Comparison of switching surges and basic lightning impulse surges at transformer in MV cable grids Tarik Abdulahović #, Torbjörn Thiringer # # Division of Electric Power Engineering, Department of Energy and Environment, Chalmers University of Technology, 9 Gothenburg, Sweden tarik.abdulahovic@chalmers.se torbjorn.thiringer@chalmers.se Abstract The electric transients generated during the switching of a breaker placed near the transformer in a system with a substantial amount of cables can have a very short rise time and can reach very high magnitudes. In practice, the magnitude of the voltage surges is usually limited using the surge arresters while the rise time of the surge to which the apparatus is exposed, remains unchanged. In this paper, the characteristic of the switching transients generated in the MV cable grids are analyzed. For this purpose, cases with both dry-type and oil insulated transformers are studied. These cases are typical for a MV industrial grid and a wind park (WP) collection grid. Furthermore, the case with and without surge arrester protection is analyzed in order to observe the impact of the surge arresters on the voltage surges. Simulations show that the magnitude of the voltage surges exceeds the basic lightning impulse insulation level (BIL) defined by present standards. Furthermore, the rise time of the voltage surges is much shorter compared to the rise time of the lightning impulse especially in the case of the dry-type transformers and the WP collection grid. When surge arresters are used, the magnitude of the surges is limited to the BIL but the rise times of the surges can be twenty five times shorter compared to the BIL. Only when surge arresters are used with other fast transient mitigation equipment such as RC protection or surge capacitor protection, the magnitude of very fast voltage surges are brought down. I. INTRODUCTION - THE STANDARD LIGHTNING IMPULSE TEST The results obtained in this work are compared to standard values of the basic lightning impulse voltage level (BIL). The results obtained in this section are measured at a non-standard voltage level which is set below the rated voltage of the transformers, cables and other equipment. This is performed in order to avoid damages on the equipment used for testing since the simulations showed that transient overvoltages of very high magnitudes are expected during some tests. The standards for both dry-type and oil filled transformers define for each voltage level the BIL at which the transformer will not show any signs of insulation damages. These voltage impulse tests are considered as the strongest stress that can occur to the insulation of a transformer and therefore, if the transformer is able to withstand this voltage without any damages, its insulation will most probably survive other high frequency transients. The BIL defined by standards for dry-type and oilimpregnated transformers is given in Table I. TABLE I NOMINAL SYSTEM VOLTAGE AND BASIC LIGHTNING IMPULSE INSULATION LEVELS (BIL) FOR DRY-TYPE AND OIL-FILLED TRANSFORMERS [], [] BIL (kv) 0 9 0 0 00 U nom 8. (DT) S (DT) S (DT) S. (DT) S 8. S S S. S Where S is referred as the standard value, as an optional higher level where the transformer is exposed to high overvoltages, is the case where surge arresters are used and found to provide appropriate surge protection and DT is dry-type transformers. This data is used for comparison with strikes obtained by measurements and simulations. For the presentation of the voltage strikes in the scatter plots, the magnitudes of the voltage strikes are shown in per unit, where pu presents the nominal voltage of the transformer. This is done because, the maximum magnitudes of the voltage strikes obtained at any voltage level with properly calculated surge arresters is the same when presented in per unit. However, the BIL for each voltage level has a different value when expressed in pu making the direct comparison meaningless. For example, for the 8.kV level, the basic lightning insulation level is.pu for dry-type transformers and 8.pu for oil insulated transformers while for the.kv level the BIL values are.pu (with surge arrester used) and.8pu respectively. For this reason, the minimum value in per unit is used as the reference value for the dry-type and the oil insulated transformers. The comparison is done for both standards since the magnitude of the voltage strikes is the same for both dry-type and oil insulated transformers, while the difference is in the rise time of the strike.
II. SWITCHING TRANSIENTS As mentioned before, the case with only surge arresters connected to the transformer terminals is taken as the base case. The reason for having surge arresters connected all the time is just a precaution in order to avoid any damage of the transformers during experiments performed for the verification purpose. However, a simulation case without any surge protection devices is presented to illustrate the magnitude of the generated surges. This case is simulated only for the opening operation of the breaker since the highest magnitudes of surges are obtained during re-ignitions due to the voltage escalation. It is important to underline that an interruption of an inductive load is an extreme case performed to obtain the worst case scenario. However, it does not represent a normal operation in a MV cable grid where it should not be normal practice to open the breakers and switches when the transformer is fully loaded. This case could still happen accidentally, because of malfunction or other reasons. Furthermore, even if such an accident happens, the risk of generation of multiple reignitions and the voltage escalation is quite low since this phenomenon can appear only if the contacts open during a very narrow time window when the current is below the current chopping level. A. Test Setup An experimental setup is built in ABB Corporate Research, Västerås, Sweden for the purpose of high frequency transient analysis. The simulation models are developed and the simulation results are verified with good accuracy [], []. The same setup is used for the analysis in this article. Additionally, a test case with dry-type transformers is presented. The test setup is presented in Fig.. Cable SC, 0kV XLPE single core cable with the same conductor cross section; Transformer TX, 0./0.kV/kV,.M V A, Dyn Z k =.%; Transformer TX, 0/0.9kV/kV, MV A, Dyn Z k =.