By Chris Francis, BC Hydro, and Lawrence Johnson, J-Tech Engineering
At BC Hydro there is a strong desire to add visibility of the distribution system into the control center. Since adopting DNP3 (Distributed Network Protocol version 3) in 1998, the utility has been adding DNP3-capable devices into its distribution SCADA system. As a protocol developed for the utility market with readily available devices supporting it, DNP3 was the logical choice. The devices that have been incorporated thus far into BC Hydro’s distribution SCADA system include: GE DART RTUs, Cooper F5 & F6 Reclosers, Cooper CL6 Voltage Regulators, EnergyLine M-Operators, S&C 5800 Series switches, and the ABB Power Control Device.
BC Hydro distribution SCADA dial-up polling schemes typically call the site one to three times per day to check the integrity of the recloser, switch or voltage regulator. However, a key requirement is that these far-end devices must “cry out” when an event or problem occurs.
A shortcoming of most commercial implementations of DNP3 arises when the use of unsolicited report by exception (URBE) is needed and a dial-up line is the only available means of communication. While suitable for dedicated links down to 2,400 bps, and polled-only dial-up connections, DNP3 provides the ability to communicate a URBE, but not the means to dial a modem.
Most commercial implementations of the serial DNP3 protocol do not incorporate any modem control functions at all. They are based on the principle that the channel is always available. The problem with this assumption is that many communication channels, such as telephone, require the link to be established only when needed. This is also true of satellite communication systems such as Globalstar and Cellular Digital Packet Data (CDPD) when used with TCP.
One solution to this “connection” problem is to purchase a modem incorporating the intelligence to establish the link when an unsolicited report by exception must be made. Generally speaking, modems with this capability are hard to find. Some modems will dial-out when they receive a contact closure as from a PLC. However, this contact closure would still have to be initiated by the device, and as such is very dependent on using a device with contact output capability.
In addition, the geography of British Columbia is generally quite mountainous and heavily forested. And, with the exception of a few dense urban areas, the distribution system is quite spread out. A common source of power disruption is trees falling on power lines. As all utilities in treed regions are well aware, keeping current with the tree trimming around power lines is a top priority, so the more efficiently tree trimmers can work, the better. For safety reasons, BC Hydro tree trimmers require the disabling of reclosing before they can begin work. In some remote island locations, this has meant that a line truck must wait for a ferry in the morning, sail across to flip a switch, return and repeat the trip in the same afternoon. For this reason, using an autodialer with DNP3 definitely improves the bottom-line.
To bridge the gap between the desired functionality and the available off-the-shelf technology, BC Hydro engineers Chris Francis and Russel Chore developed the original Automatic Dialer, or autodialer device. To streamline manufacturing and add new features, J-Tech Engineering became involved and redesigned the board, incorporating surface mount technology and making the solution DIN-rail mountable. The embedded firmware for the application was rewritten in C (originally it was written in assembly language), permitting the addition of more features, such as different baud rates, and a more advanced user-interface.
When inserted between the DNP3 control equipment and the modem, the autodialer performs the missing modem control function and solves the basic problem. It watches the DNP3 control box located in a pole top enclosure out on a remote feeder for attempts to communicate over the unestablished communication link. If an attempt to communicate is made when no link is present, the autodialer takes control of the modem and establishes the communication channel. While establishing the connection, the device blocks all traffic between the modem and DNP3 control that might cause errors. Once the communication link is established, the autodialer becomes a pass-through device. The DNP3 control equipment can then send its report or other data.
The beauty of this solution is that not only does it enable the use of any off-the-shelf modem, but it needn’t be limited to a plain old telephone system (POTS) modem. On a site-by-site basis, the choice of what to use becomes a trade-off between what is available, what is economical, or, more importantly, what is robust. If a land-line is available, any POTS modem may be used provided it meets the environmental requirements of equipment in a pole top enclosure. Other options include a cellular modem or a CDPD modem. If the plant is too remote for any of these options, a Globalstar modem can be used.
To date, over 60 pole top distribution devices have been added into the BC Hydro distribution SCADA system using the autodialer.
Autodialer Technical Details
The autodialer hardware is implemented using mostly surface mount technology, and is based around a Microchip PIC processor. The device chosen is a PIC16F870, which includes internal Flash memory for storing and running the firmware, a small amount of RAM required by the firmware for variable storage, EEPROM space for holding dial strings and configuration details (field-updateable). The serial port on the CPU is implemented as a software port, and connects through a dual RS232 voltage level shifter, allowing the device to monitor most of the standard RS232 port data and control lines.
The firmware was originally written in the processor’s native assembly language, but was upgraded to C as more requirements were added to the device. The core of the application is a state-machine, operating as shown in Figure 1.
Click here to enlarge image
The autodialer is an extremely simple-to-use device. It is DIN-rail mountable, accepts power input from 9 to 36 VDC, consumes minimal current, and is field-configurable using a serial port and any terminal software under any operating system, on virtually any standard computing platform.
Users simply need to install a programming jumper and reset the autodialer. Then, the user connects the terminal equipment (DTE) serial port to the programming device of choice using a standard 9-pin serial cable. The serial port (assuming factory default settings) should be set to 9,600 bps, 8 data bits, no parity, one stop bit, and no flow-control. Upon resetting the autodialer, a self-explanatory menu will appear, prompting for an AT command dial string (e.g. atdt16045551212), a number of attempts to be made in establishing the communication link, a time between retries should the channel fail to be established, and finally, a speed for communication. Most standard speeds from 110 bps up to 19,200 bps are supported.
Chris Francis, P.E., has been a SCADA engineer with BC Hydro for 16 years. He has been involved in designing and implementing real-time control systems both for both distribution and transmission assets. Recently, Francis gave an IEEE presentation on using broadband satellite for monitoring very remote IPPs.
Lawrence Johnson, P.E., owns and operates J-Tech Engineering Ltd., which specializes in embedded processor based product development including design architecture, design for manufacturability, schematic capture, PCB layout, enclosure design, and software development. Johnson is also a registered consultant with Microchip Technology.