Remote - Special SCADA issue 2012 - (Page 14)
Advantages of the DNP3 Communications Protocol in Water and Wastewater Telemetry Systems
Vishal Prakash, Business Manager CSE-Semaphore DNP3, or Distributed Network Protocol, has been around for close to two decades now. It was developed by Westronic (now part of GE) in 1993 primarily for the power industry. From its inception, DNP3 has been as an open protocol, allowing end users to use a common protocol across many hardware platforms. The DNP3 protocol has a number of features and advantages. However, the following features are particularly useful for water and wastewater applications. • Open protocol • Classification of field data • Report by exception • Time-stamped data • Support for time synchronization • Secure authentication • Diagnostic information for each I/O point • Communication to multiple masters Why are these features important? How do they enhance a water/ wastewater telemetry system? What are the benefits for the end user? Let’s look at each of the features in detail and try to answer the questions posed above. From its inception, DNP3 has strived to be an open protocol. There are many open protocols, including Modbus, probably the most widely used open protocol. So, why is it that being an open protocol is important? The guardian of the DNP3 protocol is the DNP3 Users Group Technical Committee, which was formed in 1995 and is vendor independent. The committee’s charter not only includes specifying protocol enhancements and new functionality called “DNP Subset Definitions,” but ensures there is backward compatibility and absolutely makes sure there are no vendor specific variants of the protocol. The firm compliance requirement ensures that hardware and software manufacturers who do support DNP3 do so within very well defined parameters. It should be noted that a vendor need not support all DNP3 protocol functionality. The compliance to DNP3 can be via three different levels: 1, 2 and 3. All vendors who support DNP3 must make available a device profile document that clearly lists the DNP3 features supported by the product and thus the compliance level. But how does this help customers? What are the benefits? DNP3, being a true open protocol, allows the end customer to use hardware and software from different vendors with the complete confidence that they will be able to communicate with each other and that a common top-end SCADA software package can be used to bring in the data from the different pieces of hardware. This is beneficial not just technically but also commercially, as it provides the end users vendor independence, allowing them to add equipment from different suppliers without the need to replace the entire system every time they want to expand and or change. In a water or wastewater system there are various assets that require monitoring and control. These include treatment plants, pump stations, valve sites, canals, weirs, tanks, ponds, dams, reservoirs, pipelines and much more. At these locations, a great deal of normal day-to-day activity exists. For example, at a pump station, it is very normal for the pump to go ON or OFF when the configured conditions are true or false. The question is, do you want every event reported to the top end? The answer is no, because this will take up bandwidth and possibly prevent information on critical events from getting through to the DNP3 master device. To prevent this scenario, DNP3 protocol allows users to classify their
data into different groups called “Class.” The protocol currently supports four classes of data: 0, 1, 2 and 3. Class 0 data is real-time data. Classes 1, 2 and 3 are reserved for objects that require time stamp information (event data). Each class of data is independent from the other. A class of data also has variation parameters which allow the user to select the type of value, time, and diagnostic information to be recorded.
Figure 1. Various DNP3 objects and their different classifications: 0, 1, 2 and 3.
This DNP3 protocol feature is extremely useful especially in multilayered systems as it allows the user to categorize field data. For example, normal conditions at a pump station site, like pumps starting and stopping, may be configured as Class 2 type events. Thus, when the pump changes state, an event will be created as a Class 2 type event and stored in memory. Since this is normal operation, an unsolicited report (or Report by Exception) need not be initiated to a DNP3 master device. When the master device performs a routine background poll, it can recover all Class 2 events. But assume at the same pump station site there is a pump fault indication. This is, in most cases, a critical alarm of which the DNP3 master needs to be aware, so that appropriate action can be taken. Thus, the pump fault status indication can be configured as a Class 1 event, thereby triggering an unsolicited report to the DNP3 master device. DNP3 protocol’s fundamental support for data classification allows the end user to design and operate an efficient telemetry system irrespective of the type of communications media Figure 2. An example multi-layered telemetry system. being used.
Classification of Field Data
Report by Exception (Unsolicited Reporting)
Report by Exception refers to a remote device’s ability to initiate communications to a top-end master. Traditional telemetry systems are typically poll-only systems. In medium-to-large systems, a polling protocol can lead to loss of data. To prevent the loss of data, communications bandwidth would have to be increased constantly. Taking the example described in the section “Classification of Data,” with DNP3 protocol’s inherent support for unsolicited reporting, the pump fault alarm can be sent to the DNP3 master immediately upon occurrence. If the telemetry system were a poll-only system using a different commu-
Table of Contents for the Digital Edition of Remote - Special SCADA issue 2012
Remote - Special SCADA issue 2012
Re-Inventing SCADA for the Next Generation Enterprise
Securing Automation within Critical Infrastructures for SCADA Security
As the Brain, So the Body: The PLC Market and Automation
Reducing Labor Costs and Increasing Investor Confidence With Real-Time Monitoring of Photovoltaic Energy Network
Advantages of the DNP3 Communications Protocol in Water & Wastewater Telemetry Systems
Hardwire vs. Wireless Failsafe Control Systems
Advantech Introduces IEC 61850-3/IEEE 1613 Compliant Substation Automation Platform
CalAmp Launches Fusion-LTE 4G Broadband Router Platform for Smart Grid Applications
El Paso Water Utilities Implements Trimble Fieldport Mobile Software to Streamline Customer Field Service
Remote - Special SCADA issue 2012