Synchronous Machine-I

Synchronous Machine-I

Contents

Recall the Synchronous Machines and Types of Synchronous Machines 1

Describe the working of Synchronous Generator 3

Describe the working of Synchronous Motor 5

Describe the Construction of Synchronous Machines 6

Describe the Rotor Construction of Synchronous Machines 8

Recall the concept of the excitation system and prime mover of synchronous machines 9

Recall the windings and types of Synchronous Machine windings 9

Recall the concept of armature reaction in Synchronous Machines 9

Describe the equivalent circuit of a Cylindrical Rotor Generator 9

Draw the phasor diagram of a cylindrical rotor generator 9

Describe the various Tests performed on Synchronous Generator i. DC Resistance Test ii. Open-Circuit Test iii. Short-Circuit Test iv. Concept of Short-Circuit Ratio 9

List the methods to determine Voltage Regulation and explain the concept of Voltage Regulation 9

Explain the Voltage Regulation by EMF Method 9

Explain the Voltage Regulation by MMF Method 9

Recall the Concept of Zero Power Factor Characteristics or Potier Triangle Characteristics 9

Determine the Voltage Regulation by Zero Power Factor or Potier Triangle 9

Determine the Voltage Regulation by ASA (American Standards Association) Method 9

Recall the conditions to be satisfied for Parallel operation of Synchronous Generators 9

Recall Advantages of Parallel operation of Synchronous Generators 9

Describe the methods of Synchronising the Generators for Parallel Operation 9

Recall the Synchronizing Power of: i. Two identical machines on No-Load, floating with respect to each other ii. Machines connected to Infinite Bus 9

Recall the concept of two machines in Parallel Operation 9

Recall the Synchronous Machines and Types of Synchronous Machines

Learning Outcome:

At the end of this learning outcome, learners should be able to recall the basic concepts of synchronous machines and understand the different types of synchronous machines.

Synchronous Machines:

Synchronous machines are electric machines that operate at synchronous speed, which is the speed at which the rotating magnetic field in the stator of the machine rotates. The speed of the rotating magnetic field is determined by the frequency of the supply voltage and the number of poles in the stator winding.

Synchronous machines can operate either as generators or as motors. When a synchronous machine operates as a generator, it converts mechanical energy into electrical energy, while when it operates as a motor, it converts electrical energy into mechanical energy.

Types of Synchronous Machines:

There are two main types of synchronous machines: synchronous generators and synchronous motors.

  1. Synchronous Generators:

Synchronous generators are electric machines that convert mechanical energy into electrical energy. They operate at synchronous speed and are used to generate electrical power in power plants. Synchronous generators are also used in renewable energy systems such as wind turbines and hydroelectric power plants.

Synchronous generators are characterized by their ability to produce a constant voltage at a fixed frequency, which makes them suitable for use in power systems. They are also highly efficient and have a high power factor.

  1. Synchronous Motors:

Synchronous motors are electric machines that convert electrical energy into mechanical energy. They operate at synchronous speed and are used in applications that require constant speed such as large industrial fans, pumps, and compressors.

Synchronous motors are characterized by their ability to operate at a constant speed, which makes them suitable for applications that require precise speed control. They are also highly efficient and have a high power factor.

Synchronous motors can be further classified into two types: non-excited synchronous motors and excited synchronous motors. Non-excited synchronous motors have a permanent magnet rotor and do not require a separate excitation source, while excited synchronous motors have a wound rotor and require a separate excitation source.

Example:

An example of a synchronous generator is a hydroelectric power plant. In a hydroelectric power plant, water is used to turn a turbine connected to a synchronous generator. The synchronous generator converts the mechanical energy from the turbine into electrical energy.

An example of a synchronous motor is a large industrial fan used in a manufacturing plant. The synchronous motor operates at a constant speed and provides precise control of the airflow generated by the fan.

In conclusion, synchronous machines are electric machines that operate at synchronous speed and can operate as generators or motors. The two main types of synchronous machines are synchronous generators and synchronous motors, which are characterized by their ability to produce a constant voltage or operate at a constant speed, respectively. Synchronous machines are widely used in power systems and industrial applications due to their high efficiency and precise control capabilities.

Describe the working of Synchronous Generator

Learning Outcome:

At the end of this learning outcome, learners should be able to describe the working of a synchronous generator.

Synchronous Generator:

A synchronous generator is an electrical machine that converts mechanical energy into electrical energy. It operates at synchronous speed and is widely used in power systems to generate electrical power. The basic components of a synchronous generator are the stator, rotor, and excitation system.

Working Principle:

The working principle of a synchronous generator is based on Faraday’s law of electromagnetic induction. According to this law, when a conductor is moved through a magnetic field, a voltage is induced in the conductor. In a synchronous generator, the rotor rotates in a magnetic field produced by the stator windings, which induces a voltage in the stator windings.

The voltage induced in the stator windings is an alternating voltage that varies sinusoidally with time. The frequency of the voltage is determined by the speed of the rotor and the number of poles in the stator winding. The voltage magnitude and phase angle of the induced voltage depend on the magnetic field strength and the position of the rotor relative to the stator.

Excitation System:

The excitation system of a synchronous generator is responsible for producing the magnetic field in the rotor. The magnetic field in the rotor can be produced by either a DC excitation system or an AC excitation system.

In a DC excitation system, a DC current is passed through the rotor winding, which produces a magnetic field. The DC current is supplied by a separate DC source such as a battery or a rectifier.

In an AC excitation system, the rotor winding is connected to a separate AC source, which produces a magnetic field. The AC source can be either a separate generator or a transformer connected to the stator winding of the synchronous generator.

