Function, Operating Principle, and Setting Calculation of Definite Time Overcurrent Protection

I. Purpose of Definite Time Overcurrent Protection
        Backup Protection Core Function: Serves as backup for primary protection systems (e.g., differential, distance protection), operating reliably when primary protection fails or circuit breakers malfunction.
        Fault Coverage: Clears phase-to-phase short-circuit faults and severe overloads on protected lines and adjacent equipment (e.g., transformers, busbars).
        Equipment Security: Prevents thermal stability failure or insulation degradation caused by sustained fault currents.
        Selectivity Assurance: Achieves precise fault isolation through graded time coordination, avoiding over-tripping.

II. Operating Principle of Definite Time Overcurrent Protection
Activation Criteria
        Current Measurement: Real-time acquisition of phase currents (I_a,I_b,I_c).
        Threshold Comparison: Activates timing logic when any phase current exceeds the preset pickup current setting (I_op.set).

Time Coordination Mechanism
        Fixed Time Delay Characteristic: Triggers a time-delay relay upon activation, issuing a trip command after a preset operating time (t_op).
        Selectivity Principle: Operating times increase stepwise (Δt=0.3∼0.5s) toward the power source, ensuring coordination (Fig. 1).

Execution Logic
        If fault current persists beyond t_op→ Trip circuit breaker + send fault signal.
        If fault current clears before t_op→ Timer resets, protection returns.

        Key Characteristic: Operating time t_op is constant, independent of fault current magnitude (vs. inverse-time protection).

III. Setting Calculation for Definite Time Overcurrent Protection
(1) Pickup Current Setting (I_op.set)
        Basic Principle: Must discriminate against maximum load current (I_L.max):

I_op.set=K_rel*K_ss⋅(I_L.max/K_r)

        K_rel: Reliability coefficient (1.15–1.25);
        K_ss: Self-starting coefficient (motor group inrush multiplier, 1.5–3.0);
        K_r: Reset ratio (0.85 for electromechanical relays; 0.95 for digital relays).

(2) Time Setting (t_op)
Graded Time Coordination:t_n=t_n−1+Δ_t
       t_n: Operating time of current protection stage;
       t_n−1: Longest operating time of downstream adjacent protection;
       Δ_t: Time grading margin (≥0.5s for electromechanical; ≥0.3s for digital relays).

(3) Sensitivity Verification (K_sen)
       Verification point: Minimum two-phase fault current (I⁽²⁾_k.min) at line end under minimum system configuration:

K_sen=I⁽²⁾_k.min/I_op.set≥1.5

       If unsatisfied, reduce I_op.set or switch to inverse-time characteristic.

(4) Special Condition Adjustments
      Branching Factor Correction: Adjust I_op.set based on branch current ratio with infeeds;
      Directional Restraint: Apply directional element (e.g., 90°-connection power directional relay) in multi-source networks.

IV. Engineering Application Notes
      Application Scope: Medium/low-voltage distribution networks (≤35kV), radial power systems.
      Limitation: Longest operating time at source-end faults requires coordination with instantaneous overcurrent protection.
      Standard Compliance: Settings must comply with IEEE C37.112 / IEC 60255-3 and DL/T 584 (for Chinese grids).

      Note: Final settings require iterative optimization using grid parameters (fault levels, impedance ratios, load profiles) and validation via EMT simulation (e.g., PSCAD/EMTP).