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This manual uses a systems approach to troubleshooting and is designed to encourage readers to take a new look at the methodology of fault finding and rectification on their plant. Having covered the types of equipment, the manual then looks at first line troubleshooting, then the advanced level and finally works through some typical examples.

Introduction & Basics to Practical Troubleshooting of Instrumentation, Electrical and Process Control

1       Introduction & Basics

This course is not intended to be an encyclopaedia of electricity and instrumentation but rather a training guide for gaining experience in this fast changing environment.  It is aimed at providing engineers, technicians and any other personnel involved with process measurement, more experience in that field.  It is also designed to give students the fundamentals on analyzing the process requirements and selecting suitable solutions for their applications.

1.1          Basic Measurement and Control Concepts

The basic set of units used on this course is the SI unit system. This can be summarized in the following table 1.1.

Table 1.1

SI Units

1.2          Measurements

Measurement of a given quantity is essentially an act or the result of comparison between the quantity (whose magnitude is not known) and a predefined standard. Since two quantities are compared, the result is expressed in numerical values. So measurement can be defined as the process by which physical parameters can be converted to meaningful numbers. Measuring process is the process where comparison is done between the property of an object or system under consideration and to an accepted standard unit, a standard defined to that particular property. In order to have the result of a measurement to be meaningful, there are two basic requirements:

• Standard used for comparison purposes must be accurately defined and commonly accepted.
• Apparatus used and method adopted must be provable.

1.2.1          Significance of measurement

Advancement in Science and Technology is dependent on the progress in measurement techniques. As Science and Technology move ahead, new phenomena and relationships are discovered, which makes new types of measurements a necessity. In addition to confirming the validity of a hypothesis, measurements also add to its understanding. The results in modern Science and Technology being associated with sophisticated methods of measurement.

1.2.2          Methods of measurements

The methods of measurement can be broadly classified as:

• Direct Methods: Here the unknown quantity (also called the measurand) is directly compared against a standard and the result is expressed as a numerical number and a unit. Most common examples for direct methods are measurement of physical quantities like length, mass and time.
• Indirect Methods: Measurement by direct methods is always not possible, feasible and practicable. More over in most cases direct methods are inaccurate as they involve human factors and are also less sensitive. In engineering applications measurement systems are used that require indirect methods for measuring purposes.

1.2.3          Instruments

Measurements involve the use of instruments as a physical means of determining quantities as variables. A simple instrument consists of a single unit that gives an output reading or signal according to the unknown variable applied to it. In a complex measurement system the instrument may consist of several separate elements. These elements consist of transducing elements that converts the measurand to an analogous form. This analogous signal would be then processed by some intermediate means and then fed to end devices that present the result of measurement for display or control. The modular nature of elements in measuring instruments makes it to be referred to as measurement system.

It would be advantageous if we classify the instruments indicating their importance. There are many ways for classifying instruments. Let’s see an example of instrument classification. Here the instruments are basically classified as follows:

Critical: Instruments that are critical and would compromise on product or process quality if it does not conform to the specifications are put in this category.

Non-critical: These are instruments that are not so critical; whose functionality would be more of operational significance. Example of such an instrument would be the ones used for recording in operating logs but are not critical.

Reference Only: These are instruments that are neither critical nor significant to equipment operation and are not used for making quality decisions.

OSHA: Occupational Safety and Health Administration mandates the calibration of an instrument.

These classifications are useful in assigning the calibration frequencies. We might assign a calibration frequency of six months to a critical temperature transmitter and assign a calibration frequency of twelve months for a non-critical temperature transmitter.

1.3          Basic Measurement Performance Terms and Specifications

There are a number of criteria that must be satisfied when specifying process measurement equipment.  Below is a list of the more important specifications.

1.3.1          Accuracy

The accuracy specified by a device is the amount of error that may occur when measurements are taken. It determines how precise or correct the measurements are to the actual value and is used to determine the suitability of the measuring equipment.

Accuracy can be expressed as any of the following:

• error in units of the measured value
• percent of span
• percent of upper range value
• percent of scale length
• percent of actual output value.

Figure 1.1

Accuracy terminology.

Accuracy generally contains the total error in the measurement and accounts for linearity, hysteresis and repeatability.

Reference accuracy is determined at reference conditions, i.e. constant ambient temperature, static pressure, and supply voltage. There is also no allowance for drift over time.

The range of operation defines the high and low operating limits between which the device will operate correctly, and at which the other specifications are guaranteed.  Operation outside of this range can result in excessive errors, equipment malfunction and even permanent damage or failure.

1.3.2         Budget/Cost

Although not so much a specification, the cost of the equipment is certainly a selection consideration. This is generally dictated by the budget allocated for the application. Even if all the other specifications are met, this can prove an inhibiting factor.

1.4          Advanced Measurement Performance Terms and Specifications

More critical control applications may be affected by different response characteristics. In these circumstances the following may need to be considered:

1.4.1         Hysteresis

This is where the accuracy of the device is dependent on the previous value and the direction of variation.  Hysteresis causes a device to show an inaccuracy from the correct value, as it is affected by the previous measurement.

