Profibus Protocol in PLC and Automation Technology

Profibus is an industry-standard communications bus protocol used in process automation and sensor networks using programmable logic controllers. Understanding how the networks function will be beneficial to any plant engineer having to deal with PLC problems.

Profibus (from process field bus) is a protocol for field bus communication in automation technology. Profibus links automation systems and controllers with decentralized field devices such as sensors, actuators, and encoders. Profibus networks exchange data using a single bus cable.

Types of Profibus network

Two alternative versions of Profibus are generally used in automation: Profibus DP and Profibus PA. PA and DP contain the same protocols and both can be linked using a coupler device.
Profibus DP (Decentralized Peripherals) is normally used for data exchange with field devices like sensors and actuators through a programmable logic controller in production automation field applications.
Profibus PA (Process Automation) is used to interface measuring instruments through a process control system in process automation applications.
An early version of Profibus was Profibus FMS, for "Fieldbus Message Specification." Profibus FMS was intended to interface between Programmable Controllers and PLCs, sending complex data information between them.

Reference Standards

The Profibus communication is specified in IEC 61158 and IEC 61784. These standards set out the details of how each device can communicate and describe data exchange safety.

• IEC 61158 (Digital data communications for measurement and control – Communication Layers.)
• IEC 61784-1 (Communication Profiles)
• IEC 61784-2 (Realtime Ethernet RTE)
• IEC 61784-3 (Safety Communication)
• IEC 61784-4 (Security)
• IEC 61784-5 (Installation)

Importance of Profibus Protocol in Industries

Many products like PLC, drives, and instruments manufacturers offer Profibus. The advantage is that for each application, one solution, the same inventory, and the same knowledge portion can be used. Profibus can interface with 12 Mbps, which is the fastest field bus in the automation world. The network works on the master-slave scheme in which a master passes the token with its slaves and then to next station in a closed system.

Profiles

Profiles are pre-defined configurations of the functions available from Profibus for different devices and applications. The end user can interchange the same product from different vendors because of profile interoperability. Profiles are available for encoders, linear transducers, CNC machines, Drives, PLC, SCADA (Supervisory Control and Data Acquisition), various digital instruments, etc.

Profibus Troubleshooting

The configuration and programming software package is required to set up a Profibus network.
Diagnostic trouble rectification includes process related responses, built-in diagnostics mechanisms to indicate troubles on SCADA or PLC, and a Profibus interfacing break-through cable itself. Continuous monitoring of Profibus communication can identify some of the critical problems before they lead to major production loss. The diagnostics from different devices are event-triggered and generate a recordable alarm report.
Generally an engineer needs two tools to rectify the errors in network: a bus analyzer to determine the protocol quality and an oscilloscope to confirm the signal quality of the data communication, e.g. short circuit, wire breaks, termination error, etc.
A short circuit in the Profibus cable will disconnect the data communication from the master. The instrument on that node will not be destroyed in this case. The short circuit and the distance to the trouble point can be detected with an oscilloscope.
Diagnostic Function Blocks
Function blocks are capable of diagnosing each slave device. The diagnostic information can be stored in a data block. This data block further can be displayed on a SCADA PC.
Diagnostics Repeater
The repeater acts as a slave on the network and can interface diagnostics information to the master. This diagnostic data contains various fault types like Profibus cable break, conductor short circuit with the shield, terminator resistor break, nodes diagnostics, etc. The location and type of the fault on the cable can be identified in text and a graphical representation.
Power Cable Disturbance
Profibus interfacing can be disturbed by interference caused by a power cable laid near the Profibus cables. At least 10 cm air space between the Profibus and power cable should be maintained. Also proper shielding is important to prevent interference.

Important Parameter Check List During Commissioning of a Profibus Network

• Profibus wire polarity (A=green, B=red) should be correct.
• Ensure maximum 32 devices or less per segment.
• Avoid branch lines.
• Use only Profibus cable and connectors.
• Adequate baud rate is set with respect to cable length.
• Ensure that the address of each node is defined correctly.
• Protect the cable from short circuit and breaks.
• In case of higher transmission speed, the minimum distance between two devices shall be at least one meter.
• Multiple Profibus cables laid inside the same metal conduit gives good behavior when EMC is involved.
• One slave device can’t have two masters.