Synchronisation:

Before a synchronous generator can be connected to a power system, it must be synchronised with the system. Synchronisation is the process of matching the frequency, phase angle, and voltage magnitude of the generator with the power system.

During synchronisation, the speed of the generator is adjusted until it matches the frequency of the power system. The phase angle and voltage magnitude of the generator are adjusted until they match the phase angle and voltage magnitude of the power system.

Example:

An example of a synchronous generator is a thermal power plant. In a thermal power plant, steam is used to turn a turbine connected to a synchronous generator. The synchronous generator converts the mechanical energy from the turbine into electrical energy.

In conclusion, a synchronous generator is an electrical machine that converts mechanical energy into electrical energy. Its working principle is based on Faraday’s law of electromagnetic induction. The excitation system is responsible for producing the magnetic field in the rotor, and synchronisation is required before the generator can be connected to a power system. Synchronous generators are widely used in power systems to generate electrical power.

Describe the working of Synchronous Motor

Learning Outcome:

At the end of this learning outcome, learners should be able to describe the working of a synchronous motor.

Synchronous Motor:

A synchronous motor is an AC motor that runs at a constant speed and is synchronised with the frequency of the power system. It is a type of electric motor that operates on the same principles as a synchronous generator. The basic components of a synchronous motor are the stator, rotor, and excitation system.

Working Principle:

The working principle of a synchronous motor is based on the interaction of a magnetic field and a magnetic flux. The stator windings of the motor produce a magnetic field, while the rotor winding is excited by DC current to produce a magnetic flux. The magnetic field produced by the stator and the magnetic flux produced by the rotor interact to produce a torque that causes the rotor to rotate.

The speed of a synchronous motor is determined by the frequency of the power system and the number of poles in the stator winding. The rotor of a synchronous motor must rotate at the same speed as the rotating magnetic field produced by the stator winding. If the rotor speed is different from the speed of the magnetic field, the motor will not produce any torque and will not start.

Excitation System:

The excitation system of a synchronous motor is responsible for producing the magnetic flux in the rotor. The magnetic flux in the rotor can be produced by either a DC excitation system or an AC excitation system.

In a DC excitation system, a DC current is passed through the rotor winding, which produces a magnetic flux. The DC current is supplied by a separate DC source such as a battery or a rectifier.

In an AC excitation system, the rotor winding is connected to a separate AC source, which produces a magnetic flux. The AC source can be either a separate generator or a transformer connected to the stator winding of the synchronous motor.

Example:

An example of a synchronous motor is a hydroelectric power plant. In a hydroelectric power plant, water is used to turn a turbine connected to a synchronous motor. The synchronous motor runs at a constant speed and is synchronised with the frequency of the power system. The synchronous motor drives the generator, which produces electrical power.

In conclusion, a synchronous motor is an AC motor that runs at a constant speed and is synchronised with the frequency of the power system. Its working principle is based on the interaction of a magnetic field and a magnetic flux. The excitation system is responsible for producing the magnetic flux in the rotor. Synchronous motors are widely used in power systems to drive various types of equipment such as pumps, fans, and compressors.

Describe the Construction of Synchronous Machines

Learning Outcome:

At the end of this learning outcome, learners should be able to describe the construction of synchronous machines.

Synchronous Machines:

Synchronous machines are electrical machines that convert mechanical energy into electrical energy or vice versa. They are widely used in power systems for power generation and transmission. The two main types of synchronous machines are synchronous generators and synchronous motors.

Construction:

The construction of a synchronous machine consists of two main parts: the stator and the rotor.

Stator:

The stator is the stationary part of the synchronous machine and consists of a cylindrical frame made of steel laminations. The frame is designed to provide support for the stator core and to provide a path for the magnetic flux. The stator core is made of laminations of high-grade silicon steel and is designed to minimize iron losses due to hysteresis and eddy currents.

The stator winding is made of copper or aluminium conductors and is wound around the stator core. The winding can be either a distributed winding or a concentrated winding. In a distributed winding, the winding is spread over the entire surface of the stator core, while in a concentrated winding, the winding is concentrated in a few slots.

Rotor:

The rotor is the rotating part of the synchronous machine and consists of a cylindrical shaft made of steel laminations. The rotor core is also made of laminations of high-grade silicon steel and is designed to minimize iron losses. The rotor winding is made of copper or aluminium conductors and is wound around the rotor core.

The rotor winding can be either a salient pole winding or a non-salient pole winding. In a salient pole winding, the rotor poles are made of steel laminations and are bolted to the rotor shaft. In a non-salient pole winding, the rotor poles are made of a solid piece of steel and are integral to the rotor shaft.

Excitation System:

The excitation system is responsible for producing the magnetic field in the rotor. The excitation system can be either a DC excitation system or an AC excitation system. In a DC excitation system, a DC current is passed through the rotor winding, which produces a magnetic field. In an AC excitation system, the rotor winding is connected to a separate AC source, which produces a magnetic field.

Examples:

Synchronous generators are used in power plants to generate electrical power. They are also used in wind turbines and hydroelectric power plants to convert mechanical energy into electrical energy.

Synchronous motors are used in industries for driving large machinery such as pumps, compressors, and fans. They are also used in power systems for power factor correction and load balancing.

In conclusion, synchronous machines are electrical machines that convert mechanical energy into electrical energy or vice versa. The construction of a synchronous machine consists of two main parts: the stator and the rotor. The stator is the stationary part of the machine, while the rotor is the rotating part. The excitation system is responsible for producing the magnetic field in the rotor. Synchronous machines are widely used in power systems for power generation and transmission.

