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scada system overview |
Introduction
EPA and other government regulations have mandated that public utilities provide ever increasing oversight of their systems. Utility departments are learning that a good radio telemetry system (The technology of automatic radio transmissions of data from a remote source to a receiving station for recording and analysis.) can provide the backbone for remotely monitoring and controlling their freshwater, wastewater, and collection systems.
Because utility systems are typically spread throughout a municipality, city or county, radio telemetry is the most reliable and cost effective means to tie each of the remote locations back to the plant or administrative office. By using a common radio channel, a computer at the water plant or other central site can query each of the remote locations for the status of the pumps, valves, pressures, etc. The computer can also control the operation of each of those components through the same radio channel.
The Central Computer at the plant or office, and the units at each of the remote locations share the radio channel. One at a time, the computer at the water plant polls for information from each of the remote locations. To do this, each remote location has a unique radio address so it knows when the central computer is talking to it. The total poll loop takes from 15 seconds to 2.5 minutes, depending on the number of remote sites in the system.
All radio based telemetry systems must obtain a frequency and radio license from the FCC (Federal Communications Commission.) Because the frequency application process can be complex and time consuming, NDS aids its customers in preparing the paperwork and coordinating the application.
What Can Telemetry Do For Me ?
Telemetry also allows the utility to discover alarm conditions before they become critical events. The utility can monitor high-well conditions and power outages occurring at remote sites in "real time."
Another advantage of telemetry is in preventive maintenance. By utilizing the reports provided by our TAC II Software, the utility can diagnose pump problems long before they become failures. These reports allow the user to compare run times of pumps from day to day. These run times normally are very constant, so a variation in run times can indicate a problem. By monitoring pump performance, many customers have virtually eliminated overtime pump repairs.
Another valuable use of radio telemetry is being able to relay system status across town to initiate automatic control functions. The system can transmit status such as "pump run," "tank level," or "water pressure" from one remote location to another to activate control functions such as turning on chlorine injectors, or turning off well field pumps.
System Overview
Remote Terminal Units (RTU) are located at the wells, pumping stations, tanks, treatment plants, metering stations, and other remote sites scattered throughout the utility's district.
The heart of the central computer is the TAC II software that monitors the communications between the Central Site and each of the remote sites.
Design Philosophy
Our goal was to build a total solution package designed specifically for the average utility to operate and maintain. All of its features and functions are the result of specific needs and recommendations received from Water Industry personnel. We continuously enhance TAC II based on customer input.
Each system is a mix of standardized, off-the-shelf components configured to support the unique applications of each customer. We tailor the design to maintain a highly reliable system with minimum down time.
TAC II Communications
The system transmits the time-tagged information to the central site computer, where it is analyzed for alarm conditions, used to update instructions to other sites, and stored for use in detailed reports.
There are three bands normally used for TAC II system communications. They are VHF in the 150 to 170 MHZ range; UHF in the 450 to 470 MHZ range; and the 900 MHZ band. We recommend the 450-470 MHZ for use in telemetry because of its resistance to atmospheric skip interference compared with the 150-170 MHZ band. VHF and UHF radios are significantly lower in cost when compared to radios in the 900 MHZ Band. Other frequencies may be used based on frequency availability.
The area of coverage varies due to terrain, antenna height, frequency band and vegetation. You can expect coverage of 6-10 miles radius in relative flat areas with light vegetation. Area coverage may be extended through taller antennas and the use of repeaters.
TAC II also allows an innovative capability of message hopping or "digipeating" messages from one remote station to another. Through this technique, the system can support a small number of remote stations outside the main coverage area without the utility having to acquire a second frequency and purchase a radio repeater.
NDS helps each customer analyze their applications to determine which implementation best suits the utility's requirements.
The Central Site
The Central Site Computer is a Pentium based computer running our TAC II software. It is a standard, desk-top PC that does not require any special operating environment apart from connection to a 30 minute reserve Uninterruptable Power Supply (UPS). The system has been designed so the addition of more RAM memory, mass storage, or input/output devices is possible and easy. The computer runs under a multitasking/multi-user operating system such as UNIX. This allows several remote terminals to access the TAC II software simultaneously.
Connected to the central computer is a fiber optic modem used to communicate with the central site radio. Also connected are printers, LAN networks, backup storage, remote terminals, and any other computer equipment and software required to support the utility's SCADA requirements.
The central computer system can be partitioned so the fresh water, wastewater, and/or collections departments can simulate having their own system. This, in effect, allows the utility to operate separate systems on the same frequency using the same central site equipment.
It is important to note that the operation of the utility's equipment at the remote sites is not directly dependent on the TAC II system. The utility's equipment at a remote site will continue to operate in the event of a major communications or central computer failure. Naturally, data logging and alarm annunciation will be interrupted.
Equipment operations at critical, but isolated, remote sites can also be controlled by installing "intelligent" DFS modules at the site. These modules contain preprogrammed logic and firmware instructions or parameters written to the customer's specifications. During normal operations, the parameters can be changed or updated by the telemetry system.