Other Related Information

Color of the Profibus DP cable

Generally the Profibus DP standard cable color will be violet, but it can have another color- for example for shipboard use it is black and for robust applications it is green.

Pin configuration on the DB9 connector

• Pin 3= red colored B line Data+ (input/output)
• Pin 8= green colored A line Data- (input/output)
• The metal casing of the connector= shield

Profibus DP to Serial Gateway
A serial gateway allows the user to communicate with any serial device with a Profibus network. It can support RS232, RS422, and RS485 serial formats without any modification in hardware.

Meaning of “GSD” File
GSD stands for “General Station Description.” The manufacturer of the devices is responsible for providing the GSD file, which describes the Profibus functionality of the device. During hardware configuration, the GSD file is required in order to have the device recognized by the controller. Integration of a new device in a configuration is done by importing a GSD file and synchronizing the address of the device.
DP Slaves in a Network
A total of 124 DP slaves can be configured for data exchange.
Each device on a Profibus network shall be assigned by address. For specifying the address, most devices have either rotary switches (decimal or hexadecimal) or DIP switches. Also a configuration tool is used for some devices to set the address.
If a slave device fails and needs replacement with the same type, the master recognizes the replaced device and with the same Profibus address.
Transmission Speed
1.5 Mbps is widely used default transmission speed. For long length cables speed should be lower to minimize the disturbances.
Bus Termination
Profibus cable ends should be terminated to stop reflections (signal resonance on the cable). Two ends of each segment should be isolated by active termination. Normally a Profibus connector comes with a built-in switch that can terminate the end.

NetTest Analysis Tool

To detect errors in Profibus DP segments, the NetTest analysis tool is widely used for line analysis. It provides information on short circuits, cable breaks, shield damage, termination mistakes, termination activated mistakes, baud rate, active node list, signal quality of slaves, etc.
A “NetTEST II” handheld diagnostic device is also available in the market.

Trasformer testing and fault rectification.

Power transformer failure results in production loss, unavailability of critical services, and loss of revenue. Routine testing and performing diagnostics can minimize loss and down time.

Reliable and continual performance of power transformers is the key to beneficial generation and transmission of electric power.
Generally, reasons for failure include external factors such as lightning strikes, system overload, short circuits, and internal factors such as insulation deterioration, winding failure, overheating, and the presence of oxygen, moisture, and solids in the transformer oil.
To minimize unexpected outages, periodic transformer testing and diagnostics is necessary.
Three categories can be defined for transformer testing:

• Performance acceptance test after installation and commissioning of the transformer.
• Predictive maintenance plan-based test during normal operation of the transformer to verify that electrical properties have not changed from design specifications.
• Failure test for identify breakdown cause of the transformer.

These tests are required to determine electrical, thermal, and mechanical characteristics.

Visual Inspection

A daily checklist procedure should be established to perform the visual routine test. It should contain oil temperature, winding temperature, oil level, humming (noisy operation), and oil leakage checks. An annunciation window (an indicator that announces which electrical circuit has been active) displays alarm and trip signals generated from the load.
Buchholz Relay
A Buchholz relay is a safety device normally mounted at the middle of the pipe connecting the transformer tank to the conservator. It is a gas detection relay used to detect minor and major faults in the transformer. A Buchholz relay operates by detecting the volume of gas generated in the transformer tank. Gas produced by faults accumulates over time within the relay chamber. Whenever the volume of gas exceeds a certain safe level, the float moves lower, closes the contact, and generates an alarm. The fault alarm can be displayed on an annunciation window and the master trip relay will cause the circuit breaker to open.

Thermal Imaging (Thermography)

Thermal imagers capture images of infrared energy or temperature. They can detect heat patterns or temperature changes in equipment. The engineer can determine problems prior to an expensive down time by analyzing these temperature changes. Conveniently, one can measure and compare heat readings for each part of the equipment without disrupting the transformer's operation.
Prevention, diagnosis, and repair benefits can be obtained for transformers by introducing Infrared thermography into your predictive maintenance plan.

