STUDY AND COMPARISON OF THE EFFECT OF CONVENTIONAL , LOW LOSSES AND AMORPHOUS TRANSFORMERS ON THE FERRORESONANCE OCCURRENCE IN ELECTRIC DISTRIBUTION NETWORKS

Ferroresonance phenomenon is one of the grid transients, which has devastating effects on electric distribution networks. On the other hand, considering the increasing application of low-loss transformers, especially the amorphous transformers, it is essential more than ever to study the effects and consequences of application of this type of transformers comprehensively. In this paper the effects of the ferroresonance phenomena on transformers with commonly used, low-loss and amorphous cores are investigated. Firstly, a segment of a real case distribution system is simulated in PSCAD software, and then the core characteristics are changed without any change in the other system elements. In each step, other than recording the voltage and current waveforms, harmonic analysis on all phases have been conducted and results are briefly presented and discussed. Also critical lengths for the occurrence of ferroresonance phenomenon are calculated for all transformers with different capacities. The effects of the transformers type on the ferroresonance phenomenon and also on the critical length for the occurrence of such phenomenon are presented. Key wordsAmorphous transformer, ferroresonance, power loss, transformers, transient states. INTRODUCTION The ferroresonance phenomenon is typically referred to the series resonance between saturated magnetizing inductance of transformers and capacitance of transmission lines and distribution network cables [1]. In many real-case networks, ferroresonance phenomenon may cause significant over-voltages. Over-voltages caused by this phenomenon can include a wide range of frequencies, which in turn result in degradation of power quality [2]. Detection of characteristics and features of the waveforms associated with the ferroresonance phenomenon highly depend on the accurate simulation of distribution network transformer models [3]. Given the complexities, difficulties and considerable consequences in the event of ferroresonance phenomenon, several methods have been proposed in order to identify its characteristics [4]. FUNDAMENTALS OF FERRORESONANCE Fig. 1 shows a simple RLC circuit. Based on the oscillation frequency, different modes during the ferroresonance phenomenon can be divided into 4 categories. Fig. 1. RLC circuit a) Basic mode: in this mode voltage and current follows a periodic pattern at the system frequency. In this case the voltage waveform contains the fundamental frequency and the harmonics. b) Sub-harmonic mode: as shown in Figure 3, in this mode, the signals are periodic with a period of n times the main source period (T). This is known as sub-harmonic n. Sub-harmonic ferroresonance mode usually contains the odd harmonics. c) The quasi-periodic mode: in this mode the waveforms are not periodic. The frequency spectrum can be expressed by nf1 + mf2, where n and m are integers and f1/f2 is non-integer. d) Chaotic mode: as shown in Figure 2, in this mode, frequency spectrum is continuous and covers different sections of the plane. Fig. 2. Chaotic mode. 24th International Conference on Electricity Distribution Glasgow, 12-15 June 2017


INTRODUCTION
The ferroresonance phenomenon is typically referred to the series resonance between saturated magnetizing inductance of transformers and capacitance of transmission lines and distribution network cables [1].In many real-case networks, ferroresonance phenomenon may cause significant over-voltages.Over-voltages caused by this phenomenon can include a wide range of frequencies, which in turn result in degradation of power quality [2].Detection of characteristics and features of the waveforms associated with the ferroresonance phenomenon highly depend on the accurate simulation of distribution network transformer models [3].Given the complexities, difficulties and considerable consequences in the event of ferroresonance phenomenon, several methods have been proposed in order to identify its characteristics [4].

FUNDAMENTALS OF FERRORESONANCE
Fig. 1 shows a simple RLC circuit.Based on the oscillation frequency, different modes during the ferroresonance phenomenon can be divided into 4 categories.Fig. 1.RLC circuit a) Basic mode: in this mode voltage and current follows a periodic pattern at the system frequency.In this case the voltage waveform contains the fundamental frequency and the harmonics.b) Sub-harmonic mode: as shown in Figure 3, in this mode, the signals are periodic with a period of n times the main source period (T).This is known as sub-harmonic n.Sub-harmonic ferroresonance mode usually contains the odd harmonics.c) The quasi-periodic mode: in this mode the waveforms are not periodic.The frequency spectrum can be expressed by nf1 + mf2, where n and m are integers and f1/f2 is non-integer.d) Chaotic mode: as shown in Figure 2, in this mode, frequency spectrum is continuous and covers different sections of the plane.The transformer core structure plays a substantial role in occurrence of ferroresonance phenomenon, can affect the intensity of this phenomenon and bring it to any of the aforementioned modes.Generally, the decrease in core loss and reduction in hysteresis curve area and its inner loop increase the ferroresonance phenomenon intensity and its occurrence in the chaotic mode.This feature, which can be seen specifically in amorphous transformers' cores, is investigated in the upcoming discussions.

