Importance of Transformer Demagnetization

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1 Available online at ScienceDirect Procedia Engineering 00 (2017) th International Colloquium "Transformer Research and Asset Management Importance of Transformer Demagnetization Edis Osmanbasic a *, Igor Krajisnik b a DV Power, Stockholmsvägen 18, Lidingö 18150, Sweden b a Elektroprivreda RS-IRCE a.d., Vuka Karadzica 17, Lukavica , Bosnia and Herzegovina Abstract This paper provides information about the influence of remanent or residual magnetism in power transformers. It addresses causes of this phenomenon, such as winding DC resistance testing and a consequence of the remanent flux. The residual magnetism can cause various problems such as erroneous measurements on a transformer, an increased inrush current at the start-up of a power transformer, or incorrect operation of protective relays due to magnetized CT cores. The process and different methods of performing demagnetization, detection and verification of the demagnetization process using Sweep Frequency Response Analysis Test (SFRA), are also described. The SFRA and excitation current tests are performed when the transformer is in natural state (out of service for a long period), when the transformer core is magnetized, and then demagnetized. The field tests are performer in order to show the remanent magnetism influence on those results. The transformer SFRA graphs are different when the transformer core is magnetized (using DC current source) and demagnetized (after demagnetization process). The two different approaches of the demagnetization process are compared: all three transformer core legs are demagnetized, and only the middle core leg is demagnetized The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of ICTRAM Keywords: Remnant magnetism, Magnetization, Demagnetization, Transformer core, FRA test, Excitation current test * Edis Osmanbasic. Tel.: ; fax: address: eddie@dv-power.com The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of ICTRAM 2017.

2 2 Edis Osmanbasic/DV Power 00 (46) Introduction Remanence or remanent magnetization is the magnetic flux left behind in a ferromagnetic material (such as iron) after an external magnetic field or magneto-motive force (mmf) is removed. After a DC current test, such as a winding resistance measurement, the magnetic core of a power or instrument transformer may be left magnetized (residual magnetism). Also, when disconnecting a transformer from service, some amount of magnetic flux trapped in the core could be left present. In a demagnetized state of the core all atomic magnetic moments or dipoles are randomly oriented to cancel each other. Once they are forced to take one direction by the external mmf (e.g. the current in the winding), another force (opposite mmf) would be needed to return them into the original demagnetized state. 2. Remanent magnetism causes 2.1. DC testing Figure 1. Current and flux relationship A winding resistance measurement of a power transformer is performed using DC current in the order of 10% rated current of the winding under test. The proper winding resistance measurement technique requires bringing the magnetic core into saturation, in order to speed up the process of result stabilization. This level of DC current makes the transformer core magnetized. Once the test is completed, the transformer is discharged, but not demagnetized. This test is normally performed as the very last due to effects of remanence to other AC test results Transformer shutdown As in most cases voltage and current are not in phase because the transformer is inductive object and does not supply only resistive load (cosφ<1). Interrupting the current leaves the remaining flux in the non-zero state at the point where the alternating current was interrupted by a circuit breaker. The Figure 1 shows relationship of current, voltage, and flux in an unloaded power transformer. It can be observed that interrupting current at zero crossing will leave a significant amount of flux trapped in the core. Also, certain amount of remanent flux still remains after relays open a circuit carrying very high short-circuit currents. Considering the previous causes the transformer core is almost always magnetized to a certain degree when transformer is out of service. However, the DC winding resistance test is most common and the worst culprit in this process. 3. Remanent magnetism effects The remanent magnetism can cause several problems in electrical system. Also it can influence the wrong decision regarding the measurements performed for the purpose of transformer condition assessment. The most frequent remanent magnetism effects are listed below.

