The openness of the standard will make the Automatic Meter Reading (AMR) and interoperability a reality. One among the many open protocols is the DLMS/COSEM. This open protocol based on IEC 62056 helps to overcome most of the challenges for data acquisition from energy meters. The difficulties faced by the utilities to have successful remote metering deployments and the consequent difficulty in accounting energy losses due to proprietary protocols are well known. The Indian Companion Specification (ICS) is intended to provide a field level basis for efficient and secure transfer of electricity metering data in an open manner with judicious application of features and protocols of the International Standard. The Indian Companion Specification (ICS) was evolved by BIS as an Indian Standard IS 15959 which inherits the IEC 62056 series of standards. This article presents the qualitative testing of DLMS/COSEM ICS compliant Energy meters.
The advent of electronic energy meters with data download capability made Metering Billing Collection (MBC) process to move away from human to a machine dependent. Further the deregulation raised the need for integrating meter data with the business processes of stakeholders.
The MBC process which begins with meter reading / downloading was the starting point of contention due to variety of protocols. With different makes of meters, the data collection for an automated process was a challenge. In order to bring in homogeneous environment and to optimise the business processes, structured data and also embracing all makes of meters was found to be an urgent necessity. This directed to the quest for open communication protocols so that AMR / AMI systems can become a reality. The need for open protocol was very much felt. One among the many open protocols is the DLMS/COSEM which has evolved over a period of time and is adopted by IEC as IEC 62056 series of standards and are adopted in India.
DLMS/COSEM is based on a strict client-server structure. The philosophy is to “model”, “message” and “transport” with an object oriented approach. The server is meant to be within the meter while the client accessing the meter could be a gateway or the central office. Other use cases where the server is within the gateway and the client is in the central office are also feasible. Before the actual metering information can be exchanged an association has to be build up, which is initiated by the client. The DLMS client can then access the interface object model inside the server. Once an association exists the DLMS server is also able to send notifications to the client without an explicit request.
DLMS/COSEM supports clock synchronization and transmission of measurement data and profiles. The DLMS/COSEM has been improved and the current version facilitates digital signature and firmware upload. Data objects for firmware updates are already part of the Blue Book Ed. Both are relevant for smart meters.
DLMS/COSEM includes authentication and confidentiality services based on symmetric encryption. The continuous development of DLMS/ COSEM open communications protocols for data retrieval, updation and reconfiguration of metering devices has enabled diverse operators to access safely and quickly data from metering equipment provided by diverse manufacturers.
Among the many open protocols like MODBUS, ANSI, DLMS, the DLMS/COSEM (IEC 62056) was chosen by India.MODBUS was more towards process instrumentation and ANSI the meter protocol used in North America.
The IEC 62056 is an open protocol exclusively evolved for Metering purpose. It operates at meter level. A wide range of data types can be read either selectively or in groups. The parameters are identified through unique codes. The protocol can work with many common communication medium. The protocol implementation can be verified for conformance with the dedicated test tool.
India has been aligning with IEC for standardization. In the present effort to bring in uniform protocol and based on the relative merits and demerits, IEC 62056 protocol has become the natural choice for metering purposes in India.
The Indian Companion Specification (ICS) was formulated and released by Bureau of Indian Standards (BIS) as IS 15959 : 2011 “Data exchange for electricity meter reading, tariff and load control – Companion specification”. This standard supplement the IEC 62056 series of standards. This companion specification spelt out the Indian specific requirements for designing open energy meters based on DLMS/COSEM. With this standardization a homogeneous environment would prevail at metering end for data collection.
The Companion Specification refers to latest updates of Interface classes and OBIS codes available in 12th edition (2014) of the Blue Book of DLMS UA. This also refers to updates of procedures and services available in 8th edition (2014) of the DLMS UA Green Book. These revised Technical Reports of DLMS UA, considered as pre-standards for several parts of IEC 62056, will be adopted by IEC in due course and subsequently by BIS.
