40 Questions
What is the reason for ignoring the effect of salient-pole shape on the operation of synchronous machines?
All machines are assumed to have cylindrical rotors
What is the terminal voltage in the synchronous generator electrical equivalent circuit?
V
What is the synchronous reactance (XS) composed of?
Armature leakage reactance and reactance of armature reaction
What type of load is represented in the phasor diagram of a synchronous generator at UPF?
Purely resistive load
What is the significance of the phasor diagram of a synchronous generator at lagging power factor?
It represents an inductive load
What is the internal generated voltage produced in one phase represented by?
EO
What is the effect of armature reaction allowed in the induced e.m.f?
It modifies the induced e.m.f.
What type of rotor shape is assumed in the analysis of synchronous machines?
Cylindrical rotor
What happens to the rotor velocity of a synchronous generator to maintain frequency for a stable system?
It remains constant
What is the difference between the input mechanical power and the output power of a synchronous generator?
Losses of the machine
What is the assumption made to simplify the phasor diagram?
The armature resistance is negligible
What is the term for the maximum power indicated by the equation?
Static stability limit
What is the relationship between the input mechanical power and the output power of a synchronous generator?
Input power is always greater than output power
What are the losses in a synchronous generator composed of?
Copper losses, core losses, and stray losses
What type of load is connected to the synchronous generator, as per the assumption?
Lagging load
What is the expression for the input mechanical power of a synchronous generator?
Pm = τs ωm
What is the primary condition for connecting an alternator to the infinite bus-bars?
The r.m.s line voltages of the incoming alternator and the infinite bus-bar must be equal.
How can the frequency of the incoming alternator be controlled?
By controlling the speed of the prime mover driving the alternator.
What is the purpose of a synchroscope?
To measure and display the frequency difference and phase shift angle between two power systems.
What happens when two unsynchronized AC power systems are connected together?
High currents flow, which can severely damage equipment.
What is the purpose of the three lamps method?
To check the phase sequence of the incoming alternator and the bus-bar.
What happens if the lamps brighten in succession in the three lamps method?
The systems have opposite phase sequence.
How is the process of paralleling a new generator to the line done in large generators?
Automatically by a computer.
What is the purpose of automatic voltage regulation (AVR)?
To adjust the generated voltage of the incoming alternator.
What happens to the armature current when the load on a synchronous generator is increased at a lagging power factor?
It increases
What is the characteristic of an infinite bus in a power system?
Its voltage and frequency do not vary regardless of the power drawn from it
What happens to the terminal voltage and frequency of the system when the governor mechanism set point of a generator is increased?
Both the terminal voltage and frequency remain constant
What is the voltage regulation of an alternator defined as?
The change in terminal voltage from no-load to full-load
What happens to the terminal voltage when the load on a synchronous generator is increased at a leading power factor?
It increases
What is the effect of increasing the mechanical input power to the prime mover on the power angle of the generator?
It increases the power angle
What is the condition of the generator when it is operating at a slightly leading power factor?
It is supplying negative reactive power
Why should a stationary alternator not be connected to live bus-bars?
Because the induced e.m.f. is not zero
How can the generator be adjusted to supply some reactive power to the system?
By increasing the field current of the alternator
What is the effect of increasing the load on a synchronous generator at unity power factor?
The terminal voltage decreases
What is the role of the governor set point in a generator operating alone?
It controls the operating frequency of the power system
What is the condition for paralleling an alternator with infinite bus-bars?
The alternator must be synchronized
What is the sign of voltage regulation for a lagging load?
Positive
What is the effect of increasing the field current on the reactive power output of the generator?
It increases the reactive power output
What is the condition of the system when a generator is operating alone and supplying the system loads?
The real and reactive power supplied by the generator are equal to the load demand
What is the effect of increasing the load on a synchronous generator at a lagging power factor?
The armature reaction voltage increases
Study Notes
Synchronous Generator: Equivalent Circuit
- A synchronous generator has an internal generated voltage (Eo) which is produced in one phase.
- The induced e.m.f (E) is the induced voltage after allowing the effect of armature reaction.
- The terminal voltage (V) is the voltage at the generator terminals.
- Synchronous reactance (XS) is the sum of armature leakage reactance (XL) and reactance of armature reaction (XAR).
Phasor Diagram of a Synchronous Generator
- The phasor diagram shows the relationship between the internal generated voltage (Eo), terminal voltage (V), and current (I) of a synchronous generator.
- The phasor diagram is different for different power factors (UPF, lagging, and leading).
- At unity power factor (UPF), the current is in phase with the terminal voltage.
- At lagging power factor, the current lags the terminal voltage.
- At leading power factor, the current leads the terminal voltage.
Power Relationships for S.G.
- Not all mechanical power (Pin) going into a synchronous generator becomes electrical power (Pout) out of the machine.
- The difference between input power and output power represents the losses of the machine.
- The input mechanical power is the shaft power in the generator.
- The rotor velocity must remain constant to maintain frequency for a stable system.
Power and Torque in Synchronous Generators
- The maximum generated power occurs when the internal generated voltage (Eo) is equal to the terminal voltage (V).
- The maximum power is called the static stability limit of the generator.
The Effect of Load Changes on Operating of Synchronous Generator
- When the load increases at the same power factor, the armature current increases, and the armature reaction voltage is larger than before.
- At lagging power factor, the terminal voltage (V) decreases as the load increases.
- At unity-power factor, the terminal voltage (V) decreases as the load increases.
- At leading power factor, the terminal voltage (V) increases as the load increases.
Voltage Regulation of Alternator
- Voltage regulation is defined as the change in terminal voltage from no-load to full-load divided by full-load voltage.
- For lagging loads, VR would be positive.
- For leading loads, VR would be negative.
- For unity loads, VR would be positive.
Conditions for Paralleling Alternator with Infinite Bus-bars
- The r.m.s line voltages of the incoming alternator and the infinite bus-bar must be equal.
- The frequency of the generated voltage of the incoming alternator must be slightly higher than the frequency of the running system.
- The incoming alternator and the infinite bus-bar must have the same phase sequence.
Operation of Generators in Parallel with Large Power System
- When a generator is connected in parallel with a large system, the generator will be essentially “floating” on the line, supplying a small amount of real power and little or no reactive power.
- The behavior of alternators connected to an infinite bus-bar is controlled by the governor mechanism and the field excitation.
- Increasing the mechanical input power to the prime mover will not change the speed ultimately, but it will increase the power angle.
- Increasing the field current causes increasing the reactive power output of the generator.
This quiz covers the equivalent circuit of synchronous generators, a crucial topic in electrical engineering.
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