Figure 1.2

Hysteresis.

1.4.2         Linearity

Linearity is how close a curve is to a straight line. The response of an instrument to changes in the measured medium can be graphed to give a response curve. Problems can arise if the response is not linear, especially for continuous control applications. Problems can also occur in point control as the resolution varies depending on the value being measured.

Linearity expresses the deviation of the actual reading from a straight line. For continuous control applications, the problems arise due to the changes in the rate the output differs from the instrument. The gain of a non-linear device changes as the change in output over input varies. In a closed loop system changes in gain affect the loop dynamics. In such an application, the linearity needs to be assessed. If a problem does exist, then the signal needs to be linearised.

Figure 1.3

Linearity.

1.4.3         Repeatability

Repeatability defines how close a second measurement is to the first under the same operating conditions, and for the same input. Repeatability is generally within the accuracy range of a device and is different from hysteresis in that the operating direction and conditions must be the same.

Continuous control applications can be affected by variations due to repeatability. When a control system sees a change in the parameter it is controlling, it will adjust its output accordingly. However if the change is due to the repeatability of the measuring device, then the controller will over-control. This problem can be overcome by using the deadband in the controller; however repeatability becomes a problem when an accuracy of say, 0.1% is required, and a repeatability of 0.5% is present.

Figure 1.4

Repeatability.

Ripples or small oscillations can occur due to overcontrolling. This needs to be accounted for in the initial specification of allowable values.

1.4.4         Response

When the output of a device is expressed as a function of time (due to an applied input) the time taken to respond can provide critical information about the suitability of the device. A slow responding device may not be suitable for an application. This typically applies to continuous control applications where the response of the device becomes a dynamic response characteristic of the overall control loop. However in critical alarming applications where devices are used for point measurement, the response may be just as important.

Figure 1.5

Typical time response for a system with a step input.

1.5          Definitions of Instrumentation Terminology

Below is a list of terms and their definitions that are used throughout this manual.

Accuracy

How precise or correct the measured value is to the actual value. Accuracy is an indication of the error in the measurement.

Ambient

The surrounds or environment in reference to a particular point or object.

Attenuation

A decrease in signal magnitude over a period of time.

Calibrate

To configure a device so that the required output represents (to a defined degree of accuracy) the respective input.

Closed loop

Relates to a control loop where the process variable is used to calculate the controller output.

Coefficient, temperature

A coefficient is typically a multiplying factor. The temperature coefficient defines how much change in temperature there is for a given change in resistance (for a temperature dependent resistor).

Cold junction

The thermocouple junction which is at a known reference temperature.

Compensation

A supplementary device used to correct errors due to variations in operating conditions.

Controller

A device which operates automatically to regulate the control of a process with a control variable.

Elastic

The ability of an object to regain its original shape when an applied force is removed. When a force is applied that exceeds the elastic limit, then permanent deformation will occur.

Excitation

The energy supply required to power a device for its intended operation.

Gain

This is the ratio of the change of the output to the change in the applied input. Gain is a special case of sensitivity, where the units for the input and output are identical

and the gain is unitless.

Hunting

Generally an undesirable oscillation at or near the required setpoint. Hunting typically occurs when the demands on the system performance are high and possibly exceed the system capabilities. The output of the controller can be overcontrolled due to the resolution of accuracy limitations.

Hysteresis

The accuracy of the device is dependent on the previous value and the direction of variation.  Hysteresis causes a device to show an inaccuracy from the correct value, as it is affected by the previous measurement.

Ramp

Defines the delayed and accumulated response of the output for a sudden change in the input.

Range

The region between the specified upper and lower limits where a value or device is defined and operated.

Reliability

The probability that a device will perform within its specifications for the number of operations or time period specified.

Repeatability

The closeness of repeated samples under exact operating conditions.

Reproducibility

The similarity of one measurement to another over time, where the operating conditions have varied within the time span, but the input is restored.

Resolution

The smallest interval that can be identified as a measurement varies.

Resonance

The frequency of oscillation that is maintained due to the natural dynamics of the system.

Response

Defines the behavior over time of the output as a function of the input. The output is the response or effect, with the input usually noted as the cause.

Self Heating

The internal heating caused within a device due to the electrical excitation. Selfheating is primarily due to the current draw and not the voltage applied, and is typically shown by the voltage drop as a result of power (I2R) losses.

Sensitivity

This defines how much the output changes, for a specified change in the input to the device.

Setpoint

Used in closed loop control, the setpoint is the ideal process variable. It is represented in the units of the process variable and is used by the controller to determine the output to the process.

The difference between the maximum and minimum range values. When provided in an instrument, this changes the slope of the input-output curve.

Used in closed loop control where the process no longer oscillates or changes and settles at some defined value.

Stiction

Shortened form of static friction, and defined as resistance to motion. More important is the force required (electrical or mechanical) to overcome such a resistance.

Stiffness

This is a measure of the force required to cause a deflection of an elastic object.

Thermal shock

An abrupt temperature change applied to an object or device.

