Demagnetisation of CT cores under exposure of operating currents

Transcription

1 Demagnetisation of CT cores under exposure of operating currents by Dr. Fred Steinhauser, Omicron Saturated CT cores and their behavior during fault conditions have been and still are an important topic for protection engineers, in particular when fault currents with transient components occur. Many papers have been written and presented about this. Typically, the CT cores contain some residual magnetism when the current flow is interrupted. Especially at reclosing, this residual magnetism may cause undesired effects. In this context, the question "How will the magnetisation in the CT develop when power is restored and the CT is exposed to normal operating currents?" is frequently raised. Surprisingly, the answer is often based on opinion, it is even said this may be a "matter of philosophy". Statements like "some say the saturation level remains there forever, some say it will go away over time" can be heard. Much of what is said is based on assumptions, models and simulations. But for engineers, there must be a better way than relying "on philosophy". The recognised method is to look for plain facts and align them with proven theories. For this paper, systematic measurements have been made to put some light on this matter. Fig. 1: Injecting current on the secondary winding of the CT for magnetising the core. Practical implications Measuring residual flux can be a cumbersome procedure [6]. In the past, it was hardly feasible to perform a large number of such measurements in reasonable time. Also, for establishing a relationship between a measured residual flux and the conditions that caused this residual flux, the full previous history (the anamnesis) of the CT must be known. Even if the fault records were available for some arbitrary cases of saturated CTs, this is not a systematic approach for reliably determining a correlation of effects. Another problem is that the established methods for determining the residual flux result also in a destruction of the residual flux [3]. It is not possible to resume the test at the most recent point. Thus, every single measurement has to start from the de-magnetised state, first bringing the CT into a defined state of magnetisation (applying a defined residual flux). Then, some other condition which may alter the residual flux may be applied. Then, this residual flux is to be measured. Each of the steps involved must be performed with adequate precision and repeatability. Modern test equipment allows for precise application of the test quantities for each of these steps and performing the measurements in reasonable time to make a systematic series of tests feasible. Establishing a defined magnetisation The first step for performing measurements with residual magnetism is to apply a defined initial magnetisation to the CT core. It is assumed that we start with a demagnetised core. In our case, this condition is fulfilled without further efforts, since the measurement device for the residual flux leaves the core demagnetized after the measurement. Then, a DC current is applied as shown in Fig. 1. The primary side of the CT is an open circuit, so the injected current equals the excitation current through the main inductance of the CT. As its magnitude is increased, the magnetisation will develop along the initial magnetisation curve. This residual flux is then measured. This Fig. 2: Magnetisation along the initial magnetisation curve and the hysteresis curve. Fig. 3: Setup for measuring the residual flux. can be conveniently achieved with a test set with dedicated residual magnetism measurement features as shown in Fig. 3. Since the values scatter slightly from measurement to measurement, some averaging has to be applied to get reliable values. Fig. 4 shows how the residual magnetism depends on the excitation current applied for magnetising the core. Using this method, it is possible to establish a well defined magnetisation of the core as a starting condition for the tests. To make the results more independent from the actual ratings of the specific CTs, the quantities are normalised. The currents are energize - November Page 28

2 Fig. 4: Residual flux vs. magnetisation current. Fig. 5: Equivalent circuit of the CT with burden. Fig. 6: Reduction of the residual magnetism when applying nominal current. referred to the kneepoint current I k, which is the exciting current at the knee point voltage. The residual flux Q res is referred to the saturation flux Q s and expressed as the residual magnetism. The residual magnetism is defined as : = 100% / (1) The reference values I k and Q s used here are those obtained according to IEC [1]. These values are measured under special conditions and vary depending on the standard and method used for determining them. But the actual standard which is used to determine these reference values is not so crucial; some other values would just lead to a different scaling of the diagrams, leaving the essence of the statements unaffected. Demagnetisation The goal of this work is to investigate how the magnetisation of the saturated core is affected by normal operating currents. Therefore, conditions as under nominal currents shall be applied. Applying nominal currents at the primary side is difficult. While it might be feasible for steady state conditions, it is not possible to produce such currents with the dynamic behavior as required for the following tests, e.g. for a defined time or even for a defined number of cycles. The current further modifying the magnetisation of the core is again applied on the secondary side as shown in Fig. 1 for the magnetisation. As the primary side is an open circuit, the current injected equals energize - November Page 29 Branches: Cape Town Gauteng KNZ Head Office: Tel: ESETA & SABS ACCREDITED

