In power system protection, relays must be precisely calibrated to distinguish between normal load fluctuations and genuine abnormal conditions like faults or extreme overloads. Two fundamental parameters govern this behavior: Current Setting and Time Setting. Together, they ensure the relay operates accurately and maintains “selectivity” (coordination) with other protective devices.
Table of Contents
Current Setting
The Current Setting, also known as the Plug Setting or Pickup Current, is the minimum current threshold at which the relay initiates its operation. Since relays cannot handle high-voltage line currents directly, they operate via a Current Transformer (CT).
The relay’s secondary pickup current is determined by the plug setting percentage, which then relates back to the primary line current:
Example
Given:
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CT Ratio: 400:1
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Plug Setting: 125%
Calculation:
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Secondary Pickup: $1 \text{ A} \times 1.25 = 1.25 \text{ A}$
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Primary Pickup: $1.25 \text{ A} \times 400 = 500 \text{ A}$
Result: The relay will trigger once the line current exceeds 500 A.
Time Setting (TMS/TDS)
The Time Setting dictates the delay between fault detection and the sending of a trip signal to the circuit breaker. This delay is vital for coordination—ensuring the relay closest to the fault trips first before the backup relays intervene.
In Inverse Definite Minimum Time (IDMT) relays, the operating time is influenced by the magnitude of the fault and the Time Multiplier Setting (TMS).
The actual operating time ($T_{op}$) is calculated by scaling the reference time from the relay’s characteristic curve:
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$T_{op}$: Actual operating time.
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$TMS$: Time Multiplier Setting (usually a value between 0.1 and 1.0).
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$T_{curve}$: The standard time value taken from the relay’s specific curve at a given Plug Setting Multiplier (PSM).
Example
If the relay’s characteristic curve indicates an operation time of 2 seconds for a specific fault, and the TMS is set to 0.5:
Importance of Proper Settings
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Reliability: Ensures the relay only trips when a real fault occurs.
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Selectivity: Prevents “sympathetic tripping” of healthy circuits.
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Equipment Safety: Minimizes thermal and mechanical stress on transformers and conductors by clearing faults quickly.