c i f i c a DIgSILENT Pacific P Power system engineering and software T N Transformer saturation and reactive E power loss – a case study L Umberto Cella I DIgSILENT Pacific seminar S 28/11/2019 g I D
c i f i c a P Presentation outline • Saturation in iron core of transformers T • Saturation: • Reactive power N • Harmonics E • Case study: inverter transformers in solar farm • Reactive power generation requirement L • Tests on site I • Simulations of saturation S • Conclusions g I 2 D
c i f i c a P Saturation in iron core • Occurs when iron is “too full” (saturated) with magnetic flux: T • The material cannot magnetize more than that • The H field to obtain the same B is similar to what is needed in air N • Magnetization current increases a lot quicker Some equations: E • Flux is imposed by voltage: φ = 𝑤 𝑢 𝑒𝑢 + 𝜒 0 L • Flux in limb of transformer: φ = n ∗ B ∗ 𝐵 𝑑𝑝𝑠𝑓 (hyp.: B is uniform across core) I S • N = number of turns, A = area of core cross-section g I 3 D
c i f i c a P Saturation in iron core • Example: curve from a steel manufacturer T N E B [T] L I S g H [A/m] I 4 D
c i f i c a P Saturation in iron core • Flux is imposed by the voltage T • If the source impedance is high enough, flux is sinusoidal N • Magnetizing current is not E • There are harmonics in the current • There are harmonics in V if the source impedance is high L • In general saturation causes: I • Distortion S • Increase in reactive power absorbed by transformer From: Inrush current mitigation in three-phase g transformers with isolated neutral Ramón Cano-González, Alfonso Bachiller-Solera, José Antonio Rosendo-Macíasa, Gabriel Álvarez-Cordero I 5 D
c i f i c a P Saturation: reactive power and distortion • The current absorbed by a saturating transformer is distorted T • The 50 Hz component causes increase in Q absorption N • The harmonic components cause distortion of the voltage • Spectrum: E • The waveform is symmetrical with respect to time axis • Hence only odd harmonics L • How about inrush then? LR transient I • There is a large amount of 𝜒 0 , or residual flux S • 𝜒 0 causes the zero of the waveform to move up/down • Current is asymmetrical now: even (order 2n, n=1,2…) g harmonics can be high (“chopped” appearance) I 6 D
c i f i c a P Saturation: transformer connection • The waveform (or the harmonics) of a saturation current can change with T the connection of the windings: N • Δ : voltage (flux) is imposed on the From: https://www.electronicshub.org/wp-content/uploads/2017/07/Star-and- Delta-Connections.jpg limb; currents are added E • Y: voltage (flux) results from limb impedance, current in limb is line current L • Different waveforms of current I • Flux is in common to primary and S secondary, unless distortion is very high g I 7 D
c i f i c a P Case study: reactive power capability in solar farm • Analysis of a problem occurred to a solar farm T • Could not meet Q capability N • Saturation was suspected to cause the problem E • DIgSILENT conducted analysis of test data to find the root cause: • Test data L • PowerFactory simulations I • Conclusions S g I 8 D
c i f i c a P Reactive power capability requirement • Clause S5.2.5.1 of National Electricity Rules (NER) T • Power characteristic for a generator N • Marked point: Q>0 at max P • Ideally: characteristic for E voltage at connection point between 0.9 and 1.1 pu L • Injecting max Q at 1.1 pu makes inverter terminal voltage I rise above 1.1 pu S g I 9 D
c i f i c a P Reactive power capability test • Capability required by electricity rules (NER) T • Transmission network provider (TNSP) to check capability N • Requirement: • Demonstrate Q supply to grid capability for a point of connection voltage of 1.1 pu E • Issue: • Supplying Q increases the voltage L • The voltage at grid was high (not 1.1 pu, but as high as practical) I • The voltage at the inverters had to be even higher… S • …saturation occurred? • Question: saturation absorbed too much Q? g • Fact: solar farm could not supply agreed amount I 10 D
c i f i c a Connection of inverters to transformers P • Each inverter has its own transformer • HV: 33 kV T • LV: 575 V N • S = 2 MVA • Group: Dyn 11 E • Each pair of inverters and transformers L is connected to 33 kV solar farm cables • Cables terminate on a 33 kV bus I • 33/132 kV transformer connects bus to the S grid • Point of connection (POC) at 132 kV g I 11 D
c i f i c a P Power balance test: meter connection • P and Q measured either side of main transformer and inverter transformer T • Only one inverter and one inverter transformer are connected N • All other cables and plant disconnected E L I S g I 12 D
c i f i c a P Power balance test • P and Q measured either side of main transformer and inverter transformer T • Main trf: N E L I S g I 13 D
c i f i c a P Power balance test • P and Q measured either side of main transformer and inverter transformer T • Inverter trf: N E L I S g I 14 D
c i f i c a P Power balance test • P and Q measured either side of main transformer and inverter transformer T • Inverter trf: N • Q error high with higher Q supply E L I S g I 15 D
c i f i c a P Power balance test, repeated • Q error high with higher T Q supply and higher voltage • This indicates that the inverter N transformer changes its Q absorption significantly E • Is it saturation? • Check for distortion L • Compare Q absorption with I trf V/I no-load curves S g I 16 D
c i f i c a P V/I no-load curves • Curves relating RMS current and voltage at no-load T • Difference between type test and field N test data • Steel data and factory E test coincide • Site measurement differs L • Transformers saturating more than expected? I S g I 17 D
c i f i c a P Test at night • Transformers gradually disconnected and reconnected during the night T • To make sure that distortion: • Was there if inverters were off N • Disappeared if transformers were off • Was not due to the grid E • Test results were also compared to simulations L I S g I 18 D
c i f i c a Test at night P T N E L I S g I 19 D
c i f i c a Test at night P • P and Q T N E • V 50 Hz L I S • I 50 Hz g I 20 D
c i f i c a Test results P • The test demonstrated that: T • The transformers are absorbing more reactive power than expected N (total Q absorbed / transformer number > Q type-test • Distortion is caused by the transformers, not inverters and not grid E • It is necessary to: • Demonstrate that there is a mismatch between V/I characteristic of L installed transformer and V/I supplied in test data • Power Factory time-domain simulation of transformer with: I • declared V/I curve from type test S • modified V/I curve, where B values are increased by a factor, to model higher saturation g I 21 D
c i f i c a Simulation P T N E L I S g I 22 D
c i f i c a Simulation P T N E L I S g I 23 D
c i f i c a Simulation results P • The simulation demonstrated: T • Good agreement between model with “modified” V/I curve and N measured waveforms • That type-test V/I curve was not compatible with measured data E • Waveform simulation is a tool suitable for saturation investigation: • Waveform is closely related to curve L • Oscillations between stray capacitance and non-linear inductance can be reproduced I • Peak amplitude of current can be reproduced and checked (peak A/m) S • Eventual ferro-resonance phenomena can be predicted g I 24 D
c i f i c a Conclusion P • Transformers will be replaced – awaiting for feedback T N • Usage of extensive site tests and simulation technology: • Identified a problem E • Provided information on which all parties could discuss and agree • Made a strong case for a solution L • Questions? I S g I 25 D
c i f i c a P T N E L I S DIgSILENT Pacific g Power system engineering and software I D
Recommend
More recommend
