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Frequently Asked Questions

What is a SCADA System?

SCADA is a term that is frequently tossed about with little or no explanation. SCADA is a mnemonic for Supervisory Control and Data Acquisition. The key word in the term is supervisory. This indicates that decisions are not directly made by the system. Instead, the system executes control decisions based on control parameters entered by the agency staff. The system monitors the health of the process and generates alarm notifications when conditions are out of tolerance. It is also tasked with placing the process in a safe mode. It waits for user inputs to correct problems. The supervisory mode is designed to operate the system in a manner that avoids out of tolerance conditions. In a water / wastewater process, pumps are started and stopped by the system according to limits assigned by operations. As long as the system responds correctly to the control commands, the system remains in control. Supervisory mode also provides operations staff the capability to override control decisions.

 

Another significant task of a SCADA system is Data Acquisition. Information gathered by the system is available to operations in order to evaluate the effectiveness of operations. This information is available in real-time and recorded for review.

What is a Distributed Control System?

How does a SCADA system differ from a Distributed Control System (DCS)?

For all intents and purposes, a SCADA system is a DCS. In the past, SCADA was centrally located and all control decisions were made by one central entity. As technology has evolved, the decision making functions have been distributed throughout the system. Now, remote stations make control decisions locally based on control parameters and values that may or may not be collected at the local station.

What telemetry options are available?

The choice of telemetry options is primarily dependent on the distance between stations and the physical topography of the system to be monitored. Typically, the answer to this question must be separated based on the type of control system, remote telemetry, such as a water distribution and storage or wastewater collection and transportation (remote telemetry) or a treatment plant with short distances and manageable communications paths.

Remote Telemetry

As with computer technology, telemetry options have expanded in recent years. Originally, the primary information transport mechanism was tone based telemetry over analog circuits leased from the local phone utility. As technology changed, tone was replaced by digital modems and the rate increased from 30 characters per seconds up to 1,920 characters per second (and in some cases even 5,600 characters per second. Nowadays, higher communications speeds are available. In all these cases, the weak link was the need for land based lines owned and supported by the local phone company.

Radio based telemetry provides an organization independence from the local phone company. Radio based telemetry has grown through the years and has experienced growth pains. There were two significant obstacles: Licensing and physical path barriers. Both these obstacles have been significantly reduced with the advent of spread spectrum technology. Spread Spectrum technology is unlicensed and, depending of the sophistication of internal algorithms, can be resistant to interference. Although physical path boundaries still exist, the ability to easily incorporate repeaters can be used as a solution. With the advent of Gigahertz frequencies, Ethernet speeds (~10-54 Mbps) are currently supported. Fiber Optic and Internet connections are also currently available.

 

Fiber Optic communications provide security and noise immunity but can be cost prohibitive for long distances. In general, long distance Fiber should only be considered if there is some other physical construction being performed. Internet connections are in use by some agencies as well. Using Virtual Private Networks, firewalls and proprietary protocols, adequate security at the RTU locations can be achieved.

 

Plant Local Telemetry

Within a treatment plant, telemetry options are considerably more flexible. In all cases, the telemetry media (hardwire serial, vendor networks, Ethernet copper / Ethernet fiber and radio) is owned and supported by the end-user. Typically, the primary concern is data throughput and reliability. Therefore, redundant data highways are preferred. Fiber Optic cable avoids issues of interference. Radio can be a solution when conduits are not readily available, especially within existing facilities.

What is the best PLC?

What is the best Human Machine Interface System?

As with a PLC, there are many Human Machine Interface (HMI) packages available. At any given time, each package supports particular features. Features that are of proven value are quickly integrated into competing packages. Therefore, the selection of one package over another doesn't guarantee the best package over time.

Selection of a package should be based on the availability of integrators, support, manufacturer training and proven experience as well as overall cost of ownership through product's useful life.

What level of security is necessary for water & wastewater SCADA / DCS Systems?

Security has become a major issue with any computer based control system. Because water systems have significant concerns over public safety, the security of SCADA systems is of primary concern. There are some powerful tools and procedures that can significantly reduce the threat.

As with any security system, physical security is the first line of defense. Therefore it is critical that a SCADA system monitor the physical premises and report unauthorized activity.

At the remote stations, RTUs should be programmed and configured in a way that does not allow changes to the primary program over the communications media. In addition, the RTU should be programmed with fixed control logic to identify and react when program parameters are modified or unusual conditions are present. In critical circumstances (such as sewage lift stations), hardwired backup should also be included in the control logic. All these tools will serve to ensure a station reports to the central computer when a problem is identified. In addition, the central computer should be programmed to recognize when the remote station fails to communicate, as this is an indication that there is a need for operator intervention. Note that most of these requirements should be followed as a general rule, not just in response to security concerns.

In most cases, the security of the telecommunications is not of major concern at the remote station end. Without intimate knowledge of the site, it would be difficult to intercept a communications link and cause a specific event to occur. More likely, the station would shut down and a communications failure would be detected.

