E . f n Bmax A4 44= × × × ×EQUATION898 V2 EN (Equation 76)The relative excitation M is therefore according to equation 77.( ) ( ) ( )E fM p.u. = Ur frIECEQUATION2296 V1 EN (Equation 77)Disproportional variations in quantities E and f may give rise to core overfluxing. Ifthe core flux density Bmax increases to a point above saturation level (typically 1.9Tesla), the flux will no longer be contained within the core, but will extend into other(non-laminated) parts of the power transformer and give rise to eddy currentcirculations.Overexcitation will result in:• overheating of the non-laminated metal parts• a large increase in magnetizing currents• an increase in core and winding temperature• an increase in transformer vibration and noisePotection against overexcitation is based on calculating the relative volt per hertz (V/Hz) ratio. The protection might initiate a reduction of the generator excitation (in caseof a step-up transformer), and if this fails, or if this is not possible or implemented theTRIP signal will disconnect the transformer from the source after a delay ranging fromseconds to minutes, typically 5-10 seconds.Overexcitation protection may be of particular concern on directly connectedgenerator unit transformers. Directly connected generator-transformers are subjectedto a wide range of frequencies during the acceleration and deceleration of the turbine.In such cases, OEXPVPH (24) may trip the field breaker during a start-up of amachine, by means of the overexcitation ALARM signal. If this is not possible, thepower transformer can be disconnected from the source, after a delay, by the TRIPsignal.The IEC 60076 - 1 standard requires that transformers operate continuously at notmore than 10% above rated voltage at no load, and rated frequency. At no load, theratio of the actual generator terminal voltage to the actual frequency should not exceed1.1 times the ratio of transformer rated voltage to the rated frequency on a sustainedbasis, see equation 78.Ef---- 1.1£ Urfr------×EQUATION900 V1 EN (Equation 78)or equivalently, with 1.1 · Ur = V/Hz> according to equation 79.Section 9 1MRK 504 135-UEN AVoltage protection224Technical manual
