SCADA is a system based on blocks of electronic and communication evolutions, it stands for Supervisory control and data acquisition, it comprises of plants where sensors and actuators are allocated and connected to either PLCs ( Programmable Logic Controllers ) or microprocessors like RTUs ( Remote Terminal Units), which control the sensors and actuator according to a software program and send this information to a SCADA master via a means of communication, this communication could be wired or wireless. This system was developed to automate plants, cut cost and decrease labor.
SCADA systems (Supervisory control and data acquisition) is design that controls plants using computers, communication e.g. (Data networks, Fiber Optics) and GUI (Graphical user interface) to achieve an elevated level of process supervision and management, in order to interface with plants and machinery it also uses devices like PLCs (programmable logic Controllers), RTUs (Remote Terminal units) and PIDs (Proportional–Integral–Derivative), an interface helps the operator to monitor the process and issue commands e.g. (set point changes) through the SCADA master computer, which then communicates this command to the logic controller of microprocessor that perform calculations and connects to plant sensors and actuators.
This system is was built on the concept that helps an operator have remote access to various controllers that are local to the plants, to access standard automation protocols. They are able to control large scale processes at different locations, this has made it into a common method often used by industries and manufacturers.
Functionality of SCADA systems is accounted to its fortitude in supervision over various other devices.
The following diagram is a generalized model of a SCADA system showing the stages of control that SCADA systems go through, the control is greatly computerized so that operators can interface with the machinery.
• Stage 0 consists of plant level devices like sensors and actuators, depending on the type of application these sensors may be temperature sensors, pressure sensors or any other type, same goes to the actuators, they may be valves, electric arms etc…
• Stage 1 consists of logic control modules such as PLCs and RTUs (Microcontrollers) which is where the sensors and actuators are connected to.
• Stage 2 consists of computers that have the SCADA software which supervises the operation and can override process, these computers have software that is pre-programmed to control and supervise the processes of the sensors and actuators via the logic controllers, they receive information from processor nodes and have interfaces for controlling and changing set points and alarms, further a historian records all data to enable data trending and analysis at higher stages.
• Stage 3 This stage is largely concerned with controlling the production level, it does not directly control like stage 2, however it monitors production rate to check wither target production is going to be met or not.
• Stage 4 At this stage the production schedule is created and monitored.
Since SCADA systems are typically large scale, it would be hard to follow every piece of equipment, hence a “tag” system is used for controlling every piece of equipment by referencing to the tag identification.
SCADA systems are based on the connection between complex devices, In order to match a large scale of instruments protocols are needed to set communication parameters between connecting devices, Some of the common protocols are listed below:
• Modbus: A standard for Logic controller communications, a Modbus network protocol allows devices to recognize other devices address and determine messages sent to it so action may be taken, the main disadvantage is that it cannot transfer large positive and negative numbers.
• Modbus X : fixes the Modbus issues and has a larger resolution or up to 9 digits and plus or minus 99 decimals.
• Distributed network protocol (DNP) : this protocol is commonly used in electric power systems, the only shortcoming is that it has many rules which has made industries hesitant to implement it, however the protocol is actively upgrading and is up to version 3.0.
• American Standard Code for Information Interchange (ASCII) : an old and tested protocol for computers, modems, printers and many sensors and actuators.
• IEEE 60870 : This protocol is mainly used in power transmission and distribution. It has become an international protocol and is spreading through many regions.
4.0 COMMUNICATION CONT.
SCADA systems use these protocols to communicate either wirelessly or hard wired, some of the communication methods used are as follows:
Type Advantages Disadvantages
Telephone Line Readily exists on site Usually needs third party to fix line problems, and has slow speed
Ethernet Good for low distances like local sites Signal is lost further than 1 kilometer and needs boasting.
Fiber Optic Very fast data transmission rate and high bandwidth High cost of implementation and monthly fees
Coaxial Cable Readily exists on site, good bandwidth Monthly fees
Type Advantages Disadvantages
UHF and VHF Voice Radio
Low maintenance and easily repaired Frequency license needed
900Mhz spread spectrum and
2.4Ghz Data Radio No license needed and has a high data transmission rate Needs to be within line of sight
Wi-Fi Good choice for short distances, good transmission rate Range is limited
Microwave Good for connecting high places such as mountains Hard to implement
Cellular Most popular option because costs are declining, good transmission speed Area coverage must be inspected for good cellular connection
Satellite Good for harsh condition like remote areas Very costly
5.0 History and Generations
In 1950, computers were first invented, they were used by industries for control purposes, this has initiated the need for further supervision and control. In the 1960s, telemetry was established for monitoring, which allowed for automated communications to transmit measurements and other data from remotes sites to monitoring equipment. The term “SCADA” was coined in the early 1970s, and the rise of microprocessors and PLCs during that decade increased enterprises’ ability to monitor and control automated processes more than ever before.