Insulation Resistance Test

Insulation ages and deteriorates because of moisture, dust, and electrostatic stress. Insulation should be monitored continually to avoid sudden failure of the equipment.
An insulation resistance test detects insulation quality within the transformer. The conductive impurities or mechanical flaws in the dielectric can be analysis based on this test. The instrument used to measure insulation resistance is known as the "megger." Normally meggers have a test voltage of 500V, 2500V, or 5000V.

Each winding should be short circuited at the bushing terminals. The resistance value should be measured between each winding and with respect to ground also. The winding should be discharged after the test is completed by connecting to the ground.
The insulation resistance value measured is usually in the order of mega-ohms. Generally the value should be greater than 1 megohm for every 1kV rating of the equipment.
Insulation resistance values decrease with increase in the temperature. Therefore the values should be normalized for a standard temperature. It is necessary to have the insulation resistance as high as possible.

Transformer Turns Ratio Test

Each winding of a transformer contains a certain number of turns of wire. The "transformer turns ratio" is the ratio of the number of turns in the high voltage winding to that in the low voltage winding. The ratio is calculated under no-load conditions.
The transformer ratio can change due to several factors like physical damage because of faults, deteriorated insulation, contamination of oil etc. If a transformer ratio changes more than 0.5 percent from the rated voltage ratio, it needs immediate attention.
The turns ratio will establish the proper relationship between the primary and secondary winding impedances. The turns ratio is the square root of the impedance ratio, i.e.

iZ_pri/Z_sec = (N_pri/N_sec)²

Z_pri = Primary Impedance

Z_sec = Secondary Impedance

N_pri = Number of turns on the primary coil

N_sec = Number of turns on the secondary coil

Dissolved Gas Analysis (DGA)

Transformer overloading, overheating, corona, sparking, and arcing can cause thermal degradation of the oil and paper insulation within the tank. Thermal and electrical faults can accelerate the decomposition of dielectric fluid and solid insulation. Gases generated by this process include hydrogen, methane, ethane, acetylene, carbon monoxide, and carbon dioxide, all which will dissolve in the transformer oil.
The DGA test involves extracting the gases from the oil and injecting it into a gas chromatograph. Gas concentrations are detected using a flame ionization detector and a thermal conductivity detector.
Diagnostic and analysis of the specific proportions of each gas shall help to identify the fault type (thermal conditions involving the oil or the paper, partial discharge, sustained arcing, etc.).
A DGA test study can minimize damage by taking precautionary actions at an early stage.

Magnetic Balance Test

The magnetic balance test is conducted on transformers to detect inter-turn faults and magnetic imbalance. It gives an idea about the flux distribution in the core. It is only an indicative test and does not reduce the need for other tests in diagnostics.
The magnetic balance test is carried out on a three phase transformer by applying a two phase supply across the phases (i.e. one winding say U and V) with a lower than rated voltage. Other phases should be kept open circuit. The sum of voltage measured between V-W and U-W should be equal to U-V. A voltage measured in the secondary side will also be proportional to the voltage measured at the primary.
A very low voltage will induce in defected winding because it will not allow flux to pass in the magnetic path around the core. It may result in the sum of the two voltages not being equal to the applied voltage.

Tan Delta Test

Degradation of the insulation takes place because of mechanical vibration, over temperature operation, and gaseous and metallic impurities in the transformer. This may cause insulation ageing and breakdown. It is very important to study the insulation quality of the machine. The dissipation factor Tan or Power Factor Cos Ø is considered to indicate the quality of insulation. It is also known as the loss angle test or the dissipation factor test.
A clean insulation acts as a capacitor. The current should lead the voltage by 90 degrees in a pure capacitor. The pure insulation should also conduct similarly. If the insulation is deteriorated, the current will also have resistive factor. This will cause the angle of the current to be less than 90 degrees. This measured difference in the angle is described as the loss angle. The tangent of the angle (i.e. opposite/adjacent) indicates the condition of the insulation. A greater loss angle value points to a high contamination of the insulation.

Transformer Oil Break Down Test

The BDV test measures the dielectric strength of the oil using an oil tester. During this test, spherical electrodes having a 2.5 mm gap shall be gradually applied voltage until the oil loses its dielectric strength. This test should be performed for one minute, and the breakdown voltage displayed on the oil tester meter should be considered the BDV. Normally it may be 60 kV and over for one minute or as per ASTM D877-82 or IS-335.
A low value in this test indicates the presence of impurities in the oil. In this case it should be filtered to remove impurities and moisture.


