Research on 3 Phase Overload Protection Technology

1. Theoretical analysis of overload capacity
The overload capacity of three-phase transformers is mainly constrained by the following parameters:
Thermal time constant (τ): The typical value for oil immersed transformers is 120-180 minutes, while for dry-type transformers it is 30-60 minutes
Insulation material thermal grade: Significant difference in allowable temperature rise from Class A (105 ℃) to Class H (180 ℃)
Load loss ratio (Pk/P0): The national standard GB/T 6451 specifies that the short-circuit impedance Uk% directly affects the dynamic overload capacity
Experimental data shows that under 1.3 times the rated load, the top oil temperature rise of oil immersed transformers is exponentially related to time t
Δθoil = Δθ∞(1-e^(-t/τ))
Among them, Δ θ ∞ is the steady-state temperature rise, with a typical value not exceeding 65K (IEC 60076-7)
2. Design of critical protection systems
Modern protection systems adopt a multi-level protection architecture:
Primary protection
Differential protection (87T): action threshold set to 1.2In, time delay of 100ms
Overcurrent protection (51): The inverse time characteristic curve complies with IEC 60255 standard
Secondary protection
Fiber optic temperature measurement system: Real time monitoring of hotspot temperature (θ h) with an accuracy of ± 1 ℃
Dissolved Gas Analysis (DGA): Trigger warning when detecting C2H2 content>1 μ L/L
3. Collaborative control of cooling system
Control strategy of forced oil circulation cooling (OFAF) system during overload:
Q_{cool} = k \cdot (θ_{oil} - θ_{amb}) \cdot v^{0.8} 
Where k is the heat dissipation coefficient and v is the oil flow velocity (m/s). Suggest using a variable frequency pump to dynamically adjust the flow rate, which can increase the overload duration by 15% -20%
(Note: All technical parameters are referenced from IEEE C57.91-2018 guidelines)