In the 80s and 90s, SCADA continued to evolve thanks to smaller computer systems, Local Area Networking (LAN) technology, and PC-based HMI software. SCADA systems soon were able to be connected to other similar systems. Many of the LAN protocols used in these systems were proprietary, which gave vendors control of how to optimize data transfer. Unfortunately, these systems were incapable of communicating with systems from other vendors. These systems were called distributed SCADA systems.
In the 1990s and early 2000s, building upon the distributed system model, SCADA adopted an incremental change by embracing an open system architecture and communications protocols that were not vendor-specific. This iteration of SCADA, called a networked SCADA system, took advantage of communications technologies such as Ethernet. Networked SCADA systems allowed systems from other vendors to communicate with each other, alleviating the limitations imposed by older SCADA systems, and allowed organizations to connect more devices to the network.
5.0 History and Generations Contd.
First generation: “monolithic”
Historically speaking the first SCADA systems operated using minicomputers, no networks have yet come to exist. That’s why early SCADA systems were not connected to other systems and therefore independent. All protocols of that time were managed by the SCADA developers, a backup main frame system was usually used in case the primary main system failed. Some first generation SCADA systems were developed as “turn key” operations that ran on minicomputers such as the PDP-11 series made by the Digital Equipment Corporation.
Second generation: “distributed”
At this stage the LAN (Local area network) came into play, SCADA implemented LAN in order to distribute data to multiple stations all connected to LAN, Real time communication was performed, to utilize this network each particular station was tasked independently, at this point all protocols were still developed mainly by the SCADA developers.
Third generation: “networked”
More communication protocols were developed by both independent developers and by the original SCADA developers, the evolution in networking has made it able to connect several networks by LAN in a Process and Control network (PCN), these networks may be remote to each other while still be connected to one single supervisor that’s also able to store and analyze data (historian), it has become a very powerful tool in system control.
Fourth generation: “Internet of things”
In the advent of Cloud computing SCADA systems have started using the Internet of things so that it may be able to reduce costs, elevate security, increase compatibility, ease maintenance and increase speed. To be able to adapt to this technology, the main concept or central data allocation needs to be decentralized, a new concept have been developed based on object oriented programming, this concept is called “data modeling”, data modeling uses a virtual representation of each device which contains rel time information about each device.
6.0 Benefits and issues
• The Following are the benefits or advantages of SCADA:
1. Collect data that otherwise may be hard to collect and use.
2. The system is very customize able and flexible.
3. It can interface a virtually unlimited amount of activity which makes it versatile.
4. Can obtain real time simulation.
5. Easy to interface to preexistent PLCs and RTUs.
6. The advancement of protocols allows for data supervision from remote locations..
7. Usually have back-up systems which renders it more robust.
8. It is scalable and flexible in adding additional resources.
9. Wide range of applications available.
10. cuts the cost of production.
11. Allows us to study and analyze data using the historian so we may find trends and increase production quality and rate.
• Following are the issues or disadvantages of SCADA:
1. Implementation may be complex depending on the plant.
2. As the system is complex, it requires skilled operators, analysts and programmers to maintain SCADA system.
3. Installation costs are high.
4. The system increases unemployment rates.
5. Compatibility issues exist.
The base of any SCADA system is its supervisory computer, this is where the data pools in from the logic controllers, data can come from processes and events that take place in the plant where filed devices like sensors and actuators are located. Depending on the size of the system there may be several work station connected to the logic controllers, theses work station all have HMI (human machine interface) which allows the operator to supervise the station, a master station may also include a single or several computers with HMIs, several servers may be used for data acquisition, distribution of software application and disaster recovery locations. To strengthen the system both primary and backup servers are often set on ready standby in case of system server crashes or malfunction.
Remote terminal units or Programmable logic controllers
RTUs Remote terminal units or PLCs is where sensors and actuators are connected to the I/O (input/output), PLCs are more easily configured, more versatile, and cost effective than RTUs, using protocols they communicate process information to the supervisory computers, they are always programmed and tested prior to production, some commonly used software to program PLCs are ladder logic, which is easily programmed and modified by the supervisory computers.
As we discussed before in the communication section, using protocols logic controllers are connected to supervisory computers, this communication be in wired or wireless form, the supervisory computers monitor the process either in real time, or on a periodic basis, in case the communication is cut the plant does not stop working, and when communication is resumed the supervisory computer may resume monitoring, to prevent this from happening industries use dual redundant communication forms so if one fails the other continues to transfer data.
7.0 Components Contd.
HMI stands for human machine interface, it’s the window where operator may supervise the SCADA system, the HMI shows the site in a graphical representation often using schematics to view the instruments and devices in action, alarms and logs of trending data using graphs are also presented using the HMI, further the HMI is capable of creating reports, and sending notifications.
Graphical diagrams are lines and symbols that present process of devices, some installation may include an actual digital image with symbols drawn over it to show the actual process location and ease understanding.
The operator that supervises the plant using the HMI may perform overriding of the process in cases of alarm, or change a set point for an actuator or sensor simply by pointing a cursor and clicking on the symbol of event in real time.
