Multifunction meters for seamless integration with DCS

September 13, 2013 7:56 pm

Ram Sinha of Siemens highlights the need of multifunction meters on profibus DP and Profinet for flawless integration with DCS in the process industry
In process industries like cement, metals, food and beverage, oil and gas etc, integration of distributed control system (DCS) with motor control centres (MCC) or power control centres (PCCs) is largely done using Profibus and Profinet fieldbus. But, these industries predominantly use Modbus RTU over RS485 technology for the communication of multifunction meters (MFMs) with DCS.With the advancement of technology, now we have options of using the meters with Profibus or Profinet protocols directly which can communicate at higher speed vis-à-vis conventional Modbus RTU meters and ensure precise and reliable measurement of power values for infeed, outgoing feeders and individual loads.Basic requirement for meters integration with DCSIn conventional integration, we use PCCs and MCCs connected with DCS through hard wiring via remote input or outputs. In intelligent MCCs and PCCs, we use the products which can communicate directly with DCS and should support the same protocol.Be it conventional or intelligent switchboard, the communication capability of multifunction meter is the vital aspect in selection of multifunction meter for the following reason:•  When DCS is communicating with MFMs, protocol must be ‘Deterministic’ in nature. •  Speed of data transfer should be very high (in MBs) which plays a significant role for the communication of critical parameter from Meters to DCS and then DCS to protection device for better control.Seamless integration of meters with DCS on a common communication protocol will give the insight of the switchgear feeder constantly.Conventional way of meters integration with DCSUsually while selecting a MFM with communication capability, we end up in choosing a meter having RS485 port for MODBUS RTU communication.Let us see how the connection of such meters happens in an i-MCCs having intelligent motor protection devices on Profibus protocol.All the switchgear will be connected to DCS / PLC on Profibus whereas since the meters have Modbus protocol we will need an additional communication process (CP) card or a converter to get it integrated. Modbus RTU inherently is a non deterministic protocol with the average baud rate 9.6 kbps.Limitations of such systemsNumber of meters connected in one loop behind a gateway or CP: There is a limitation to the number of meters that can be connected in one loop; else it will result in a delay in real-time data communication from each meter.Number of variables to be transmitted from each device: Data traffic will be very high even if one wants to monitor only the basic parameters like voltage, current, power, power factor, energy, harmonics etc in each polling cycle. Hence the refresh rate over the DCS will be poor.Additional cost for setting up such network: Users have to lay down separate Modbus cabling for meters communication, and of course required to keep CP cards and drives in inventory for future purpose.Use Common protocol for meters and DCS Selection of same protocol for meters and DCS can solve the problem of seamless integration of each other.For example, if the DCS hardware communicates on Profibus then by using meters with Profibus we can get the real time data without using any converters.Major benefits to the user•  Single communication protocol throughout the plant: No compatibility issues between devices.•  Faster Communication through Profibus: Improves real time data availability to the EMS/SCADA and brings Determinism.•  MODBUS RTU cables are completely avoided:  Reduction in cabling costs•  Modbus to Profibus converters avoided:  •  Saving in cost of the converter•  Possibility of redundancy via provision of Y- links•  A converter failure can lead to data loss of all the devices – this is avoided•  Reduction in size of the CPU as the corresponding CP cards are avoided.Future of metering communication: ProfinetWith the technological innovation in communication, devices which carry relatively larger size of data are moving towards Ethernet.Hence, multifunction meters also require such a protocol which can use Ethernet platform to communicate with DCS and having deterministic in nature. Thanks to Profinet which has made the things much easier.Benefits to the user•  Meters having Profinet protocol, gives flexibility to the user to adapt any system architecture like line, star or tree etc.•  Real time data is available all the time at DCS directly from the meters without using any converters•  Device level redundancy can be achieved with Profinet architecture•  Important decision like load management etc can be taken on the basis of data given by meters to DCS.ConclusionProfibus DP or Profinet protocols are known for their reliable data handling. So, MFMs selected should support these protocols for its integration with DCS. This will help users for transmitting real time power values for main distribution boards, electrical feeders or individual loads precisely and reliably, and important measured values are supplied in addition for assessing the actual state of the plant and appropriate decisions can be taken by DCS. Siemens offer its PAC 3200/ 4200 meters on Profibus DP and Profinet for direct integration with DCS to the process industry users.About the author:-Ram Sinha, Business Development Manager, Energy Management System, Infrastructure and Cities Sector, Siemens – SAS Cluster. He is responsible for business development of energy management system in IC LMV division of Siemens India.