Maintenance Modem
Automatic Dialer/Voice Unit
Once someone answers the phone, the computer will ask for an authentication code before announcing the alarm condition. The computer will continue to dial down the list until it gets a correct code response. The functionality of the unit is completely configurable by the user from the Central Site Computer. The computer records all voice unit activity in the system log.
The dialer/voice feature is extremely powerful when used in conjunction with a cellular phone and a laptop computer. If desired, utility personnel can monitor and operate their entire system from home, or from a remote field location. This is an excellent option for utilities that are short on personnel.
Central Terminal Unit
The CTU antenna tower height can range from 20 feet to well over 100 feet depending upon the distance between the Central Site and the RTU sites. The antenna is usually a high gain, omni-directional antenna selected specifically for the frequency of operation.
The data transmission connection between the Central Site Computer and the CTU is made by fiber optic cable and modems. DFS incorporated optical fiber connections as its standard in order to isolate the central computer from high voltage spikes induced by close lightning strikes.
The functional modules used at the CTU and RTUs are the same, and are interchangeable throughout the system. This provides for increased redundancy, and reduces the spare parts requirements.
What Can I Monitor And Control ?
The functional modules used with TAC II can monitor and control the following kinds of I/O (input/output) points and signals within the utility's system:
Digital inputs (DI) monitor a voltage/no voltage condition. We can even furnish the required bias voltage through a set of dry contacts. Examples of DI monitoring points include:
Commercial power loss
Digital outputs (DO) can be used to control anything from relays to motor starters. The utility's equipment must provide a source voltage between 3-60 VDC or 60-280 VAC. Examples of DO control points include:
Energize a relay
Analog inputs (AI) monitor devices that have an output of 1-5 VDC, or 4-20mA. DFS can furnish the 24 VDC to power the device if required. Examples of AI monitoring points include:
Measuring pressure in pipes
Analog outputs (AO) control devices that require 4-20mA. DFS modules generate their own 4-20 mA. source to feed the device. Examples of AO control points include:
Control a variable position valve
Once the above signals are communicated to the central computer, TAC II can do several things:
Analyze the information to see if it matches pre-set alarm conditions, e.g., a loss of commercial power alarm followed by a high well alarm means send out the generator truck!
Log the information into the TAC II history files used to produce management reports, e.g., a day-by-day report of run times by each pump at each site; and derived flow reports.
Transmit a control command back to the site that sent in the information, e.g., if the low level float is tripped, turn off the pump.
Forward information to multiple remote sites (called point-to-point) to activate control functions, e.g., if the tank level at Site A reaches 50 feet, turn on the pump a Site B.
Logically control the pump runs for a whole system or branch, e.g., control the head pressure of a line by allowing only a set number of pumps in the line to run at the same time (called flow equalization). Naturally, high well and other alarms are given priority.
Using a mixture of DFS functional modules, any remote site can be configured to perform a broad range of digital and analog monitoring and control operations.
The Remote Terminal Unit
The antenna is a high gain yagi selected for the frequency used. The basic mast is a 21 foot galvanized or aluminum 1 1/4" pipe. The tower is a Rohn 25AG top section mounted in a concrete base. The tower, mast and antenna assembly is properly grounded through the use of a ground rod.
The RTU is wired into the motor control circuits and signaling circuits within the utility's control panel through electrical conduit. It communicates with the Central Site via the two-way radio link.
RTU Enclosures
The white painted exterior is not to protect the metal; its purpose is to help keep the interior cool.
All fittings, mounts, brackets, latches, and fasteners used in fabricating and mounting the box are stainless steel. All other components are at least hot-dipped galvanized, or otherwise certified for outdoor and electrical use.
The three basic sizes of RTUs are determined by the number of active modules that can be mounted. Each RTU contains a power supply module, a back-up battery, a radio module, and up to 4, 10, or 15 input/output modules. Each module is a separate, plug-in, functional unit, making service and repair very easy.
The RTU Modular Backplane
The MBP bolts into the RTU enclosure, serving as both a mother board and backplate onto which the active modules are plugged and wiring connections made. There are no active components permanently mounted to the MBP, making it highly reliable.
RTU Modules
All RTUs contain two required base modules: The Power Supply Module (PSM), and the Radio Interface Module (RIM). All other modules are optional, as their use depends on what equipment functions the utility wants to monitor or control at the remote site. We mix and match them as the application requires. There are four basic types of RTU modules:
1. Support - Modules which do not perform any input/output functions.
2. Digital - Modules having only two I/O states (i.e., On/Off, Open/Closed, etc.)
3. Analog - Modules that monitor or control based on a numerical value (i.e., 25 ft., 128 psi, 147 gpm, etc.)
4. Controllers - "Logic" modules which perform complete, closed loop, automatic control functions
The following features are common to all of our functional modules:
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