Need for Protocol Testing
Testing of energy meters for communication protocol unlike any other equipment is very much essential to make sure that the meter is complying with the said protocol standards. This is to ensure interoperability and easy acquisition of data from the meters using any third party client tool which makes the device operate vendor neutral. This enables every meter reading system to understand a value by its identification system, irrespective of the meter make. The DLMS/COSEM also allows for uploading the Objects supported by a meter, and then reading the meter values, which enable any DLMS/COSEM Compliant Client software to read any manufacturer meter.
Compliance Testing as per is 15959: 2011 Indian Companion Specification
The ICS standard is intended for use as companion to IEC 62056 series of standards on “Electricity Metering – Data Exchange for Meter Reading, Tariff and Load Control”, adopted from various parts of IEC 62056. The metering device is evaluated in order to gain confidence about implementation of declared features of ICS and generic standards IEC 62056.
The compliance testing evaluation is carried out to verify whether the DLMS protocol are implemented correctly in the metering device. The test setup is as shown in Fig. 1. Meters claiming compliance to this Companion Specification will be required to:
- Conform to DLMS/COSEM (IEC 62056) protocol requirements as certified by the Conformance Test Tool (CTT)
- Parameter verification as a part of this:
- All mandatory parameters applicable to the category of the meter under test are implemented.
- All data types where specified are conforming to ICS.
- All Application Associations are implemented as specified in ICS with all specified services supported.
- Association object lists conform to ICS with access rights and OBIS codes as specified here and
- Event related DLMS objects are implemented with Event identifiers as specified in the Event reference tables in ICS.
Fig 1: Compliance Test Setup
DLMS/COSEM Conformance
The objective of conformance testing is to establish whether the Implementation Under Test (IUT) conforms to the relevant specification(s).
The IUT is to be verified for three layers viz.,
- COSEM layer: Association, Request/Response handling.
- The COSEM Interface objects: Testing for COSEM Interface object model implies that the protocol stack is in order.
- The COSEM Application layer: Testing for COSEM Application Layer implies that the mapping and modelling is in order.
- HDLC layer: Addressing, Data integrity & verification, Flow Control, Sequencing.
- The data link layer using HDLC protocol:-Testing for data link layer using HDLC protocol implies that the physical layer is in order.
- Physical Layer: Communication services.
CTT takes as input a text file called the Conformance Test Information (CTI) file that describes the relevant device parameters used during the test.
The CTT test consists of sending messages to the IUT and observing the responses. The conformance assessment process is the complete process of accomplishing all conformance testing activities necessary to enable the conformance of the IUT to be assessed.
The open protocol validation is done using a Conformance Test Tool (CTT).
The preparation for testing phase involves:
- Preparation of the IUT
- Production of the CTI file
- Preparation of the CTT
- The test operations include:
- Review of the CTI
- Test selection and parameterization
- One or more “test campaigns”
- Positive & negative cases.
- The CTT automatically generates three documents:
- The conformance test report
- The conformance log and
- The line traffic.
Line traffic which gives the actual communication taking place between the CTT and IUT. The line traffic window displays the bytes (in hex-string notation) sent and received over the serial line. This information helps the designer to debug the code whenever required.
A flowchart of the compliance assessment process is given in Fig. 2.
Fig 2: Compliance assessment process overview
The conformance test report is automatically generated during each test campaign. It is a digitally signed text file, containing the following elements:
- Date of testing
- Identification of the test tool and license owner
- Identification of the manufacturer as declared in the CTI file
- Identification of the IUT as declared in the CTI file
- A summary of results for each test suite
- The result of each test case
- A copy of the CTI file
- Digital signature, allowing to check the authenticity of the conformance test report.
The test verdict will be Passed, Failed or Inconclusive:
- Passed: Means that the observed test outcome gives evidence of conformance to the conformance requirement(s) on which the test purpose of the test case is focused, and is valid with respect to the relevant specification(s);
- Failed: Means that the observed test outcome either demonstrates non-conformance with respect to (at least one of) the conformance requirement(s) on which the test purpose of the test case is focused, or contains at least one invalid test event, with respect to the relevant specification(s);
- Inconclusive: Means that the observed test outcome could not be judged by the automatic test tool this requires human intervention and the test engineer in this case will analyse the results and declare whether the test is pass or fail.
