The Effect of Various Types of DG Interconnection Transformer on Ferroresonance

Transcription

1 The Effect of Various Types of DG Interconnection Transformer on Ferroresonance M. Esmaeili *, M. Rostami **, and G.B. Gharehpetian *** * MSc Student, Member, IEEE, Shahed University, Tehran, Iran, E mail: m.esmaeili@shahed.ac.ir **Associate Professor,Member, IEEE, Shahed University, Tehran, Iran, Rostami@shahed.ac.ir *** Professor, Senior Member, IEEE, Amirkabir University of Technology, Tehran, Iran, grptian@aut.ac.ir Abstract: When switching occurs in a distribution network, in the presence of distribution generators, overvoltage is a possibility due to ferroresonance especially in unbalanced switching. This paper focuses on ferroresonance phenomenon in distribution networks in the presence of Distributed Generation (DG). The main goal of this article is to determine the effect of various types of DG interconnection transformers on balanced and unbalanced ferroresonance. The studies are performed based on a digital computer simulation approach using the PSCAD/EMTDC software package. Fig. 1 shows a single line diagram of the studied system. Keywords: Distributed generation, ferroresonance, interconnection transformer, island, overvoltage 1. Introduction Ferroresonance is a non-linear resonance phenomenon that can affect distributed networks [1]. It can cause abnormal rates of harmonics and transient or steady state overcurrent and overvoltage which are great threats to the electrical equipment[2,3]. Single or more-phase switching or disturbances such as short-circuits are the factors which can initiate this phenomenon [4]. When an isolated generator is connected to a network whose capacitance is equal to, or greater than the magnetizing reactance requirement, ferroresonance would occur. The ferroresonance may happen with both induction and synchronous machines [5]. The ferroresonance can result in significant overvoltage where peak voltage can reach 3 to 4 per unit [6].The following conditions are necessary for ferroresonance to occur: 1. The DG must be separated from the utility source 2. The KW load in the island must be less than three times the rating of the DG. 3. Adequate capacitance must be available on the island to resonate (typically % of the generator rating). 4. There must be a transformer in the circuit in order to provide nonlinearity [7]. The direct connection of a distributed generator with the power network is risky since generators possess an isolation level which is incompatible with the utility system [8]. The phenomena of balanced and unbalanced ferroresonance may occur in several transformer connection patterns. The impact of five common coupling transformer winding configurations on ferroresonance will be discussed in this article. 2. System Description DG Fig. 1: Single-line diagram of the studied system To investigate different operational scenarios, the PSCAD/EMTDC software package is used to develop a time domain simulation model of the studied system in Fig. 1. The main grid is represented by a threephase voltage source and DG is modeled as a synchronous machine. The parameters of electrical equipment are shown in Table I. Electrical Components Generator frequency total load capacitor Transformer1(T1) Transformer DG(T2) Transformer3(T3) Transformer3(T4) TABLE I Parameters of the system Parameters 38 kva, 7 kv 60Hz 10 kw 100 kva 300kVA 13.8kV/0.4kV 200KVA 7kV/13.8KV 300kVA 13.8kV / 0.48kV 100kVA 0.48kV / 0.228kV Table II shows the transformers parameters used in the PSCAD simulation.

2 TABLE II Parameters of the Distribution transformers characteristic Parameters Impedance no load losses copper losses. Air core reactance Knee Voltage Magnetizing current The simulation was run for 0.2 seconds, with 5 μs steps. recloser opens at 0.1 second. 3. Analytical Investigation 3.1 Delta on DG side and solidly-grounded wye on system side This connection causes the DG system to introduce an effectively grounded source to the utility distribution system also this interconnection blocks the DG generated harmonics by the delta connection and to avoid the ferroresonance. Figs. 2-4 illustrate the waveforms of phase to for delta/grounded wye connection during balanced and unbalanced switching. Fig. 5 illustrates the waveform of phase to ground voltage ( ) for delta/grounded wye connection when ground fault occurs in the main feeder after breaker (between breaker and DG in Fig. 1). single-phase Fig. 3: Voltage waveform of phase to ground voltage ( ) ( ) Three phase switching single-phase switching Fig. 4: Voltage waveform of phase to ground voltage ( ). Three phase breaker opens at 0.1 second Fig. 5: Voltage waveform of phase to ground voltage ( ) during fault condition. Fault occurs at 0.1 second in phase B. Fig. 2: Voltage waveform of phase to ground voltage ( ) ( ) Two phase switching 3.2 Delta on DG side and isolated-neutral wye on system side Due to the lack of ground reference in ungrounded connections, these connections can continue supplying an isolated portion of the system after a ground fault occurrence in the main feeder that has caused ferroresonance. Figs. 6-8 illustrate the waveforms of phase to ground voltage ( healthy phase, interrupted phase) for

