What is synchronization and effects of poor synchronization in power plant

Synchronization of Turbo Alternator or an AC generator is the process of connecting the generator with grid power supply which is an interconnection of large pool of generators and power consumption loads. Simply the grid is parallel operation of some number generators with same frequency. So to connect the Generator in power plant in this pool of parallel running generators, The incoming generator parameters like frequency, phase angle and voltage should be matching with the existing grid frequency.

Before going detail description first let us understand what is the need of synchronization of generator. Generator is connected with the prime mover which provides the rotating magnetic field and hence this rotating magnetic field will induces the voltage in the stationary part. The frequency and phase angle of the voltage signal is controlled by the prime mover speed and magnitude of the voltage signal is controlled by the generator excitation. 

To understand the phenomenon let us correlate the entire operation with the person wants to catch a running travel bus. Consider the travel bus is grid power supply and the person is incoming generator. Now if the person wants to get in to the bus then he should equally or little faster than the bus same the generator tries connect to the grid should run equally or little faster than the grid. Here the speed is measured with the frequency because speed is proportional to the frequency( 50 Hz, 60 Hz). The person is now running with the same speed of the bus but the bus door is one end of the bus and he is at another end of the bus so he needs to match with the door to get in to the bus. Like the same if the generator is running at the same frequency of grid it cannot be synchronized until unless the phase of the two voltages matches.

What is the need of synchronizing two different power sources?

"Suppose you have a trolley that can only drawn by either pushing or pulling it ,two workers are there to drive it if one of them is pushing in one direction but the other one is in another direction ..What will happen?

they can't move it with different speed or different direction."

Similar is gonna happen with power source.

Phase angle indicates the direction, If there is a phase difference between two power sources it can't operate any load.

In order to synchronize a generator to the grid, four conditions must be met:

1. Phase Sequence

The phase sequence (or phase rotation) of the three phases of the generator must be the same as the phase sequence of the three phases of the electrical system (Grid).

The generator or transformer power leads could actually be interchanged during maintenance orthe potential transformer leads could be interchanged during maintenance..

2. Voltage Magnitude

The magnitude of the sinusoidal voltage produced by the generator must be equal to the magnitude of the sinusoidal voltage of the grid.

If all other conditions are met but the two voltages are not the same, that is there is a voltage differential, closing of the AC generator output breaker will cause a potentially large MVAR flow.

3. Frequency

The frequency of the sinusoidal voltage produced by the generator must be equal to the frequency of the sinusoidal voltage produced by the grid.

The synchroscope would be rotating rapidly counter clockwise. If the generator breaker were to be accidentally closed, the generator would be out of step with the external electrical system. It would behave like motor and the grid would try to bring it up to speed.

In doing so, the rotor and stator would be slipping poles and damage (possibly destroy) the generator as described previously. The same problem would occur if the generator were faster than the grid.

4. Phase Angle

As previously mentioned, the phase angle between the voltage produced by the generator and the voltage produced by the grid must be zero.

The phase angle (0 to 360°) can be readily observed by comparing the simultaneous occurrence of the peaks or zero crossings of the sinusoidal waveforms..

Effects of poor synchronization:

Prime mover damages if the speed and rotor angle is not matches with grid voltage frequency and phase angle due to rapid acceleration or deceleration. Let us suppose generator has to connected to the grid frequency of 60 Hz. But the breaker has closed with poor synchronization at the generator frequency of 58Hz (i.e for two pole generator speed is 3480 out of 3600 rated), now once the breaker closes the generator is connected in the pool of parallel generators which forces the incoming generator to rotate at the same grid frequency. Due to this sudden acceleration of the rotor from 3480 to 3600 rpm and a sudden break at 3600 rpm damages the rotor mass. Same way in the reverse when the generator is running higher frequency than the grid frequency.

A large currents may suddenly flow through the Generator windings and Generator transformer windings due to poor synchronizations which damages the windings.

There will be power and voltage oscillations because of this sudden acceleration and deceleration of the rotor.

It may leads to activation of the generator protective relays which causes the major interruption so the process should be started once again after clearing the protection. 

Source 1: https://www.quora.com/What-is-the-need-of-synchronizing-two-different-power-sources

Source 2: http://www.ijraset.com/fileserve.php?FID=4512

Source 3: http://grouper.ieee.org/groups/scc21/1547.8/email/pdfX2pEArHjt0.pdf

Mundo Siemens

How to select Level Measuring technology?