Time constant

Typically a unit of measure which defines the response of a device or system. The time constant of a first order system is defined as the time taken for the output to reach 63.2% of the total change, when subjected to a step input change.

Transducer

An element or device that converts information from one form (usually physical, such as temperature or pressure) and converts it to another (usually electrical, such as volts or millivolts or resistance change). A transducer can be considered to comprise a sensor at the front end (at the process) and a transmitter.

Transient

A sudden change in a variable which is neither a controlled response nor long lasting.

Transmitter

A device that converts from one form of energy to another. Usually from electrical to electrical for the purpose of signal integrity for transmission over longer distances and for suitability with control equipment.

Variable

Generally, this is some quantity of the system or process. The two main types of variables that exist in the system are the measured variable and the controlled variable. The measured variable is the measured quantity and is also referred to as the process variable as it measures process information. The controlled variable is the controller output which controls the process.

Vibration

This is the periodic motion (mechanical) or oscillation of an object.

The zero in an instrument is the output provided when no, or zero input is applied. The zero adjustment produces a parallel shift in the input-output curve.

1.6          Introduction to Industrial Troubleshooting

Industrial troubleshooting any device or set of devices requires a systematic and systemic approach. This requires understanding at two levels:

• The general concepts of troubleshooting
• Operation and behavior of the industrial instrumentation systems.

The course assumes a basic working knowledge of industrial electrical, instrumentation and communications applications.

1.6.1          The general concepts of troubleshooting

Operation and behavior characteristics of the industrial instrumentation systems

It is assumed that course delegates have a basic working knowledge of various industrial applications, especially within the electrical, instrumentation and communication disciplines.

In a nutshell, one could go about trouble-shooting / problem-solving using the following guidelines:

• Collect all the date (process values, operators stops, etc.)
• Analyze the data, in order to define the problem
• Identify all the possible causes (and here it is important not to criticize)
• Eliminate all the unlikely causes
• Determine probable causes
• Decide on a plan of action
• Implement your repairs / corrections
• Evaluate whether the problem has been resolved.
• Start over, if necessary.

1.7          Industrial Instrumentation Basics

Industrial instrumentation covers a wide variety of systems that are used in automated industrial processes. These systems may include:

• Logic controllers
• Sensors
• Actuators
• Communication Devices
• I/O devices
• Electrical and power systems that support them

The purpose of these systems is to monitor some certain aspects of a process (e.g. by using sensors) and are very often used to implement one or other form of control.  In the case of a simple control loop, information from sensors in the field is conveyed to a logic controller, which in turn implements an activation/deactivation signal to an output device such as an actuator. In many instances, the required control output is quite simplistic.  However, in other instances, instrumentation systems may require specialized ways and means to accept command inputs and outputs.  This could, for example, be through a man machine interface, and could include details such as statuses, alarms, set points, etc.

Complexity of industrial instrumentation systems can vary considerably and would normally depend on factors such as the type and criticality of the process being controlled, the degree of accuracy required, the safety aspects, etc. Regardless of the type of the complexity, all industrial controllers have one common underlying theme. It is crucial that this common theme be fully understood and the different pattern of its variation. Only once this has been achieved, will the person, working on the system, be able to conceptualize the systems and approach their troubleshooting in a systemic and systematic manner. This will be explained, in the following sections, in greater detail.

1.8          Basic Concepts of Industrial Controllers

The following terms assist in understanding the basic concepts of industrial controllers.

1.8.1          Process

In the real world, process can be described as an activity (or sequence of activities) that the industrial controller monitors in order to ensure the proper operation of that operation. The diversity of processes can be huge, both in terms of complexity as well as scale. Consider a solution of liquid, in a storage vessel, being heated to a preset temperature.  This is an example of a very simple process. On the other hand, the assembly of different components onto the chassis of a motor vehicle on a factory assembly line could be considered a complex process, as it often consists of very many varying sub-processes.

A process essentially comprises of:

• The environment
• The initial conditions of the environment
• The object / material on which the process is working
• The specified conditions before the action on the object / material can begin
• The exit conditions which qualify that the process has successfully been completed
• The final conditions that should exist once the process has come to its conclusion.

In the case of a simple process, the environment is easy to achieve and control. At the other extreme, one may come across processes that consist of a very controlled environment.  In this instance, many conditions may need to be satisfied, before the object in the process can be exposed to the environment under which control takes place.

Industrial controllers can play a role in each of the above constituents of the process. In practice, they may be used to create the environment, or they may be used to control the environment in such a way that it conforms to the specifications required for the process.  These devices may also control the actual object’s entry and exit from the environment, and then restore the environment back to its normal (original) state in order to prepare it for the next cycle.

1.8.2          Process Variables

Process variables are the criteria or indicators used by the controller, to check whether the environment or the object is at the required / correct phase with respect to the overall process cycle. Examples of some of the variables under consideration may be temperature, pressure, flow, position, thickness, etc. The controller monitors this variable by viewing signals, which are usually electrical in nature. It is important to understand that the controller maps an electrical value to a corresponding environment variable (by means of internal calculations). The controller has no way of observing the actual environmental condition of the process, but gets indirect electrical signals. Thus, for example, a controller may convert a 4 to 20 mA signal, and interpret this as a pressure ranging from 0 to 350 kPa.  This concept will often be used in the troubleshooting techniques.

