Alexander Dierks – Alectrix (Pty) Ltd, South Africa Innovative Test Techniques & Diagnostic Measurements to Improve the Performance and Reliability of Power System Transformers Dr. Michael Krüger, Alexander Kraetge – Omicron Electronics GmbH, Austria Alexander Dierks – Alectrix (Pty) Ltd, South Africa 9/17/2018

The Switch Quiz Room with one light Three switches outside room Which switch operates the light? You are only allowed to open and enter the room ONCE! 9/17/2018

The Switch Quiz Turn Switch 1 & 2 ON and wait Turn Switch 2 OFF, enter the room and check: If light is ON  Switch 1 If light is OFF and bulb is warm  Switch 2 If light is OFF and bulb is cold  Switch 3 9/17/2018

Sources of Transformer Faults 9/17/2018

Classical Tests of Transformers Ratio Measurement (Tap changer dependant) Excitation Current Measurement (Open Circuit Impedance) Short Circuit Impedance Winding Resistance (static) 9/17/2018

New Test Techniques: Summary Onload Tapchanger (OLTC) Testing SFRA Measurement Capacitance and Tan Delta Measurement Di-electric Response Measurement Partial Discharge Measurement 9/17/2018

Functioning of Tap changer 9/17/2018

OLTC Principle (1) A B Tap Selector Diverter Switch Diverter Resistors 9/17/2018

OLTC Principle (2) A B Tap Selector Diverter Switch Diverter Resistors 9/17/2018

OLTC Principle (3) A B Tap Selector Diverter Switch Diverter Resistors 9/17/2018

OLTC Principle (4) A B Tap Selector Diverter Switch Diverter Resistors 9/17/2018

OLTC Principle (5) A B Tap Selector Diverter Switch Diverter Resistors 9/17/2018

OLTC Principle (6) A B Tap Selector Diverter Switch Diverter Resistors 9/17/2018

Measurement of Winding Resistance Measuring Lines for I Measuring Lines for U 9/17/2018

New Voltage Selector Contact 9/17/2018

Testing of a new 500MVA Transformer 9/17/2018

220:110 kV 100MVA TRF: A-Phase 9/17/2018

Faulty Tap Selector Contact 9/17/2018

Winding Resistance A-Phase After Repair 9/17/2018

Delta R for U-Mp „UP“-„DOWN“ Before repair After repair 9/17/2018

Transient Current during Switching Process 3 A 1 Slope  Ripple 4 2 1 = Diverter switch switches to the first commutating resistor 2 = Both commutating resistors are in parallel 3 = Final contact of the diverter contact B is reached 4 = Current control of the CPC 100 regulates the current to the nominal test current again 9/17/2018

Ripple of a Diverter Switch in a Good Condition (1100MVA) 9/17/2018

Ripple of an Aged Diverter Switch 9/17/2018

Aged Diverter Switch 9/17/2018

New Test Techniques: Summary Onload Tapchanger Testing SFRA Measurement Capacitance and Tan Delta Measurement Di-electric Response Measurement Partial Discharge Measurement 9/17/2018

Transformer Model for SFRA 9/17/2018

Sweep Frequency Response Analysis (SFRA) Magnitude Phase This slide explains the so-called sweep frequency method. In contrast to the impulse method, here we work with a sinusoidal signal, which is swept between 10 Hz and 10/20 kHz (or more). Vin – represented in red – is the input signal, which is standardized to 1 or 100%. We sweep the signal over the whole frequency range and measure the signal Vout - represented in blue - at the other end of the winding. We see that the blue signal (output signal) compared to the red signal (input signal) shows both a damping as well as a phase shift. The transfer function (H) in dB is the result of the calculation according the shown formula: H(dB) = .... Advantages of the sweep FRA method: The input amplifiers of the equipment can be realized with a very small bandwidth; by that it can be avoided that other distortion signals superimpose the measurement signal Over the whole frequency range we have a constant amplitude of the oscillator signal and by that no limitation of / a constant energy at higher frequencies. This allows a more sensitive measurement of particularly at higher resonance frequencies. 9/17/2018

SFRA Measurement: Test Set Up This slides shows the standard FRA measuring method: The yellow cable (generator output) is connected to the beginning of the winding, with the red cable the injected voltage is measured back to compensate wiring influences (reference channel). With the blue cable (output) the damped signal at the other end of the same winding is fed back to equipment to the measurement input. This is the standard FRA method which should be applied as a minimum for all FRA measurements. The measurement is performed for each phase: If a transformer has two voltage levels this means that six measurements are performed, if we have a three-winding transformer (upper, medium, and low voltage winding), nine such measurements are necessary. 9/17/2018

SFRA: Healthy transformer 33kV 10MVA 9/17/2018

SFRA: Damaged Winding 400kV 700MVA 9/17/2018

What can SFRA detect? Fingerprint (Reference Measurement) is important!!! Geometrical changes in the core and/or windings due to transport or electrical faults: Core deformation / Core movement (clamping) Faulty core groundings Fault on magnetic cores (shorted laminates) Bulk winding movement Coil deformation - axial & radial Buckling of windings Broken or open internal connections Inter-winding / Inter-turn short circuits (list is self explaining) Bucklings: Certain kind of winding deformation (see picture later in this presentation) Winding faults: Can also be localized in some cases 9/17/2018

Radial Deformation (Buckling) Source: G. Bertagnolli, Short Circuit Duty of Power Transformers, ABB Trasformatori, Legnano 9/17/2018

