What Influence Does Residual Magnetism Have on the Transformer Core?

IntroductionMBA lc trong h thng in l mt thit b v cng quan trng. Nu mt MBA b s c, thit hi v mc kinh t gy ra v mt in v chi ph ca bn thn MBA l v cng ln. V thc t l MBA vn hnh lu nm, vic gim st cht lng ca MBA tr nn ngy cng quan trng. nh gi tnh trng cht lng ca MBA, c nhiu phng php khc nhau c p dng k c cc phng php truyn thng v chn on chuyn su. Trong c mt s phng php chn on b nh hng ca t d dn n kt qu b sai khc v khng c tn tin cy cao. T d cn nh hng n dng in khi ng. Dng in khi ng cao c th gim tui th ca MBA do cc lc c hc tc ng ln h thng giy cch in. Chnh v th cn c khuyn co kh t d cho MBA trc khi thc hin cc php o chn on v ng in tr li cho MBA.Power transformers are key elements in the electrical grid. If a power transformer fails, the financial impact of the outage time is in most cases considerably larger than the damage on the transformer itself. Due to the fact that the transformer fleets are getting older, the condition assessment and the appropriate maintenance strategy of power transformers is getting more and more important. To assess the condition of a transformer, various electrical measurement methods can be used. But a large number of diagnostic measurements are affected by residual magnetism. Therefore, it is difficult to analyse a reliable condition assessment of transformers. Residual magnetism can also have an impact on the inrush current. A too high inrush current can reduce the lifetime of a transformer due to the mechanical forces on the insulating paper. It is therefore recommended to demagnetise the transformer before performing diagnostic measurements and re-energising it.

Influence of residual magnetism on electrical routine and diagnostic measurementsnh hng ca t d n cc php o chn on, nh gi cht lng MBAT d c th cao n 90% mt t thng (B) trong sut qu trnh vn hnh. Vic nh hng ca t d n kt qu o l iu tt yu. Thc t trong cc php o dng in khng ti, php th cn bng t hoc phn tch p tuyn tn s xc nh s c trong li thp, t d s nh hng n kt qu v tr nn kh chn on hn.The residual magnetism can be as high as 90% of the magnetic flux density (B) during operation. In the event of a fault or during routine tests, various electrical diagnostic techniques can be used for analysing the condition of a transformer. Residual magnetism influences certain diagnostic measurements in such a way that a reliable and meaningful analysis becomes difficult. Particularly, when performing exciting current measurements, the magnetic balance test, or sweep frequency response analysis for localisation of faults in the core, residual magnetism may have such a negative effect that results become unintelligible.Figure 1: Residual Flux BR and hysteresis loop at different Flux densities1.2 Influence on sweep frequency response analysis measurements (SFRA)SFRA hoc FRA s dng k thut phn tch tn s qut m t c tnh ng hc ca mt h thng, trong trng hp i vi chng ta l MBA, da vo so snh tn hiu u vo v u ra. Cc php o SFRA c m t ch yu trong cc tiu chun IEC -18, IEEE C57.149, CIGRE v DL914. The Sweep Frequency Response Analysis (SFRA or FRA) uses frequency response analyses to describe the dynamic characteristics of an oscillating network, which is a transformer in our case, based on its input and output signals. The SFRA measurement method is described in the IEC 60076-18 and IEEE C57.149-2012 and has become increasingly accepted as a diagnostic method.Mt MBA s cha cc im cng hng song song v ni tip khc nhau tng ng vi cc thnh phn in cm L, in dung C v in tr R. Khi mt trong cc thng s ny thay i v d in hnh nh in cm chnh do c vn vi li thp hoc dch chuyn cun dy th mt hoc nhiu im cng hng s b thay th hoc dch chuyn n mt v tr khc. Mi mt mng in (MBA) u c c tnh p ng tn s ring, thng gi l du vn tay. gii thch mt php o SFRA th thng da vo vic so snh vi kt qu u tin hoc so snh vi cc MBA cng loi. Gin SFRA u tin ca MBA s khng thay i trong sut chu k lm vic ca MBA. Khi thc hin php o SFRA tt c cc nh hng n kt qu o phi c loi b v chng c th dn hiu nhm kt qu thu c t php o.A transformer reflects an oscillating system consisting of various series and parallel resonances with corresponding inductances (L), capacitances (C) and resistances (R). 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 are also displaced or shifted. Every electrical network has a unique frequency response, its so-called fingerprint. Interpretation of an SFRA measurement is based on a comparison of measurements, for example with the initial fingerprint or with other transformers of the same type. The plot of a fingerprint should not change throughout the entire life cycle of a transformer. All influences which could affect SFRA measurements must therefore be avoided, as they could lead to misinterpretation of the obtained test results.Khi t d nh hng n p ng tn s c bit trong min tn s thp ni m in cm chim phn ch o trong p ng, khi iu chc chn rng MBA phi c kh t trc khi thc hin php o. iu cng ng ngha l php o SFRA s hiu qu trong vic kim chng t d.Since residual magnetism influences the frequency response particularly at lower frequencies, where the magnetisation inductance dominates the response, it is vital to ensure that the transformer has been demagnetised before performing the measurement. Meanwhile, because of this pronounced and well understood influence at the lower frequencies, an SFRA measurement is effective in verifying residual magnetism.Cc kt qu o SFRA phn nh in cm chnh thng qua cc im cng hng u tin. Hnh 3 th hin cc im cng huonwrgh in hnh ca MBA vi 3 tr t. 2 im cng hng song song v ni tip c th c nhn thy r rng 2 pha ngoi cng.The SFRA measurement reflects the main inductance through the first resonance points. Fig. 3 shows those typical resonance points of a three-limb transformer's main inductance. Two significant parallel and series resonance points can clearly be seen on the outer windings. This can be described to the two magnetic paths with different lengths. In comparison with this, the winding on the middle limb displays only one characteristic single resonance point.Figure 2: SFRA connection diagram for NV-wiringKhi MBA lc hoc phn phi c cch ly ra khi h thng, dng nh n vn cn duy tr t d trong li thp. T d chc chn s nh hng n kt qu o v chn on v vy nn kh t trc khi thc hin php o.When power or distribution transformer is isolated from power system, it is likely that residual magnetism remains in the core.Residual magnetism influences certain diagnostic measurements in such a way that a reliable and meaningful analysis becomes difficult.