1.8.3          Measurement of Process Variables

The environment and other aspects of the process are measured using devices called sensors. There are different types of sensors, depending upon the environment variable being monitored and the type of signal being produced. For example the temperature of a chamber can be measured i) by the mechanical movement of a metallic piece that expands or contracts with change in temperature, ii) by the change in voltage across a thermocouple or iii) by change in resistance of an RTD.  Each of the methods, described, refers to a different type of sensor. Different sensor types produce different types and levels of signals.  In addition to this, the sensors also require appropriate circuits to condition their information before the controller can make use of this information.

1.9          Control Systems Basics

Once a controller has received information about the process (and its environment, for that matter), it takes decisions based on the logic stored within its program.  These types of decisions may include some of the following:

• Stop the process
• Start a new process
• Reset/restart the process
• Perform calculations and co-relations
• Start an additional activity in the process
• Raise an alarm
• Do nothing and wait for the next event or wait for a while.

This list is not exhaustive, and would vary from process to process.

1.10       Electrical Basics

A significant proportion of industrial electricity is about single-phase and three-phase transformers, AC and DC machines. In this context, we will study the electrical circuits and their construction, design, testing, operation, and maintenance.

For troubleshooting electrical equipment and control circuits, it is important to understand the basic principles on which the electrical equipment works. The following sections will outline the basic electrical concepts.

1.10.1        Basic electrical concepts

In each plant, the mechanical movement of different equipment is caused by an electric prime mover (motor). Electrical power is derived from either utilities or internal generators and is distributed through transformers to deliver usable voltage levels.

Electricity is found in two common forms:

• AC (alternating current)
• DC (direct current)

Electrical equipment can run on either of the AC/DC forms of electrical energies. The selection of energy source for equipment depends on its application requirements. Each energy source has its own merits and demerits.

Industrial AC voltage levels are roughly defined as L.V., (Low Voltage) and H.V., (High Voltage) with frequency of 50 Hz. to 60 Hz.

An electrical circuit has the following three basic components irrespective of its electrical energy form:

• Voltage (volts)
• Ampere (amps)
• Resistance (ohms)

Voltage is defined as the electrical potential difference that causes electrons to flow. Current is defined as the flow of electrons and is measured in amperes.

Resistance is defined as the opposition to the flow of electrons and is measured in Ohms.

All three are bound together with Ohm’s law; which gives the following relation between the three:

Where, V=Voltage

I = Current

R = Resistance

1.10.1.1     Power:

In DC circuits, power (watts) is simply a product of voltage and current.

For AC circuits, the formula holds true for purely resistive circuits; however, for the following types of AC circuits, power is not just a product of voltage and current.

Apparent power is the product of voltage and Ampere, i.e., VA or kVA is known as apparent power. Apparent power is total power supplied to a circuit inclusive of the true and reactive power.

Real power or true power is the power that can be converted into work and is measured in watts.

Reactive power: If the circuit is of an inductive or capacitive type, then the reactive component consumes power and cannot be converted into work. This is known as reactive power and is denoted by the unit VAR.

1. Relationship between powers:

Apparent Power (VA) = V x A

1.10.1.2     Power factor

Power factor is defined as the ratio of real power to apparent power. The maximum value it can carry is either 1 or 100(%), which would be obtained in a purely resistive circuit.

Power factor =                       True Power

Apparent Power

Watts

kVA

1.10.1.3     Percentage Voltage Regulation

1.10.1.4     Electrical energy

This is calculated as the amount of electrical energy used in an hour and is expressed as follows:

Kilowatthour  = kW x h

Where,      kW = kilowatt

h = hour

1.10.1.5     Types of circuits:

There are only two types of electrical circuits – series and parallel.

A series circuit is defined as a circuit in which the elements in a series carry the same current, while voltage drop across each may be different.

A parallel circuit is defined as a circuit in which the elements in parallel have the same voltage, but the currents may be different.

1.10.2        Transformer

A transformer is a device that transforms voltage from one level to another. They are widely used in power systems. With the help of transformers, it is possible to transmit power at an economical transmission voltage and to utilize power at an economic effective voltage.

Basic principle:

Transformer working is based on mutual emf induction between two coils, which are magnetically coupled.

When an AC voltage is applied to one of the windings (called as the primary), it produces alternating magnetic flux in the core made of magnetic material (usually some form of steel). The flux is produced by a small magnetizing current which flows through the winding. The alternating magnetic flux induces an electromotive force (EMF) in the secondary winding magnetically linked with the same core and appears as a voltage across the terminals of this winding. Cold Rolled Grain Oriented (CRGO) steel is used as the core material to provide a low reluctance, low loss flux path. The steel is in the form of varnished laminations to reduce eddy current flow and losses on account of this.