Level Measurement Technology

Two Groups of Level Measurement -
1.Point Measurement (Discrete)
2. Continuous Measurement (Analog)
Common Types of Level Measurement Technologies
1.Plumb Bob
This is one of the oldest level measurement technology.
A worker drops a length of pre-measured rope until a float contact the material surface.
Advantages -
Simple
Disadvantages -
Human Non- reputability
Hazard exposure
Process damage from broken cable
Not instantaneous

2.Ultrasonic (Non-Contacting)
Measure time of flight from transmission to received echo.This determines distance and calculates level.
Advantages of Ultrasonic Level Measurement-
Non-Contact
Well Proven
Solids,Liquids,or Slurries
Control Capabilities (Pumping and alarm)
Potted/robust transducer for vibration and shock (No electronics in vessel)
Remote display
Disadvantages of Ultrasonic Level Measurement-
Sensitive to vapor changes in medium
Can be sensitive to foam
Limited pressure and temperature range

3.Radar (Non-Contacting)
Measure time of flight from transmitted signal to return signal for distance measurement.
Advantages of Radar Level Measurement-
Non-Contacting
Insensitive to vapor and dust
Unaffected by temperature and pressure
Very long ranges (up to 100 m)
Disadvantages of Radar level measurement-
Can be sensitive to foam
Can be sensitive to very heavy condensate
Display and antenna are integral

4.Guided Wave Radar (GWR) or Capacitance (Contacting)
Both technologies use a rod or cable which extends into the material being measured.
Advantages of Guided Wave Radar or Capacitance level Measurement
Able to measure the liquid-liquid interface of two imiscible liquids
Wide temperature / Pressure range up to 427 deg C or up to 431 bar
Disadvantages of Guided Wave Radar or Capacitance level Measurement
Can be sensitive to material build up
Wear in solids
Pull forces on roof-solids applications
Equipment damage from broken rods.

5. Hydrostatic Level (Contacting)
Measure head pressure of material in vessel
Advantages of Hydrostatic Level Measurement
Easy to use
Complex internal geometry possible
Suitable for high temperature and pressure
Most common level measurement in all industries
Disadvantages of Hydrostatic Level Measurement
Contacting - chemical compatibility with seals
Susceptibility to specific gravity changes may require recalibration
Adds fitting/piping to system if not submersible

Source: https://automationforum.in/t/how-to-select-level-measuring-technology/882

Temperature Sensor Installation for Best Response and Accuracy

The installation of the sensor can introduce errors, noise, and dynamics causing poor measurement and control loop performance. Here we look at best practices to get the most out of the inherent capability of the sensor. My next post will provide guidance on the communication of the sensor signal to the control room to provide the best total installation.

Thermowell Length

To minimize conduction error (error from heat loss along the sensor sheath or thermowell wall from tip to flange or coupling), the immersion length should be at least 10 times the diameter of the thermowell or sensor sheath for a bare element. Thus, for a thermowell with a 1 inch (2.54 cm) outside diameter, the immersion length should be 10 inches (25.4 cm). For a bare element with a ¼ inch (6.35 mm) outside diameter sensor sheath, the immersion length should be at least 2.5 inches (63.5 mm). This is just a rule of thumb. Computer programs can compute the error and do a fatigue analysis for various immersion lengths and process conditions. For high velocity stream and bare element installations, it is important to do a fatigue analysis because the potential for failure from vibration increases with immersion length.

The choice of thermowell length, location, and construction determines whether the temperature measurement is representative of the process, how much process noise is seen, how much delay and error is introduced, and the potential failure rate. This post provides some general guidance. This post provides some general guidance. For more details including the equations to predict eight sources of measurement error see Greg McMillan’s ISA book Advanced Temperature Measurement and Control, Second Edition

Thermowell Location

The process temperature will vary with process fluid location in a vessel or pipe due to imperfect mixing and wall effects. For highly viscous fluids such as polymers and melts flowing in pipes and extruders, the fluid temperature near the wall can be significantly different than at the centerline (e.g., 10 to 30°C; 50 to 86°F). Often the pipelines for specialty polymers are less than 4 inches (101.6 mm) in diameter, presenting a problem forgetting sufficient immersion length and a centerline temperature measurement. The best way to get a representative centerline measurement is by inserting the thermowell in an elbow facing into the flow (position 1 in the figure below). If the thermowell is facing away from the flow, swirling and separation from the elbow as can create a noisier and less representative measurement (position 2 in figure). An angled insertion (position 3 in figure) can increase the immersion length over a perpendicular insertion (position 4 in figure) but the insertion lengths shown for both are too short unless the tip extends past the centerline. A swaged or stepped thermowell can reduce the immersion length requirement by reducing the diameter near the tip.

