Difficulty: Easy
Correct Answer: Instrument transformers (current and potential transformers)
Explanation:
Introduction / Context:Power systems require devices that simultaneously support accurate metering and reliable protection. Instrument transformers scale high currents and voltages to safe, measurable levels and provide galvanic isolation. They are indispensable at the interface of protection relays and revenue-grade meters.
Given Data / Assumptions:
Concept / Approach:Protection requires reliable reproduction of system quantities during faults (high primary values, transient behavior), while metering needs accuracy near nominal conditions. Special classes of instrument transformers (e.g., metering-class CTs, protection-class CTs) address each need; in practice, both protection relays and meters are fed by instrument transformers, sometimes dedicated, sometimes shared via separate cores.
Step-by-Step Solution:
Recognize that wattmeters, energy meters, and PF meters measure quantities but cannot directly connect to high primary levels safely.Instrument transformers step down quantities and isolate circuits, serving both metering inputs and protection relays.Therefore they are the common element used for both purposes.Verification / Alternative check:
Typical substation schematic shows CTs feeding both relays and meters via separate secondary cores.Why Other Options Are Wrong:
Wattmeters, energy meters, and PF meters are metering devices only; they do not provide isolation or scaling for protection.Common Pitfalls:
Confusing the measuring instruments themselves with the auxiliary transformers that enable safe connections.Final Answer:
Instrument transformers (current and potential transformers)
Discussion & Comments