Often the HMI package includes a drawing program, where operators may change the interface in a way that suits them.
Further the HMI collects data but does not delete it, it saves it in the historian, this way it can create trends and analyze the information so it may help production and understanding.
8.0 Power Distribution
Most cutting edge technology these days depends on computers to control solutions, this way it can cut the cost of production and improve reliability. Using computers allows for optimized operation, controlled decision making as well as damage management of a power system network. Being able to collect data SCADA system have proved to be an efficient solution in power operations.
Usually, SCADA systems used for electrical power distribution are used to automate a network by providing remote supervision, coordination, controlling and operating distribution components.
They replace manual labour with automated equipment, to improve efficiency of a power distribution system, SCADA operators use real-time view, data trends and logging, maintain optimal voltage and current, maintain power factors as well as generate alarms.
8.0 Power Distribution Contd.
SCADA does the following tasks in a power distribution system, automatic supervision, protects and controls equipment using logic controllers like (RTUs), it resumes power service in case of error condition, further it maintains optimal operating conditions.
Moreover, SCADA enhances the redundancy of supply by reducing duration of outages and also gives the cost-effective operation of distribution system. Therefore, distribution SCADA supervises the entire electrical distribution system. The major functions of SCADA can be categorized into following types.
• Substation Control
• Feeder Control
• End User Load Control
8.0 Power Generation and Distribution Contd.
• Substation Control using SCADA
In substation automation system, SCADA performs the operations like bus voltage control, bus load balancing, circulating current control, overload control, transformer fault protection, bus fault protection, etc.
SCADA system continuously monitors the status of various equipments in substation and accordingly sends control signals to the remote control equipments. Also, it collects the historical data of the substation and generates the alarms in the event of electrical accidents or faults.
8.0 Power Generation and Distribution Contd.
The above figure shows the typical SCADA based substation control system. Various input/output (I/O) modules connected to the substation equipment gathers the field parameters data, including status of switches, circuit breakers, transformers, capacitors and batteries, voltage and current magnitudes, etc. RTUs collect I/O data and transfers to remote master unit via network interface modules.
The central control or master unit receives and logs the information, displays on HMI and generate the control actions based on received data. This central controller also responsible for generating trend analysis, centralized alarming, and reporting.
The data historian, workstations, master terminal unit and communications servers are connected by LAN at the control center. A Wide Area Network (WAN) connection with standard protocol communication is used to transfer the information between field sites and central controller.
Thus, by implementing SCADA for substation control eventually improves the reliability of the network and minimizes the downtime with high speed transfer of measurements and control commands.
• Feeder Control using SCADA
This automation includes feeder voltage or VAR control and feeder automatic switching. Feeder voltage control performs voltage regulation and capacitor placement operations while feeder switching deals with remote switching of various feeders, detection of faults, identifying fault location, isolating operation and restoration of service.
In this system, SCADA architecture continuously checks the faults and their location by using wireless fault detector units deployed at various feeding stations. In addition, it facilitates the remote circuit switching and historical data collection of feeder parameters and their status. The figure below illustrates feeder automation using SCADA.
8.0 Power Generation and Distribution Contd.
In the above typical SCADA network, different feeders (underground as well as overhead networks) are automated with modular and integrated devices in order to decrease the number and duration of outages. Underground and overhead fault detection devices provide accurate information about transient and permanent faults so that at the remote side preventive and corrective measures can be performed in order to reduce the fault repeatability.
Ring main units and Remote Control Units (RTUs) of underground and overhead network responsible for maintenance and operational duties such as remote load switching, capacitor bank insertion and voltage regulation. The entire network is connected with a communication medium in order to facilitate remote energy management at the central monitoring station.
• End User Load Control Automation by SCADA
This type of automation at user end side implements functions like remote load control, automatic meter reading and billing generation, etc. It provides the energy consumption by the large consumers and appropriate pricing on demand or time slots wise. Also detects energy meter tampering and theft and accordingly disconnects the remote service. Once the problem is resolved, it reconnects the service.
The above figure shows a centralized meter data-management system using SCADA. It is an easy and cost-effective solution for automating the energy meter data for billing purpose.
In this, smart meters with a communication unit extract the energy consumption information and made it available to a central control room as well as local data storage unit. At the central control room, AMR control unit automatically retrieves, stores and converts all meter data.
Modems or communication devices at each meter provide secure two-way communication between central control and monitoring room and remote sites.
8.0 Power Generation and Distribution Contd.
• Advantages of Implementing SCADA systems for Electrical Distribution
Due to timely recognition of faults, equipment damage can be avoided
Continuous monitoring and control of distribution network is performed from remote locations
Saves labor cost by eliminating manual operation of distribution equipment
Reduce the outage time by a system-wide monitoring and generating alarms so as to address problems quickly
Improves the continuity of service by restoring service after the occurrence of faults (temporary)
Automatically improves the voltage profile by power factor correction and VAR control
Facilitates the view of historian data in various ways
Reduces the labor cost by reducing the staff required for meter reading