Programmable Controllers in Power Plants
Power plants or otherwise power stations have many operations and tasks that require real time management. It is for this reason that these power plants across the globe have incorporated programmable controllers. Programmable controllers can also be referred to as programmable logic controllers. These are digital computers thus solid-state members. They are used in many industries and machines for electromechanical process automation. They function in such a way that they can store instructions for instance timing, arithmetic, sequencing, counting, data manipulation and communication.
According to Bryan and Bryan, programmable controllers can also be termed as industrial computers consisting of specially designed central processing unit and a comprehensive interface that links to the relevant field devices. The power industry employs Programmable Controllers in efficiency control of compressors, the process of coal fluidization, management of energy and the entire power plant.
1968 saw the year when the design criteria for the first programmable controller was proposed and therefore specified by General Motors. Power plants have since been known to be very dependent on this ever improving development. The traditional relay systems were known to be inflexible and very expensive. They relied on electrically operated devices to switch electrical circuits mechanically. The advent of Programmable controllers meant that these relay systems had to be eliminated. This was therefore a good opportunity for power plants to seize. Among the specifications forwarded by General Motors included: system reusability, simple method of programming, easily replaceable output and input interfaces, relay of data to the central system by the control system, cost effectiveness and ability to sustain the dynamic industrial environment.
As it was anticipated, the first programmable controllers exhibited the desired functionality thus eliminating the relay systems. The requirements for instance expandability, saving on space, reusability, programmability and ease of adapting to the complex power plant environment and installation were all met.
The ladder diagram format was used for controller programming. As much as it was tedious to program, it had a power plant standard that was easily recognizable.
Drbal et al. reveal that power plants among other industries began to use programmable controllers by 1971. The other industries included manufacturing, metals, pulp and paper and food and beverage among several others. The ever changing technology means that programmable controllers are also being modified to suite the wide range of customer needs. The various changes have a focus on both software and hardware. The hardware relates to the controllers physical components while the software comprises of the control programs.
Several steps have been made in terms of hardware enhancement. The scanning times are faster. The fact that the programmable controllers are smaller means that they occupy less space in addition to having more power. This is an aspect of efficiency since it is not logical for a power plant to be involved in occurrences that lead to wastage of power. The input and output systems have comparatively high density. The input/output interfaces are microprocessor based thus the level of intelligence. The host computer, PID and network are examples of the interfaces. The precise mechanical design has been modified to consist of input/output systems that appear like a single integral unit and rugged input/output enclosures. Direct connectivity to the controller has been enhanced by special interfaces. These devices are strain gauges and thermocouples. The operator interface techniques have also been improved by the peripheral equipment.
The programmable controllers’ software has also been enhanced in a number of ways to suit the needs of power plants irrespective of their locations. The object-oriented language along with its tools has been used in the system. This means that powerful instructions have been incorporated. In addition to this, BASIC and C as high-level programming languages have also been used. The ladder diagram instruction has been designed in such a way to have advanced functional block instructions.
The ladder diagram language with respect to a power plant can be described as a set of symbolic instructions that are used to generate programs for the programmable controllers. This particular instruction set is commonly known as contact symbology. To perform its operations, ladder diagrams normally use rungs. Fault detection and diagnostics of the controllers have also been expanded. Complex calculations in a power plant have been made possible by employing the use of floating-point math. In general, manipulation of data, information and instructions has been made much easier.
According to Krishna further advancements have seen programmable controllers being established into what is commonly known as computer aided design (CAD) systems, computer aided manufacturing (CAM) systems, computer-integrated manufacturing systems and robots. Improved user friendly interfaces whether graphic, human oriented man/machine or operator are major indicators of advancing PLC technology in power plants. Like any other industry, power plants aim at achieving improved service delivery and general efficiency. The ever improving programmable controller technology for power plants reliably guarantees this need. The interfaces are meant to facilitate communication of the various components of the system including the hardware, software and equipment.