Followings are other tests that can be used to detect oil based faults:

• Acidity test
• Electric strength test
• Color test
• Polychlorinated Biphenyl Analysis (PCB) test
• Fiber estimation
• Furfuraldehyde analysis test
• Metal in oil test
• Resistivity test
• Furan analysis
• Frequency Response Analysis

Diameter Calculation for Winder application.

Diameter Computation is generally used for the winder application in Paper, Textile and Strip industries as a winder, beamer or coiler.

Recoiler rotation speed shall reduce to maintain the line surface speed.

When Line will run then as per diameter increment in recoiler, its speed should
reduce based on the reference of Bridle and Recoiler.

Since motors regulate RPM and web handling is concerned with producing web at a controlled speed measured in FPM or MPM, we must know the winding diameter accurately. There are many methods to derive the diameter. The trivial case is for a roller of fixed diameter. In this case the manufacturer’s or the roll grinder’s micrometer measured diameter is used.

In winders and unwinds, the diameter changes whenever the line is running. The drive system needs the diameter for RPM and torque calculations. To save costs, the drive system often calculates diameter using existing instruments necessary for speed control and already paid for.

Diameter calculation involves a calculation based on the ratio of two tachometer RPM feedbacks. This is the most common method of diameter calculation provided by drive system integrators.

One tachometer determines the line speed in MPM. The second determines the RPM of the winding roll. As long as the web is tight and the web path length is not changing, the speed of the web matches the speed at the winder. Note this method does not depend on the gauge of the web, but only on speeds as measured by tachometers on the motors.

RPM_web * Diam_web = RPM_wdr * Diam_wdr

There are a few compromises when using this diameter calculation. The first is that the calculation will not work when the line is stopped (division by zero RPM). This means the diameter is not self starting and the core diameter must be entered prior to starting the winder.

Accuracy is decreased below 10% speed since the calculation divides by a low RPM. Accuracy is also decreased during acceleration and deceleration because there is often a filter on both the web speed and winder RPM signals. The filtering may have a different time constant for each signal.

The best results using an ultrasonic or laser sensor to constantly measure spool diameter as it grows and adjust spool RPM accordingly.

Harmonics Analysis and Remedy

Power distribution systems are designed to function at 50 or 60 Hz frequencies. But some types of connected loads generate current and voltage with different frequency which may be in multiples of the 50 or 60 Hz basic frequency. These abnormal frequencies cause electrical distortion which is called power system harmonics.

Power quality problems can produce equipment malfunctioning, voltage fluctuations, power outage and system distortions. It’s prime responsibility of maintenance engineer to understand the causes of these problems and to find out the solution to prevent them.

The sources of harmonics generation are power electronic loads such as variable frequency drives (VFDs), computers, printers and switching power supplies. The abundant use of non-linear (such as a rectifier) loads also results harmonics. 

Introduction:

Usually the voltage varies sinusoidal at a fundamental frequency of 50 or 60 Hz. While we connect linear load it draws a sinusoidal current but in case of non-liner load it is not perfectly sinusoidal.

Generally because of the third harmonics, the neutral conductor current increases so an electrical engineer should take care in the design of an electric system considering non-linear load. Besides the increased line current, other part of electrical machine may bear ill effects because of harmonics in the power system. Normally 1^st to 25^th harmonics are considered, in which the majority problems are due to 3^rd, 5^th and 7th harmonics.

Generation of Harmonics in power system:

At the point of power generation, the power distortion may be very less i.e 1% to 2% only.

Linear loads: When a sinusoidal voltage is connected with a load, the current drawn by the load is proportional with the voltage. e.g. resistive heaters, incandescent bulbs, constant speed motors etc.

Nonlinear loads: The non-sinusoidal current and voltage waveforms

In contrast, some loads cause the current to vary disproportionately with the voltage during each half cycle. These loads are classified as nonlinear loads, and the current and voltage have waveforms that are non-sinusoidal, containing distortions, whereby the 60-Hz waveform has numerous additional waveforms superimposed upon it, creating multiple frequencies within the normal 60-Hz sine wave. The multiple frequencies are harmonics of the fundamental frequency.

