Transformer protection, selection and vector group

Unit protection scheme of a power transformer

The main unit protection of a power transformer is percentage-biased differential protection (87T), backed by restricted earth fault, Buchholz and others. It compares the currents entering and leaving the transformer. Inside the zone the difference trips the relay; for through faults the currents balance.

   HV bus         Power transformer        LV bus
  ---[CT1]-----+====( Yd11 )====+-----[CT2]---
       |       |   Buchholz(63) |        |
       |      REF(64)           |        |
       |                                 |
       +---> [ 87T  bias + operate ] <---+
               2nd harmonic block
               (inrush), 5th (overflux)
                    |
               Master trip 86 -> HV & LV CBs

Points to note:

• CT ratios and connections (or software settings) correct for the ratio and the phase shift of the vector group (for Yd11, CTs on the Y side connected in delta, or numerical compensation).
• Bias (restraint) handles CT mismatch and tap-changer range.
• 2nd harmonic restraint blocks tripping on magnetising inrush. 5th harmonic blocking is used for overfluxing.
• Supplementary protections: REF (64), Buchholz (63), oil/winding temperature (26/49), pressure relief (63PR), overcurrent and earth-fault backup (50/51, 51N), overfluxing (24).

Selection criteria for a power transformer

• Rated MVA: from generator rating (e.g. generator MVA plus margin), with cooling class ONAN/ONAF/OFAF.
• Voltage ratio: generator voltage to transmission voltage (e.g. 11/132 kV); tap changer range (OLTC or off-circuit, e.g. ±10 % in 1.25 % or 2.5 % steps).
• Vector group to suit the system and earthing (GSU usually YNd11 with HV star grounded).
• Impedance (% Z): balance between fault level limit and voltage regulation/stability; usually 10–14 % for GSU.
• Number of phases: three-phase unit or a bank of single-phase units (transport limits on hilly roads in Nepal).
• Insulation level: BIL and power-frequency withstand.
• Losses (no-load and load), efficiency and capitalised loss cost.
• Cooling, temperature rise, altitude derating, noise, transport size and weight.
• Standards: IEC 60076.

What the vector group reflects

The vector group (e.g. YNd11, Dyn11, Yy0) tells:

1. The winding connection of HV (capital letter: Y, D, Z) and LV (small letter: y, d, z).
2. Whether the neutral is brought out (N or n).
3. The phase displacement of LV with respect to HV by the clock number, each hour being $30^\circ$. For example "11" means LV leads HV by $30^\circ$ (11 o'clock position), and "1" means LV lags by $30^\circ$.

It is essential for parallel operation (only transformers with the same phase shift can be paralleled), for differential CT compensation, and for earthing and the third-harmonic path (a delta winding traps triplen harmonics).