3 the magnetisation current through the main inductance L m Estimating the nominal current Fig. 5 shows the circumstances in the loaded CT at nominal conditions. The secondary current flows through the secondary CT impedances and the burden, producing a voltage drop U m,nom which occurs at the main inductance. With the total secondary impedance =, + + (2) the magnitude of the magnetisation current at a given burden ZB can be easily calculated:, =, (3) As it has to be expected for a reasonable CT, this is a fairly small current, only a few percent of the knee-point current. In reality, this means only a few milliamperes. Demagnetisation with injected current The current source injects a sinusoidal current into the secondary winding. As long the compliance voltage of the current source is not exceeded, the voltage adapts to the load impedance and the current is kept constant. As the secondary impedances are almost negligible compared to the impedance of the main inductance, the voltage measured at the terminals is almost identical to the voltage at the main inductance and according to hysteresis curve. It must be noted that this injection of a defined current is different from the procedures described in most standards, such as IEC , where a sinusoidal voltage is applied and a current with potentially non-sinusoidal waveform is obtained. This current I m, nom was now applied to the CT for a few seconds and the remaining residual magnetism was measured. It was significantly reduced from the value established through the magnetisation. This was a first finding, but further measurements with longer exposure time of the nominal current showed the same result, the residual magnetism did not drop further. Consequently, the exposure time was reduced until a change of the measured residual magnetism could be observed. It turned out that the time interval to be considered is less than 1 s or only a few cycles. Fig. 6 shows how the residual magnetism develops under the application of a current of about nominal magnitude. The process is more or less completed after about 20 cycles and the residual magnetism is roughly reduced to half of the initial value. This result could be used to get to an answer to the question raised in the motivation. And as in most cases, the extremes (stays forever / goes away) do not apply and the actual circumstances might depend on many other parameters which were not considered in the measurement. The insight summarised in Fig. 6 is not Fig. 7: Reduction of the residual magnetism at different frequencies. Fig. 8: Voltages (top and middle trace) and current (bottom trace) when applying nominal current to the magnetised CT. unique and not new. There are several publications [2, 3, 4], one of them dating back almost 70 years that describe similar findings. Some of these statements shall be briefly reviewed here. Reference [2] contains a statement, called a hypothesis, which nicely resembles what was concluded above: "This residual flux remains in the current transformer core Fig. 9: Magnetisation changing during the first cycle at a frequency of 10 Hz. practically undiminished unless the circuit is loaded, in which case it is decreased gradually but not usually removed entirely." It describes one specific case where a CT was magnetied to a certain value and then exposed to a current of 5 A for 24 hours. After this, the residual flux had been reduced to about 31% of the initial energize - November Page 30

4 magnetisation. At the time when this experiment was made, it would have been probably very difficult to find out that the same reduction would have occurred most likely after 24 cycles and not only after 24 hours. Reference [3] displays some oscillographic recordings which indicate that most of the change of the residual magnetism takes place in a few dozen cycles. Reference [4] states that under the flow of rated current the residual flux in CTs is reduced to 60% to 70% of the remanent flux within a few seconds. So far, the measurements and results described are just in line with the state of the art. Frequency dependency of the effect For the measurements described above it was implicitly assumed and therefore not explicitly mentioned that the current applied for changing the magnetisation of the core had nominal frequency, in this case 50 Hz. But it turns out that the reduction of the magnetisation is essentially more significant if the frequency of this current is lowered. Fig. 7 shows how the residual magnetism develops for different frequencies of the applied current. The initial magnetisation was established with I mag = 2 I k, so the yellow curve (for 50 Hz) is identical to the lower curve in Fig. 6. The residual magnetism is already heavily reduced at 20 Hz (red curve), and it is essentially removed at 10 Hz after only 5 to 10 cycles (blue curve). The observed differences in the levels of the residual flux are remarkable. Obviously, the magnetic properties of the core material are very differently involved at the different frequencies. A further attempt to visualise this is to look at the voltages that occur at the CT when the current is applied. The current is injected from a current source, imposing the sinusoidal waveform of the current. Any influence from the core material will become visible in the waveform of the voltage. Recorded waveforms from two cases are shown in Fig. 8. The lower (yellow) trace shows the injected current. As explained above, it is sinusoidal and the burst contains exactly five cycles. The special scaling axes in Fig. 8 must be kept in mind. The horizontal axis shows the angle (T t ), giving the same distance for a cycle independent of frequency. As the frequencies are different by a factor of 5, the voltage ranges are also different and not to scale. The upper (red) trace is the voltage at 50 Hz. After the first cycle, the waveform is close to sinusoidal, not showing much of a visible effect from the hysteresis of the core material. The middle (blue) trace is the voltage at 10 Hz. Its waveform is visibly affected from the hysteresis. The first cycle at 10 Hz is where the largest change in the residual magnetism occurs. This is illustrated in Fig. 9 in detail. By performing an integration of the voltage and appropriate scaling, the approximate course of the residual magnetism over the applied current can be constructed. The current value on the horizontal axis is referred to I max, which is the peak value of the sinusoidal current applied. During the positive half wave of the current, the magnetisation curve describes a small hysteresis loop superimposed to the remanence point established by the initial magnetization. At the end of the positive half wave, the magnetization is more or less back where it started off when the current was applied. During the negative half wave, the magnetisation is heavily eroded as long the magnitude of the current is increasing. But even when the current moves back toward zero after reaching its maximum value, the magnetization becomes further decreased until the positive zero crossing of the voltage takes place. An attempt for an explanation So, why can such different behaviour be observed at frequencies not higher than the rated frequency? Or is the rated frequency already a "considerably high" frequency when it comes to the demagnetizing effect of a magnetic field caused by a rated energize - November Page 31

