Power Transformer

Could a 400/33kV 120MVA power transformer with a YNyn0 vector group be applicable to a renewable generation project as the main transformer?

Im concerned that the vector group is a star-star winding that could cause zero sequence impedance, unbalanced loads and Harmonics issues. Could any of the potential issued be mitigated by introducing an NER or Earthing Transformer?

  • Is the renewables project stepping up from the low side to the high and  how is the renewable source organized -
    1) does it have a neutral capable of sourcing and sinking significant current for a start ?

    2) what sort of waveforms are expected - is it just rectangular current pulses, resonant conversion or something  more sophisticated?

    At  least in the UK HV transmission is normally neutral free, and to that end star - delta transforming is used and as well as making for easier wiring, it also means that 3rd order harmonics run round the delta but don't get sent out over the wire.

    Generally NER wont help supress harmonics on its own, but suitably placed inductance might.


  • Yes  the energy park will be the 33kV side and will step up to 400kV for connection to the transmission system. 

    1) The Neutrals will be Solidly grounded, built to IEC 60076 / IS 2026.

    2) The wind turbine rectifies the wave to 50Hz sin wave. There may be an opportunity to introduce reactive compensation and a harmonic connection at 33kV. Further system studies would need to be undertaken.

  • will step up to 400kV for connection to the transmission system

    That feels rather unusual. I thought that very little got direct connections to the supergrid in the UK - is this a massive project? or outside of the UK?

    Most of the stuff around here gets connected to the 11kV distribution system.

       - Andy.

  • Renewable energy projects connecting to 132kV and above are common. Stepping from 33kV to 400kV is not so common but it still works. 

    The project is above 100MW connecting into NGET so not huge in today's terms. 

    • Because of the direct connection to the EHV transmission system, a high impulse strength is required
    • The transformer must produce zero-phase displacement
    • Star-Star fails to meet the requirement that the transformer should have one winding connected in the delta to eliminate third-harmonic voltages.
    • Star-star is seldomly used because of difficulties with the third harmonic exciting current
    • The station transformer is almost invariably star/star connected since both HV and LV windings must provide a neutral connection to Earth.
    • Until the late 1970s, a star/star-connected station transformer would automatically have been provided with a delta-connected tertiary for the elimination of the third harmonic. However, as auxiliary systems and the transformers feeding them became larger, fault levels increased and it became clear these could be effectively reduced and third harmonic remain at acceptable levels if a three-limb transformer without a tertiary winding was used.
    • If the tertiary is omitted, zero-sequence impedance will be greatly increased and it is necessary to be sure that, in the event of an 11 kV system line-to-earth fault, there will be sufficient fault current to enable the protection to operate. Works testing of the Littlebrook station transformer showed that the actual value of zero-sequence impedance was low enough to meet the auxiliary system protection requirements. It was also necessary to ensure that the absence of a tertiary would not give rise to excessive third-harmonic currents circulating in the system neutral. Such currents flow whenever the system has more than one neutral earthed where an auxiliary gas turbine generator with its neutral earthed is operated in parallel with the station transformer supply, thus setting up a complete loop for circulating currents.
    • The impedance of this loop to third-harmonic currents can be increased by connecting a third-harmonic suppressor in series with the gas turbine earth connection. This is an iron-cored reactor whose design flux density is carefully chosen to be fully saturated at 50 Hz, thus having a low impedance at normal supply frequency, whereas at 150 Hz it operates below the knee point and, being unsaturated, has a high impedance, effectively equal to the magnetising reactance.
    • To ensure that protection problems are not encountered when deciding to omit the tertiary from a star/star transformer it is good practice to specify that the zero-sequence impedance should fall within a band from, say, 0.9 to 6 times the positive-sequence value.