24 th International Conference on Electricity Distribution Glasgow, June Paper 0790

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1 STUDY AND COMPARISON OF THE EFFECT OF CONVENTIONAL, LOW LOSSES AND AMORPHOUS TRANSFORMERS ON THE FERRORESONANCE OCCURRENCE IN ELECTRIC DISTRIBUTION NETWORKS Mehrdad Hajizadeh Iman Safinejad Nima Amirshekari ABSTRACT 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 words- Amorphous 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 []. Given the complexities, difficulties and considerable consequences in the event of ferroresonance phenomenon, several methods have been proposed in order to identify its characteristics []. 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 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, 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.

2 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 DIN 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 are also categorized in three categories A, B and C. Categories B and C have the highest and lowest 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. On average, in these transformers no-load and load are % and 2% lower than those for conventional transformers, respectively. AB' Transformers AB' transformers have an average load and no-load with core sheets of code m1 s based on standard ICE1 with maximum loss of (1. w/kg-1. T). The loss values are presented in Table 1, for the conventional and low-loss transformers of class AB', with different capacities []. Figs. and show magnetic flux density against core and saturation curve of the transformers of class AB' with core of type m1 s, in logarithmic scale, respectively []. Table1. Compare conventional and low-loss transformers Capacity (KVA) Load (W) Normal Trans No load (W) Total Low_loss Trans load (W) No load (W) Core Type: M1 - S Fig.. magnetic flux density against core 1 2 Exciting Force(A/m) Core Type: M1 - S Fig.. saturation curve of the transformers of class AB' Amorphous transformers Core Loss(W/Kg) Amorphous transformer cores are made of amorphous alloy. This alloy, with a non-crystalline property, accelerates the reversal of the magnetic flux and change in the direction of magnetic poles. This means that less energy is consumed in each cycle for the magnetization and demagnetization, which reduces the core. The area restricted by the hysteresis curve of the amorphous core transformers is smaller comparing to conventional transformers used in distribution networks. This reduces the no-load of the transformer []. The saturation point of the common transformers core is about 2T. This point reduces to 1. T in amorphous cores, which restricts the operating point of these transformers to 1. T. Amorphous cores saturation point generally occurs in % of nominal load []. The hysteresis curve of amorphous core of type 2As1 and the inter loop are shown in Fig.. The magnetic flux density in Tesla against the core in watts per kilogram (W/kg) is shown in Fig. []. Fig.. hysteresis curve of the amorphous core

3 Core Type: 2sA R2 R R R R1(core inner radius )= R2(core outer radius)=/ cm R(shell inner radius)=1/ cm R(shell outer radius)=1/1 cm R(secend insulation outer radius)=1/ cm µ11 (innermost insulation permeability coefficient )= 1 µ12 (outermost insulation permeability coefficient )= 1 ℇ11 (innermost insulatio Dielectric coefficient)=2/ ℇ12 (outermost insulationdielectric coefficient) = AC -2-1 Core Loss(W/Kg) Fig.. core loss curve As can be seen in Fig., the amorphous core loss is lower comparing to the class AB' and conventional transformers. THE SYSTEM UNDER STUDY Fig. shows a single-line diagram of the system under study which is a 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 km of kv underground cables. A 1 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. Circuit Breaker Z eq =Z ground cable +Z ohead line 1 KVA Distribution Trans Open Circuit Fig.. single-line diagram of the system under study Fig. shows the system simulated using PSCAD software. Figs. and show the information on the cables used in the underground network and the conductors used in the overhead lines, respectively []. Fig.. the system under study in PSCAD software µs(shell Relative permeability coefficient )=1 µc(core Relative permeability coefficient) =1 ρs(shell resistivity)=1/ ˣ - ρc (core resistivity)=1/ ˣ - Pole Height=12 m Flash between two pole=2.2m Fig.. parameters of kv cables / cm 1 cm R=/ Ω/Km X=/1 Ω/Km conductor outer radius= / cm Fig.. 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. Ferroresonance in conventional transformers Conventional core transformers have the highest no-load loss. Given the flux density of 1. T, the core loss is 1.2 w/kg. The knee point voltage is considered to be 1. pu. [12]. Considering the saturation curve and the loss curve, Figs. 11 and 12 give the current and voltage waveforms for phase b during the ferroresonance phenomenon, respectively.