Describe the Rotor Construction of Synchronous Machines

Learning Outcome:

At the end of this learning outcome, learners should be able to describe the rotor construction of synchronous machines.

The rotor is the rotating part of the synchronous machine and is responsible for producing the magnetic field that interacts with the stator winding to produce torque. The rotor construction of synchronous machines can be classified into two types: salient pole and non-salient pole rotors.

Salient Pole Rotor:

A salient pole rotor is also known as a projected pole rotor or a spider rotor. It is a type of rotor construction that consists of a number of poles that are bolted to the rotor shaft. The poles are made of laminated steel, and their shape is similar to a salient pole, hence the name.

The rotor poles are designed to produce a strong magnetic field, which is necessary for the efficient operation of the machine. The poles are separated from each other by slots, which are used to hold the rotor winding. The winding is made of insulated copper or aluminum conductors and is wound around the pole faces.

The winding can be either a concentrated winding or a distributed winding. In a concentrated winding, the winding is concentrated in a few slots, while in a distributed winding, the winding is spread over the entire surface of the pole faces.

Non-salient Pole Rotor:

A non-salient pole rotor is also known as a cylindrical rotor or a smooth rotor. It is a type of rotor construction that consists of a solid cylindrical rotor that rotates inside the stator winding. The rotor is made of laminated steel, and its surface is smooth and cylindrical.

The rotor winding is made of insulated copper or aluminum conductors and is embedded in slots that are cut into the surface of the rotor. The winding can be either a distributed winding or a concentrated winding.

The non-salient pole rotor is used in high-speed applications because it has a low inertia, which makes it easy to accelerate and decelerate. It is also used in applications where a smooth and uniform torque is required, such as in electric vehicles and aircraft.

Examples:

Salient pole rotors are commonly used in low-speed applications, such as hydroelectric generators, where the rotor speed is low and the torque is high. They are also used in large turbo generators used in power plants, where the power output is high.

Non-salient pole rotors are commonly used in high-speed applications, such as in aircraft generators and electric vehicles, where the rotor speed is high and the torque is low. They are also used in small- to medium-sized generators and motors where a smooth and uniform torque is required.

In conclusion, the rotor construction of synchronous machines can be classified into two types: salient pole and non-salient pole rotors. Salient pole rotors consist of a number of poles that are bolted to the rotor shaft, while non-salient pole rotors consist of a solid cylindrical rotor with a smooth surface. The rotor winding is made of insulated copper or aluminum conductors and is either a distributed or concentrated winding. Salient pole rotors are commonly used in low-speed applications, while non-salient pole rotors are used in high-speed applications.

Recall the concept of the excitation system and prime mover of synchronous machines

Excitation System:

The excitation system of a synchronous machine is used to supply the direct current to the rotor winding of the generator or motor. The magnetic field produced by the rotor winding interacts with the stator winding and induces voltage or current in the stator winding. The excitation system can be either static or rotating.

Static excitation systems use solid-state components such as diodes, thyristors, and transistors to control the excitation voltage. These systems provide more precise control of the excitation voltage and are more efficient.

Rotating excitation systems use a DC generator mounted on the shaft of the synchronous machine to provide the excitation voltage. The output of the DC generator is connected to the rotor winding through slip rings and brushes. These systems are simpler and less expensive than static systems, but they are less efficient and require regular maintenance.

Prime Mover:

The prime mover of a synchronous machine is the source of mechanical energy that drives the rotor of the machine. The prime mover can be any device that converts mechanical energy into rotational energy, such as a steam turbine, gas turbine, water turbine, or diesel engine.

The choice of prime mover depends on the application and the availability of fuel or energy sources. For example, a hydroelectric power plant may use a water turbine as the prime mover, while a gas-fired power plant may use a gas turbine. In some cases, multiple prime movers may be connected to a single synchronous machine to provide redundancy or backup power.

Overall, the excitation system and prime mover are essential components of synchronous machines that work together to produce electrical power. The excitation system provides the necessary current to the rotor winding to create the magnetic field, while the prime mover supplies the mechanical energy to drive the rotor.

Recall the windings and types of Synchronous Machine windings

Windings:

A synchronous machine has two types of windings, namely stator winding and rotor winding. The stator winding is stationary, while the rotor winding rotates with the rotor. The stator winding is used to produce a magnetic field that interacts with the rotor winding to produce the output voltage or torque.

Types of Synchronous Machine Windings:

There are two types of windings in synchronous machines: salient pole winding and non-salient pole winding.

  1. Salient Pole Windings:

A synchronous machine with a salient pole winding has a rotor with poles that are projected outward from the rotor surface. The rotor winding is wound around these poles. Salient pole machines are typically used in applications where low-speed operation is required, such as hydroelectric generators.

  1. Non-Salient Pole Windings:

A synchronous machine with a non-salient pole winding has a rotor with smooth cylindrical surface, and the rotor winding is embedded in the rotor slots. Non-salient pole machines are typically used in high-speed applications, such as gas turbine generators.

Both types of windings can have two types of connections: star (Y) and delta (Δ). In a star connection, the winding ends are connected to a common point, while in a delta connection, the winding ends are connected in a closed loop. The choice of connection depends on the application and the desired output voltage.

In summary, synchronous machines have two types of windings, and the type of winding and its connection type are chosen based on the application and the operating conditions of the machine.

Recall the concept of armature reaction in Synchronous Machines

Armature reaction is the magnetic field produced by the stator current that interacts with the main magnetic field produced by the rotor. In a synchronous machine, the stator winding produces a magnetic field that rotates at the synchronous speed. The rotor winding, which is excited by a DC current, produces a magnetic field that interacts with the stator magnetic field to generate the output voltage or torque.