Typically, the coil connected to the source is known as the primary and the coil applied to the load is the secondary.

A schematic diagram of a single-phase transformer is shown in the Figure 1.6.

Figure 1.6

Schematic Diagram of a Single-Phase Transformer.

A single-phase transformer consists mainly of a magnetic core on which two windings, primary and secondary, are wound. The primary winding is supplied with an AC source of supply voltage V1. The current Iå flowing in the primary winding produces flux, which varies with time. This flux links with both the windings and produces induced emfs. The emf produced in the primary winding is equal and opposite of the applied voltage (neglecting losses.) The emf is also induced in the secondary winding due to this mutual flux. The magnitude of the induced emf depends on the ratio of the number of turns in the primary and the secondary windings of the transformer.

Potential induced:

There is a very simple and straight relationship between the potential across the primary coil and the potential induced in the secondary coil.  The ratio of the primary potential to the secondary potential is the ratio of the number of turns in each and is represented as follows:

The concepts of step-up and step-down transformers function on similar relation.  A step-up transformer increases the output voltage by taking N2 > N1 and a step-down transformer decreases the output voltage by taking N2 < N1.

Current induced:

When the transformer is loaded, then the current is inversely proportional to the voltages and is represented as per follows:

EMF equation of a transformer:

r.m.s.value of the induced emf in the primary winding is:

r.m.s.value of the induced emf in the secondary winding is:

Where,

N1 = Number of turns in primary

N2 = Number of turns in secondary

Øm = Maximum flux in core

f  = Frequency of AC input in Hz.

1.10.3        Types of transformers

1.10.3.1     As per the type of construction:

Core type: Windings surround a considerable part of the core.

Shell type: Core surrounds a considerable portion of the windings.

1.10.3.2     As per cooling type:

Oil filled self-cooled: Small and medium sized distribution transformers.

Oil filled water-cooled: High voltage transmission line outdoor transformers.

Air-Cooled type: Used for low ratings and can be either of natural air circulation (AN) or forced circulation (AF) type.

1.10.3.3    As per application:

1. a)Power transformer:These are large transformers used to change voltage levels and current levels as per requirement. Power transformers are usually used in either a distribution or a transmission line.
2. b)Potential transformer:These are precision voltage step-down transformers used along with low range voltmeters to measure high voltages.
3. c) Current transformer:These transformers are used for the measurement of current where the current carrying conductor is treated as a primary transformer. This transformer isolates the instrument from high voltage line, as well as step down the current in a known ratio.
4. d) Isolation transformer: These are used to isolate two different circuits without changing the voltage level or current level.

A few important points about transformers:

• Used to transfer energy from one AC circuit to another
• Frequency remains the same in both the circuits.
• No ideal transformer exists.
• Also used in metering applications (current transformer i.e. CT, potential transformers, i.e., PT)
• Used for isolation of two different circuits (isolation transformers)
• Transformer power is expressed in VA (Volt amperes)
• Transformer polarity is indicated by using dots. If primary and secondary windings have dots at the top and bottom positions or vice versa in diagram; then it means that the phases are in inverse relationship.

1.10.4        Connections of single-phase transformer

Depending on the application’s requirement, two or more transformers have to be connected in a series or parallel circuits. Such connections can be undertaken as depicted in the following diagram examples:

1. Series connection of two single-phase transformers:

Figure 1.7

Series connection of two single phase Transformers

As shown in the above Figure 1.7, two transformers can be connected in a series connection. If both are connected as above then voltage times two of voltage rating of the individual transformer can be applied. Their current rating must be equal and high enough to carry load current. Precaution should be taken to connect transformers windings keeping in mind the polarity. In the above example, primary total turns to secondary total turns are in the 2:1 ratio, leading to half voltage.

1. Parallel connection of two single-phase transformers:

Figure 1.8 Parallel connection of two single phase Transformers

As shown in the above figure 1.8, two transformers are connected in series on the primary side while the secondary sides are connected in parallel.

On the primary side, the number of turns is added while on the secondary side they remain as it is due to their parallel condition. L.V.D.T. (Linear Voltage Differential Transformer) is the best practical example of the basic transformer and its series connection.

Use of transformers with such connections can however pose problems of safety and load sharing and are hardly used in practical power circuits. It is possible to deploy these connections while designing control transformers if such use will have any specific advantage. Parallel operation of two separate transformers is possible under specific conditions to meet an increased load requirement but the risks involved must be properly evaluated.

1.10.5        3-phase Transformers

Large-scale generation of electric power is generally 3-phasic with voltages in 11 or 32 kvolts. For such high 3-phasic voltage transmission and distribution requires use of the 3-phase step-up and step-down transformers.

Previously, it was common practice to use three single-phase transformers in place of a single 3-phase transformer. However, the consequent evolution of the 3-phase transformer proved space saving and economical as well.

Still, construction-wise a 3-phase transformer is a combination of three single-phase transformers with three primary and three secondary windings mounted on a core having three legs.

