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Power Transformer

MacIntyre

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?

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    • 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.

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  • 0
    • 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.

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