Analyze Eddy Current Loss in the Three Phase 100 kVA Transformer Core with the Mix 60°-0° T-joint Core

MATERIALS AND METHODS

The main apparatus consists of a model cores three-phase 100 kVA transformer assembled with three limbs core with T-joint cutting angle the mix 60°-0° assembled from CRGO (M5 grades) 3% Si-Fe material. The core has 550x580 mm with the limbs and yokes 100 mm wide as shown in Fig. 3. The experimental cores assembled with the mix 60°-0° T-joint, mitred overlap corner joints with staggered yoke and overlap length is 5 mm as shown in Fig. 4 and assembled from 0.3 mm thick laminations of M5 grain-oriented silicon iron (CRGO). Associated instruments are used to measure fundamental and third content of the power loss (Maizana, 2011).

Fig. 3:	Dimension (mm) of 100 kVA transformer model

Fig. 4:	Transformer core type with the mix 60°-0° T-joint

Fig. 5:	Diagrams of the methods that used to measure the eddy current losses, Ns: Secondary winding turns, Np: Primary winding turns, R: Red colour cable, R: Yellow colour cable, B: Blue colour cable and V1, V2, V3: Voltmeter

Fig. 6:	The actual circuit

The testing process is done by using the no-load test frame. The no-load test frame consisting of three windings for each three phase core is designed in order not only to avoid introducing stress to the laminations but also to keep the magnetism exactly constant in all limbs of the cores. Each winding only extends along 85% on each limb to enable the stagger length of the three phase core to be varied. An extra softwood base 200 mm high is used to raise the overall height of the core, to minimize the effect of the stray flux on the localized measurements. The core could be energized 1-1.8 T at 50 Hz with less than 1.5% third harmonic distortion and also with variable frequency at 1.5 T and the power loss is measured with repeatability better than ±1% using a three phase power analyzer as shown in Fig. 5 and the actual circuit is shown in Fig. 6.

RESULTS AND DISCUSSION

From the measurement results obtained some data such as the loss of the core for each different frequency. Data presented in graph form such the following shows that picture. Signal input to this study similar to that used in the study by Mayuri et al. (2010).

Figure 7 shows the variation of overall power loss with flux density in the three phase cores. The core with adjusted at frequency of 50 Hz has lowest loss over the complete range of flux density. This situation also applies to the investigation conducted by the Yao et al. (2007). At this frequency, the loss that occurs in the transformer core is assumed as hysteresis loss only. The result of this investigation shows the core loss is 1.508 W kg^-1. Figure 8 is shown the variation of power loss with frequency in the three phase core. The graph shows that the losses will increases with increases of the frequency. The loss that produces in this investigation is classical eddy current. As shown in concept by Zurek (2005). The eddy current loss is rise to be 39.1% at frequency of 55 Hz, flux density of 1.5 T. The variation of eddy current loss that obtains from measurement as indicated in Table 1.

Table 1:	Eddy current losses refer frequency

Fig. 7:	The variation of overall power loss with flux density at different frequencies

Fig. 8:	The variation of hysteresis loss (Ph) and eddy current loss (Pe) with frequencies at 1.5 T flux density

Fig. 9:	The flux flow in transformer core

Fig. 10:	The flux flow in core lamination

Where frequency supply increases more than 50 Hz, these will makes eddy current loss increase and at the same time as showed in Eq. 2, the flows of flux through the lamination plate also increase as showed in Fig. 8. During the investigation, there is a lot of thing regarded to the eddy current losses can be explain clearly. The results obtained show the variation of the value is dependent on the variable flux density and frequency respectively as showed in Eq. 1. For example, the flow of the flux at the joint and what happen to the movement of flux when the value of flux density and frequency increased. Since there are air gap among the joint, the movement of flux stuck on the joint and this will makes the flux flows to the other lamination. In this condition not all the flux flowing towards into the middle limb or other limb at difference instant in time of transformer core because there is still flux left in the area of butt-joint as showed in Fig. 9. The flux will be circulated in the area as showed in Fig. 10. The circulating flux then will present hot spot on the joint and it is known as eddy current. These phenomena occurred due to the laminate as described by Mthombeni (2006) and Mazurek et al. (2010).

CONCLUSION

From the result of this investigation is found that the smallest power loss of transformer core at the core when adjusted in frequency of 50 Hz which are the rotational fluxes make large contributions to the total power loss in three phase transformer core.

With increase of the frequency will cause the increase of power loss in the transformer core. It caused not all the flux flowing towards into the middle limb or other limb at difference instant in time of transformer core because there is still flux left in the area of butt-joint and the core loss that produced at the frequency more than 50 Hz will know as classical eddy current loss.