LOW-LOSS TRANSFORMERS
Low-loss transformers are designed and manufactured in accordance with standard DIN42500 and are divided to nine different categories.Based on this standard, the transformer no-load loss falls into one of three categories A', B' and C'.In this classification, category A' has the highest and C' has the lowest no-load loss.Load losses are also categorized in three categories A, B and C. Categories B and C have the highest and lowest losses in full load condition, respectively.Design AA' has an average load loss and relatively high no-load loss and design CC' has low no-load and load losses.On average, in these transformers no-load and load losses are 35% and 25% lower than those for conventional transformers, respectively.

AB' Transformers
AB' transformers have an average load and no-load losses with core sheets of code "m150_30_s5" based on standard ICE1998 with maximum loss of (1.07 w/kg-1.7 T).The loss values are presented in Table 1, for the conventional and low-loss transformers of class AB', with different capacities [5].Figs. 3 and 4 show magnetic flux density against core losses and saturation curve of the transformers of class AB' with core of type m150_30_s5, in logarithmic scale, respectively [6].As can be seen in Fig. 6, the amorphous core loss is lower comparing to the class AB' and conventional transformers.To study the ferroresonance phenomenon the transformer secondary is left open and the transformer core is saturated.One second after initiation of the system operation, switch b switches from the closed position to the open position.In order to investigate the ferroresonance phenomenon more accurately, it is assumed that the effect of inrush current caused by the transformer magnetic flux is completely damped during the first second.In the upcoming discussions, the voltage and current waveforms during the ferroresonance phenomenon are presented and discussed for conventional, low-loss and amorphous transformers.

Ferroresonance in conventional transformers
Conventional core transformers have the highest no-load loss.Given the flux density of 1.65 T, the core loss is 1.2 w/kg.The knee point voltage is considered to be 1.55 pu.[12].Considering the saturation curve and the loss curve, Figs.11 and

Ferroresonance in low-loss class AB' transformers
According to the loss comparison table for the conventional and low-loss transformers, it can be concluded that the load, no-load and total losses are respectively 30, 15 and 27% lower for transformer of class AB' comparing to the conventional transformers.Considering this fact and also the regarding saturation and loss curves, and with other network components unchanged, the current and voltage waveforms of phase b during the ferroresonance phenomenon are presented in Figs. 14 and 15, respectively.

CONCLUSIONS
In this paper, a test distribution system was used to investigate the ferroresonance phenomenon.With changing the system 315 kVA transformer core characteristics, the effects of the conventional, low-loss (AB' class) and amorphous transformers on the occurrence of ferroresonance were compared and investigated.Due to the high no-load loss, the conventional transformers have least impact on the occurrence of ferroresonance phenomenon.The amorphous core transformers with minimum no-load losses have the highest impact on the occurrence of ferroresonance.The impact of the low-loss transformers of class AB' is lower than amorphous transformers and higher than the conventional transformers.

Fig. 7 Fig. 7 .
Fig.7shows a single-line diagram of the system under study which is a 20 kV power distribution network.At the sending end of the network, power switches have been installed, which can be used to connect and disconnect each phase at certain times.This network includes 5 km of 20 kV underground cables.A 315 kVA transformer has been installed at the receiving end of the network.To better understand the ferroresonance concept and to investigate the effects of this phenomenon more precisely, the transformer secondary is left open

Fig. 10 .
Fig. 10.conductors' parameters for overhead lines INVESTIGATION OF FERRORESONANCE PHENOMENON One of the most common situations in ferroresonance phenomenon is disconnection of one phase of transformer, which in the case of open circuit secondary and core saturation, intensifies the ferroresonance [11].To study the ferroresonance phenomenon the transformer secondary is left open and the transformer core is saturated.One second after initiation of the system operation, switch b switches from the closed position to the open position.In order to investigate the ferroresonance phenomenon more accurately, it is assumed that the effect of inrush current caused by the transformer magnetic flux is completely damped during the first second.In the upcoming discussions, the voltage and current waveforms during the ferroresonance phenomenon are presented and discussed for conventional, low-loss and amorphous transformers.
12 give the current and voltage waveforms for phase b during the ferroresonance phenomenon, respectively.

Fig. 13 .
Fig. 13.THD for the voltage of phase b

Fig. 16 .
Fig. 16.THD for the voltage of phase bFerroresonance in amorphous transformersResults of occurrence of the ferroresonance phenomena in amorphous transformers are shown in Fig.17

Table 2 .
Critical cable lengths for different capacities