Motors:

Electric motors experience hysteresis loss caused by eddy currents set up in the iron core of the motor. These are proportional to the frequency of the current. Since the harmonics are at higher frequencies, they produce more core loss in a motor than the power frequency would. This results in increased heating of the motor core, which (if excessive) can shorten the life of the motor. The 5th harmonic causes a CEMF (counter electromotive force) in large motors which acts in the opposite direction of rotation. The CEMF is not large enough to counteract the rotation, however it does play a small role in the resulting rotating speed of the motor.

VFD (Variable Frequency Drives):

There is an increasing use of variable frequency drives (VFDs) that power electric motors. The voltages and currents emanating from a VFD that goes to a motor are rich in harmonic frequency components. Voltage supplied to a motor sets up magnetic fields in the core, which create iron losses in the magnetic frame of the motor. Hysteresis and eddy current losses are part of iron losses that are produced in the core due to the alternating magnetic field. Hysteresis losses are proportional to frequency, and eddy current losses vary as the square of the frequency. Therefore, higher frequency voltage components produce additional losses in the core of AC motors, which in turn, increase the operating temperature of the core and the windings surrounding in the core. Application of non-sinusoidal voltages to motors results in harmonic current circulation in the windings of motors. The net rms current is [I.sub.rms] = [square root of [([I.sub.1]).sup.2] + [([I.sub.2]).sup.2] + [([I.sub.3]).sup.2] +] ..., where the subscripts 1, 2, 3, etc. represent the different harmonic currents. The [I.sub.2]R losses in the motor windings vary as the square of the rms current. Due to skin effect, actual losses would be slightly higher than calculated values. Stray motor losses, which include winding eddy current losses, high frequency rotor and stator surface losses, and tooth pulsation losses, also increase due to harmonic voltages and currents.

The phenomenon of torsional oscillation of the motor shaft due to harmonics is not clearly understood, and this condition is often disregarded by plant personnel. Torque in AC motors is produced by the interaction between the air gap magnetic field and the rotor-induced currents. When a motor is supplied non-sinusoidal voltages and currents, the air gap magnetic fields and the rotor currents contain harmonic frequency components.

The harmonics are grouped into positive (+), negative (-) and zero (0) sequence components. Positive sequence harmonics (harmonic numbers 1,4,7,10,13, etc.) produce magnetic fields and currents rotating in the same direction as the fundamental frequency harmonic. Negative sequence harmonics (harmonic numbers 2,5,8,11,14, etc.) develop magnetic fields and currents that rotate in a direction opposite to the positive frequency set. Zero sequence harmonics (harmonic numbers 3,9,15,21, etc.) do not develop usable torque, but produce additional losses in the machine. The interaction between the positive and negative sequence magnetic fields and currents produces torsional oscillations of the motor shaft. These oscillations result in shaft vibrations. If the frequency of oscillations coincides with the natural mechanical frequency of the shaft, the vibrations are amplified and severe damage to the motor shaft may occur. It is important that for large VFD motor installations, harmonic analyses be performed to determine the levels of harmonic distortions and assess their impact on the motor.

Transformers:

The harmful effects of harmonic voltages and currents on transformer performance often go unnoticed until an actual failure occurs. In some instances, transformers that have operated satisfactorily for long periods have failed in a relatively short time when plant loads were changed or a facility's electrical system was reconfigured. Changes could include installation of variable frequency drives, electronic ballasts, power factor improvement capacitors, arc furnaces, and the addition or removal of large motors.

Application of non-sinusoidal excitation voltages to transformers increase the iron losses in the magnetic core of the transformer in much the same way as in a motor. A more serious effect of harmonic loads served by transformers is due to an increase in winding eddy current losses. Eddy currents are circulating currents in the conductors induced by the sweeping action of the leakage magnetic field on the conductors. Eddy current concentrations are higher at the ends of the transformer windings due to the crowding effect of the leakage magnetic fields at the coil extremities. The eddy current losses increase as the square of the current in the conductor and the square of its frequency. The increase in transformer eddy current loss due to harmonics has a significant effect on the operating temperature of the transformer. Transformers that are required to supply power to nonlinear loads must be de-rated based on the percentages of harmonic components in the load current and the rated winding eddy current loss.