3 3.1. High inrush current and incorrect operation of relays Edis Osmanbasic/DV Power 00 (46) The inrush current is always present during a transformer start up. The high inrush current value is several times higher than transformer nominal current value. This increased current value additionally heats the transformer windings, the significant force is generated which compresses the windings. The remanent magnetism present in the transformer core can cause the significantly higher inrush current value (when the polarity of the applied voltage is in the same direction as the residual magnetism inside the core). In worst case the value can be close to the faulty short circuit current peak value. The highest inrush current occurs when the voltage is applied near the zero crossing with the polarity in the same direction as the residual magnetism in the core or the corresponding limb. The transformer core becomes saturated, thus the inductance L is significantly reduced. The high inrush current occurred because it is limited only by the winding resistance and transmission line impedance. The increased current duration period can give falsely signal to the protection system which can undesirably operate and take the transformer out of service. This is the most worrying effect, the protection system operation and false failure alarm. Over-current caused mechanical shocks may damage the coils and release the clamping pressure, that would eventually lead to loose windings and a transformer failure. The windings are exposed to mechanical stress proportional to the square of the current. The Figure 2 shows a situation where the inrush current was switched off after 5 cycles of transformer energization. The A phase current exceeded 400A in the first half cycle, while the other two phases show great asymmetry, and a presence of higher harmonics in the current waveforms Incorrect measurement test results Figure 2. Relay operating after 5 cycles on Inrush current of phase A The remanent magnetism influences transformer diagnostic tests which results depend on transformer core condition. The magnetization or the no load or excitation current measurement, and the Frequency Response Analysis (FRA) test are significantly influenced by remanent magnetism in the transformer core. The remanent magnetism can causes wrong decisions about transformer condition, or make results impossible to analyze. The excitation current test can indicate a problem in transformer windings or core. These test results should be compared with reference results and also between phases. On a core form three phase transformer, the three single-phase excitation current results should demonstrate a pattern of two similar (for the phases on the outside transformer legs) and one lower value (for the phase on the middle transformer leg) [1]. These values are very voltage dependent and do not have a linear behavior to the applied voltage, thus for the results comparison, tests should be performed always at the same voltage level. The table 1 shows typical excitation current values obtained at 100V on a transformer when magnetized, and after the demagnetization procedure. The single-phase currents form a pattern of two similar (4.8-5 ma) and one lower value result (3.9 ma) for a demagnetized unit. The much higher current of the

4 4 Edis Osmanbasic/DV Power 00 (46) phase A before demagnetization indicates that the core leg associated with the phase A was magnetized. The threephase current is influenced by various construction factors and their pattern was not proven to be effective for this evaluation. Modern transformers today have very high capacitive component of the no load current that may influence the increase or decrease of the total excitation current on a magnetized transformer. The excitation current being inductive would compensate this capacitive component and the resultant current can take different values. Sometimes the capacitive component exceeds the inductive and then the magnetized leg may exhibit lower excitation current than when demagnetized [2]. Table 1. The excitation current results before and after the demagnetization process I ex (ma) Before demagnetization After demagnetization 1~ 3~ 1~ 3~ Phase A Phase B Phase C FRA test uses frequency response analysis to describe the dynamic characteristics of an oscillating RLC network based on its input and output signals. When one parameter is changed, for example the main inductance due to a core problem or the geometric shift of a winding, one or more characteristic resonance points is/are also displaced or shifted. This problem in SFRA measurement method is described in literature [3], [4]. The remanent magnetism influences the FRA graph around 1kHz region. In this frequency region the core inductance dominates the response. Hereafter in this paper the results of these tests are presented when transformer core is magnetized and demagnetized. 4. Demagnetization procedure There are several methods of transformer core demagnetization: Variable Voltage Constant Frequency Source (VVCFS), Constant Voltage Variable Frequency Source (CVVFS), Decreasing Amplitude of an Alternating Polarity dc Current (DAAPC), and Constant Alternating Voltage with Decreasing Time (CAVDT). The VVCFS - Variable Voltage Constant Frequency Source is the best method, performed in the factory, and requires very large energy source. Impractical for field operation it provides industrial frequency (50 or 60Hz) source powerful enough to magnetize the magnetic core. Another requirement is the possibility to control and slowly reduce this voltage. By reducing the voltage and current from nominal to zero, the core is demagnetized; however, this is impossible to do in the field on even a medium size power transformer. The paper published by Deleon et al. [5] explains simple and quick method to demagnetize the iron core of a power transformer measuring and calculating coercive force to be applied in a single application. This is a variation of the CVVFS (Constant Voltage Variable Frequency Source). The experiment in that publication shows results for single phase and three phase units. The method has been verified on Y-Y connections for three-and five-limb transformers as well. The IEEE Standard (section ) directs one to alternate the polarity of a fixed voltage with decreasing application time per alternation of polarity. This is the CAVDT method. With each alternation, the voltage is applied until the current flow has reversed and is slightly lower in absolute magnitude than the current in the previous application. The explanation is clear: with decreasing time you should obtain slightly lower magnitude of the current. However, this is easier said than done. Once the current reaches the knee-point of the saturation curve, with applied fixed voltage it changes too fast to manually control with precision. Experience has

