3 was as specified in Figure 2 then we can compare the difference between
the actual customer load (primary side) and that measured on the meter (secondary side). Further we
assume that the phase to neutral voltage is 230V (with no voltage transformer being used).

                      Quantity                         Customer Load     Metered Load
                                                       Primary Side      Secondary Side

                      Current Transformer Ratio        200               1

                      Current Per Phase                1000A             5A

                      Voltage Per Phase                230V              230V

                      Watts Per Phase                  230kW             1.15kW
                      (assuming unity PF)

                      Watt-Hours consumed in           16.56MWh          82.8kWh
                      24 hours
                           Table 1: Worked Example of Primary and Secondary ratings
From these figures it is clear that this is a large load, the required metering type specified in Chapter 7 of
the National Electricity Rules is based on the annual energy consumption. There is no simple relationship
between peak consumption and annual consumption. The development of the SMI Minimum Functional
Specification has often considered the requirements of Type 4 (remotely read interval meters) which are
defined for annual loads up to 750MWh (which would support a business pulling the specified current for
4 hours per working day).
It is acknowledged that the majority of meters meeting the SMI Functional Specification will be installed for
small consumers where the annual consumption is 100MWh per annum. While it is still not possible to
determine the CT ratio used on these installations it is certainly likely to be smaller than the value shown in
Table 1.




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      3.2         Relevance to the Functional Specification
The effects of CT metering need to be clarified in several functions covered in the functional specification.

            3.2.1        Meter Resolution
The functional specification states:
      The resolution of energy measurement is at least 0.1kWh or 0.1kVArh.
The associated footnote adds:
      For CT meters this is the final resolution that must be supported taking into account scaling factors
      associated with transformer ratios (primary rating)
The functional specification is therefore requiring the meter to measure the customer load to 0.1kWh.
When using higher transformer ratios this requirement might not be possible. For the meter shown in
Table 1 to meet this requirement the actual meter resolution must be 0.0005kWh (or 0.5Wh). Clearly
higher CT ratios would place an even higher meter resolution requirement.
To determine if this is really an issue will require each jurisdiction to consider the maximum CT ratios that
they are likely to employ during the deployment of their Smart Meters.

            3.2.2        Auto-off function for the Supply Contactor
The Functional Specification supports a 2A relay in the CT meter to control an external supply contactor.
This is shown in the following figure:




                          Figure 4: CT connected meter with External Supply Contactor

As such we need to clarify use of the safety feature which monitors the load flowing through the meter for
a short time after turning on the supply contactor. Taking the wording from Version 14 of the Functional
Specification:

    When the Supply Contactor is switched to the On position

    For safety, the meter shall support an auto-off function if load is detected flowing through the meter
    when switching the Supply Contactor to the On position.



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    Meter will automatically turn off if more than “X” Watts of load is detected flowing through the meter for
    more than “Y” seconds during the measurement period of “Z” seconds after the Supply Contactor is
    turned on, where:
    (i). “X” range: 20 W – 25 kW per element, per phase, remotely and locally programmable in 20 W
    increments;
    (ii). “Y” range: 1- Z seconds, remotely and locally programmable in 1 second increments;
    (iii). “Z” range: 1- 3,600 seconds, remotely and locally programmable in 1 second increments;
    (iv). Enabling/disabling of load checking after turning the Supply Contactor on is configurable;
    (v). Remote alarming to the NMS that the meter has automatically turned off due to exceeding the
    programmable thresholds;

    For CT connected meters (with 2A relay)
          The setting of “X” will be the value measured by the meter, the meter does not need to know the
          CT ratio and is not required to calculate the customer load
A similar footnote has beeb added to Function 13 Supply Capacity Limiting.
    For CT connected meters (with 2A relay)
          The setting of Y, V and L will be the value measured by the meter, the meter does not need to
          know the CT ratio



4           Recommendation
The BRWG has resolved that the Smart Meter does not need to store the CT ratio so that all
measurements relate to the meter measured load (secondary side).
The BRWG has also clarified that the meter resolution will be 0.1kWh and has removed the footnote which
required CT meters to take into account the CT ratio.




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5           Appendix A – Glossary
The following acronyms are in the NSMP Glossary.

ACOSS           Australian Council of Social Services
AEMC            Australian Energy Market Commission
AEMO            Australian Energy Market Operator
AER             Australian Energy Regulator
AMI             Advanced Metering Infrastructure (Victorian smart metering program)
AS              Australian Standard
B2B             Business to Business
BPRG            B2B Procedures Reference Group (established under the IEC)
BPPWG           Business Processes and Procedures Working Group (established under the NSSC)
BRDRG           Business Requirements Definition Reference Group (established under the AMI
                program)
BRWG            Business Requirements Working Group (established under the NSSC)
CATS            Consumer Administration and Transfer Solution
COAG            Council of Australian Governments
DNSP            Distribution Network Service Provider
DRET            Commonwealth Department of Resources, Energy and Tourism (also referred to as
                RET)
EEEC            Equipment Energy Efficiency (E3) Committee
ENA             Energy Networks Association
ERAA            Energy Retailers Association of Australia
ESC             Essential Services Commission
FRC             Full Retail Contestability
FRMP            Financially Responsible Market Participant
HAN             Home Area Network
IEC             Information Exchange Committee (established under section 7.2A.2 of the Rules)
IHD             In-home Display
LNSP            Local Network Service Provider
MCE             Ministerial Council on Energy (established under the COAG)
MDA             Metering Data Agent
MDF             Metering Data File
MDFF            Metering Data File Format
MOU             Memorandum of Understanding
MRG             Metrology Reference Group (established under the RMEC)
MSATS           Market Settlement and Transfer Solution
MTWG            Metering Technology Working Group (established under the AMI program)
NCRE            National Consumer Roundtable on Energy

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NECF            National Energy Consumer Framework
NEL             National Electricity Law
NEM             National Electricity Market which excludes Western Australia and Northern Territory
NEMMCO          National Electricity Market Management Company
NEO             National Electricity Objective (as set out in section 7 of the NEL)
NER             National Electricity Rules
NSMP            National Smart Metering Program
NSSC            National Stakeholder Steering Committee (National Smart Metering Program)
NT              Northern Territory
OMRV            Operating Model Requirements Version (Victorian AMI Program)
PDRG            Business Process & Data Reference Group (established under the RMEC)
PwC             PricewaterhouseCoopers
PTWG            Pilots and Trials Working Group (established under the NSSC)
RET             Commonwealth Department of Resources, Energy and Tourism
RIS             Regulatory Impact Statement
RFP             Request for Proposal
RMEC            Retail Market Executive Committee (an advisory committee to AEMO)
RP              Responsible Person
RPWG            Retail Policy Working Group (established under the MCE)
RWG             Regulation Working Group (established under the NSSC)
SCO             Standing Council of Officials (as established under the MCE)
SM              Smart Metering
SMCN            Smart Metering Communication Network
SME             Subject Matter Expert
SMI             Smart Metering Infrastructure
SMMS            Smart Metering Management System
SMWG            Smart Metering Working Group (established under the SCO)
SWIS            South Western Interconnected System in Western Australia
TFWG            Testing Framework Reference Group
TOR             Terms of Reference
TRWG            Technical and Regulatory Working Group (established under the AMI Program)
WA              Western Australia
WAIMO           Western Australian Independent Market Operator
WEM             Wholesale Electricity Market (Western Australia)
WIGS            Wholesale Inter-connector Generator and Sample
WG              Working Group




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