When the machine is loaded, the armature current flows through the stator winding, creating a magnetic field that interacts with the rotor magnetic field. The interaction between these two fields causes a shift in the position of the resultant magnetic field. The direction and magnitude of the shift depend on the power factor of the load and the excitation level of the rotor winding.

If the load power factor is lagging, the armature reaction magnetic field will weaken the rotor field and shift it in the opposite direction of rotation. This results in a decrease in the output voltage and an increase in the armature current. Conversely, if the load power factor is leading, the armature reaction magnetic field will strengthen the rotor field and shift it in the direction of rotation. This results in an increase in the output voltage and a decrease in the armature current.

To compensate for the armature reaction effect, synchronous machines are equipped with an excitation system that regulates the DC current supplied to the rotor winding. This ensures that the rotor magnetic field remains constant, and the output voltage and current are stable under varying load conditions.

In summary, armature reaction is the interaction between the stator magnetic field and the rotor magnetic field in a synchronous machine. It can cause a shift in the position of the resultant magnetic field, affecting the output voltage and current. The excitation system is used to compensate for this effect and maintain stable machine performance under varying load conditions.

Describe the equivalent circuit of a Cylindrical Rotor Generator

The equivalent circuit of a cylindrical rotor generator is a simplified representation of the generator that enables the calculation of the machine’s electrical performance. The equivalent circuit consists of three parts: the armature circuit, the field circuit, and the core losses.

The armature circuit includes the stator winding, the armature resistance, and the armature reactance. The stator winding is modelled as a voltage source, which produces the output voltage, and the armature resistance represents the losses due to the flow of current in the stator winding. The armature reactance represents the inductive reactance of the stator winding.

The field circuit includes the rotor winding, the field resistance, and the field reactance. The rotor winding is excited by a DC current, which produces the magnetic field in the rotor. The field resistance represents the losses due to the flow of current in the rotor winding, and the field reactance represents the inductive reactance of the rotor winding.

The core losses include the hysteresis losses and the eddy current losses. Hysteresis losses occur due to the magnetization and demagnetization of the core material, while eddy current losses occur due to the flow of current in the core material.

The equivalent circuit is used to calculate the machine’s performance under different operating conditions. For example, the output voltage can be calculated by subtracting the voltage drop due to the armature resistance and reactance from the voltage produced by the stator winding. The output power can be calculated by multiplying the output voltage with the output current.

In summary, the equivalent circuit of a cylindrical rotor generator consists of the armature circuit, the field circuit, and the core losses. It is a simplified representation of the generator that enables the calculation of the machine’s electrical performance under different operating conditions.

Draw the phasor diagram of a cylindrical rotor generator

The phasor diagram of a cylindrical rotor generator shows the relationship between the electrical quantities of the stator and rotor windings. The phasor diagram is a graphical representation of the complex numbers used to represent the voltage, current, and impedance of the generator.

Let us assume that,

  • Ef=Excitation voltage
  • V= Terminal voltage per phase applied to the armature
  • Ia=Armature current per phase drawn by the motor from the supply
  • Ra= Effective armature resistance per phase
  • XS=Synchronous reactance per phase of armature winding
  • Cosφ= Power factor
  • δ=Torque angle

The voltage equation of a cylindrical rotor synchronous motor is

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The phasor diagrams of a 3-phase cylindrical rotor synchronous motor operating at different power factors can be drawn with the help of equation 2.

Phasor Diagram at Lagging Power Factor

The phasor diagram of the synchronous motor operating at a lagging power factor Cosφ is shown in Figure-1.

9K=

Consider the synchronous motor is taking a lagging current from the supply. Here, the supply voltage (V) is taken as the reference phasor along OA such that OA = V. For lagging power factor cosφ, the armature current (Ia) lags behind the supply voltage (V) by an angle φ along OB where OB = Ia.

The voltage drop in the armature resistance is IaRa which is in phase with the armature current. The phasor IaRa is represented by CD.

The voltage drop per phase in the synchronous reactance is IaXS .The phasor is in a direction perpendicular to the phasor IaRa and is represented by DA.

Therefore, the phasor V is equal to the phasor sum of Ef, and IaRa. The angle δ between V and Ef is called the torque angle. It plays an important role in the power transfer and the stability of the synchronous motor operation.

Phasor Diagram at Unity Power Factor

The phasor diagram of the synchronous motor operating at unity power factor is shown in Figure-2.

9K=

Suppose that the synchronous motor is drawing the current (Ia) from the supply at unity power factor. Here, the supply voltage (V) is taken as the reference phasor along OA such that OA = V.

For unity power factor, the armature current (Ia) drawn by the motor is in phase with the supply voltage (V) and is represented by OB where OB = Ia.

The voltage drop in the armature resistance is IaRa which is in phase with the armature current. The phasor IaRa is represented by CD.

The voltage drop per phase in the synchronous reactance is IaXS. The phasor is IaXS in a direction perpendicular to the phasor IaRa and is represented by DA.

Therefore, the phasor V is equal to the phasor sum of Ef,IaRa and jIaXS.The angle δ between V and Ef is called the torque angle. It plays an important role in the power transfer and the stability of the synchronous motor operation

Phasor Diagram at Leading Power Factor

The phasor diagram of the synchronous motor operating at leading power factor cosφ is shown in Figure-3.

9K=

Suppose that the synchronous motor is drawing the current (Ia) from the supply at leading power factor cosφ. Here, the supply voltage (V) is taken as the reference phasor along OA such that OA = V.

For leading power factor, the armature current(Ia) drawn by the motor leads the supply voltage (V) by the phase angle φ and is represented by OB where OB = Ia.

