Protective Relay
A protective relay is a
sophisticated electrical apparatus designed to identify and respond to abnormal
or fault conditions in an electrical circuit. It vigilantly monitors electrical
parameters and, upon detecting irregularities, commands a circuit breaker to
disconnect, thereby protecting the system from potential harm. This device acts
as a critical safeguard, ensuring the stability and safety of electrical
infrastructures.
Consider a protective relay as the
vigilant sentinel of your electrical network. It’s tasked with identifying
potential issues and initiating preventative measures to avert harm. Picture a
light switch equipped with a safeguard. A standard switch toggles the light on
or off, whereas a switch integrated with a protective relay may include a fuse.
Should the current exceed the designated limit for the switch (akin to a bulb
drawing more power than its rating), the fuse would activate, severing the
electrical flow to forestall a potential blaze.
How it works:
- Surveillance:
The relay perpetually scrutinizes parameters such as the electrical
current coursing through a conductor.
- Recognition:
Upon detecting an abnormal surge in current (indicative of a potential
short circuit), the relay is alerted to an anomaly.
- Intervention:
Subsequently, the relay engages a mechanism (comparable to activating a
circuit breaker) to disrupt the electrical supply and segregate the
affected zone. This precautionary step shields the conductors and apparatus
from thermal overload and consequent damage.
To put it in perspective: leaving a
cooking pot unattended on a high flame could lead to overheating and possibly
ignite a fire. A smoke alarm (mirroring the relay’s function) would detect the
smoke (analogous to the excessive current) and set off the alert (similar to
the breaker’s trip function) to notify you (and interrupt the electrical
circuit) to mitigate the risk of a more severe incident.
MCQ on protective relay. Answers are given at the end:
- Q: What is the main role of a protective relay in
electrical systems?
- A) To isolate the faulty section
- B) To trip the circuit breaker
- C) To provide power supply
- D) To monitor power usage
- Q: Which type of relay is specifically designed to
detect faults within a particular zone?
- A) Distance relay
- B) Overcurrent relay
- C) Differential relay
- D) Voltage relay
- Q: Feeder protection relays are primarily used for what
kind of fault protection?
- A) Underfrequency
- B) Overvoltage
- C) Overcurrent
- D) Reverse power flow
- Q: What feature allows an overcurrent relay to be set
for specific fault currents and response times?
- A) Reset mechanism
- B) Adjustable settings
- C) Trip coil
- D) Auxiliary contacts
- Q: What is a drawback of using ground fault relays?
- A) They can’t detect phase faults
- B) They are too sensitive
- C) They are ineffective for high-impedance ground
faults
- D) They have a slow response time
- Q: Why do ground fault relays have a time delay
feature?
- A) To prevent nuisance tripping
- B) To allow for device coordination
- C) To save energy
- D) To increase sensitivity
- Q: Ground fault relays detect faults by measuring which
type of current?
- A) Direct current (DC)
- B) Alternating current (AC)
- C) Zero-sequence current
- D) Phase current
- Q: What is a significant benefit of digital protective
relays over analog ones?
- A) Lower cost
- B) Simpler design
- C) Communication capabilities
- D) No need for calibration
- Q: Current transformers (CTs) are used in relay
operations to:
- A) Increase current levels
- B) Step down current levels
- C) Convert AC to DC
- D) Provide isolation
- Q: Which relay is utilized to safeguard generators from
imbalanced loads?
- A) Voltage relay
- B) Frequency relay
- C) Differential relay
- D) Phase relay
- Q: Distance relays operate by measuring what parameter
to identify faults?
- A) Voltage drop
- B) Current rise
- C) Impedance
- D) Power factor
- Q: Overcurrent relays detect faults based on:
- A) Temperature increase
- B) Current exceeding set limits
- C) Voltage fluctuations
- D) Frequency deviations
- Q: The purpose of a directional relay is to:
- A) Measure voltage direction
- B) Detect the direction of fault currents
- C) Control the direction of power flow
- D) Indicate the direction of load movement
- Q: Voltage transformers (VTs) are paired with
protective relays to:
- A) Step up voltage levels
- B) Step down voltage levels
- C) Convert DC to AC
- D) Regulate voltage
- Q: An instantaneous overcurrent relay is characterized
by:
- A) Delayed tripping
- B) Tripping without intentional delay
- C) Gradual tripping
- D) Periodic tripping
- Q: Buchholz relays are capable of detecting:
- A) External transformer faults
- B) Internal transformer faults
- C) Transformer overloads
- D) Transformer underloads
- Q: The function of a recloser in power distribution is
to:
- A) Disconnect power permanently
- B) Automatically close the breaker after a fault
- C) Reduce power consumption
- D) Increase power efficiency
- Q: Backup protection is typically provided by which
type of relay?
- A) Distance relay
- B) Voltage relay
- C) Overcurrent relay
- D) Frequency relay
- Q: The ‘pick up’ setting of a relay refers to:
- A) The minimum voltage required
- B) The value above which the relay activates
- C) The energy consumption rate
- D) The physical weight of the relay
- Q: ‘Trip circuit supervision’ ensures that:
- A) The trip circuit is energy efficient
- B) The trip circuit is continuously operational
- C) The trip circuit is easily accessible
- D) The trip circuit is cost-effective
Answers:
- B) To trip the circuit breaker
- C) Differential relay
- C) Overcurrent
- B) Adjustable settings
- C) They are ineffective for high-impedance ground
faults
- B) To allow for device coordination
- C) Zero-sequence current
- C) Communication capabilities
- B) Step down current levels
- C) Differential relay
- C) Impedance
- B) Current exceeding set limits
- B) Detect the direction of fault currents
- B) Step down voltage levels
- B) Tripping without intentional delay
- B) Internal transformer faults
- B) Automatically close the breaker after a fault
- C) Overcurrent relay
- B) The value above which the relay activates
- B) The trip circuit is continuously operational
Some Mathematical Problems of Protective Relay:
1. Problem:
Determining Plug Setting Multiplier (PSM) A protective relay with a
rated current of 5 A is set to operate at 125% of the rated current. Calculate
the Plug Setting Multiplier (PSM).
Solution:
2. Problem:
Calculating Time Setting Multiplier (TSM) Given a relay with a
standard inverse time characteristic and a time dial setting of 0.7, calculate
the Time Setting Multiplier (TSM) if the fault current is 8 times the rated
current.
Solution: For a standard inverse time characteristic, the TSM can be calculated as:
3. Problem:
Determining the Operating Time of a Relay A relay has an inverse time
characteristic with a formula ( T = TSM \times \left(\frac{M}{PSM}\right)^{-1}
), where ( T ) is the operating time, ( TSM ) is the time setting multiplier, (
M ) is the multiple of rated current, and ( PSM ) is the plug setting
multiplier. If ( TSM = 0.1 ) and ( PSM = 2 ), calculate the operating time for
a fault current that is 10 times the rated current.
Solution:
4. Problem:
Calculating the Required PSM for a Desired Operating Time If a relay
with a definite time characteristic requires an operating time of 2 seconds for
a fault current of 10 times the rated current, and the TSM is 0.2, calculate
the required PSM.
Solution:
5. Problem:
Determining the Fault Current from PSM and TSM A relay with a TSM of
0.3 and a PSM of 1.5 is set to trip for a certain fault condition. Calculate
the fault current if the rated current is 5 A.
Solution: we can solve for the
fault current:
0 Comments