The distance of the thermowell in a pipeline from a heat exchanger, static mixer, or desuperheater outlet should be optimized to reduce the transportation delay but minimize noise from poor mixing or two phase flow. Generally 25 pipe diameters are sufficient to ensure adequate mixing from turbulence if there is a single phase, turbulent flow, and no great differences in the viscosity of streams being combined. Two phases exist for desuperheaters, split ranged transitions from cooling water to steam in jackets, the use of lime ammonia as a reagent for pH control due to flashing and whenever slurries are involved.

The transportation delay will increase with distance adding more dead time to the loop. Consequently, there is a compromise between getting enough mixing to achieve a representative low noise measurement and creating too much additional dead time. In general, the transportation delay should be less than 10% of the PID reset time setting.

 Insight: Generally a distance of 25 pipe diameters between the equipment outlet and the temperature sensor is sufficient to provide a relatively uniform temperature profile of a single phase fluid. The presence of different phases (e.g. bubbles or solids in liquids and droplets in steam) and high viscosity fluids will require longer distances.

For desuperheaters, the distance from the outlet to the thermowell depends upon the performance of the desuperheater, process conditions, and the steam velocity. To give a feel for the situation there are some simple rules of thumb for the length of piping from the desuperheater to the first elbow known as straight piping length (SPL) and the total piping length from the desuperheater outlet to the sensor known as sensor total length (TSL). Actual SPL and TSL values depend on the quantity of water required with respect to the steam flow rate, the temperature differential between water and steam, the water temperature, pipe diameter, steam velocity, model, type, etc. and are computed by software programs.

SPL (feet) = Inlet steam velocity (ft/s) x 0.1 (seconds residence time)
SPL (m) = Inlet steam velocity (m/s) x 0.1 (seconds residence time)
TSL (feet) = Inlet steam velocity (ft/s) x 0.2 (seconds residence time)
TSL (m) = Inlet steam velocity (m/s) x 0.2 (seconds residence time)

Typical values for the inlet steam velocity, upstream of the desuperheater range from 25–350 ft/s (7.6 to 107 m/sec). Below 25 ft/s there is not enough motive force to keep the water suspended in the steam flow. Water tends to fall out and run down the pipe to a drain. When this happens the water no longer cools the steam and the system thinks it needs to add more water, which compounds the problem. Problems can also include pipe wall erosion and high thermal stress gradients in the pipe wall (i.e., a hot top and cold bottom, which can crack welds or warp the pipe to an egg-shaped cross-section). Current technology has an inlet velocity limitation of 350 ft/s (107 m/sec). Velocities higher than 350 ft/s cause the desuperheater to vibrate and damage the unit to the point where it breaks apart.

Thermowell Construction

The stem of a thermowell is the part that is inserted into the process stream. Stems can be tapered, straight, or stepped. The performance of a thermowell varies with its stem design. In general, a tapered or stepped stem provides a faster response, creates less pressure drop, and is less susceptible to conduction error and vibration failure.

If the thicknesses of the thermowell walls and the fit of the sensing element are identical, thermowells with straight stems have the slowest time response because they possess the most material at the tip (largest diameter). Thermowells with stepped stems have the fastest time response because they possess the least material at the tip (smallest diameter). A small diameter also results in the least amount of drag force. Thermowells with stepped stems also provide the maximum separation between the wake frequency (vortex shedding) and the natural frequency (oscillation rate determined by the properties of the thermowell itself). If the wake frequency is 80% or more of the thermowell natural frequency, resonance and probably damage can occur. Generally, thermowells with tapered stems are slightly more expensive as a result of a more complicated manufacturing process.

Insight: Swaged, stepped, and tapered thermowells offer a faster response, lower pressure drop, and less possibility of vibration damage from resonance with wake frequencies.

The tip of the sensor must touch the bottom of the thermowell. Spring loaded sensor designs help ensure this is the case despite different installation practices and orientation. The fit of the sensor should be as tight as possible to reduce the annular clearance since air acts as insulator. The sensor lag can increase by an order of magnitude for a sloppy fit. For liquid systems, the additional lag effectively becomes an additional equivalent dead time in the measurement.

Insight: The tip of the temperature sensor must touch the bottom of the thermowell and the fit must be tight to prevent introducing a large sensor lag due to the low thermal conductivity of air.