Nh gii thch t trc i vi dng in khi ng, in cm thay i ph thuc vo mc t ha li thp, khi m L kh t>L. Mt im cng hng cha mt h thng c L, C v c th c m t bi phng trnh sau:As previous explained for the inrush current, the inductuance changes depending on the degree of core magnetization, whereby Ldemagnetised>L. A resonace point comprises a network of capacitances and inductances and can be describles using equation (1):Fo=1/2pi sqrt(LxC)in cm thp c phn x bi tnh trng t ha cao, nhiu im cng hng dch chuyn v pha tn s cao hn.The lower the inductance becomes as reflected by a state of higher residual magnetism, the more the resonace points move toward higher frequencies.1.3 Influence on exciting current measurementsPhp o dng t ha c th cung cp kt qu ng tin cy trong vic xc nh c s c trong li thp. Cc s c s dn n tng dng in t ha. Nu c gi tr dng in t ha tham kho, vic nh gi c th d dng thc hin. V dng in t ha khng c tnh cht tuyn tnh theo in p t vo nn cc php o so snh nn thc hin vi cng mt in p. Bin ca dng in t ha s ph thuc vo chiu di ng t ha nn dng in t ha ca pha A, C thng cao hn B nh hnh 4. Nu c b t d pha B, iu ny s dn n kt qu khng tin cy nh hnh 5.Measuring exciting current can provide evidence for potential signification faults in the core. Faults in the core lead to increasing exciting current. If reference values fof the exciting current are available, these can use for the assessment. Since exciting currents do not have a linear behavior to the applied voltage [2], measurements fir comparation with the reference values must be performed at the same voltage. The assessment is performed based on a typical pattern of a three-phase transformer or based on reference measurements if they are available. The magnitude of the magntismation current depends on the length of magntised path. This is virtually identical for the windings in the outer limbs (A, C) but lower for the windings on the middle limb (B) on fig 4. If there is for example, residual magnetism on the middle limb, this is easily lead to incorrect interpretations and a reliable diagnosis becomes impossible fig 5 YNyno 22.5/0.4 5.3MVA.