Damaged Winding (Axial Movement) 9/17/2018

Grounding of the Measurement Cables Important for reproducible measurements Braids for a low-inductive connection cable screen - flange This slides shows the standard FRA measuring method: The yellow cable (generator output) is connected to the beginning of the winding, with the red cable the injected voltage is measured back to compensate wiring influences (reference channel). With the blue cable (output) the damped signal at the other end of the same winding is fed back to equipment to the measurement input. This is the standard FRA method which should be applied as a minimum for all FRA measurements. The measurement is performed for each phase: If a transformer has two voltage levels this means that six measurements are performed, if we have a three-winding transformer (upper, medium, and low voltage winding), nine such measurements are necessary. 9/17/2018

New Test Techniques: Summary Onload Tapchanger Testing SFRA Measurement Capacitance and Tan Delta Measurement Di-electric Response Measurement Partial Discharge Measurement 9/17/2018

C / Tan Delta Progression of a Bushing Source: RWE 9/17/2018

Bushing Fault Source: RWE 9/17/2018

Bushing after Explosion Source: RWE 9/17/2018

Tan Delta / Dissipation Factor Bushings: Aging and decomposition of insulation Water content Bad contacted electrodes or capacitive layers Cracks in the insulation Transformers: Aging Water content in oil and paper Contamination by particles 9/17/2018

Capacitance Measurement Bushings: Partial breakdowns between layers Oil ingress into cracks of solid insulation (RBP) Change of Geometry between: Transformer windings Windings and tank Windings and core Core and tank 9/17/2018

CPC 100 / TD 1 Tan-delta & Capacitance Measurement 9/17/2018

220kV RIP Bushing Stored Outside 9/17/2018

220kV RIP Bushing Stored Outside Minimum Minimum Minimum 9/17/2018

New Test Techniques: Summary Onload Tapchanger Testing SFRA Measurement Capacitance and Tan Delta Measurement Di-electric Response Measurement Partial Discharge Measurement 9/17/2018

Design of the Winding Insulation Measurement of Water Content in Paper & Pressboard Source: Weidmann AG 9/17/2018

Frequency Domain Spectroscopy (FDS) X (t) N Z 1 2 C X, L Reference path Measurement I CN CX 9/17/2018

PDC Analysis Polarisation-Depolarisation-Current Principle Recording of current 9/17/2018

Dirana: Combination of PDC and FDS FDS  11 h Meas. Time PDC  Meas. only up to 1 Hz 0.1 Hz – 5 kHz 0.1 mHz – 0.1 Hz FDS+PDC  0.1 mHz – 5 kHz in less than 3 hours meas. time 9/17/2018

PDC up to 0.1Hz und FDS above 0.1Hz 9/17/2018

FDS / PDC Measurements on a 800 kVA Distribution Transformer 1969 Furan content: 12.27 mg/kg !!  DP  130 Decision: transformer was taken out of service 9/17/2018

FDS curves for different temperatures FDS / PDC Measurements on a 800 kVA Distribution Transformer FDS curves for different temperatures 50°C 25°C 12°C 9/17/2018

FDS / PDC Measurements on a 800 kVA Distribution Transformer 9/17/2018

New Test Techniques: Summary Onload Tapchanger Testing SFRA Measurement Capacitance and Tan Delta Measurement Di-electric Response Measurement Partial Discharge Measurement 9/17/2018

Fault on a 25 MVA Furnace Transformer 9/17/2018

Overpressure valve released 200l of oil Fault on a 25 MVA Furnace Transformer Overpressure valve released 200l of oil 9/17/2018

DC insulation resistance Fault on a 25 MVA Furnace Transformer DC insulation resistance 9/17/2018

Fault on a 25 MVA Furnace Transformer Taps No-load current Taps 9/17/2018

Fault on a 25 MVA Furnace Transformer Z0 (f) = R0 (f) + j X0 (f) 9/17/2018

Fault on a 25 MVA Furnace Transformer FRA (log view) 9/17/2018

Fault on a 25 MVA Furnace Transformer Faulty B phase FRA (lin view zoomed) 9/17/2018

Partial discharge measurement Fault on a 25 MVA Furnace Transformer Partial discharge measurement Optical fiber MPD 600 Measuring electrode 9/17/2018

Partial discharge measurement Fault on a 25 MVA Furnace Transformer Partial discharge measurement 9/17/2018

Fault on a 25 MVA Furnace Transformer Opened transformer 9/17/2018

Fault on a 25 MVA Furnace Transformer Melted screw 9/17/2018

Melted steel with copper marks Fault on a 25 MVA Furnace Transformer Melted steel with copper marks 9/17/2018

Conclusions / Summary Defective contacts of the OLTC can be detected by measuring the static winding resistance over all taps up- and down. Switching problems of the diverter switch of the OLTC can be found through dynamic resistance measurements. The Sweep Frequency Response Analysis (SFRA) is a powerful tool for detecting winding displacements, winding deformations and faulty core and screen connections. Reproducible connection techniques are important to ensure reliable and reproducible measurements. Tan Delta frequency sweeps are much more sensitive for investigating insulation problems compared to measurements at 50Hz only. Polarization / Depolarization Current (PDC) and Frequency Domain Spectroscopy (FDS) measurements are useful tools for determining the water content in cellulose for deciding on further actions (e.g. drying). Partial Discharge (PD) measurements are an important tool for the troubleshooting of insulation problems. 9/17/2018

Thank you for your interest ! 9/17/2018