Take advantage of general guidelines on thermowell insertion length, location, construction, and fit to make sure the sensor is seeing the actual process temperature with a low probability of vibration failure and minimal noise, delay and lag.

Source: http://automation.isa.org/2015/02/temperature-sensor-installation-best-response-accuracy/
https://es.scribd.com/document/236572732/Piping-Handbook-Hydrocarbon-Processing-1968 (Chapter Thermowell Design For Process Piping - John A. Masek Philadelphia)

Understanding Modbus Serial and TCP/IP

P&ID Diagram - Online Drawing Tool Draw n the browser with Google Docs

FREE online P&ID diagram drawing template - enabled for the FREE online Google Docs.

Make your own P&ID diagrams with this FREE online drawing tool.

Log in to your Google Account (Google Accounts are free) and copy ("File > Make a copy") this online P&ID drawing template to start making your own drawings.

• Select, copy and paste the components you want to use. Customize existing components and make new ones
• Share and collaborate online with others - export and publish diagrams to the public  
• Combine Google Documents and Spreadsheets with Drawings to make shared functional descriptions, items lists and more 

The template is FREE to use - but we appreciate a credit to the Engineering ToolBox  on your drawing.

Related Topics

• Documentation - Documentation of process control systems - Block Flow Diagrams (BFD), Process Flow Diagrams (PFD), Piping and Instrumentation Diagrams (P&ID) and more
• Process Control - Instrumentation and process control systems, design and documentation
• Water Systems - Hot and cold water service systems - design properties, capacities, sizing and more
• Piping Systems - Dimensions of pipes and tubes, materials and capacities, pressure drop calculations and charts, insulation and heat loss diagrams
• 2D Schematic Drawings - Create and share online schematic diagrams and drawings - P&ID, HVAC, Process Flow .. - using templates with Google Docs
• Drawing Tools - 2D and 3D drawing tools

Related Documents

• Electric Circuit Diagram - Template - Use Google Drive to make online shareable electric circuit diagrams
• PFD - Process Flow Diagram - Online Drawing - Draw Process Flow Diagrams - Online with Google Docs
• P&ID - Piping and Instrumentation Diagram - P&ID is a schematic illustration of a functional relationship between piping, instrumentation and system components
• PFD - Process Flow Diagram - The Process Flow Diagram - PFD, a schematic illustration of the system
• ISA codes for Process Instrumentation - ISA process instrumentation codes - and combinations
• HVAC Diagram - Online Drawing - Draw HVAC diagrams - Online with the Google Drive drawing tool
• Signal Transmission - Standard Systems - Standard signals used for transmission from sensor to controller
• Drawing Scales - Commonly used scales for blueprint drawings

Source: http://www.engineeringtoolbox.com/p-id-drawing-d_1639.html

SCADA Interview. Questions & answers.

SCADA (supervisory control and data acquisition) is a type of software application and hardware control that defines the way data and real-time processing is controlled. You mostly find SCADA in every industries such as power plants and oil and refining businesses. You can also find SCADA rules in telecommunications and transportation.

What is the Scada system?

SCADA (supervisory control and data acquisition) is a system, operates with coded signals over communication channels so as to provide control of remote equipment (using typically one communication channel per remote station.

Acronym for supervisory control and data acquisition, a computer system for gathering and analyzing real time data. SCADA systems are used to monitor and control a plant or equipment in industries such as telecommunications, water and waste control, energy, oil and gas refining and transportation. A SCADA system gathers information, such as where a leak on a pipeline has occurred, transfers the information back to a central site, alerting the home station that the leak has occurred, carrying out necessary analysis and control, such as determining if the leak is critical, and displaying the information in a logical and organized fashion. SCADA systems can be relatively simple, such as one that monitors environmental conditions of a small office building, or incredibly complex, such as a system that monitors all the activity in a nuclear power plant or the activity of a municipal water system or other plants.

What is meant by PLC Scada?

Programmable Logic Controller or PLC is a computing system used to control electromechanical processes. SCADA stands for Supervisory Control and Data Acquisition. It is a type of industrial control system that is used to monitor and control facilities and infrastructure in industries.

What is Scada network?

Acronym for supervisory control and data acquisition, a computer system for gathering and analyzing real time data. SCADA systems are used to monitor and control a plant or equipment in industries such as telecommunications, water and waste control, energy, oil and gas refining and transportation.

What is meant by supervisory control?

Supervisory control is a general term for control of many individual controllers orcontrol loops, whether by a human or an automatic control system, although almost every real system is a combination of both.

