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