Commonly used 3-phases are:

• 3-phase three wire (Delta)
• 3-phase four wire (Star)

1.10.5.1     Delta connection:

It consists of 3-phase windings connected end-to-end (figure 1.9) and are 120 degrees apart from each other electrically. Generally, the delta 3-wire system is used for an unbalanced load system. The 3-phase voltages remain constant regardless of load imbalance.

Figure 1.9

3- phase transformer delta connection on Primary side.

Relationship between line and phase voltages:

VL = Vph

Where,

VL = Line voltage

Vph = Phase voltage

Relationship between line and phase currents:

IL = Ö3 Iph

Where,

IL = Line current

Iph = Phase current

3-Phase 4-wire star connections:

The star type of construction (figure 1.10) allows a minimum number of turns per phase (since phase voltage is 1/Ö3 of line voltage) but the cross section of the conductor will have to be increased as the current is higher compared to a delta winding by a factor of  . Each winding at one end is connected to a common end, like a neutral point – therefore, as a whole there are four wires.

A 3-wire source as obtained from a delta secondary winding or a star secondary winding without a neutral wire from the supply may cause problems when feeding to a star connected unbalanced load. Because of the unbalance, the load neutral will shift and cause change of voltage in the individual phases of the load. It is better to use a star-connected 4-wire source in such cases. 3-wire sources are best suited for balanced loads such as motors.

Figure 1.10

Three phase 4-wire transformer star connection.

Relationship between line and phase voltages:

VL = Ö3 Vph

Where,

VL = Line voltage

Vph = Phase voltage

Relationship between line and phase currents:

IL = Iph

Where,

IL = Line current

Iph = Phase current

Output power of a transformer in kW:

Where,

VL = Line voltage

IL = Line current

Cos f = power factor

1. Possible combinations of Star and Delta

The primary and secondary windings of three single-phase transformers or a 3-phase transformer can be connected in the following ways:

• Primary in delta – secondary in delta
• Primary in delta – secondary in star
• Primary in star – secondary in star
• Primary in star – secondary in delta

Figure 1.11 shows the various types of connections of 3-phase transformers. On the primary side, V is the line voltage and I the line current. The secondary sideline voltages and currents are determined by considering the ratio of the number of turns per phase (a = N1/N2) and the type of connection. Following Table 1.2 gives a quick view of primary line voltages and line currents and secondary phase voltages and currents.

 Connection Line Voltage Line Current Phase Voltage Phase Current (a) Delta-Delta Primary Delta V I V I / 1.732 Secondary Delta V/a Ia V/ a Ia / 1.732 (b) Delta-Star Primary Delta V I V I / 1.732 Secondary Star 1.732V/a Ia / 1.732 V/a Ia / 1.732 (c) Star-Star Primary Star V I V / 1.732 I Secondary Star V/a Ia V / 1.732 a Ia (d) Star-Delta Primary Star V I V / 1.732 I Secondary Delta V/1.732a 1.732 Ia V / 1.732 a Ia

Table 1.2

Voltage and current transformation for different 3-phase transformer connections.

The power delivered by the transformer in an ideal condition irrespective of the type of connection  = 1.732 VL, IL assuming cosf = 1.

Figure 1.11

Types of Connections for 3-phase Transformers.

1.10.6        Testing Transformers

The following tests are carried out on transformers:

• Measurement of winding resistance
• Measurement of Voltage ratio
• Test Phasor voltage relationship
• Measurement of impedance voltage, short circuit impedance and load loss.
• Measurement of insulation resistance
• Dielectric test
• Temperature rise.

Why is transformer rating defined in kVA?

A transformer, unlike a motor, has no mechanical output (expressed in kW). The current flowing through it can vary in power factor, from zero PF lead (pure capacitive load) to zero PF lag (pure inductive load) and is decided by the load connected to the secondary. The conductor of the winding is rated for a particular current beyond which it will exceed the temperature for which its insulation is rated irrespective of the load power factor.

Similarly, the voltage that can be applied to a transformer primary winding has a limit. Exceeding this rated value will cause magnetic saturation of the core leading to distorted output with higher iron losses. It is therefore usual to express the rating of the transformer as a product of the rated voltage and the rated current (VA or kVA). This however does not mean that you can apply a lower voltage and pass a higher current through the transformer. The VA value is bounded individually by the rated voltage and rated current.

Why is power transmitted at higher voltages?

When a particular amount of power has to be transmitted over a certain distance the following aspects need to be considered to decide the best voltage.

A lower voltage will need higher size conductors to withstand the high current involved. There is a physical limitation to the size of conductor. Also, the percentage voltage drop may become excessive.  A higher voltage will make the conductor size manageable and reduce the voltage drop (% value) but the cost of the line becomes high due to larger clearances needed.

The best voltage will be one in which the total operational cost which is the sum of the annualised capital cost (of the line) and the running cost due to power loss in the line is the lowest. In practice, it is found that transmitting bulk power over long distances is economical if done in the HV range. The actual voltage will vary based on the distance and quantum of power. Distribution circuits where typically the amount of power and distance involved are both lower, the best voltage is in the MV range (11, 22 or 33 kV). For the same reason, low voltage circuits are found only in local sub-distribution circuits.