1.4 Influence on the magnetic balance testiu ny c th gii thch nh sau v d nu t in p 100v vo pha gia, cc kt qu o in p ca 2 pha ngoi c th xp x 50V. iu ny l do ng t ha c chiu di bng nhau. Nhng nu a in p 100V vo pha ngoi, th s cho kt qu khc do cc ng t ha c chiu di khc nhau. T c th a ra cc nhn nh v cc vn c th c i vi li thp.This should results in the following typical pattern: if, for example, a voltage 100V is applied to the winding on the middle limb, the measured voltages on the other windings should each display a value of approximatedly 50V. This can be explained by the two magnetic paths of the same length. When voltage is applied to the one of the windings on the outer limbs, it results ub a different pattern as the magnetic paths have different lengths. If the recorded pattern this can indicate either problems in the core or can be related to undesirable effects of residual magnetism.V t d nh hng n p ng tn s c bit trong min tn s thp, ni m nh hng ca in cm tc ng mnh n p ng, cn phi kh t trc khi thc hin php o.

1.4 Influence of residual magnetism on the inrush currentKhi MBA c ng in tr li in p nh mc, mt dng in khi ng c th vt qu dng in thng thng ca MBA trong mt vi chu k u. Nu li thp MBA vn cn t d, nh dng in c th tin n mc gn vi gi tr ngn mch. Dng in ny c th gy ra cc lc c hc lm bin dng cun dy v lp giy cch in, gy kch hot thit b bo v khng ng, tng ng sut v dc in p ln li. Ch c thnh phn tr ca cun dy mi kh nng lm gim dng khi ng cao n mc n nh trong vi chu k u. hnh 8When a trans is re-energised directly to its rated voltage, an inrush current occurs that can greatly exceedd the normal current for a few periods. If trans core still contains residual magnetism, the first peak current can even reach a level which can be close the the short-circuit at maximum. These high currents can cause undesirable effects such a mechanical deformation of the windings and its insulating paper, incorrect triggering of protect equipment, increased stress for the installation and voltage dips in the grid. Only the ohmic components such as the winding resistance are capable of attenuating the high inrush currents to a stable level within just a few cycles fig 8.

The highest inrush current occurs when the voltage is applied near the zero crossing and the polarity of the voltage is applied in the same direction as the residual magnetism in the core or the corresponding limb fig 9 equation 2-4

If the core reaches saturation, the transformers inductance is greatly reduced. The current is now only limited by the winding resistance on the high-voltage side and the impedance of the connected transmission line.

2. Cc phng php kh tThe following three methods are available for demagnetising magnetic materials: Demagnetisation through vibration Demagnetisation through heating up Curie temperature Electrical demagnetizationKhi 2 phng php u khng th c s dng kh t cho MBA, th phng php in tr nn phng php kh thi nht. Cc NSX c th t vo MBA mt in p nh mc tn s cng nghip v thay v ngt in p mt cch t ngt, th c th gim in p mt cch t t v qu trnh kh t s thc hin hnh 10. kh t d hin trng, thng ch c kh nng s dng tn hiu in p v tn s thp hn. Trong nhiu trng hp, khng th iu chnh ngun in p cao, m cung cp in p bnh thng ca MBA, c th c s dng kh t MBA ti hin trng. Since the first two methods cant be used for a transformer, the electrical method becomes the sole option. Manufactures can apply nominal voltage at nominal frequency on transformers and instead of shutting down the voltage suddenly, it could gradually reduce the voltage, the core is then progressively demagnetized (fig 10). To demagnetize transformer core on site, it is often only possible to use reduced voltage and frequency signals. In many cases, no adjustable high voltage source, which can provide the normal voltage of the transformer, can be used to demagnetize transformer core on site. Only a single phase source can be used.

