RET615 provides main protection for two-winding power transformers and power generator-transformer blocks. The eight standard configurations offer comprehensive protection functions for detection and elimination of operational disturbance conditions and power transformer faults.
The standard configurations A, C, E and G are intended for power transformers with earthed HV side neutrals. The A and E configuration features low-impedance REF protection while the C and G configurations offer high-impedance REF protection. The B, D, F and H configurations fit transformers with solidlyearthed LV side neutral or the LV side with a neutral earthing resistor. The B, D, F and H configurations also fit applications including a separate earthing transformer located within the area of protection. The A, B , E, and F configurations also suit applications, where the turns ratio of the CT of the neutral earthing circuit differs from the turns ratio of the line CTs. The C, D, G and H configurations require that the turns ratios of the CTs are equal.
Standard configurations E, F, G and H include phase-voltage protection and measurement functions which provide two-stage power transformer overvoltage and undervoltage protection and/or supervision.
Two optional RTD/mA modules are available. The optional RTD/mA module offered for the standard configurations A-D allows up to eight analog signals to be measured via the six RTD inputs and the two mA inputs using transducers. The RTD and mA inputs can be used for measuring the oil temperature at the bottom and top of the transformer. Furthermore, the RTD/mA inputs can be used for measuring the ambient air temperature. The RTD inputs also offer thermal protection of dry-type power transformers fitted with Pt-100 temperature sensors. Temperature measurement over the RTD/mA inputs extends the function of the three-phase thermal overload protection of the relay. Further, one of the RTD inputs can also be used as a direct resistance measuring input for position information of the on-load tap changer. Alternatively, the tap changer position indication can be obtained via a mA-transducer.
The optional RTD/mA module offered for the standard configurations E-H allows up to three analog signals to be measured via the two RTD inputs and the one mA input using transducers. The RTD inputs can be used for measuring the oil temperature and the ambient air temperature. Temperature measurement over the RTD inputs extends the function of the three-phase thermal overload protection of the relay. For on-load tap changer, the tap changer position indication can be obtained via a mA transducer. If required, the analog temperature or tap changer position values can be sent to other devices using analog horizontal GOOSE messaging. Analog values can also be received from other devices over the station bus, thus improving the extent of relevant information.
Figure 1 illustrates the protection of a two-winding power transformer with RET615 standard configuration A. The main protection used in this application example is transformer differential protection. Additionally, restricted earth-fault protection with numerical low-impedance principle is applied on the transformer's HV side.
Figure 2 illustrates the protection of a two-winding power transformer with RET615 standard configuration D. The main protection used in this application example is transformer differential protection. Additionally, restricted earth-fault protection with high-impedance principle is applied on the transformer's LV side.
Figure 3 illustrates the protection of a two-winding power transformer with RET615 standard configuration E. The main protection used in this application example is transformer differential protection. Additionally, restricted earth-fault protection with numerical low-impedance principle and voltage based protection are applied on the transformer's HV side.
Figure 4 illustrates the protection of a two-winding power transformer with RET615 standard configuration. The main protection used in this application example is transformer differential protection. Restricted earth-fault protection with numerical low-impedance principle is applied on the transformer's HV side. The RTD inputs of RET615 are utilized for monitoring the oil temperature and oil level of the transformer. Additionally, REU615 with the standard configuration B is used as automatic voltage regulator for the on-load tap changer of the transformer.