Iec 60076-5 ((install)) -
A Helpful Guide to IEC 60076-5: Power Transformer Short-Circuit Withstand Capability
4. Short-Circuit Current Calculation (Clause 4)
The rated short-circuit apparent power at the transformer terminals is:
[ S_sc = S_r \cdot \frac100z_k ]
Where:
- (S_r) = rated power of transformer
- (z_k) = short-circuit impedance (%)
The symmetrical short-circuit current:
[ I_sc = \fracI_r \cdot 100z_k ]
The asymmetrical peak current (making current):
[ i_peak = \kappa \cdot \sqrt2 \cdot I_sc ]
Where (\kappa) depends on the X/R ratio.
1. Introduction
IEC 60076-5 is a pivotal part of the IEC 60076 series, which governs power transformers. Specifically, this part specifies the requirements for power transformers to withstand the mechanical and thermal stresses resulting from external short circuits without damage. It applies to all types of power transformers as defined in the scope of IEC 60076-1. iec 60076-5
The primary goal of IEC 60076-5 is to ensure that a transformer, after experiencing a short circuit at its terminals (or within the specified limits), remains operational and does not suffer permanent deformation, displacement, or overheating that would impair its future service.
6.2 Pass/Fail Criteria After Test
Post-test evaluation includes:
- Visual inspection: No visible deformation, buckling, or displacement.
- Measurement of short-circuit impedance: Change ≤ 2% from pre-test value.
- Measurement of no-load loss and current: No significant change.
- Dielectric tests (repeated): Must withstand specified voltage tests (e.g., induced voltage, lightning impulse).
- Dissolved Gas Analysis (DGA) and winding resistance: No evidence of hot spots or broken conductors.
2. Clamping System Design
IEC 60076-5 implicitly mandates a robust clamping system. Leading designs use:
- End rings made of high-density pressboard or steel.
- Belleville spring washers to maintain pre-stress even after winding shrinkage.
- Through-bolts with torque monitoring.
3. Key Definitions
| Term | Meaning | |------|---------| | Short-circuit current (Iₛ꜀) | RMS symmetrical current during a short circuit | | Asymmetry factor | Accounts for DC offset (√2 for worst-case making current) | | Dynamic stability | Ability to withstand peak electromechanical forces | | Thermal stability | Ability to withstand heating effect without exceeding temperature limits | | Test current | Actual applied current during short-circuit test (must be ≥75% of calculated Iₛ꜀) | A Helpful Guide to IEC 60076-5: Power Transformer
3. Conductor Selection and Reinforcement
For radial forces, manufacturers use:
- Hard-drawn copper (not annealed) for inner windings.
- Continuous transposed conductors (CTC) with epoxy bonding to prevent conductor movement.
- Interleaved windings to reduce axial force imbalance.
2. Scope
The standard covers:
- Three-phase and single-phase transformers
- All rated powers (from small distribution to large power transformers)
- Ability to withstand external short circuits without damage
- Both symmetrical and asymmetrical short-circuit currents
It does not cover:
- Instrument transformers
- Step-voltage regulators
- Traction transformers mounted on rolling stock
The Role of Symmetrical and Asymmetrical Currents
A nuanced but crucial aspect of the standard is its treatment of the DC offset component. At the moment a short circuit occurs, if the voltage waveform is at zero, the resulting current can be completely asymmetrical for the first few cycles, reaching a peak amplitude approaching ( k \times \sqrt2 ) times the RMS symmetrical current (where k can be as high as ~2.55 for a pure inductive circuit). IEC 60076-5 explicitly requires that the mechanical design withstand this first peak, while the thermal design uses the symmetric RMS current over the rated duration. This distinction is vital because forces depend on peak current, while heating depends on RMS current. (S_r) = rated power of transformer (z_k) =