1.10.7         Basic Principles of Electrical Machines

1.10.7.1      Electromechanical Energy Conversion

The electromechanical energy conversion device is a link between electrical and mechanical systems.   When the mechanical system delivers energy through the device to the electrical system, the device is called a generator.

 Mechanical System

 Electrical System

When an electrical system delivers energy through the device to the mechanical system, the device is called a motor.

 Electrical       System

The process is reversible; however, the part of energy converted to heat is lost and is irreversible. An electric machine can be made to work either as a generator or as a motor.

The electromechanical conversion depends on the interrelation between:

• Electric and magnetic fields
• Mechanical forces and motion.

In rotating machines, power is generated by the relative motion of the coils.  In the case of a generator, the winding is rotated mechanically in the magnetic field. This causes the flux linkages with the windings to change causing induced voltages.

In the case of a motor, the current-carrying conductor is allowed inside a magnetic field.  Mechanical force is exerted on a current-carrying conductor in a magnetic field and hence a resultant torque is produced to act on the rotor.

In both a generator as well as a motor, the current-carrying conductor is in the magnetic field. The conductors and flux travel with respect to each other at a definite speed. In rotating machines, both voltage and torque are produced. Only the direction of power flow determines whether the machine is working as a generator or a motor. For a generator, e and i are in the same direction.

Tm = Te + Tf

Where,

Tm = Mechanical Torque

Te = Electrical Torque

Tf  = Torque lost due to friction

For a motor, e and i are in opposite direction.

Tm = Te + Tf

In a generator, the power is supplied by the prime mover. Electrical power is produced by the action of the generator and the resultant power produced due to friction is lost. Whereas in the case of a motor, the power is supplied by the electrical power supply inputs, and there is a slight loss of the resultant mechanical power produced due to friction.

1.11        Meters used in Electrical Troubleshooting

For troubleshooting electrical circuits and systems, the following meters are used depending on the requirements of the parameters to be measured or detected for faultfinding:

• Multi-range voltmeters
• Clip around or clamp-on ammeters
• Electrostatic voltmeters (high voltage measurements)
• Multimeter or volt-ohm-milli-ammeter (voltage, resistance, current, etc)
• Thermocouple meters (indirect current measurement)
• True wattmeters (directly measurement of power in watts)
• Pseudo wattmeters
• Digital voltmeters
• Heterodyne wavemeter (analogue measurement of frequency)
• Digital frequency meters
• Continuity testers
• Analog ohmmeters
• Digital ohmmeters
• Insulation testers
• Digital capacitance meters
• Q meter (measuring inductance and capacitance)
• Oscilloscope (measuring wave forms, amplitude, frequency, phase, et cetera)
• Logic level probe testers
• Logic analyzers  (diagnosing logic systems problems)
• Spectrum analyzers.

Certain important meters from the above list are explained in detail in the following chapters as and when required.

1.12        Electrical Devices and symbols

Any electrical drawing representing an electrical installation or a circuit takes the help of specific symbols to represent various electrical devices in shorthand. This provides a quick idea to the reader about a circuit or installation and is particularly useful while troubleshooting.

Therefore, it is important to familiarize oneself with various symbols. Some of the commonly used device symbols are detailed in the following section and in Figure’s 1.12 and 1.13.

Figure 1.12

Electrical Devices and Symbols.

Figure 1.13

Electrical Devices and Symbols.

1.13        Troubleshooting Basics

Under ideal circumstances, the logical options within a control system are designed to cover all possible events and permutations that may occur.  If well designed, it will control a process to function normally, and will assist the process to recover safely from any abnormal condition that may occur.  At the same time, whilst all this is occurring, the control system may also raise the required alarms and generate error reports. As previously mentioned, this all occurs under perfect conditions.  In the real world, this may not necessarily be the case. Their may be some process situations that are not properly handled by the logic functionality within the device and/or the controller may not able to recover from an abnormal situation in a stable manner.

The causes of fault situations may include:

• Real component failures
• Intermittent faults
• Incompatible inputs
• Change in process environment that deems the programmed logic to be incorrect.

Attention will no be drawn to basic strategies that can be followed, whilst investigating industrial instrumentation. Troubleshooting of any electronic system should follow the following phases:

• Gaining System Familiarity
• Symptom Analysis
• Developing diagnostic strategy
• Fault Diagnosis
• Fault Validation
• Verifying correct Operation.

The time and rigor devoted to each of these steps will depend upon the system and the type of fault under investigation. Some faults are quite trivial and can be diagnosed straightaway by having a quick look at the symptoms. For example, if a fuse blown alarm lamp is illuminated, the first diagnosis would be to investigate the fuse.  At the other extreme, problems may occur which would require a good understanding of the system, careful analysis and diagnosis of the symptoms, and all this needs to be followed up by a step-by-step investigation of the suspect systems and devices.

