What is the primary cause of magnetizing current in a transformer?
A Voltage imbalance
B High frequency
C Magnetic flux
D Load condition
Magnetizing current is the current required to establish the magnetic flux in the transformer core. It flows through the primary winding and is typically small compared to the load current. This current is essential for the transformer’s operation.
What is inrush current in a transformer?
A High starting current
B Continuous current
C Low-frequency current
D Zero current
Inrush current is the initial surge of current that occurs when a transformer is first energized. It can be several times higher than the normal operating current and is caused by the magnetizing of the transformer’s core.
What is the purpose of transformer protection systems?
A Prevent overheating
B Increase efficiency
C Control voltage
D Detect faults
Transformer protection systems are designed to detect faults such as short circuits or overloads. These protection schemes help isolate faulty parts, preventing further damage to the transformer and the entire electrical system.
Which fault is most commonly detected by a differential protection relay in transformers?
A Inrush current
B Overload
C Short circuit
D Ground fault
Differential protection relays are commonly used to detect short circuits in transformers by comparing the current entering and exiting the transformer. Any difference indicates a fault, and the relay can isolate the transformer to prevent damage.
What is the main function of a Buchholz relay in transformer protection?
A Control voltage
B Detect gas accumulation
C Measure temperature
D Detect oil level
A Buchholz relay is used in oil-filled transformers to detect the accumulation of gases caused by internal faults. When gases are generated by overheating or electrical faults, the relay triggers an alarm or shuts down the transformer.
What is the effect of inrush current on a transformer?
A Prevents fault detection
B Increases efficiency
C Causes transformer heating
D Reduces magnetizing current
Inrush current can cause significant heating in the transformer due to the large current drawn during energization. While brief, this surge of current can stress the transformer’s insulation and winding, potentially leading to damage if not properly managed.
How can inrush current be reduced in a transformer?
A Apply pre-magnetization
B Use smaller transformers
C Increase voltage
D Use resistors
Pre-magnetization involves gradually energizing the transformer to build up the magnetic field before full voltage is applied. This method reduces the inrush current by preventing a sudden change in the magnetic flux when the transformer is energized.
What is the impact of magnetizing current on transformer operation?
A Decreases load capacity
B Increases efficiency
C Increases core losses
D Decreases power loss
Magnetizing current, although small, causes core losses such as hysteresis and eddy currents. These losses, while small in magnitude, contribute to the overall power loss in the transformer and can affect efficiency.
Which protection device is used to prevent transformer from overloads?
A Differential relay
B Buchholz relay
C Oil pump
D Thermal relay
A thermal relay is used to protect transformers from overloads by sensing temperature rise caused by excessive current. It disconnects the transformer from the system if the temperature exceeds a safe limit, preventing overheating and damage.
What is the role of the primary winding in the magnetizing current of a transformer?
A Control frequency
B Create magnetic field
C Generate voltage
D Generate heat
The primary winding of a transformer generates a magnetic field when energized with AC. This field induces voltage in the secondary winding and causes the magnetizing current to flow through the primary, which is essential for transformer operation.
Why is a transformer’s magnetizing current typically small?
A High voltage
B High resistance
C Low flux density
D Low frequency
The magnetizing current is small because the flux density in the transformer core is low relative to the amount of current needed to establish the magnetic field. This ensures minimal power loss in the magnetizing current during normal operation.
What is the function of an overcurrent relay in transformer protection?
A Detect overload conditions
B Prevent short circuits
C Control voltage
D Reduce inrush current
An overcurrent relay is used to detect excessive current (overload) in a transformer, triggering the protection system to disconnect the transformer if the current exceeds safe operating limits, preventing damage to the windings or insulation.
What happens when a transformer experiences a short circuit fault?
A Magnetizing current decreases
B Voltage increases
C Inrush current reduces
D Excessive current flows
When a transformer experiences a short circuit fault, excessive current flows through the windings, potentially damaging the transformer. Protection mechanisms like differential relays are used to quickly detect and isolate the fault to prevent further damage.
What does the differential protection scheme of a transformer detect?
A Internal faults
B Temperature rise
C Voltage fluctuations
D Ground faults
The differential protection scheme detects internal faults by comparing the current entering and leaving the transformer. If the currents differ, it indicates a fault inside the transformer, prompting the system to disconnect the transformer from the network to prevent damage.
How does an over-voltage relay protect transformers?
A Detects short circuits
B Disconnects the transformer under excessive voltage
C Prevents overheating
D Measures load current
An over-voltage relay protects transformers by detecting when the voltage exceeds a preset limit. If the voltage is too high, the relay disconnects the transformer to prevent damage to the insulation and other components due to excessive voltage.