What are some different levels of SCADA machines and systems?

SCADA machines have several different forms. You can have field level devices such as sensors. You can have remote terminal units (RTUs), a main controller or “master station,” or you can have a simple computer system interface that displays data for the user.

Why SCADA systems are Implemented?

Some of the reasons or advantages why SCADA systems are implemented are.

SCADA systems improves the performance of the operation of the plant

SCADA systems provides better protection to the equipment of the plant

SCADA systems improves productivity of the personnel

Information receives very fast, process the information and display it to opertor in graphs and plots. Hence helps the operator to take the decisions fast.

Provides better energy savings and saves economy

Why do companies use SCADA?

SCADA is a methodology that defines performance and protection of data for the plant or communication center. It helps improve productivity for employees and helps the economy save energy.

What voltage does an RTU operate with?

A remote terminal unit operates at 110V/240V.

What is an HMI?

An HMI (human machine interface) is the system that connects to a SCADA database that displays data for the user. The HMI allows the operator to review diagnostic data and trending graphs.

What is DDE?

DDE is the acronym for dynamic data exchange. DDE provides a communication protocol that allows devices to send and receive communication signals. This protocol was developed by Microsoft.

What are some advantages of SCADA over other protocols?

SCADA allows users to create object-oriented graphs, they can offer trending graphs to review data in real-time. SCADA also deals with big data, so you can develop a database system that displays trends over a number of years. This allows the business quick access to important data that can be used for future growth and enhancements.

How does SCADA handle data?

SCADA systems gather data and send them to a central machine that processes the data. The SCADA software is used to manage the data and display it to the user. SCADA also logs information about who runs reports and the data that is used.

How does SCADA handle issues?

SCADA has an alarm or alert system that interfaces with personnel to send them a warning when a system is not working properly.

What is the “Historian” in SCADA?

The historian is the name given to the software service that collects events and logs them into the database for future use.

Have you been having trouble setting yourself apart from other candidates in your SCADA interviews? 

If so, you should consider SCADA training to set yourself apart from the crowd. Fill out the form below for a course syllabus and pricing information on our instructor lead, live online and self paced training options.

Is this Control and Monitoring system supplied as a single package?

Yes

How much does each additional driver cost?

Most are included

Does the Control & Monitoring system use Client/Server distributed processing?

Yes

Can the system be expanded, without re-engineering, to handle future requirements?

Yes

Can I make changes to the system without shutting down?

Changes take place immediately as there is no compiling. If there are redundant servers all changes are automatically sent to the Standby servers.

How can we exchange data with other applications?

Exchange of data with other applications and systems is done with Ethernet, Modbus, OPC, ODBC (SQL), or through an API. The OPC interface supports OPC-DA (Real-time Data Access), OPC-HAD (Historical Data Access), and OPC-AE (Alarms and Events).

Can other automation systems, like a DCS, communicate using industry standard Communication drivers like Modbus or DNP3?

The system supports being a Modbus or DNP3 Slave so other systems can connect and access real-time data. This is accomplished by configuring a virtual device which only the points required for the other system are mapped.

What external databases does the Control and Monitoring system support?

Any ODBC compliant (SQL) database.

How can we provide for data integrity and system control in the event of hardware failure?

Data integrity & system control is maintained through server mirrored redundancy. The system supports triple mirrored server redundancy.

Are multiple copies of the configuration database required to enable redundancy?

No, the redundancy is mirrored. Mirrored redundancy means that no programming or extra database configuration is required to make the redundancy work. All functions are available from the standby or redundant server.

What happens if a computer or IO node or client fails?

If a server fails, then automatically connect to the standby server as soon as the client detects that Main server has failed, transparent to the user.

How can we provide backup for critical tasks such as plant-floor I/O, alarms and trends?

All server functions are mirrored to the redundant server. The system provides for Data backups to Local or Storage on a network client with backup device (like large hard drive or tape).

When the primary Server fails, is there any loss of monitoring or control before the redundant I/O Server assumes control?

No, the transfer is transparent to the users and all functions are as normal.

When the primary and standby Servers are in operation, are they both polling the I/O Devices (PLCs)?

No, only the server acting as Main will communicate to the IO

What happens to alarms monitoring if the primary alarms server fails?

The servers are mirrored and all functions are transferred transparently to the users or functions ongoing.

What happens if the LAN fails?

If redundant LANs are installed the transfer is transparent to the system

Can we create descriptive tag names to incorporate a more meaningful tag naming strategy?