The voltage drop in the armature resistance is IaRa which is in phase with the armature current. The phasor IaRa is represented by CD.

The voltage drop per phase in the synchronous reactance isIaXS. The phasor is IaXS in a direction perpendicular to the phasor IaRa and is represented by DA.

Therefore, the phasor V is equal to the phasor sum of Ef,IaRa and jIaXS.

The phasor diagram of a cylindrical rotor generator provides a visual representation of the electrical quantities and their relationship with each other. It is useful in understanding the behavior of the generator under different operating conditions, such as varying loads or changes in excitation current.

Describe the various Tests performed on Synchronous Generator i. DC Resistance Test ii. Open-Circuit Test iii. Short-Circuit Test iv. Concept of Short-Circuit Ratio

Synchronous generators are essential electrical machines used to produce electrical energy. To ensure the proper functioning and performance of these generators, various tests are conducted before their installation and during their operation. Here are the different tests conducted on synchronous generators:

i. DC Resistance Test:

DC resistance test is performed to measure the resistance of the field winding and armature winding of the generator. The purpose of this test is to ensure that the winding resistance is within the specified range to avoid excessive losses and overheating of the machine. The test is conducted by passing a DC current through the winding and measuring the voltage drop across the winding using a voltmeter and ammeter.

ii. Open-Circuit Test:

The open-circuit test is performed to determine the no-load magnetization characteristics of the synchronous generator. In this test, the generator is run at synchronous speed with the field winding excited at a rated voltage and the armature winding left open-circuited. The field current and voltage are varied to obtain the open-circuit characteristic curve of the generator. This curve helps in determining the voltage regulation and efficiency of the generator.

iii. Short-Circuit Test:

The short-circuit test is performed to determine the copper losses and leakage reactance of the synchronous generator. In this test, the field winding is excited at a rated voltage and the armature winding is short-circuited with the help of a switch. The current is then gradually increased until the generator is operating at its rated current. The voltage drop across the armature winding is measured, and the short-circuit characteristic curve is plotted. The curve helps in determining the impedance and efficiency of the generator.

iv. Concept of Short-Circuit Ratio:

The short-circuit ratio (SCR) of a synchronous generator is defined as the ratio of its open-circuit voltage to its short-circuit current at a given power factor. It is a measure of the ability of the generator to deliver current under short-circuit conditions. The SCR is an important parameter in determining the suitability of a generator for a particular application.

In conclusion, these tests are crucial for determining the performance and efficiency of synchronous generators. The results obtained from these tests help in identifying any faults or defects in the machine, ensuring that the generator operates safely and efficiently.

List the methods to determine Voltage Regulation and explain the concept of Voltage Regulation

Voltage regulation is an important parameter of a synchronous generator that determines the ability of the generator to maintain a stable voltage output under different load conditions. It is defined as the change in voltage from no load to full load expressed as a percentage of the rated voltage. A low voltage regulation indicates that the generator can maintain its output voltage within a narrow range, even under varying load conditions.

There are several methods to determine the voltage regulation of a synchronous generator. Some of the commonly used methods are:

  1. Direct Load Test Method: This method involves applying a load to the generator and measuring the terminal voltage at different load conditions. The voltage regulation can be calculated by comparing the no-load voltage and the voltage at full load.
  2. EMF (Electromotive Force) Method: This method involves measuring the generated EMF of the generator at no-load and full-load conditions. The voltage regulation can be calculated by dividing the difference between the generated EMFs at no-load and full-load by the rated voltage of the generator.
  3. ZPF (Zero Power Factor) Method: This method involves connecting a load to the generator that draws a current at unity power factor. The voltage regulation can be calculated by measuring the voltage drop between no-load and full-load conditions and dividing it by the rated voltage of the generator.
  4. Potier Reactance Method: This method involves inserting a reactance in series with the generator and adjusting its value until the generator output voltage at full load is equal to the rated voltage. The voltage regulation can be calculated by dividing the reactance value by the synchronous reactance of the generator.

The voltage regulation of a synchronous generator is an important parameter that affects the stability of the power system. A low voltage regulation indicates that the generator can maintain a stable voltage output under varying load conditions, while a high voltage regulation indicates that the generator may not be able to maintain a stable voltage output and may cause instability in the power system.

Explain the Voltage Regulation by EMF Method

Learning Outcome: ALO:

  1. Perform an open-circuit test on the synchronous generator to determine the open-circuit voltage and the field current required to produce it.
  2. Calculate the synchronous reactance of the generator using the open-circuit test results.
  3. Perform a short-circuit test on the synchronous generator to determine the short-circuit current and the field current required to produce it.
  4. Calculate the synchronous impedance of the generator using the short-circuit test results.
  5. From the synchronous impedance, synchronous reactance and the resistance of the generator can be calculated.
  6. Calculate the voltage drop in the armature resistance at full-load current.
  7. Calculate the full-load voltage of the generator.
  8. Calculate the voltage regulation using the following formula:
    Voltage regulation = (E0 – Vf) / Vf x 100%
    where E0 is the open-circuit voltage of the generator, Vf is the full-load voltage of the generator.

The EMF method is widely used as it is relatively easy to perform and gives accurate results. However, it is important to note that the EMF method assumes a constant power factor and neglects the armature reaction effects, which may lead to inaccurate results in certain situations.

In conclusion, the voltage regulation of a synchronous generator is an important parameter, and the EMF method is one of the methods used to determine it. The method is based on the open-circuit and short-circuit tests of the generator, and it assumes a constant power factor and neglects the armature reaction effects.