The new programmable logic controller instructions are crafted with the aspect of the entire system’s intelligence in mind. The entire programmable controller system has to make valid decision and execute instructions with minimal operator interference during normal operations in a power plant. The future of programmable logic controllers obviously promises many improved modifications in an effort to achieve maximum efficiency of this necessary invention especially in power plants. This is so because programmable controllers are quickly becoming the main component of most factories.
Components and Systems
Bryan and Bryan assert that the components and systems of a programmable controller are similar irrespective of the area of application. It follows that in a power plant also, the central processing unit is the main component of the controller. The Central Processing Unit being the brain of the controller consists of the processor, memory system and power supply. The central processing unit stands out as the most important component of the programmable controller. It functions as a brain. On the other hand, the central processing unit is made up of a processor, the main memory for storage and finally the power supply. Power supply determines the integrity and reliability of the entire system thus the first-line manager. As an element it ensures that voltages are sufficiently regulated.
Different manufacturers have different central processing units in terms of architecture. The microprocessors which are wired into integrated circuits have amazing control and generally computing capability. Personal computers and manual/ handheld or mini-programmers are the devices that are commonly used as programming devices by manufacturers. Mini-programmers are cheaper and portable as compared to personal computers. They look like calculators with a larger display and a special keyboard.
In power plants for instance, the programmable controller is associated with two kinds of memory. Volatile memory is characterized by loss of contents when the system is disconnected while nonvolatile memory stores its contents under these circumstances. The central processing unit and the field devices are physically linked by the discrete input/output system. Implementation of the desired control is sometimes made possible by special and analogue input/output modules.
Principles of operation
A power station is known for its many operations all geared towards generating electrical energy to be used for various purposes at respective points of needs. The installed programmable controller therefore has to effectively coordinate all these operations including monitoring the optimum functioning of the generator. It is important to note that there various kinds of fuel that a power station can use. This issue is related to the type of energy source. The entire system of a programmable controller, as mentioned earlier, is made up of inputs, the central processing unit and the output. A combination of the input/output systems makes up the interface which functions to connect the field devices to the controller. The interface therefore facilitates the conditioning of a couple of signals that are transmitted during a particular communication process. This communication is usually between the controller and the respective external field devices like turbines or generators.
All the fields in the power plant are fitted with sensors which send relevant signals to the programmable logic controller. The input terminals connect with the terminal of the sensors through a wire system. Examples of sensors include: selector switches, push buttons, limit switches, thumbwheel switches and analogue sensors. It follows that the terminals of the output interfaces will be connected to devices that should be controlled. These devices include: solenoid valves, position valves, pilot lights and motor starters.
The power supply component ensures that the required power is provided at the various points of need. Common sense dictates that there can never be any operation without power thanks to the fact that it is a power plant that is in question.
The scan cycle is such the first stage is to read the instruction before execution and finally the writing stage. This process forms a loop.
Programmable Logic Controller Languages
There are three commonly used programming languages that are commonly used when designing programmable controllers for power plants. They are Ladder, Boolean and Grafcet. Both Ladder and Boolean programming languages are similar in terms of implementation of operations. The only difference between these two programming language is in the way of entering information into the system and manner in which instructions are represented. Grafcet implements its instructions on the basis of actions and steps using programs that are graphic-oriented.
Programmable controllers in power plants also use ladder diagrams. The ladder diagram is a representation of electrical operations in a comprehensive sequence. It shows the connection of the controller to the field devices. A predetermined sequence ensures that when a device is activated, another one also activated depending on the desired or intended instruction.
Number systems and codes
Toshiba asserts that number systems are very essential to the program controllers. Their operation fully depends on numbers in order to store, manipulate and even represent data or information to and from the connected field devices. The several quantities and codes within the system are usually represented by binary numbers. At times it is important for an operator to be familiar hence appreciate these numbers. It is important to note that the binary concept is a very old idea. In the world of binary digits, several things exist in the ‘1’ and ‘0’ states. The binary concept translates in to the vital logic functions. Logical statements such as AND, OR, NOT are used for decision making. The controller is capable of determining whether a function is FALSE or TRUE prior to effecting any operation.