Yes

If we need to make a configuration change to one data point, for example, does that changed need to be made on all nodes?

The system has only one configuration database and any change made are automatically propagated to all servers and clients. Nothing else is required.

Can we build display pages on one node and display them any node in the system?

Yes assuming that the users on the other clients have permissions to see the display page.

How do we backup/archive the system configuration information?

The system has an Export function which allows saving the database to any media.

How do we restore the system configuration and history in an event of data loss?

Import the backup file or database

How do we set up communication with an I/O Device (PLC)?

The system uses a objects/addresses for defining IO devices. Define the object, and configure the individual parameters in the IO window. This can be used as a template object so it can be reused in the system.

Can we control how the system polls the I/O Devices (PLCs)?

Yes and the polling configuration is easily controlled by adjusting communications parameters of the device.

Can we retain system values on disk at shutdown and restore them on system restart?

Yes the system maintains all the last values of every point in the system with a timestamp.

How many separate security areas can we define?

As many groups and-or individual as is needed, no limit

What elements of the system can we associate with security areas?

The security level can be defined down to the database point level.

How do we configure a system that only uses an industrial keyboard?

The system primary input is through the pointing device. A screen keyboard can be implemented for function not on an external keyboard.

How many colors does the package have for building graphics displays?

True Color

How do we draw complex entities such as 3D pipes?

The 3D effects are done with shading and sizing techniques like most artists do.

How easily can we construct intelligent objects that will save drawing and configuration time?

Objects can be made into templates from which instances are created. Each instance contains all database entries. Typically only addressing and selection of the physical IO device are necessary.

Can we edit library objects and intelligent objects supplied with the package?

Yes, but it is recommended to copy/paste the original object into the library and modify it as a new object; thereby, keeping the original library.

If we change a library object or template object, are the changes reflected throughout the system—or do we have to change every occurrence of the object?

The changes will take affect immediately to all other instances of object or template in the system.

Can we use Boolean and other arithmetic expressions when animating data?

Yes

Can we apply multiple animations to the same object?

Yes

Can we disable command buttons on a graphics page?

Yes

Can we create custom sliders?

Yes

What graphics file formats can we import into the graphics builder?

The AutoCAD, DXF, and all other graphics formats supported by the particular vendor software can be imported or copied and pasted into the drawing builder.

How do we recover from accidental deletion or moving of objects when editing a complex display page?

The use of undo function is used to reestablish deletions. It also support layers so graphics objects can be put on separate layers to prevent accidental changes.

How do we align objects on the page?

Use a snap to grid or align the object with the alignment tool

How can we add special effects to text and other objects?

Special effects are done with colors, lines, shading etc.

How can we make a global change of one color for another in a object?

Special color tags may be created. When these are referred to for colors, changing the tag color will change all instances that use that color.

How can we edit a group of objects?

Objects with common attributes can be selected, and the changes will apply to all in the selected group.

How are Alarms and hardware communications failures reported by the Control and Monitoring system?

Alarms are reported and are always visible on any page through the alarm banner which is normally at the bottom of the page. There is an Alarm Management system within the system that provides for alarm redirection, sorting, filtering, filed editing, etc. Custom alarm lists can be embedded on any page that is filtered for that specific page. For instance, you only what to see the alarms that pertain to a specific substation or breaker.

Do we have to configure alarms for all items of hardware?

No but we can also configure as per requirement

When an alarm is acknowledged at one operator station, is it automatically acknowledged at all nodes (workstations) without having to write programs or scripts?

Yes. This is true even on cluster servers.

Can we provide help about specific alarms that the operators can access easily?

Yes

Can we disable alarms?

Yes

How many logical alarms groups can we create?

The system provides for as many alarm groups as desired or individual & group.

How can we prioritize alarms?

Alarms can be prioritized by severity, time, device, etc.

Can we send alarms to a printer and file as well as display them on the screen?

Yes and alarms can be sent out to groups or individuals via Email, pager, and/or text messages to cell phones.

What flexibility does the Control and Monitoring system provide for defining trend data?

Any database object may be configured to be placed in the Historian and trended. Any point in the system can also be viewed using a feature called Current Trending which allows operators to select a point to view the current trend of a point without it being configured in the Historian.

What facilities are provided for handling trend data on the screen?

All trending is accessed through the Historian using standard menus items. Data can be viewed in a list or graphically without custom configuration. If needed, custom trend pages can be configured in the system.

How do we display and extract historical data?