Explain the Voltage Regulation by MMF Method

The voltage regulation of a synchronous generator is defined as the change in voltage magnitude from no-load to full-load conditions, expressed as a percentage of the rated voltage. The voltage regulation is a critical parameter that determines the ability of a generator to maintain a steady voltage output.

The MMF (magneto-motive force) method is one of the methods used to determine the voltage regulation of a synchronous generator. The method is based on the principle that the voltage regulation of a generator is proportional to the ratio of the armature reaction MMF to the field excitation MMF.

The armature reaction MMF is the demagnetizing effect produced by the armature current on the magnetic field of the rotor. The armature reaction MMF is proportional to the armature current and the synchronous reactance of the machine. The field excitation MMF is the magnetising effect produced by the field current on the rotor magnetic field. The field excitation MMF is proportional to the field current and the field winding turns.

The MMF method involves performing two tests on the generator: the open-circuit test and the short-circuit test. In the open-circuit test, the generator is run at synchronous speed with no load connected to the output terminals. The field current is adjusted until the terminal voltage is equal to the rated voltage. The field excitation MMF is calculated from the field current and the number of field winding turns.

In the short-circuit test, the generator is run at synchronous speed with the output terminals short-circuited. The field current is adjusted until the short-circuit current is equal to the rated armature current. The armature reaction MMF is calculated from the armature current and the synchronous reactance of the machine.

The voltage regulation is then calculated using the following formula:

Voltage regulation = (Eo – Ef) / Ef x 100%

where Eo is the open-circuit voltage and Ef is the voltage across the armature terminals during the short-circuit test.

The MMF method is widely used in industry to determine the voltage regulation of synchronous generators. However, the method assumes a sinusoidal waveform for the armature current, which may not be valid in some cases. Other methods, such as the Z- or impedance method, may be more appropriate in such cases.

Recall the Concept of Zero Power Factor Characteristics or Potier Triangle Characteristics

In electrical power systems, it is essential to maintain the power factor close to unity to avoid energy losses and improve the efficiency of the system. The Zero Power Factor (ZPF) characteristics or Potier triangle characteristics is a graphical representation of the voltage-current phase relationship in synchronous generators. It is used to determine the synchronous generator’s excitation system’s operating point, which ensures that the generator operates at a power factor close to unity.

Potier Triangle:

The Potier triangle is a right-angled triangle, with the hypotenuse representing the apparent power (S) of the generator. The horizontal leg represents the active power (P) generated by the generator, while the vertical leg represents the reactive power (Q) generated by the generator. The angle between the horizontal leg and the hypotenuse is the load angle (δ), which represents the phase difference between the voltage and current in the generator.

Zero Power Factor Characteristics:

The Zero Power Factor (ZPF) characteristics or Potier triangle characteristics is a graphical representation of the generator’s performance concerning the excitation system’s operating point. The ZPF characteristic is obtained by plotting the generator’s current and voltage at zero power factor on the Potier triangle. The zero power factor condition is achieved by overexciting the generator, resulting in a leading power factor.

Importance:

The ZPF characteristics are used to determine the excitation system’s operating point that results in the generator operating at a power factor close to unity. This improves the generator’s efficiency and reduces energy losses in the system.

Conclusion:

The ZPF characteristics or Potier triangle characteristics is a useful tool for determining the excitation system’s operating point in synchronous generators. It ensures that the generator operates at a power factor close to unity, resulting in improved efficiency and reduced energy losses in the system.

Determine the Voltage Regulation by Zero Power Factor or Potier Triangle

Zero power factor characteristics, also known as Potier triangle characteristics, is a graphical method used to determine the voltage regulation of synchronous generators. It is based on the concept that when the generator is loaded with a non-inductive or unity power factor load, the power angle is 0 degrees, and the armature current is in phase with the terminal voltage. The excitation voltage is then adjusted to maintain the terminal voltage at the rated value.

The Potier triangle is constructed by plotting three points on the V-I diagram: the open-circuit voltage at the rated excitation, the short-circuit current at rated excitation, and the load current at zero power factor. These three points are connected by lines to form a triangle. The sides of the triangle represent the open-circuit voltage, the short-circuit impedance, and the load impedance. The angle between the load impedance and the short-circuit impedance is the power angle.

To determine the voltage regulation of the synchronous generator using the Potier triangle, the following steps are followed:

  1. Draw the Potier triangle by plotting the open-circuit voltage, short-circuit current, and load current at zero power factor.
  2. Measure the load impedance on the Potier triangle by drawing a line from the origin to the load current point and measuring the length of the line.
  3. Measure the short-circuit impedance on the Potier triangle by drawing a line from the short-circuit current point to the origin and measuring the length of the line.
  4. The ratio of the load impedance to the short-circuit impedance is the voltage regulation.

For example, let’s say we have a synchronous generator rated at 100 MVA, 13.8 kV, and 0.85 power factor. The open-circuit voltage is 13.8 kV, and the short-circuit current at rated excitation is 4 kA. The load current at zero power factor is 65 A. The load impedance on the Potier triangle is measured to be 190 ohms, and the short-circuit impedance is measured to be 0.1 ohms. Therefore, the voltage regulation is:

Load Impedance/Short Circuit Impedance = 190/0.1 = 1900

Voltage Regulation = (Open Circuit Voltage – Load Voltage)/Load Voltage = (13.8 kV – 12.9 kV)/12.9 kV = 6.98%

Therefore, the voltage regulation of the synchronous generator is 6.98%.

Determine the Voltage Regulation by ASA (American Standards Association) Method

The American Standards Association (ASA) method is another method used to determine the voltage regulation of synchronous generators. This method involves plotting the open-circuit characteristic (OCC) and the short-circuit characteristic (SCC) of the generator on the same graph. The ASA method is also known as the Z method.