At the central, security over the telecommunications link is only a concern if the data link uses an open protocol such as Ethernet. If the telemetry uses a proprietary protocol or even an open protocol such as Modbus, a special driver is required. This driver would not allow access to the central computer system. An Ethernet protocol is an exception to this. If an Ethernet protocol is used, the central system should be configured with firewall security and if possible, VPN protection should be included. Note that spread spectrum radio based Ethernet does offer security based on frequency hopping or direct sequence schemes and WiFi security.

With the advent of Terminal Services and SCADA connections to the Internet, new vehicles for attack are available. SCADA systems connected to the Internet must be firewall protected and should use Vertical Private Network (VPN) connections. If practical, dial-up connections, if in use, should use password access and call back technologies. The agency should have the IT department fully aware of the SCADA system and should consider it a high priority system for routine maintenance and protection management. In general, a control system should be isolated from business systems.

It should be noted that the security vulnerability of the SCADA system is not necessarily the control of the facilities. If the programming of the remote stations is protected as identified earlier, the stations should be secure in any event. Instead, the threat is the loss of use of the SCADA system and the time it might take to restore a system to operation after an attack. Therefore, the most powerful security is to maintain system backups. If a system is compromised, a backup can restore the system to full operation in a short period of time. In recent years, control system owners have and continue to become more congnizant of cyber security threats and statrted taking prudent steps to protect against potential harm.

What is the advantage of a SCADA design team with integration experience?

In any project, it is critical that the requirements included in a specification can be met. Often, specifications are written based on desires that are un-attainable or require excessive costs in order to accomplish. In some cases, specifications are developed that offer change order loopholes. Some bidders reduce their formal bid price in anticipation of additional revenues due to potential change orders.

Design teams with direct integration experience know what can and can not be done. Ideally, as each requirement in a project is identified, a possible technique will be understood by the design engineer in order to assure the requirement may be satisfied. He/she should also recognize the effort required to satisfy the requirement and can provide this information to the owner. The owner may decide the cost outweighs the benefit.

Traditionally, a cost estimate is provided with a project specification. Again, a design team with integration experience will be able to offer an estimate that is based on direct experience.

During the construction phase of a project, integration experience also benefits the owner and the contractor. Change order requests are reviewed from the Contractor's point of view to ensure the reason for the change order is justified and the cost is legitimate. Arguments regarding scope can also be more effectively reviewed as the engineer can analyze the issue from both sides. An experienced design engineer can also identify issues that may affect the project schedule. Knowledge of potential problems can be used to raise scheduling concerns with the Contractor. As the project progresses, the engineer can anticipate conditions or implications of Contractor actions.

Project Closeout also benefits by having an engineer with design experience. Typically, project closeout involves many circumstances occurring at the same time. An experienced engineer can direct Contractor efforts in order to assure outstanding issues are handled in a timely manner and will not extend the project completion unduly.

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These systems provide "real-time" monitoring and control of a given "process" within a plant. These are also referred to as "in-plant" systems. All major components of the system are usually confined to one or several close by facilities. The communications media consist of primarily coax or fiber cable. Wireless medium may be used within a plant (at low bandwidth / speed) or between buildings through microwave point-to-point connections (at higher bandwidth / speed). Examples of places where a DCS would be used include a water / wastewater treatment or power plant, a refinery, a factory or a food processing plant to name a few.

In terms of a water utility, the terms PLC and RTU are almost interchangeable. Generically, an RTU or Remote Terminal Unit is the entire unit tasked with collecting status signals and outputting control commands. This would include power supplies, communications devices and the enclosure itself. The PLC (Programmable Logic Controller) is the intelligence of the RTU.

 

It should be noted that the definition above ignores off the shelf RTUs. Some vendors market a product as an RTU. In this case, the RTU is primarily the intelligence and data interface modules. It may be a standalone unit or it may be integrated as one component of the unit. In this case you might have an RTU within an RTU.

 

The majority of water systems utilize a PLC as the basic intelligence in the RTU.

What is the difference between a PLC and an RTU?

There is no good answer to this question. Instead, the end-user must evaluate their requirements. There are many PLCs manufactured and many vendors have multiple PLCs in their line.

The best PLC is the PLC that meets the end user's needs. This may be the PLC that a chosen integrator is most familiar or most comfortable with. It might be a PLC of the same style as is currently in use by the agency. Availability of support from the manufacturer is a major criterion. Also, the availability of integrators familiar with the product should be reviewed. If the agency wishes to support the system with in-house personnel, a review of the training available from the manufacturer is essential.

Finally a continuous product line is an important consideration. The product that has been selected will, in all likelihood, be in operation for many years. During that time, new products may be installed. If the product line allows new units to be incorporated using the same communications protocol, existing stations do not have to be refurbished prematurely.