Demagnetisation of single phase and three phase transformer can be performed in the similar way. When working on three phase transformer, it is important to consider that magnetic coupling takes place between the phases. Therefore, the phase or core limb used during the demagnetization procedure is extremely important and deliberately chosen with single phase source. It also makes sence to use the high voltage side for demagnetization as there are more turns associated with winding to generate the magnetic flux. Hence, the total time for demagnetization can be reduced. Experiments have show that the middle limb is the most suitable for demagnetization with a single phase alternative source. Thereby, the flux is distributed symmertrically over the two outer limbs. To determine which winding is associated with the middle limb in a delta winding, the transformers vector group is required.2.1 The art of accurate demagnetisationThere are various approaches for electrical demagnetization. Onw of these is to reduce the voltahge or the time in predetermined steps. Depending on their type and size, small distribution transformers or lage transformer can have very different core hysteresis parameters. The disadvantage of both approaches is that it takes a long time to ensure that both types of transformer can be reliably demagnetized using the same procedure.To counteract this problem, you can additionally trigger on a current value while the test is stikk running to start the next hysteresis cycle. However, since the magnetization current increases very rapidly when transformer core reaches saturation, this process is fairly inaccurate. Various experiments have shown that small transformer in particular become re-magnetised by the final cycle, which leads to high inrush currents in return.Demagnetisation based on the measurement of the magnetic flux has proven to be the safest and most efficient approach, as it works reliably with both small and large transformers. However, this approach places very strict measuring requirements on the used equipments, as the voltage needs to be continuously measured over time and the intergral has to be derived from this equation (5)

Important to avoid any secondary hysteresis during demagnetization. The occurring residual magnetism can lead to an apparent demagnetization [[6].To demagnetize transformer cores on site, it is often only possible to use reduced voltage and frequency signals.2.2 Demagnetisation measurement procedure with current sourceSince the voltage and thereby the magnetic flux of the main winding inductance Lh can nit be measured directly, this voltage needs to be calculated, fig 11, equation (6) [7].

Therefore, the winding resistance R must be measured first and the voltage drop (Vr) due to the winding resistance then subtracted from the measured voltage (V). Equation 6 shows the calcualation of the magnetic flux on the main inductance. Thereby R(0) represent the initial flux, which corresponds to the residual magnetism.

Test setup for demagnetization is very simple. It can be done from the high as well as from the low voltage side. However demagnetization from high side is faster due to the fact that more windings are available to generate the flux .The core must saturated in both directions. The specific hysersis parameters, like the maximum flux, per transformer are then determined and the initial flux can be calculated. On the basis of these parameters, an iterative algorithm can then be used to change both the voltage and the frequency. While this is taking place, the devices nust cinstantly measure the flux in the core. Using multiple iterations, the core can be demagnetized ti below a limit of its maximum value. Following the demagnetization procedure,several magnetic domains revert back to their preferred orientation. This procedure is also referred to as magnetic viscosity. The effect can be determined when performing demagnetization once again, although it is actually neglible and therefore is not really important in practice.With this procedure a quick demagnetization can be done for small distribution transformers as well as large power transformers.

When comparing the sfra results of the individual phases, it becomes apparent that the transformer displays residual magnetism after being isolated from the power system fig 14. After the demagnetization procedure, all resonace points moved towards lower frequencies as expected and the typical sfra patern of a three-limb transformer could be seen. The transformer can therefore be considered demagnetized.Ket luanBi vit ny nhn mnh tm quan trng v s nh hng ca t d on inrush current and some electrical measurements. It should also increase the awareness of the associated risks with re-energising transformers after an outage,especially inf the transformer is already presumed to have a bad solid insulation condition.Within the last few years, the first testing devices such as CPC100 have been developed with allow a reliable on-site demagnetization of transformer without any major additional effort. Demagnetised transformer cores minimize the risk for personnel and equipment during installation. The sfra measurement method is now describeled in iec60076-18 and c57.149 and has become increasingly accepted as diagnostic method. To gain reliable and reproducible measurement results, we recomenmend degmanetising the transformer core before diagnostic measurements such as sfra measurements.

1) Ramp down the voltage when de-energizing and ramp up the voltage when energizing. This method is not practical because the voltage in a power system cannot be controlled in this way. My point for stating this first method is to illustrate that this will minimize the magnetization of the core iron.

2) Use point on wave switching to match the energization voltage (point on wave) to the stored magnetization of the core. The core magnetization is determine by the flux in the core at the instant when the transformer was last de-energized.