5 Fig. 10: Locus of the complex permeability (solid: measurement; dashed: calculation) [7]. current? This thought shall be further pursued and some reasoning shall be given. One approach is reference [7] that does not aim at the magnetic polarisation issue, but focuses on the frequency dependency of the permeability instead. The key point is that this paper gives insight how the magnetic field penetrates magnetic sheet metals as used for transformer cores. It is well known that eddy-currents occur when a magnetic material is exposed to a changing outer magnetic field. These eddy-currents build by themselves a magnetic field that is opposed to the outer field. By doing so, the eddy-currents shield the inner portions of the magnetic material from the outer field. The effects of the eddy-currents become essential when the frequency of the outer field is approaching the Wolman frequency: = 4 (4) The formula applies for sheet metals, where D is the specific resistance of the material, d is the thickness of the sheet, and A is the "overall initial permeability" which is used throughout the calculations in the mentioned paper. This Wolman frequency can assume surprisingly low values. For typical materials and thicknesses of the laminated core material, it can be as low as only a few hundred Hertz or even below. Fig. 10 from [7] shows the locus of the complex permeability [8, 9] as a function of frequency for a chromepermalloy sheet metal. The frequency is referred to the Wolman frequency. The vertical axis represents the inductive component, while the horizontal axis represents the resistive component. The resistive component is connected to the eddy-current losses. Reference [7] is very challenging, containing more than 50 numbered equations for mathematically encompassing the effects. Fig. 11 shows the local permeability over the cross section of the sheet metal. On the surface, the local permeability is zero and increases when going deeper into the material. The local permeability is essentially affected by the outer field only within a certain depth of about the value S, which characterises where the local permeability reaches the value of the overall initial permeability. The permeability remains at high levels in a large inner portion of the sheet, because it is effectively shielded by the eddy-currents and not exposed to the outer field. In the case of an outer field resulting from an operating current in a CT, this inner portion of the core material is not involved in the demagnetisation process and remains magnetised at its former level. The value of S is inversely proportional to f 2. So for lower values of f, S increases and more of the material becomes involved in the demagnetisation process, which would explain the effects shown in Fig. 7 and Fig. 8. Thus, even when the sheets in a laminated transformer core are obviously thin enough for reducing the eddy-current losses to an acceptable level, the sheets may still appear to be thick when it comes to the penetration by an outer field of rated frequency. Conclusions Referring to the question raised in the motivation section, a rough answer can be given: The magnetisation of a saturated CT core exposed to normal operating currents will neither "remain there forever", nor will it completely "go away over time". The actual degree of the demagnetisation due to operating currents depends on several parameters, which were not all investigated in detail. energize - November Page 32 Fig. 11: Local permeability over the cross section of a 36% Nickel-Iron sheet of thickness de [7]. Given the observed speed of the demagnetisation (only a few cycles in some cases), it might be realistic that saturation effects will be quickly diminished in a CT core at reclosing after successful fault clearing. The explanations and conclusions given above could be verified by systematic repetition of the measurements with other CTs, preferably with the exact knowledge of the construction. There is the question if the use of thinner sheet metals in the core yields in more effective demagnetisation due to operating currents and better recovery after saturation. This could be investigated by comparing CTs with cores with different lamination thickness at otherwise identical data. This approach may collide with a statement from reference [5] that points out that "the eddy current loss in a laminated core may increase if the lamination thickness is reduced". The overall optimum for the technical compromise may depend on how important an improvement of the recovery behaviour is regarded compared to other properties of a CT. References [1] IEC , Instrument transformers Part1: Current transformers. Ed. 1.2, IEC, Geneva [2] HT Seeley: The Effect of Current- Transformer Residual Magnetism on Balanced-Current or Differential Relays. AIEE Trans., Vol. 62, April [3] RG Bruce, and A Wright: Remanent Flux incurrent-transformer Cores. Proc. IEE, Vol. 113, No. 5, May [4] J Dickert, et al: Investigation on the Behaviour of the Remanence Level of Protective Current Transformers. MEPS'06, Wroclaw [5] SK Mukerji, et al: Eddy Currents in Laminated Rectangular Cores. Progress In Electromagnetics Research, PIER 83, 2008 [6] M Pfannenstiel: Residual magnetism of current transformers, OMICRON ITMF, [7] R Feldtkeller: Permeabilität und Wirbelströme in Blechkernen bei sehr hohen Frequenzen. Frequenz, Band 3/1949 Nr.4. [8] R Boll, Weichmagnetische Werkstoffe. Vacuumschmelze GmbH. 4th Edition, [9] L Michalowsky, J Schneider: Magnettechnik. Vulkan-Verlag GmbH. 3rd Edition, Contact Alexander Dierks, Alectrix, Tel , alexander.dierks@alectrix.co.za