Power transformers

The interconnecting transformer (ICT) of a pooling substation steps the grid's high voltage (HV) down to the medium voltage (MV) of the plant's collector bus. At a switching substation, the ICT connects two HV yards. The design sets the ICT's number and rating from the plant, and its impedance, vector group, tap changer and cooling from the voltage level and the standards set.

Read first: Bays and their equipment · Circuit breakers and isolators

1Number and ratingCopy link to Number and rating

The design takes 2 ICTs on Auto. With two units, one unit carries half the plant's output when the other is out of service. Each unit is rated for a share of the plant's apparent power with a margin. The plant's MVA is its MW over a power factor of 0.95.

The design rates each of the $n$ units for the plant's MVA times 1.1 over $n$. The rating is the next standard size of the level at or above that value (Figure 1). The user can enter the count or the rating, for example to match a client's specification.

The standard sizes, the capacity range and the impedance belong to the voltage level. A plant outside the range raises a warning, and the design still runs.

2The 400 kV split low-voltage unitCopy link to The 400 kV split low-voltage unit

At 400 kV the design takes units of 315 MVA with two 33 kV windings, 400/33/33 kV, each winding of 157.5 MVA. Each winding feeds its own MV bus section, so the MV bus has two sections per ICT.

The number of units is the plant's MVA times 1.1, divided by the unit rating and rounded up, between two and four units. The default 400 kV project of 750 MW needs 868.4 MVA, which gives 3 units and 6 MV sections. The short-circuit study models each winding as a two-winding transformer of half the rating at the unit's impedance.

3ImpedanceCopy link to Impedance

The ICT's impedance sets the fault current the grid feeds into the MV bus through it. The design takes the level's impedance from the table above. An autotransformer at a switching substation takes 12.5 %, and a two-winding unit there 10 %. The user can enter the transformer's own X/R ratio and zero-sequence ratio for the short-circuit study.

The ANSI / IEEE set takes the impedance of a transformer with load tap changing from RUS Bulletin 1724E-300, by the transformer's HV BIL: 8.5 % at 69 kV, 9.5 % at 115 kV, 10 % at 138 kV and 10.5 % at 161 kV. The bulletin's impedance table ends below the BIL of the two highest levels. At 230 kV and 345 kV the design takes the nearest IEC level's, 12.5 % and 14 %.

The bulletin gives the impedance at the self-cooled rating, and the design applies it at the top rating. On the top rating's base that impedance is lower than the transformer's own. A lower impedance raises the fault currents the MV switchgear is checked against.

4Vector group and neutral earthingCopy link to Vector group and neutral earthing

The vector group names the connection of each winding and the phase shift between them. The design's ICT at 66 kV to 220 kV is YNyn0 + d: star HV and star MV windings, both with their neutrals brought out, and a delta tertiary. The split unit at 400 kV is YNyn0yn0 + d. A 33/11 kV transformer is Dyn11, with a delta HV winding and no HV neutral.

The ICT's HV neutral is solidly earthed. Its MV neutral is earthed through a neutral grounding resistor (NGR), one per low-voltage winding, of 1000 A for 10 s by default. The resistance is the MV phase voltage over that current, 19.05 Ω at 33 kV. A surge arrester protects each low-voltage terminal.

The ANSI / IEEE set prints the vector groups in its own convention. A standard delta-wye step-down transformer has its high-side voltages leading the low side by 30 degrees (RUS Bulletin 1724E-300). So the set prints Dyn1 on the station service transformers and YNa0d1 on the autotransformers with a delta tertiary, and YNyn0 + d stays.

5On-load tap changerCopy link to On-load tap changer

The design's ICT has an on-load tap changer (OLTC) of ±10 % in steps of 1.25 %, 16 steps over the range. The ANSI / IEEE set calls it a load tap changer (LTC).

6CoolingCopy link to Cooling

The IEC + CEA / CBIP set prints the ICT's cooling as ONAN/ONAF: oil natural, air natural, and oil natural, air forced.

The ANSI / IEEE set sizes the top rating and prints the ratings of each cooling stage. RUS Bulletin 1724E-300 gives two stages of forced air above the self-cooled rating, each adding 33 % of the self-cooled rating. A 168 MVA unit therefore prints ONAN/ONAF/ONAF 101/135/168 MVA. At or below 10 MVA self-cooled, where the bulletin gives another rule, the design prints no stages.

7Switching substationsCopy link to Switching substations

A switching substation's ICTs connect two HV yards, and the design takes two units of a rating per pair of levels. At 400 kV and 220 kV upper levels they are autotransformers with a loaded delta tertiary. A 132 kV upper level takes two-winding units. The neutrals are solidly earthed, with no NGR.

The tertiary is 33 kV at 400 kV and 11 kV at 220 kV. It feeds the auxiliary transformers of the substation.

8The auxiliary transformerCopy link to The auxiliary transformer

The auxiliary transformer supplies the substation's own loads at low voltage. A pooling substation has two in total, on MV sections 1 and 2, one on duty and one on standby through a changeover. The default 220 kV project takes 2 × 250 kVA, 33/0.433 kV.

The rating follows the substation's load list: the maximum demand with a margin, rounded up to the next standard size. At a switching substation the auxiliary transformers sit on the ICT tertiaries, 33/0.433 kV at 400 kV, with an 11 kV distribution feed as the back-up. Where the ICTs have no tertiary, they take the 11 kV feed, 11/0.433 kV. The ANSI / IEEE set takes 34.5/0.48 kV, a 480Y/277 V station service.