The Historical database or Historian is a SQL accessible database. This allows SQL queries to access the data and display it on screens. There are also standard tools to extract the data so no SQL knowledge is required to extract data. For instance, display the Maximum and Average can be internal to a mimic or external to a mimic (a trend page to itself).

How do we archive and retrieve historical data?

Historical data is stored in weekly files on all the servers. The files can be archived or restored at anytime without shutting down the system.

How long and what is keep in the Historian?

All data, reports, configuration, and events etc. are stored in the one Historical database. The data is actually stored in weekly/monthly/yearly files and the system is configurable as to how many days are keep online.

What flexibility does the Control and Monitoring system provide for defining reports?

The system has a inbuilt Reports Runtime engine embedded that provides for reports from any historian parameter. Reports function is required to generate a new report design then move the report template into the Historian for general reporting; display, print, email, etc any report.

How can I include plant-floor data in a report?

Pull up a report template and select the parameters to report, click generate report, and the report data appears.

How does the Control and Monitoring system communicate with the plant floor?

Communication with devices, plant floor, and other devices is done by the selected media: RS-485 to devices converted to Ethernet, modbus, OPC, direct Ethernet, cable, fiber, radio, or any usable media.

What is the fastest method of communicating with the plant floor?

Currently the fastest method of communication to devices and the plant floor is via some media using Ethernet.

How do we ensure that data exchange with the plant-floor is maximized?

The system provides a data quality parameter with most protocols and devices.

Will the performance be maintained as the application grows?

Using managed switches and proper cabling performance is keep with growth. For example, limiting the RS-485 devices to 5 or 6 units per 485 LAN connected to the Ethernet.

What is the built in language (scripting engine) provide standard programming facilities?

The system uses the IEC61131-3 (logic engine) programming. Most customers use the “function block programming” to write the functions desired that are not already in the system or they use Visual Basic Script (VBS), but VBS requires programming code knowledge.

Can we write our own functions?

Yes

Can we use the same function in more than one place in the system?

Yes

Can we create tasks that are triggered by system events or run at certain times?

Yes the system is event driven. Can we activate other Windows applications Yes, in fact, we launch other manufacturer’s software to do many device specific functions like waveform capture and harmonics.

Author :Dr. Jay Park

Source: http://instrumentationtools.com/scada-interview-questions-answers/

DSC/PLC Flow Diagram Instrumentation Tools

A control system flow diagram represents the signal flow starting from the field transmitters to the final operator graphic display.

Here we are discussing about traditional 4-20mA analog input devices only for easy of understading the basic concept.

Signal Flow

We have thousands of field transmitters installed in a proccessplant. So it is practicaally difficult to laystraight individualcables from each field transmitter to control room for displaying the process variables on the workstation. As per design standars,particular number of field devices / transmitters are wired and terminated ina Field junction box. Cable used for connecting the field device to junction boxare called as Branch cables or field cables. Generally we use one pair cable for branches cables.

The below figure shows 5 field devices are conected to a junction box using individual branch cables. In this example, for carrying out these five signal to control room we need minimum 5 pair cable (means one cable have 5 pairs) and also we have to consider spare cable requirement for future purpose. So at least 6 pair cable or better a 12 pair cable best suits the below requirement. Say we chosose 12 pair cable and is nothing but a Main Cable or Home run cable as it interfaces the field junction box and marshallling cabinet in the control room. So finally one big main cable is required and serves our purpose.

In a process plant, we have so many junction boxes installed and connected with number of field devices.Say we have 100 no'sof junctions boxes installed that means we have 100 no's main cables are these which are coming from field to control room. So, practically it is not possible to directly wire these main cables to analog input/output card. To avoid these problems we use marshalling cabiner for terminating these 100 no's main cables.

Marshalling cabinet main purpose is to provide main cables termination and then, re-distribute the field devices to respective analog input/output card using internal wiring. Internal wiring will be used to connect from marshalling cabinet to system cabinet.

System cabinet consistsin a processor card (CPU), analog input cards,analog output cards, communication cards, etc. Once main cables are teminated in marshalling cabinet, we have to take thesefield devices to the respective analog input card channel. So, we use internal wiring to route these main cables/fielddevices from marshalling to system cabinet.

Once the main cables are connected to the analog input card via internal wiring and it converts the 4-20mA which is coming from field devices into equivalent digital signali.e.in binary codes and the same will be communicated to processorcard. the processor card performsasper the predefined or programmed instructions. the processor card may have inbuilt or separate ethernet communication link which is used to display the measured process variables on the workstation.