The OCC is obtained by conducting an open-circuit test on the synchronous generator. In this test, the synchronous generator is driven at its rated speed and the field current is gradually increased until the terminal voltage reaches its rated value. The armature circuit is kept open during the test. The voltage and field current readings are noted and used to plot the OCC.

The SCC is obtained by conducting a short-circuit test on the synchronous generator. In this test, the synchronous generator is driven at its rated speed and the armature circuit is short-circuited through an ammeter. The field current is gradually increased until the rated armature current is reached. The voltage and field current readings are noted and used to plot the SCC.

To determine the voltage regulation of the synchronous generator using the ASA method, the following steps are followed:

  1. Plot the OCC and SCC on the same graph, with field current on the x-axis and terminal voltage on the y-axis.
  2. Draw a line connecting the origin of the graph and the point where the SCC intersects the x-axis. This line is known as the Z line.
  3. Draw a line parallel to the Z line, passing through the point where the OCC intersects the x-axis. This line is known as the A line.
  4. Draw a line parallel to the x-axis, passing through the point where the OCC intersects the y-axis. This line is known as the B line.
  5. The intersection point of the A and B lines is the operating point of the generator under load conditions.
  6. The voltage regulation can be determined by calculating the ratio of the difference between the no-load voltage and the load voltage to the load voltage, as a percentage.

The ASA method is useful in determining the voltage regulation of the generator at different power factors and load conditions. However, this method is not commonly used nowadays due to the availability of more advanced methods, such as the EMF and MMF methods.

Recall the conditions to be satisfied for Parallel operation of Synchronous Generators

Synchronous generators are commonly used to provide electrical power to a power system. In order to meet the increasing demand for electrical power, it is often necessary to connect two or more generators in parallel. Parallel operation of synchronous generators is a complex process that requires careful consideration of various factors, including the operating characteristics of the generators, the load demand, and the control systems.

The conditions to be satisfied for parallel operation of synchronous generators are as follows:

  1. Voltage Level: The voltage levels of the generators must be equal. This is necessary to ensure that the loads are supplied with a constant voltage level, and to avoid overloading of any individual generator.
  2. Frequency: The frequency of the generators must be the same. This is necessary to ensure that the loads are supplied with a constant frequency, and to avoid phase differences between the generators.
  3. Phase Sequence: The phase sequence of the generators must be the same. This is necessary to ensure that the loads are supplied with the correct phase sequence, and to avoid problems such as reverse power flow and excessive voltage drops.
  4. Phase Angle: The phase angles of the generators must be close to each other. This is necessary to ensure that the generators do not oppose each other’s voltages and cause a sudden change in the power flow direction.
  5. Synchronous Impedance: The synchronous impedance of the generators must be the same. This is necessary to ensure that the generators share the load equally and prevent unequal distribution of reactive power.
  6. Power Factor: The power factor of the generators must be the same. This is necessary to ensure that the generators share the load equally and prevent unequal distribution of reactive power.
  7. Governor Response: The governors of the generators must respond in the same way to changes in load demand. This is necessary to ensure that the generators share the load equally and prevent overloading of any individual generator.
  8. Excitation System Response: The excitation systems of the generators must respond in the same way to changes in load demand. This is necessary to ensure that the generators share the load equally and prevent overloading of any individual generator.

By ensuring that these conditions are met, synchronous generators can be connected in parallel to provide the required amount of electrical power to the system.

Recall Advantages of Parallel operation of Synchronous Generators

When two or more synchronous generators are connected in parallel, they are said to be operating in parallel. This is done in order to provide increased power output, improved system reliability, and easier maintenance. Some of the key advantages of parallel operation of synchronous generators are:

  1. Increased power output: When two or more synchronous generators are operated in parallel, their combined output is greater than the sum of their individual outputs. This allows for increased power output to meet the demands of the electrical grid.
  2. Improved system reliability: Parallel operation of synchronous generators improves system reliability by providing backup power in case one generator fails. This helps to ensure continuous power supply to customers and prevents downtime.
  3. Efficient use of resources: Parallel operation of synchronous generators allows for the efficient use of resources, such as fuel and maintenance costs. By operating multiple generators in parallel, each generator can be run at its most efficient load point, reducing fuel consumption and maintenance costs.
  4. Load sharing: When synchronous generators are connected in parallel, they automatically share the load based on their individual capacities. This ensures that each generator operates at its rated capacity and prevents overloading of any one generator.
  5. Redundancy: Parallel operation of synchronous generators provides redundancy, which is essential for maintaining system reliability. If one generator fails, the remaining generators can continue to operate and meet the power demand.
  6. Flexibility: Parallel operation of synchronous generators provides flexibility in system design, allowing for the addition or removal of generators as needed to meet changing demand.

In summary, parallel operation of synchronous generators provides increased power output, improved system reliability, efficient use of resources, load sharing, redundancy, and flexibility.

Describe the methods of Synchronising the Generators for Parallel Operation

Synchronisation is the process of connecting two or more synchronous generators in parallel so that they operate together and share the load. Before synchronising the generators, they must be at the same voltage level, frequency, and phase sequence. The synchronisation process ensures that the voltage, frequency, and phase sequence of the incoming generator are in line with the system parameters.