One method of determining the capability of transformers to handle harmonic loads is by k factor ratings. The k factor is equal to the sum of the square of the harmonic currents multiplied by the square of the frequencies.

k = [([I.sub.1]).sup.2]([1.sup.2]) + [([I.sub.2]).sup.2]([2.sup.2]) + [([I.sub.3]).sup.2]([3.sup.2]) + . . . + [([I.sub.n]).sup.2]([n.sup.2]).

where [I.sub.1] = ratio of fundamental current to total rms current, [I.sub.2] = ratio of second harmonic current to total rms current, [I.sub.3] = ratio of third harmonic current to total rms current, etc., and 1,2,3, ... n are harmonic frequency numbers. The total rms current is the square root of the sum of square of the individual currents.

By providing additional capacity (larger-size or multiple winding conductors), k factor rated transformers are capable of safely withstanding additional winding eddy current losses equal to k times the rated eddy current loss. Also, due to the additive nature of triplen harmonic (3, 9, 15, etc.) currents flowing in the neutral conductor, k rated transformers are provided with a neutral terminal that is sized at least twice as large as the phase terminals.

Capacitor banks:

Many industrial and commercial electrical systems have capacitors installed to offset the effect of low power factor. Most capacitors are designed to operate at a maximum of 110% of rated voltage and at 135% of their kvar ratings. In a power system characterized by large voltage or current harmonics, these limitations are frequently exceeded, resulting in capacitor bank failures. Since capacitive reactance is inversely proportional to frequency, unfiltered harmonic currents in the power system find their way into capacitor banks, these banks act like a sink, attracting harmonic currents, thereby becoming overloaded. A more serious condition, with potential for substantial damage, occurs as a result of harmonic resonance. Resonant conditions are created when the inductive and capacitive reactance become equal in an electrical system. Resonance in a power system may be classified as series or parallel resonance, depending on the configuration of the resonance circuit. Series resonance produces voltage amplification and parallel resonance causes current multiplication within an electrical system. In a harmonic rich environment, both types of resonance are present. During resonant conditions, if the amplitude of the offending frequency is large, considerable damage to capacitor banks would result. And, there is a high probability that other electrical equipment on the system would also be damaged. If the protective device does not operate to protect the capacitor bank, serious damage will occur.

Calculation of Electrical Maximum Demand

Abstract:

The Electricity provider does charge the fixed charges on the basis of consumer’s maximum Electrical Demand.  Consumer shall restrict the power consumption under the contracted maximum demand. This article furnishes calculation for Maximum Contract Demand.

1. Introduction:

The Electricity provider does record maximum demand in pre-defined interval (e.g. 30 minutes or 15 minutes) through duly sealed and calibrated energy meter. Generally Maximum Demand denotes in kVA for billing purpose.  

Consumer need to sanction Maximum demand from Electricity Provider considering type of industry and operation pattern of the equipments.  Consumer shall pay fixed charges on the basis of Maximum Demand obtained from the provider i.e. the maximum rate at which an electrical power has been consumed during any period of defined consecutive minutes in the billing month.

2. Analysis:

General Formula to calculate the Maximum Demand is described below:

Maximum Demand= Connected Load * Load Factor / Power Factor.

Where,

Connected Load = Total Connected load in the facility in kW.

Load Factor = Utility Factor * Diversity Factor.

Power Factor = System average Power Factor.

Example:

Total connected load of facility: 6500 kW

Load Factor: 0.4 (Considering steel plant type)

Power Factor: 0.95

Maximum Demand= 6500 * 0.4 / 0.95

                          = 2737 kVA

Utility Factor and Diversity Factor can be finding out by the Time Profile of load and usage of the equipment. All equipments of facility may not operate at similar time and also may not run with full load.

Hence, Diversity Factor in percentage = Installed load / running load.

3. Conclusion:

Consumer should sanction Maximum Demand after studying the load pattern of the electrical installation. Obtaining higher Maximum Demand shall result higher minimum fixed charges plus higher deposit, and if sanctioned Maximum Demand exceed than consumer shall confront penalty.