Parameter Verification
ICS is intended to provide a field level basis for efficient and secure transfer of electricity metering data in an open manner with judicious application of features and protocols of the international standard.
IS 15959: 2011 addresses meters with open protocol for Three phase meters which are broadly classified into three categories viz., Category A for Energy Accounting and Audit Metering, Category B for Boundary/ Bank/ Ring/ ABT Metering and Category C for HV (VT/ CT) and LV (CT) consumer Metering. The parameters namely the Instantaneous parameters, Block Load Profile parameters, Daily Load Profile parameters, ToU, Billing Profile parameters, Billing period, Billing period counter, General purpose parameters, Event parameters are all clearly defined with unique OBIS codes.
The ICS also calls for parameter verification for each category of meters. The ICS lists out the applicable parameters for each category of the meters. The Metering Parameters have been identified and classified as follows:
- SNRM/UA.
- Object list download.
- Association properties, Mandatory objects & Conformance services.
- Selective access by Range & Entry.
- Simultaneous operation.
- Security.
- Instantaneous Parameters.
- Block Load Profile Parameters.
- Daily Load Profile Parameters.
- ToU.
- Billing Profile Parameters.
- Billing period.
- Billing period counter.
- General Purpose parameters:
- Name Plate Details.
- Programmable parameters.
- Event recording.
SNRM (Set Normal Response Mode)/UA
The SNRM verifies negotiation of HDLC parameters with appropriate values.
Object list download
The OBIS (Object Identification System) provides a unique identifier for all data within the metering equipment. It covers not only measurement values, but also abstract values. OBIS codes identify data items used in energy metering equipment in a hierarchical structure using six value groups A to F. This also referred as COSEM.
The association shall be addressed using the Current association OBIS code (as applicable) and attribute 2.
PC mode OBIS code 0.0.40.0.1.255
MR mode OBIS code 0.0.40.0.2.255
US mode OBIS code 0.0.40.0.3.255
For each Association, the object list parameters are downloaded and verified.
PC: No access for all objects except RTC and Sl. No.
MR: Read only for all objects
US: Read, Write for all objects
Association properties, Mandatory objects & Conformance services
This requirement will verify that the meter is communicating successfully with all the three associations (i.e. Public client – no authentication; Meter Reader – low level security & Utility Setting – high level security).
Current association shall be resolved in meter to appropriate association object (0.0.40.0.e.255) based on SAP address pair (client address, meter logical address). Value of e=1 for Public client; e=2 for Meter Reader & e=3 for Utility setting).
The following parameters of the association request are verified:
- Application Context: Logical Name without Ciphering
- Authentication mechanism: No Authentication (PC) / LLS (MR) / HLS (US)
Conformance Block containing:
- GET
- SET
- ACTION
- GET With Block Transfer
- SET with Block Transfer
- Selective Access.
Note: Item 3.1 is applicable for Public Client Association.
Item 3.1, 3.2, 3.4 & 3.6 are applicable for Meter Reader Association.
Item 3.1 to 3.6 are applicable for Utility Setting Association.
Selective access by Range & Entry
The Selective Access by Range shall be supported for Block Load profile and Daily Load profile. The companion specification requires support for Selective Access by Entry for Billing data profile and Event log profiles. The selective access requirement is to verify selective access in profile buffer.
Simultaneous operation
The meter (server) is not required to allow more than one association at any one time. Optical port shall have priority over the electrical port when both ports are accessed simultaneously.
Security
The DLMS/COSEM standard provides three different sign-on authentication mechanisms for each association’s access to meter data applied at the time of performing COSEM OPEN operation.
Lowest level security
Open access without any authentication at sign-on. This is for Public Client.
Low level security (LLS)
Password based sign-on where the client authenticates itself to the meter using a password. The Utility settings association shall provide access to write the password for all associations that utilize this authentication scheme. Meter’s LLS secret can be modified from authorized DLMS clients by writing to “secret attribute”.