4 Time(s) Fig. 11. current waveform for phase b Fig. 1. voltage waveform for phase b y Fig. 12. voltage waveform for phase b Figs. 1 present the Total Harmonic Distortion () for the voltage of phase b Fig. 1 presents the for the voltage of phase b. domain Time(s) Fig. 1. for the voltage of phase b Ferroresonance in amorphous transformers Results of occurrence of the ferroresonance phenomena in amorphous transformers are shown in Fig. 1.. Time(s) Fig. 1. for the voltage of phase b 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 are respectively, 1 and 2% 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. 1 and 1, respectively. Time(s) Fig. 1. current waveform for phase b Time(s) Fig. 1. current waveform for phase b Fig. 1. voltage waveform for phase b Figs. 1 present the for the voltage of phase b.

5 domain Time(s) Fig. 1. for the voltage of phase b THE EFFECT OF CABLE LENGTH ON THE OCCURRENCE OF FERRORESONANCE PHENOMENON Since the length of the medium voltage cable affects the capacitance needed for occurrence of the ferroresonance phenomenon, separate simulations are conducted for the three types of transformers under study with different capacities, and the critical cable lengths, which cause dangerous resonances, are calculated and provided in Table 2. Table 2. Critical cable lengths for different capacities amorphous Max Transformer type Low-loss Conventional Cable length for occurrence of ferroresonance phenomenon Min Max Min Max CONCLUSIONS Min Transformer capacities In this paper, a test distribution system was used to investigate the ferroresonance phenomenon. With changing the system 1 kva transformer core characteristics, the effects of the conventional, low-loss (AB' class) and amorphous transformers on the occurrence of ferroresonance phenomenon 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 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. REFERENCES [1] Kruno Milicevic, Zia Emin, Initiation of Characteristic Ferroresonance States Based on Flux Reflection Model,IEEETransactions on Circuits and Systems Vol., No. 1, January 1 [2] JoséC. Lacerda Ribas, Elizete M. Lourenço, JeanVianei Leite, and NelsonJ. Batistela, Modeling Ferroresonance Phenomena With a Flux-Current Jiles- Atherton Hysteresis Approach IEEE Transactions on Magnetics.Vol..No.May1 [] Amir Tokić, Jasmin Smajić, Modeling and Simulations of Ferroresonance by Using BDF/NDF Numerical Methods, IEEE Transactions on Power Delivery1 []Paul S. Moses, Mohammad A. S. Masoum, Hamid A. Toliyat, Impacts of Hysteresis and Magnetic Couplings on thestability Domain of Ferroresonance in Asymmetric Three-Phase Three-Leg Transformers IEEE Transactions on Transactions on Energy Conversion, Vol. 22, No. 2, june 11 [] M. Oladi, H. Anbiaei, N. Moghise, Technoeconomic analysis of low-loss distribution transformers class AB, th conference on distribution grids, Zahedan, May 1. [] Technical Manual of Baosteel GO Silicon Steel Products Electrical Steel Website: [] R. Hasegawa Metglas, Inc Energy Efficiency of Amorphous Metal Based Transformers Website: [] F. Firouzabadi, Introduction to structure and technology of amorphous core transformers, Vice Chancellor for Research and Development, iran transfo co. [] Metglas Magnetic Material, Amorphous Alloys for Transformer Cores april11 Website: [] A. Sedighi Anaraki, High impedance fault detection in electric energy distribution grids, Ph.D. thesis, Electrical engineering Dep., Tarbiat Modares University,. [11] Roger C.Dogan, Mark F.Mc Granaghan,Surya Santos and H.Wayne Beaty Electrical Power Systems Quality,second Edition [12] Badmanathan Tanggawelu, R. N. Mukerjee, Aznan Ezraie Ariffin Ferroresonance Studies in Malaysian Utility s Distribution Network Power Engineering Society General Meeting,, IEEE