The following are the methods used for synchronising the generators for parallel operation:

  1. Dark Lamp Method: In this method, a dark lamp is used as a visual indicator to detect the phase difference between the incoming generator and the bus bar. The dark lamp is connected between the incoming generator and the bus bar. If the phases are in synchronisation, the lamp will not glow, indicating that the phases are in line. If the lamp glows, it means there is a phase difference, and the phase sequence of the incoming generator needs to be adjusted.
  2. Synchroscope Method: The synchroscope is a device that indicates the phase difference between the incoming generator and the bus bar. The synchroscope is connected between the incoming generator and the bus bar, and it displays the relative phase angle between the two systems. The operator adjusts the phase sequence of the incoming generator by observing the movement of the synchroscope’s pointer.
  3. Three-Lamp Method: In this method, three lamps are used to indicate whether the voltage, frequency, and phase sequence of the incoming generator are in line with the system parameters. The lamps are connected between the incoming generator and the bus bar, and each lamp represents a different parameter. If all three lamps glow, it means that the parameters are not in line, and adjustments need to be made.
  4. Digital Synchronizer: The digital synchronizer is a device that automatically synchronises the incoming generator with the bus bar. The device measures the voltage, frequency, and phase angle of the incoming generator and compares them to the bus bar’s parameters. If the parameters are within the acceptable range, the digital synchronizer closes the circuit breaker to connect the generator to the bus bar.

Overall, synchronisation is an essential step in parallel operation of synchronous generators to ensure safe and efficient operation of the power system.

Recall the Synchronizing Power of: i. Two identical machines on No-Load, floating with respect to each other ii. Machines connected to Infinite Bus

i. Two identical machines on No-Load, floating with respect to each other

When two identical machines are on no-load and are floating with respect to each other, the rotor of each machine will rotate at its synchronous speed. In this situation, there is no mechanical coupling between the two rotors, and they are free to rotate independently. However, if there is a small disturbance in one of the machines, such as a sudden change in mechanical torque or a voltage dip, it can cause the rotor to deviate slightly from its synchronous speed.

When this happens, the rotor of the other machine will also experience a similar disturbance, causing it to deviate from its synchronous speed. The result is that the two rotors will tend to move closer together, as the forces between them increase, until they are back in synchronism. This phenomenon is known as the synchronising power of two identical machines on no-load floating with respect to each other.

For example, consider two identical synchronous generators that are connected to a common bus, but are not electrically connected to each other. If one of the generators experiences a sudden change in mechanical torque, it will cause a momentary change in its rotational speed. This change in speed will cause the generator to produce a transient voltage that will appear across its terminals. As the voltage across the terminals of the second generator is zero, the transient voltage produced by the first generator will appear across the gap between the two machines. This will cause a current to flow between the two machines, creating a force that will tend to bring the rotors back into synchronism.

ii. Machines connected to Infinite Bus

When synchronous generators are connected to an infinite bus, they are electrically and mechanically coupled to each other. An infinite bus is an idealised power system with an infinite amount of power available, which means that the voltage and frequency of the bus remain constant regardless of the amount of power generated or consumed by the connected generators.

When a synchronous generator is connected to an infinite bus, it experiences a constant electrical load, which keeps the rotor rotating at the synchronous speed. If there is a small deviation in the rotor speed, the electrical power output of the generator will also deviate, causing a temporary power imbalance in the system. This power imbalance will result in a torque that tends to bring the rotor back into synchronism with the infinite bus.

For example, consider a synchronous generator that is connected to an infinite bus through a transmission line. If there is a sudden increase in the mechanical torque applied to the generator, it will cause the rotor to speed up slightly. This will result in a temporary increase in the power output of the generator, which will create a power imbalance in the system. The excess power will flow through the transmission line and cause a momentary voltage drop at the generator’s terminals. This voltage drop will cause a current to flow through the line, creating a torque that tends to slow down the rotor and bring it back into synchronism with the infinite bus. Similarly, if there is a sudden decrease in the mechanical torque applied to the generator, it will cause the rotor to slow down slightly, resulting in a temporary decrease in the power output and a momentary voltage rise at the generator’s terminals. This will cause a current to flow in the opposite direction through the transmission line, creating a torque that tends to speed up the rotor and bring it back into synchronism with the infinite bus.

Recall the concept of two machines in Parallel Operation

When two or more synchronous generators are connected to a common electrical bus and are operating in parallel, they share the total electrical load and produce power together. Parallel operation is commonly used in power systems to increase the overall system capacity and provide redundancy. However, proper synchronisation and load sharing between the generators are essential for efficient and safe operation.

In parallel operation, the frequency and voltage of each generator must be identical, and the phase angle between them must be zero. Any deviation in frequency or voltage can result in an unbalanced load sharing between the generators and can cause voltage instability and other undesirable effects. Therefore, a synchronising panel is used to monitor the frequency and voltage of each generator and adjust them as necessary to achieve proper synchronisation.

The load sharing between the generators is determined by their respective droop characteristics. Droop is a characteristic that describes how the generator output voltage changes with respect to the load. In a droop system, as the load on the generator increases, the output voltage decreases, and as the load decreases, the output voltage increases. This means that the generator with a higher droop characteristic will provide more power for a given load than the generator with a lower droop characteristic.

For example, consider two synchronous generators that are operating in parallel and connected to a common bus. If the total load on the system is 100 MW, and each generator has a capacity of 60 MW, the generators will share the load in proportion to their droop characteristics. If generator 1 has a droop characteristic of 3% and generator 2 has a droop characteristic of 4%, generator 1 will provide 40 MW of power, and generator 2 will provide 60 MW of power to the load. If the load increases to 120 MW, the generators will adjust their output voltage to share the load accordingly, with generator 1 providing 48 MW and generator 2 providing 72 MW.

In summary, parallel operation of two or more generators requires proper synchronization and load sharing to ensure efficient and safe operation of the power system. The synchronising panel monitors the frequency and voltage of each generator and adjusts them as necessary, while the droop characteristic determines the load sharing between the generators.