Generators

Generator Working Principle

1. Magnetic Field Creation:
• A generator requires a magnetic field, which can be created using permanent magnets or electromagnets (field windings supplied with current).
2. Conductor Motion:
• A coil or conductive wire moves through this magnetic field, typically driven by an external mechanical energy source such as a turbine, engine, or motor.
3. Electromagnetic Induction:
• According to Faraday’s Law of Electromagnetic Induction, the movement of the conductor within the magnetic field changes the magnetic flux, inducing an electromotive force (EMF) in the conductor.
4. Current Flow:
• When the circuit is closed, the induced EMF drives an electric current, generating usable electrical energy.

This process forms the foundation of AC and DC generators, used in power generation systems worldwide.

How It Works (Hydroelectric Power Plant Working Principle)

A Power Plant is a facility that converts energy from various sources into electrical power for distribution. The image below illustrates a hydroelectric power plant, which generates electricity using the energy of flowing water.

Water Intake:

• Water from a forebay (reservoir) enters through an intake gate.
• Trash racks filter out debris to protect the turbine.

Flow Control:

• The intake structure directs water through a scroll case, ensuring smooth flow.

Turbine Rotation:

• Water flows at high pressure through the turbine, causing it to spin.
• The turbine is mechanically coupled to the generator rotor in the powerhouse.

Electricity Generation:

• The spinning turbine rotates the generator rotor inside the stator, producing electricity through electromagnetic induction.

Power Transmission:

• The generated electricity is stepped up by a transformer and transmitted through 230 kV lines to the switchyard for distribution.

Water Discharge:

• Used water exits through the draft tube into the tailrace, returning to the river at a lower level.

Stand-Alone Synchronous Generators

A stand-alone synchronous generator operates independently, supplying power directly to connected loads without relying on a power grid.

Common Applications:

1. Emergency Backup Systems – Used when the main power grid fails, ensuring continuous power supply.
2. Remote Power Generation – Ideal for isolated areas where grid access is unavailable, such as off-grid villages, islands, or industrial sites.

Key Operating Principles:

• The generator must supply the full real power (P) and reactive power (Q) demanded by the load.
• Maintaining the correct power balance ensures stable voltage and frequency, preventing fluctuations that could damage connected equipment.

This makes stand-alone synchronous generators essential for reliable, self-sustained power systems.

Benefits of Parallel Operation of Generators

1. Redundancy & Reliability 🔄
• Ensures uninterrupted power supply—if one generator fails, others continue operating.
• Allows scheduled maintenance without disrupting the power system.
2. Improved Efficiency ⚡
• Generators operate most efficiently at full load.
• By adding or removing units based on demand, fuel consumption is optimized, reducing operational costs.
3. Scalability & Flexibility 🔧
• Multiple smaller generators can be combined to meet variable load demands.
• Eliminates the need for a single large generator, making system expansion easier.
4. Optimized Resource Utilization 🌍
• Generators can be strategically placed to maximize resource availability (fuel, renewables, or cooling systems).
• Reduces geographical constraints imposed by local load requirements.

This approach is widely used in industrial power plants, hospitals, and off-grid systems, ensuring cost-effectiveness, reliability, and adaptability. 🚀

 

Conditions for Parallel Operation of Synchronous Generators

To successfully synchronize a new generator with an existing system, the following conditions must be met:

1. Equal Line-to-Line Voltages ⚡
• The RMS voltages of both the incoming generator and the existing system must be equal to prevent circulating currents and voltage instability.
2. Same Phase Sequence 🔄
• The phase sequence (A-B-C) of the incoming generator must match that of the existing system.
• A mismatch can cause severe electrical damage to equipment.
3. Equal Phase Angles ⏳
• The phase angles of the voltages should be identical, ensuring that both generators produce voltages in the same direction at the same time.
4. Slightly Higher Frequency for the Incoming Generator 🎚️
• The no-load frequency of the incoming generator should be slightly higher than the system frequency.
• This ensures a smooth transfer of power once synchronized.

These conditions ensure stable, efficient, and safe parallel operation of synchronous generators. 🔌🚀

Procedure for Paralleling Generators

1. Voltage Matching ⚡
• Adjust the field current of the incoming generator so that its line-to-line voltage matches the system voltage.
• This prevents sudden current surges when the generator is connected.
2. Check Phase Sequence 🔄
• Verify that the phase sequence (A-B-C) of the incoming generator matches that of the running system.
• Methods to verify:
• Use small test motors—if both rotate in the same direction, the phase sequence is correct.
• Use three lamps across the switches—correct sequencing causes bulbs to brighten and dim together rather than flicker randomly.
3. Frequency Adjustment 🎚️
• Measure the incoming generator’s frequency with a frequency meter.
• Adjust the prime mover speed so that the incoming generator’s frequency is slightly higher than the system frequency.
• This ensures a smooth power transfer when synchronized.
4. Phase Angle Matching ⏳
• Use a synchroscope or a three-lamp method to ensure the phase angles align.
• The breaker should be closed when the synchroscope pointer is near 12 o’clock or when the bulbs dim simultaneously (indicating phase alignment).