The IEC 1131 Standard
It is also important to mention that for the purposes of standardization, the IEC 1131 standard was developed by the International Electrochemical committee. This standard attained its international recognition in 1992. The committee aimed at coming up with a common set of instructions. Common sense dictates that there is a wide variety of controllers in use in power plants today. It is therefore very important to effect uniformity. The diversity of programmable controller manufacturer has since been the main challenge. Different manufacturers have different preferences relating to controllers. Another problem that has threatened this development has been the incompatibility of programs.
This entire standard is made up of user guide lines, programming languages, general information, communications and equipment and test requirements.
Programmable Controller System Documentation
Swainston defines documentation refers to comprehensive recording of information relating to operation of an entire system. This information can be used as reference material. It can be used during installation, repair and maintenance, debugging, design and start up. Documentation enhances easy communication in addition to answering possible design problems. This information can also be used to train operators and maintenance personnel. Documentation is the result of careful and proper compilation of software and hardware information. It is therefore a good engineering practice to always provide documentation which will in turn benefit the end user.
Documentation is made up of a number of components including: a wiring connection diagram of the input/output devices; their address assignments; variable declaration; a print-out of the control program; information about the control program; the system abstract and configuration and internal storage address assignments.
Advantages of Programmable Controllers
Programmable Controllers in power plants have various advantages in light of benefits and inherent features that are closely associated with them as compared to other controllers. The various inherent features include: software control relays, the reduced or rather small size, the wide variety of interface (input/output systems) that is modular in nature; availability of microprocessors to ensure quick responses to instructions; remote access of the input/output systems; reliable and quick disconnections of the input/output systems; memory that can be programmed; diagnostic indicators and finally the handy software counters.
The benefits are in terms of its flexibility, reliability, simplicity, expandability, cost effectiveness and neatness. Power plants across the globe have reported how Programmable controllers have better communication capability as compared to the previous relay systems. This benefit automatically translates into overall improved performance thus efficiency. The aspect of multi-functionality is also very possible. This system is capable of performing many tasks at a time thus saving on time. Shin (2004) believes that a programmable controller as a system is complete in such a way to reduce unnecessary wiring which in turn saves on costs. Another benefit is that it has been capable of controlling a wide variety of devices as long as they are well connected to it.
Programmable controllers in power plants are so handy such that they can easily be installed, wired and maintained. Servicing can easily be effected without tempering with its wiring. The monitoring of performance of a power plant has since been possible through generation of relevant reports by the controller. The operator is then able to know what is required for instance if a field device has broken down, the report will definitely point it out. As much as this feature saves on time, it also ensures that the general production of that particular power plant is not affected negatively. The fact that the breakdown of a field device will be shown on the screen means that trouble-shooting time is reduced. In the early days when relay systems were in use, breakdowns resulted into so many problems. In some cases the production lines had to be stopped.
Most of the power plants which have incorporated programmable controllers into their companies, have appreciated these systems as very vital tools for maintenance and management. This benefit is attributed to the overall improved efficiency as compared to the traditional relay systems which were known to be slow and broke down often. In other words, the relay systems were unreliable thus the need to eliminate them.
The ease of installation of the programmable controllers is closely related to its efficient design in terms of space. To install the input/output station of a programmable controller, one has to consider proper wiring to the field devices, the subsystems which are remotely located and coaxial cables for communication purposes.
A personal computer that is connected to a programmable controller is usually used for programming purposes thus a programming device. A special connector cable links the programmable controller to the personal computer. It is through this cable that these two units can communicate to each other.
Research has revealed how many power plants have since embrace the use of Programmable controllers because of the many associated benefits. The entire system is capable of handling so many tasks within the shortest time possible. More power plants should therefore feel encouraged invest in this wonderful development. As time passes by the controllers’ technology for power plants has been improving to match the ever changing needs of its clientele. More research should be done so as to improve them even further.