High voltage engineering, Network protectionPower transformer protection Overcurrent protection overcurrent relay instantaneous tripping in high-voltage side faults delayed tripping in low-voltage side faults inrush current (switching current surge) has to be taken into account differential relay Earth fault protection earthed network overcurrent relaying measurement and time delay of the neutral point current differential current relay neutral isolated network; measurement of zero-sequence voltage (delay) J.Partanen www.ee.lut.fi/fi/lab/sahkomarkkina/ 1 Network protection, Power transformer protection Winding and interturn fault protection In an interturn fault, there occur high currents in the turns experiencing an interturn short circuit. However, only small current changes can be seen outside the transformer. Buchholz gas relay also insulation failures Differential relay faster than gas relay J.Partanen www.ee.lut.fi/fi/lab/sahkomarkkina/ 2 Network protection, Power transformer protection Overload protection measurement of the oil temperature time delay with respect to the temperature rise of the winding winding temperature indicator resistance element that passes a current proportional to the load current Overvoltage protection > 10 % overvoltage damages the transformer (saturation) U/f control J.Partanen www.ee.lut.fi/fi/lab/sahkomarkkina/ 3 Network protection, Power transformer protection Transformer differential protection The protection has to be stable and sensitive: Operation: in faults inside the protection zone when switching against a fault in the protection zone when the transformer is overexcited May not operate: when switching a transformer to the grid (no fault) when the voltage is restored to normal after clearing an external fault in faults outside the protection zone when switching a parallel transformer to the grid when switching a transformer against a fault outside the protection zone J.Partanen www.ee.lut.fi/fi/lab/sahkomarkkina/ 4 Network protection, Power transformer protection P1 P2 110/20 kV P2 P1 J.Partanen www.ee.lut.fi/fi/lab/sahkomarkkina/ 5 Network protection, Power transformer protection Transformer differential protection Problems to be solved: saturation of current transformers transformer inrush current transformer overexcitation compensation of the vector group of the transformer stabilization of the operation no-load current on-load tap changer J.Partanen www.ee.lut.fi/fi/lab/sahkomarkkina/ 6 Network protection, Power transformer protection Inrush current in the transformer The iron core of the transformer saturates strongly when, under certain conditions, the transformer is connected to the network The inrush current may reach a value of 10–20 x In, 500 x I0 depends on the transformer and the instant at which the transformer is switched out (disconnected) from the network occurs always at some phase dampens in 0.1–30 s J.Partanen www.ee.lut.fi/fi/lab/sahkomarkkina/ 7 Network protection, Power transformer protection The switching current includes plenty of harmonics; compared with the excitation current of the normal state, there occurs also some 2nd harmonic. blocking based on the ratio I2/In (does not always work) blocking based on the curve form of the differential current Transformer power MVA 1 5 10 50 Supply to the high-voltage side winding 7⋅îN 5⋅îN 4⋅îN 3,5⋅îN Supply to the low-voltage side winding 12⋅îN 9⋅îN 8⋅îN 7,5⋅îN Time in which the current decreases to half / s 0.1–0.2 0.2–0.5 0.5–1.0 1.2–7.2 Measured values of the transformer inrush current. J.Partanen www.ee.lut.fi/fi/lab/sahkomarkkina/ 8 Network protection, Power transformer protection Transformer overexcitation Voltage rise of the transformer supply (load drop) Φ ∼ U/f, voltage rise causes an increase in the excitation current; an increase in harmonics in the differential current Identification from the 5th current harmonic Detection from the current curve form: During the cycle, there are two periods, during which the differential current is of the magnitude of the normal excitation current. J.Partanen www.ee.lut.fi/fi/lab/sahkomarkkina/ 9 Network protection, Power transformer protection The transformer excitation current and the current harmonics in overvoltage situation. The rated power of the transformer is 60 MVA and the rated voltage 78 kV. The transformer core material is grain-oriented steel. In the table below, U is voltage, Un rated voltage, Im transformer excitation current, In transformer rated current, I1f fundamental harmonic of the excitation current, I3f 3rd harmonic and I5f 5th harmonic. U /% Un 100 110 120 130 140 150 Im /% In <1 2 8 25 46 70 I1 f Im /% I3 f I1 f /% I5 f I1 f /% 93 76 72 76 80 84 34 67 76 73 68 62 24 48 54 44 32 22 10 J.Partanen www.ee.lut.fi/fi/lab/sahkomarkkina/ Network protection, Power transformer protection Current transformer saturation DC component there are plenty of 2nd and 3rd harmonics in the secondary current High primary current only odd harmonics In faults outside the protection zone, the current transformers of the primary and secondary may saturate at different time instants differential current Calculation of differential current right after the instant at which the polarity (sign) of the current changes removes the above problem. J.Partanen www.ee.lut.fi/fi/lab/sahkomarkkina/ 11 Protection technology, instrument transformers Saturation of the current transformer Excitation curve of the iron core non-linear and saturating knee-point voltage Saturates at a high sinusoidal overcurrent as a result of the DC component of the primary current as an effect of remanence J.Partanen www.ee.lut.fi/fi/lab/sahkomarkkina/ 12 Protection technology, instrument transformers Saturation of the current transformer Magnetic flux density / T Time / ms Magnetic flux density of the core of a current transformer, when there is a high short-circuit current including a DC component in the primary (Bs ≈ 2.1 T and J.Partanen www.ee.lut.fi/fi/lab/sahkomarkkina/ 13 Protection technology, instrument transformers Saturation of the current transformer Current / A Primary current Secondary current Time / ms Primary and secondary current of a current transformer saturated by a large DC component. The primary current is referred to the secondary of the current transformer. J.Partanen www.ee.lut.fi/fi/lab/sahkomarkkina/ 14 Network protection, Power transformer protection Vector group compensation Numerical relays: mathematical algorithms Yd and Dy compensation subtraction of phase currents problems in inrush current (phase shift ≠ 120°) and when the current transformers are saturating Delaying of samples small delay J.Partanen www.ee.lut.fi/fi/lab/sahkomarkkina/ 15 Network protection, Power transformer protection Vector group compensation by delaying the samples of the phase current and by changing the polarity (sign reversal). In the table, NS is the number of samples during one cycle. The factor –1 corresponds to the 180° phase shift of the phase current. Clock reading 0 1 2 (3) 4 5 6 7 8 (9) 10 11 Primary current Delay Factor 0 (1/12) · NS (2/12) · NS (3/12) · NS 0 0 0 (1/12) · NS (2/12) · NS (0) 0 0 1 1 1 (1) -1 -1 -1 1 1 (1) 1 1 Secondary current Delay Factor 0 0 0 (0) (2/12) · NS (1/12) · NS 0 0 0 ((3/12) · NS) (2/12) · NS (1/12) · NS 1 1 1 (1) 1 1 1 -1 -1 (1) 1 1 16 J.Partanen www.ee.lut.fi/fi/lab/sahkomarkkina/