High level security (HLS)
HLS mechanism defines a 4-pass sign-on scheme where the client and server exchange challenges (a random number or code) and then reply to the challenges with a processed response. The processing performed on the challenges is an encryption using a secret “key”.
Meter’s HLS key can be modified from authorized DLMS clients by executing “change HLS secret” method.
Instantaneous Parameters
The Instantaneous parameters are continuously updated by the meter hardware/ software as per internal sampling and computation time. The energy values are cumulative readings from the date of manufacturing or installation of meter as the case may be. Each of the parameters are readable at any instant by the HOST from remote or by HHU at site. The snap shot of all the instantaneous values of all parameters are readable by the HOST computer.
Block Load Profile Parameters
This is an array of parameters identified for capturing and storing at specified time intervals or capture times. The capture times are either 15 or 30 minutes. The capture times can be programmable by the utilities. The ICS lists the parameters whose profile (survey) is to be captured and stored in the meter as per set capture time period. The profiles are readable at any time by the HOST from remote or by HHU (MRI) at site for any specified range and time.
Daily Load Profile parameters
This is an array of load survey data captured as a profile generic at the end of 24 hours. The capture period attribute is statically fixed as 24 hours.
ToU
DLMS/COSEM provide a number of interface classes to deal with ToU metering. DLMS offers different options for modeling time based activities such as schedule, activity calendar, single action schedule etc. ICS uses activity calendar in association with script table to accomplish ToU modeling. The activity calendar is a definition of scheduled actions inside the meter, which follow the classical way of calendar based schedules by defining seasons, weeks & days. Meter follows different time based variable billing rates instead of flat rates.
Billing Profile Parameters
These are parameters identified for accounting/billing purposes. These are generated by the meter for each billing cycle and stored in the memory. The set of data for at least last 6 (six) billing cycles are stored in the memory. At the end of each cycle corresponding set of data are readable by the HOST from remote or by HHU at site.
Billing profile parameters requirement is to verify that billing energy values on time zones which is configured in activity calendar and billing is generated on the time configured scheduled time.
Billing Periods
Billing period resets are driven by an instance of the single action schedule class in conjunction with script table. The data of the billing period is stored in a profile generic object as specified in ICS. Each entry in the profile buffer captures the billing period values for a specific billing period.
Billing Period Counter
The meter shall maintain the cumulative count of all billing happened since installation.
General purpose parameters
- Name Plate Details
These parameters are electrical and non-electrical quantities and are static in nature, grouped as ‘Name Plate Details’, containing pertinent information about the supplied meter. The parameters identified and grouped as “Name Plate Details” under this classification are applicable for all categories of meters.
- Programmable Parameters
These parameters are non electrical quantities. The parameters identified and grouped as “Programmable Parameters” are programmable by the Utility engineers. For the purpose of setting / altering the values of these parameters, the security and access rights in line with the methodology described in protocol, are mutually agreed between utility and manufacturer. The parameters are programmable by HOST from remote and HHU at site.
Event recording
Any abnormal or a tamper condition is defined as an Event. Meter must have the capability to detect event and store associated information for latter communication to DLMS client. The meters identify and log both occurrence and restoration of such events. The meters also capture some of the parameters at the instance of above said log. ICS has identified the events to be logged and the parameters to be captured for each of those events. The various types of events are:
- Voltage related events.
- Current related events.
- Power Failure related events.
- Transactional events.
- Other events.
- Non rollover events.
- Control events.
The Metering Protocol Laboratory in CPRI is equipped with state-of-art facility complying to International standards for carrying out Compliance testing on DLMS energy meters as per ICS.
Fig 3: Test Setup
Conclusions
In this paper the general principle, DLMS/COSEM was described. The unique strength also were highlighted especially the testability. The adoption of DLMS/COSEM for Indian needs by way of ICS was explained. The qualitative testing of the protocol implementation was brought out. The DLMS/COSEM test methods for DLMS compliant Energy meters ensures that communication protocol implemented is conforming to Indian Companion Specification (ICS) & enhances the confidence level of the utilities.
The standardisation of meter protocol was a great step forward in rolling out the R-APDRP projects.
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