5 Edis Osmanbasic/DV Power 00 (46) shown that improper procedure very often caused other core legs to get magnetized while one was being demagnetized. Thus, no successful demagnetization could be reported in many cases. Figure 3. Demagnetization current vs. time graph during the DAAPC process Modern electronically controlled high-power dc-test instruments can control the amplitude of the applied current with high precision (the DAAPC method) and they can be programmed to perform polarity reversal and automatic demagnetization of the power transformer magnetic core [6]. As shown in the Figure 4, current is interrupted at preselected value and following a discharge process, the opposing polarity current is applied. In this subsequent step the amplitude at which the supply is interrupted is lower by 40% of the previous cycle. The process is repeated down to the lowest practically controllable current amplitude. A successful demagnetization was obtained on various sizes of power transformers up to 1100 MVA, and with HV winding rating of 1000 kv in both configurations: star or delta. 5. Case examples Several tests were performed to verify influence of remanent magnetism on the FRA graphs. The low frequency portion of the graph, around 1kHz area, is influenced by the magnetic core. Problems, defects, deformation, as well as remanence can change the position of the peak on the graph in this frequency domain. In our examples, we have started with a test of a good transformer that was out of service for short period of time. We call it as found state. Then we performed the demagnetization and repeated the FRA tests on both phases A and B. The difference shows slight change indicating that transformer was partially magnetized throughout all phases when we started the investigation. The Figure 4 (Phase A left and Phase B right) shows the FRA graphs comparison when the transformer is in as found state and when the transformer is demagnetized (blue graphs).

6 6 Edis Osmanbasic/DV Power 00 (46) Figure 4. Comparing FRA graphs: as found with demagnetized cores of phases A (left) and B The reason for magnetization of the transformer out of service is explained in Lachman s paper [7] as a need for magnetic dipoles in the core metal to take the position of minimum energy, which can be different from the completely demagnetized state. That way the core gets slightly magnetized without any excitation during the period of idling, such as transformer transport from the factory to its site. As a consequence, to obtain correct results demagnetization should be performed always before an FRA test. The next step was to magnetize the core performing a DC winding resistance test. As the test was performed using high current to saturate the core this could be considered a maximum amount of remanent magnetism that can be trapped in the core. The graph in the Figure 6 shows displacement of the first FRA peak for the winding of phase A when the core leg of the phase A is magnetized (the blue trace) and when the core leg of the phase B was magnetized (yellow trace). The reference trace (red) was a demagnetized state of the core. Figure 5. Graph of phase A in three different magnetization scenarios At the end we wanted to verify the effectiveness of demagnetization when performed only to the middle leg. As we expected, the flux that was returning through the outer two core legs demagnetized them as well. The Figure 6 shows a very good comparison of graphs for both phases A and B, when demagnetization was performed on all phases and when only the middle leg was demagnetized.

7 Edis Osmanbasic/DV Power 00 (46) Figure 6. Demagnetization of all phases and B phase only, traces of phases A and B 6. Conclusion Removing remanent magnetism is today an easy and quick task. Modern instruments for winding resistance offer this automatic function. Additionally, there are instruments manufactured with that task only provide three phase demagnetization function for large power transformers, where currents go up to 60A [6]. Eliminating the source of the problem is a first step for each asset engineer where demagnetization will help in lowering inrush current, minimize errors when performing diagnostic tests, and provide safe and correct operation of power transformers or instrument transformers. Putting demagnetized transformer into operation saves mechanical damage to the unit and eliminates system disturbances. For diagnostic purposes it is imperative to have transformer properly demagnetized before performing FRA tests, both in the field and in the factory. The difference between results in the factory and during commissioning tests in the field were incorrectly attributed to a mechanical damage in transport, when in fact it was the consequence of core magnetization without any excitation explained above. References [1] A. Rickley et al., Field Measurements of Transformer Excitation Current as a Diagnostic Tool, Power Apparatus and Systems, IEEE Transactions on (Volume:PAS-100, Issue: 4 ) Page(s): , 1981 [2] R. Proffitt, Glaring example of the effects of residual magnetism on large single phase transformers as determined by routine excitation current, Doble conference proceedings [3] IEC : Power transformers - Part 18: Measurement of frequency response [4] IEEE C57.149: Application and Interpretation of Frequency Response Analysis for Oil-Immersed Transformers [5] F.de Leon et al., Elimination of Residual Flux in Transformers by the Application of a Slowly Alternating dc Voltage Source [6] DV Power, Instruction manual: DEM60C three phase demagnetizer, Stockholm 2016 [7] M. Lachman et al, FRA of Transformers and Influence of Magnetic Viscosity Doble Client Conference paper Boston, 2010.

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