Summary

The above animation depicts a typical cabling scheme that collects and distributes signals to and from the field devices. An field junction box is used to concentrate the signals intomulticonductor home run cables or main cables. the home run cables then terminate in a remote or marshalling cabinet where the signals are marshalled (reorganized) as necessary to efficiently terminate at the I/O interface of the DCS or PLC system.

Source: http://instrumentationtools.com/dcsplc-flow-diagram/

Four aspects of good control panel design

"Good control panel design should feature strong layout and component placement, labeling, panel sizing and component spacing, and wireway design".

Most of us have seen a clean, well-organized control panel out on the factory floor—you know it when you see it. Conversely, it's also easy to spot a poorly designed panel—not well laid out, crowded, messy, wires hanging all over the place, etc.

Whereas neatness is probably the first thing that jumps out about a well-designed panel, neatness is really the by-product several other well-executed aspects of panel design. Four other aspects of good panel design include: 

• Layout and component placement
• Labeling
• Panel sizing and component spacing
• Wireway design.

Layout and component placement 

In any control panel, components or "component groups" must be laid out in a logical-functional manner. Since most panels have a main incoming power disconnect switch, most commonly located in the upper right of the panel, it makes both logical and functional sense to locate the components with the highest voltage rating at the top of the panel. From there, the power distribution down to the power components at the lowest voltage level (most commonly 24Vdc) should generally follow a left-right and top-bottom hierarchy.

Each group of power distribution components should start with a main breaker for that power level at the left, followed by distribution breakers, fuses, and terminals. This keeps each power distribution group consistent and functionally sound, and facilitates easy troubleshooting in conjunction with a good design package that reflects this hierarchy in the schematics. Enough space should be left between these groups to allow for ease of expansion. This is done when the control panel is sized appropriately for the application wherein it will be used.

The programmable logic controller (PLC) racks and input/output (I/O) terminals are typically placed below the power distribution components. This is good practice from a couple of standpoints. For one, since heat rises, it makes sense to place sensitive electronic equipment (such as a PLC) below the hotter power components at the top. A well-designed panel will incorporate the means for expelling the excess heat at the top of the enclosure. Secondly, field instrument I/O wiring is usually brought in from the bottom of the panel. When I/O terminals are located at the bottom of the panel, this makes it very easy to land the field wiring.

Labeling

Not only should every component in the panel be labeled, but the syntax on the labels must make sense, and the labels should be placed such that each one is clearly visible.

For wiring, labels must be applied at each end of the wire. For power distribution wiring that connects to a power distribution terminal, the wire is labeled per the terminal number. For all other power distribution wiring and for "general" wiring, the wire is labeled per the corresponding line number in the schematics. For PLC I/O wiring, each wire should be labeled per syntax that corresponds with its PLC address.

For panel components such as power supplies and breakers, the component is typically labeled with a standard abbreviated prefix identifying the type of component, followed by the corresponding line number for that component in the schematics. When wiring and components are labeled in this manner, it makes it easy for troubleshooting, because it's easy to identify them in the schematics as well.

Panel sizing and component spacing

For a well-designed control panel, the panel is sized to allow for "generous" component placement. Adequate horizontal room will give space for addition/expansion for components such as power distribution breakers and terminals, PLC racks, I/O terminals, etc., and will also allow for proper heat dissipation for power components. Adequate vertical room will give much needed space to land wiring into terminals neatly - and thus avoid crowding. Adequate vertical space will also allow power components to dissipate heat properly. In addition, plenty of room should be left at the bottom of the control panel to allow for coiling up spare field wiring.

Wireway design

A good control panel design incorporates the right type and the right amount of wireway. The whole purpose here is to give plenty of room for both internal panel wiring and for field I/O wiring to be routed to the I/O terminals.

Wireway must be designed to allow ease of termination of internal wiring to internal panel components. As mentioned earlier, enough space should be given so that the wiring can be brought neatly to each panel component, and such that the wire labels are clearly legible. Also, the wireway must be sized properly to allow for future wiring additions when components are added to the panel.

Wireway that conveys field I/O wiring to its respective I/O terminals must be generously sized to meet the assumption that field wiring will be brought in and terminated into every existing I/O terminal. In addition, the sizing should account for additional potential I/O wiring in the future if components are added to the panel.

Adhering to these guidelines will not only produce a great looking and organized panel, but it will also produce a highly versatile panel that is capable of expansion and is also easy to troubleshoot when problems arise.

Source: http://innovativecontrols.com/blog/what-comprises-good-control-panel-design