3 delta/isolated-neutral wye connection during balanced and unbalanced switching. Fig. 9 illustrates the waveform of phase to ground voltage ( ) for delta/isolated-neutral wye connection when ground fault occurs in the main feeder after breaker (between breaker and DG in Fig. 2). single-phase Fig. 7: Voltage waveform of phase to ground voltage ( ) ( ) Three phase switching single-phase switching Fig. 8: Voltage waveform of phase to ground voltage ( ). Three phase breakers open at 0.1 second Fig. 9: Voltage waveform of phase to ground voltage ( ) during fault condition. Fault occurs at 0.1 second in phase B Fig. 6: Voltage waveform of phase to ground voltage ( ) ( ) Two phase switching 3.3 Delta connection in both sides Overvoltage and ferroresonance occur due to lack of ground reference in delta/delta connection although the 3 rd harmonics or its multiples of 3 orders are isolated. Figs illustrate the waveforms of phase to for delta/delta connection during balanced and unbalanced switching. Fig. 13 illustrates the waveform of phase to ground voltage ( ) for delta/delta connection when ground fault occurs in the main feeder after breaker (between breaker and DG in Fig. 1). single-phase grounding fault occurs at 0.1 second in phase B and the 3.3.1single-phase switching

4 Fig. 10: Voltage waveform of phase to ground voltage ( ) ( ). Fig. 13: Voltage waveform of phase to ground voltage ( ) during fault condition. Fault occurs at 0.1 second in phase B Two phase switching 3.4 Grounded wye on DG side and delta on system side The voltage waveforms have a great distortion during balanced and unbalanced ferroresonance. Ferroresonance overvoltage reaches up to 4p.u. in transformer. Figs illustrate the waveforms of phase to for grounded wye/delta connection during balanced and unbalanced switching. Fig. 17 illustrates the waveform of phase to ground voltage ( ) for grounded wye/delta connection when ground fault occurs in the main feeder after breaker (between breaker and DG in Fig. 1). single-phase 3.4.1single-phase switching Fig. 11: Voltage waveform of phase to ground voltage ( ) ( ) Three phase switching Fig. 12: Voltage waveform of phase to ground voltage ( ). Three phase breaker opens at 0.1 second Fig. 14: Voltage waveform of phase to ground voltage ( ) ( ) Two phase switching

5 for ground wye/grounded wye connection during balanced and unbalanced switching. Fig. 21 illustrates the waveform of phase to ground voltage ( ) for ground wye/grounded wye connection when ground fault occurs in the main feeder after breaker (between breaker and DG in Fig. 1). single-phase single-phase switching Fig. 15: Voltage waveform of phase to ground voltage ( ) ( ) Three phase switching Fig. 18: Voltage waveform of phase to ground voltage ( ) ( ) Two phase switching Fig. 16: Voltage waveform of phase to ground voltage ( ). Three phase breaker opens at 0.1 second Fig. 17: Voltage waveform of phase to ground voltage ( ) during fault condition. Fault occurs at 0.1 second in phase B 3.5 Grounded wye on both sides During the event of islanding operation the ground connections impedes the ferroresonance phenomenon. This connection does not completely eliminate the ferroresonance because it is not able to eliminate 3 rd harmonics and its multiples. Figs illustrate the waveforms of phase to Fig. 19: Voltage waveform of phase to ground voltage ( ) ( ).

6 3.5.3 Three phase switching Fig. 20: Voltage waveform of phase to ground voltage ( ). Three phase breakers open at 0.1 second. [4] P. Ferracci, "Ferroresonance", Groupe Schneider: Cahier technique no190, // pp. 1-28, March [5] D. Tourn, J.C. Amatti, J.C. Gomez, and E.F. Florena, Behavior of the scheme source capacitor induction motor when voltage sags and short interruptions take place, IEEE/PES Latin America Transmission and Distribution Conference and Exposition, Caracas, Venezuela, [6] P. Barker, "Overvoltage considerations in applying distributed resources on power systems," in Power Engineering Society Summer Meeting, 2002 IEEE, 2002, pp [7] W. Feero and W. Gish, "Overvoltages caused by DSG operation: synchronous and induction generators," Transactions on Power Delivery, vol. 1, pp , [8] R. Arritt and R. Dugan, Distributed generation interconnection transformer and grounding selection, IEEE PES General Meeting, Fig. 21: Voltage waveform of phase to ground voltage ( ) during fault condition. Fault occurs in phase B at 0.1 second 5. Conclusion This paper investigates various islanding scenarios of a Distributed Generation from the main grid. The simulation results confirm that: 1) Delta / Grounded wye transformer avoids the ferroresonance when the DG becomes islanded. 2) Grounded wye/delta connection is very prone to ferroresonance in cable-fed installations, especially during the lack of one phase in the feeder. 3) Ground wye/ground wye connection has less concern over ferroresonance but it is not completely immune to ferroresonance. 4) In delta/delta connection, balanced and unbalanced ferroresonance can take place in the case of islanding. 5) The most susceptible transformer connections to ferroresonance are the ungrounded transformers because they are mostly connected to the load by high capacitance via ground cable conductors rather than overhead lines. When islanding occurs in the presence of a ground fault, the ground connection is not sufficient to avoid ferroresonance. REFERENCES [1] W. B. Gish, W.E. Feero, and S Greuel, Ferroresonance and Loading Relashionships for DSG Installations, IEEE Transactions on Power Delivery, vol. PWRD-2, no. 3, pp , July [2] G. Kumar, S. Rajan, R. Rangarajan, "Analysis of ferroresonance in a power transformer with multiple nonlinearities," International Journal of Emerging Electric Power Systems, vol. 7, [3] C. Charalambous, Z.D. Wang, J. Li, M. Osborne, P. Jarman, Validation of a Power Transformer Model for Ferroresonance with System Tests on a 400 kv Circuit, International Conference on Power Systems Transients (IPST 07), Lyon, France, 4-7 June, 2007.