Following this process ensures a safe, smooth, and efficient synchronization of generators, preventing electrical instability. 🚀

 

Procedure for Paralleling Generators

Step 1: Adjust the Voltage of Generator G2 ⚡

• Use a volt-meter to ensure the voltage of the incoming generator (G2) matches the system voltage.
• Adjust the field current of G2 to prevent voltage differences that can cause high circulating currents when connected.

Step 2: Check the Phase Sequence of Generator G2 🔄

The phase sequence of the incoming generator must match the system's sequence. This can be verified using:

1. Induction Motor Method

• Connect a small induction motor to G1 and then to G2.
• Observe the motor’s rotation direction:
  ✅ If both rotate in the same direction, the phase sequence is correct.
  ❌ If they rotate oppositely, swap any two phases on G2 to match the correct sequence.

2. Three-Light-Bulb Method

• Connect three light bulbs across the open terminals of G2.
• Observations:
  ✅ If the bulbs brighten and dim together, the phase sequence is correct.
  ❌ If the bulbs light up one by one, there is a phase mismatch. Solution: Swap two phases on G2.

Step 3: Frequency Adjustment 🎚️

• The frequency of G2 must be slightly higher than that of the running generator (G1).
• Why? This ensures that G2 acts as a generator and does not operate as a motor when connected.
• Use a frequency meter to monitor and fine-tune the frequency.

Step 4: Phase Angle Matching and Synchronization ⏳

• Once voltage, frequency, and phase sequence are set, the generators gradually synchronize
• Using a Synchroscope:
• The synchroscope needle shows the phase angle difference between G1 and G2.
• If the needle moves clockwise, G2's frequency is higher than G1.
• If the needle moves counterclockwise, G2's frequency is lower than G1.
• When the needle reaches zero degrees (0°) or 180°, close the breaker to safely connect the generators.

This method ensures safe, stable, and efficient synchronization of generators for parallel operation. 

Frequency-Power Relationship in Synchronous Generators

The frequency (f_e​) of a synchronous generator is directly related to its rotational speed (n) and the number of poles (P) by the fundamental formula:

Where:

• f​_e = Electrical frequency (Hz)
• n = Rotor speed (RPM)
• P = Number of poles

Mechanical Input and Electrical Output

• The prime mover (e.g., turbine, engine) supplies mechanical power to the rotor.
• The rotor, spinning at a synchronous speed, generates an AC voltage at a frequency determined by its speed and pole configuration.

Stability Considerations

• To maintain a stable system, the rotor speed must match the synchronous speed corresponding to the desired electrical frequency.
• Any fluctuations in mechanical input (fuel, steam, or water flow) affect the rotor speed and, in turn, the generator frequency.

This relationship is crucial in power plants and grid stability, ensuring that generators maintain the required 50 Hz or 60 Hz frequency.

Mathematical Representation of Speed Droop (𝑆𝐷)

Speed droop (SD​) is a measure of how the speed of a synchronous generator decreases as the load increases. This characteristic helps in load sharing when multiple generators operate in parallel.

Formula for Speed Droop:

 

Where:

• SD​ = Speed droop (%)
• N no_load = No-load speed (RPM)
• N full_load ​ = Full-load speed (RPM)

Power-Frequency Relationship in Droop Control:

The power (P) and frequency (f) relationship in a synchronous generator with droop control is:

f = f_rated - S_D × P

Where:

• f= Operating frequency (Hz)
• f_rated ​ = Rated system frequency (Hz)
• P = Active power output (%)
• S_D = Speed droop (%)

Key Points About Droop Control:

Ensures stable load sharing among multiple generators.

Prevents overloading by adjusting power output in response to frequency changes.

Typical droop values range between 2% and 4% in most power systems.

Role of Automatic Voltage Regulators (AVRs) ⚡

The function of AVRs:

• AVRs automatically adjust the excitation current of the generator to maintain a stable terminal voltage, even when the load varies.
• Excitation Current Regulation:

o    If the generator’s terminal voltage drops (e.g., under lagging power factor loads), the AVR increases excitation, strengthening the magnetic field and boosting voltage.

o    If the generator’s terminal voltage rises (e.g., under leading power factor loads), the AVR reduces excitation, weakening the field and lowering the voltage