Most industrial instrumentation systems, by their very nature, are expensive and complex. In addition to this, they can be controlling devices whose malfunction can have major safety implications on human beings. Even in the event of no injury to persons, the process being controlled can involve expensive production line deviations or damage to expensive equipment being produced by the process. Hence, one might make use of a more accurate pressure sensor to determine the differential pressure across a pitot tube, than would be the case of monitoring the air pressure in a small compressor used for spray-painting guns.

1.13.1        Why use a structured approach?

One may question the need to follow a rigid step based procedure to investigate a fault, the cause of which may be very apparent.  In general, 10% of all faults are so simple that they can be solved by a mere scan of the symptoms.  The remaining 90% require closer investigation.  It is this group of faults that require the step by step approach.  In this instance, the systematic approach will help to:

• Diagnose a problem more effectively, rather than a random hit and trial approach
• This approach can be documented for future references
• A track record, of actions taken, can be built up.  This record is helpful for future fault detection.  It is also useful for the current problem at hand, especially when the troubleshooting extend beyond couple of hours
• Improves the chances of finding the root cause as well as the faulty component, saving both time and money
• Improves our understanding of the system, and how it behaves and operates.

1.14        Gaining System Familiarity

The first and foremost step in troubleshooting any system should always be to gain enough understanding of the system.  This will allow you to investigate the system without compromising any safety or quality requirements.  In many cases, you will have gained some formal training on the system, which will provide you with some form of basic understanding.  In other instances, you may never have seen a system of this type before, and it may be the first time you come into contact with it.  In this chapter the term system could just as easily be replaced with plant, part of a plant, machinery, etc.  For the sake of simplicity, the following section will focus on a particular bit of machinery, which is under investigation.

Before you start troubleshooting, it is useful to gain the following information:

• What is the purpose of the machine?
• Are other processes and controllers dependent on this process?
• Does the machine require a planned outage request before taking it offline?
• What is the criticality of the machine in terms of safety, commercial standpoint etc?
• Is there a valid warranty of the machine that could be violated?
• Are there any standby units available?
• Does the supplier, user community or any other group provide any form of support, albeit telephonic or on sight?
• What is the specific process being controlled?

Not all information will be required in all cases.  It is left to your discretion to determine the extent and amount of detail required.  Various methods exist, at your disposal, to source this information, namely:

• Talk to the Operators
• Gather the maintenance history.

Each of these can get greater clarified as follows:

It is crucial that the provided manual be read and fully understood.  Depending on the type of system, this could be anything from few pages to a set of multiple volumes. Regardless of the size, most manuals cover the following:

• System Requirements
• Cautions and Precautions of Use
• Installation and Setup
• Configuration and Programming
• Operation

It is crucial that some time be devoted to going through above manual.  This will provide an understanding of the correct use and operation of the machine, the common problems that could occur as well as the essential requirements for the machine to run.

Absorb enough information, so that you can quickly locate any information, if required, during the next steps.

1.14.2        Talk to the Operators

Just as a medical patient is the best judge of how good or unwell he feels, operators of the machine, and in many cases complete plants, know the system behavior and the accompanying variations quite profoundly.  Ask him/her about recent problems, configuration changes, etc.

1.14.3        Gather the Maintenance History

Time spent gathering the past history of the machine can yield important information on aspects such as the current state of the machine. Things to look out for, in this history, are problems of a recurring nature, software/hardware upgrades, component changes, etc.

At the end of this phase, you should be able to:

• Visualize and understand the correct operation of the machine
• Know the key installation, setup and configuration parameters
• Be familiar with the commonly occurring fault categories
• Know how the system has been performing in the past
• Know whether there have been any major or minor deviations, or changes in the past few days
• Understand the various operation modes of the PLC, e.g. normal, diagnostic, safety mode, etc.
• Know the criticality of the machine for the plant and the business
• Be aware of the maintenance and fault history of the PLC as well as the machine
• Know the common occurring problems
• Know whether the organization has a substitute for the PLC or the machine.
• Know, understand and do not nullify the warranty clauses

1.15        Symptom Analysis

The next step is to understand all the symptoms of the malfunction. No matter how trivial, note down all the symptoms and deviations from the normal behavior of the system. The key here is to realize that a fault in the system will only become evident, once the machine or the associated input/output devices of the machine are operated.

All the symptoms should be carefully analyzed, in order to develop a high level diagnostics strategy. Quickly jumping to a conclusion can actually take you off in a tangent and away from the core problem.  As such, this should be avoided, unless you are extremely familiar with the equipment under consideration.

Map all the symptoms to the correct flow of operations, on the machine being controlled. In most instances, this analysis will yield the following information:

• Which phase of the process is failing?
• Which high level module is responsible for the failure in question?
• Which inputs or what data is required for the correct operation of this phase?
• What are the correct or expected outputs of this phase under normal circumstances?

1.16        Diagnostics Strategies

Based on the information collected so far, it is incumbent that you develop a strategy on how to explore the problem in greater depth.  The objective of this phase is to develop a sound strategy, in order to locate the problem as close to the faulty module or device as possible.  Knowledge gained in the previous steps has to be leveraged and combined, so that you are better able to: