Instrument Transformers PT CT Main Tasks and Errors

What are the main tasks of instrument transformers?

Transforming currents or voltages from a high value to a value easy to handle for relays and instruments (correct)
Providing possibilities of standardizing the relays and instruments to a few rated currents and voltages
Transforming currents or voltages from a low value to a high value for relays and instruments
Insulating relays, meters, and instruments from the primary HV system

Which of the following is NOT a type of error in Instrument Transformers?

Phase angle error
Ratio error
Composite error
Primary current error (correct)

What is the condition for the transformation ratio in Instrument Transformers?

Transformation ratio must be less than turns ratio
Transformation ratio must be unrelated to turns ratio
Transformation ratio must be exactly equal to turns ratio (correct)
Transformation ratio must be greater than turns ratio

What is the phase displacement in Instrument Transformers?

The angular difference between primary and secondary current vectors (D) Signup and view all the answers

What is the formula for composite error in Instrument Transformers?

$εc = 100 \frac{IP}{T} (Kn - IP)^2$ (B) Signup and view all the answers

What is the expression for current error (transformation ratio error) in Instrument Transformers?

$Current\ error% = \frac{VA(KnIs - IP)}{100%IP}$ (D) Signup and view all the answers

What is the overcurrent limit factor (FS) defined as?

The ratio of rated limiting current to primary rated current. (D) Signup and view all the answers

What does the rated short-time thermal current (Ith) represent?

The max. current that can pass through CT for 0.25, 0.5, 1, 2 or 3 sec without damage its windings or insulation. (B) Signup and view all the answers

What is the accuracy limit factor defined as?

The ratio of primary rated accuracy limit current to primary rated current. (A) Signup and view all the answers

What is the thermal rated continuous current equal to?

Primary rated current. (A) Signup and view all the answers

What is the phase displacement in the context of voltage transformer (VT)?

Angular difference between primary and secondary voltage vectors. (B) Signup and view all the answers

What are the standard values for CT with extended current measuring range?

120 %, 150 % and 200 % (A) Signup and view all the answers

What does the thermal limiting output represent?

Apparent power value – based on rated voltage – that can be drawn at a secondary winding at primary rated voltage without exceeding the limit values for overtemperature. (B) Signup and view all the answers

What does the voltage error (transformation ratio error) represent?

(KVVs − VVs) ×100% VVp ×P (D) Signup and view all the answers

What is the phase displacement error limit for error class 5P?

+/- 60 (C) Signup and view all the answers

What are the CT error limits for protection classes 5P and 10P at rated primary current?

+/- 3%, +/- phase displacement (minutes), +/- 5% (C) Signup and view all the answers

What are the Measuring VT error limits for accuracy class 0.8 – 1.2 X rated voltage?

-0.5% +/-20 minutes (B) Signup and view all the answers

What does the rated peak short-circuit current (Idyn) represent?

The max. current that can pass through VT without exceeding the limit values for overtemperature (B) Signup and view all the answers

What is the primary disadvantage of GIS systems compared to conventional AIS?

Higher system cost (B) Signup and view all the answers

What problem can arise due to particle or moisture contamination inside the compartment of GIS systems?

Flash overs (C) Signup and view all the answers

In what way does the compact design of GIS systems contribute to environmental protection?

They blend well with the surroundings (C) Signup and view all the answers

What is a critical requirement for access to live parts for maintenance in GIS substations?

Gas reclamation and disassembly of modules (B) Signup and view all the answers

Why are outages at GIS substations considered extremely long?

Severe damage to the system following faults (B) Signup and view all the answers

What is a significant challenge related to the delivery of SF6 gas to GIS substation sites?

It can be problematic (C) Signup and view all the answers

Which material is typically used for the enclosures in GIS systems?

Cast aluminum (A) Signup and view all the answers

What is the ideal pressure range for SF6 gas in GIS systems?

400-600 kPa (C) Signup and view all the answers

Why is the circuit breaker usually in a separate enclosure in GIS systems?

Due to higher gas pressure for arc extinguishing (D) Signup and view all the answers

What is the typical material used for HV conductors in GIS systems?

Aluminum (D) Signup and view all the answers

What is the purpose of gas tight compartments within modules in GIS systems?

To ensure independent gas monitoring (A) Signup and view all the answers

What is the function of the gas monitoring system in GIS enclosures?

To detect leaks and trigger alarms (B) Signup and view all the answers

Why are aluminum enclosures preferred over steel enclosures in GIS systems?

Aluminum enclosures do not require painting to prevent rusting (B) Signup and view all the answers

What type of material is recommended for enclosures with continuous currents exceeding 2000 A in GIS systems?

Stainless steel (D) Signup and view all the answers

Why is it common to silver-plate the surfaces of HV conductors in GIS systems?

To improve corrosion resistance (B) Signup and view all the answers

Why is it important for the gas tight compartments to be independent of each other in GIS systems?

To ensure independent gas monitoring (D) Signup and view all the answers