%; Breaker rated at kv,.ka; Two blocks of ZnO connected in series with continuous operating voltage COV =.kv, with characteristic points of ma@.kv and 0kA@8.kV ; Inductive load with 0.8mH inductance. Due to the limitations imposed by the breaker, tests are operated at.kv voltage at buses B, B and B. The transformers used in the experiments are of the oil-insulated type and the stray capacitances are measured [] in order to obtain a transformer model suitable for the high frequency transient analysis. However, the dry-type transformers were not available for the experiments and accordingly their stray capacitances are estimated. The value of the stray capacitances of the dry-type transformers are in order of hundreds of pico Farads [] or approximately ten times lower compared to those of the oil-insulated type. B. Switching Surge Analysis Since the worst case scenario is chosen for the opening of the breaker, voltage escalations occur. The number of reignitions is very high due to the very fast transient recovery voltage (TRV). A base case is obtained with surge arresters connected to transformer TX and the voltage plot presented in Fig. shows phase voltages at the high voltage side of transformer TX terminals. 0 U TX (kv) 0 0 0 0 Fig.. 8 0 t (ms) The base case voltage transient at transformer TX - simulation Fig.. The cable lab measurement setup The rating of the equipment installed in the cable lab is as follows: Cable SC, SC and SC, 0kV XLPE three core cable with 0mm cross section of conductor; Since the high frequency transients transients contain a large number of voltage strikes of different rise times where the steepest ones are obtained during the voltage breakdowns (BD) between the contacts of the breaker, a characterization method is introduced [], [] where voltage steps are sorted by its rise time and magnitude. For illustration, a very narrow time
interval which shows voltage escalations recorded during an experiment is presented in Fig.. U(kV) 0 0 0 0 0 Fig.. BD voltage step Voltage oscillations during HF current TRV 8.0 8.08 8. 8. 8. 8. t (ms) Characteristic voltage steps during transient - measurement The green line in Fig. present the recorded signal, while red lines are voltage steps identified by the characterization processing performed during data analysis. Once the voltage steps are identified, scatter plots are made where x-axis shows the rise time and y-axis shows the magnitude of the voltage steps. C. Switching Surges With Oil-insulated Transformers The simulation results are presented using the scatter plots where a voltage step with its magnitude and rise time are shown. The fastest voltage steps are the result of the voltage breakdown between the contacts of the breaker and in the case of the oil-insulated transformer rise time is reduced to less than half compared to the BIL. This is observed in Fig.. U TX BIL DT BIL OI 0 Fig.. Voltage strikes at TX - oil-insulated transformer with surge arrester Plots marked by BIL-DT and BIL-OI represent cases of BIL for the dry-type transformer with surge arrester used which is marked by in Table I and BIL for the oil-insulated transformer marked by S in the same table, respectively. Blue, red and green dots present voltage steps in, and, respectively. Although the magnitude of the voltage strikes never reached the level of.8pu defined by standards, the rise time of breakdown voltage steps reached 0.µs which is. times quicker than the BIL. For the case where surge arresters are not used, higher magnitudes of the voltage steps are obtained while the rise time remained the same. This is because the rise time is determined by the stray capacitance of the transformer and the surge impedance of the cable. Simulation results obtained without any surge arresters used, is presented in Fig.. U TX BIL DT BIL OI 0 Fig.. Voltage strikes at TX - oil-insulated transformer without surge arrester In Fig. it can be seen that the magnitude of the highest voltage steps surpasses the BIL set by IEEE standards for the oil-insulated transformers. Furthermore, the rise time of the highest voltage steps is approximately. times shorter than the rise time of the BIL which makes recorded voltage strikes potentially even more dangerous to the insulation according to similar studies performed on induction machines []. D. Switching Surges With Dry-type Transformers The case with dry-type transformers is also performed with and without surge arresters installed. In this case, transformer TX is a dry-type transformer and all other parameters of the test circuit are the same as in the previous case. Simulation results obtained when the transformer is protected using surge arresters are presented in Fig.. The simulation results are compared to plots marked by BIL-DT and BIL-DT which correspond to the BIL for drytype transformers with surge arrester installed and optional
BIL DT BIL DT BIL DT BIL DT U TX U TX 0 0... 0 0... Fig.. Voltage strikes at TX - dry-type transformer with surge arrester Fig.. Voltage strikes at TX - dry-type transformers without surge arrester higher level marked by and in Table I, respectively. In Fig. it can be seen that the magnitude of the voltage steps is on the limit defined by the IEEE standards. However, the rise time of the voltage strikes is much shorter now and reaches approximately 0ns which is nearly ten times quicker compared to the voltage steps generated with oil-insulated transformers. This is expected since the stray capacitance of the dry-type transformer is ten times smaller. Although standards for induction motors define for such very fast transients much lower magnitude of the voltage step when compared to the BIL [], that is not the case with standards for transformers. When the case of a dry-type transformer without any surge arresters installed is observed, magnitudes of the voltage step reach approximately the same level as in the case with an oil-insulated transformer. This is shown in Fig.. In Fig. it can be seen that the voltage steps have higher magnitudes than the optional higher level defined by the IEEE standards. Furthermore, the rise time of the voltage steps is approximately 0ns which is almost times quicker than that of the lightning impulse. E. Surge Capacitor Protection Surge capacitors have been commonly used as protection device to mitigate transients. The combination of surge capacitors and surge arresters has been used to protect medium voltage induction motor windings from steep-fronted voltage surges [8]. The purpose of using the surge capacitor is to reduce the rise time of the surge [9]. The rise time of the transient is determined by the capacitance of the surge capacitor and the surge impedance of the cable connected to the protected device. Since the capacitance of the surge capacitor is much larger than the stray capacitance of the protected device, its stray capacitance is neglected, which gives t rise =.Z cab C cap. () In order to set the rise time of the transients to be slower than the lightning impulse, the minimum capacitance of the surge capacitor is set by C cap.z cab. (). 0 In order to have a more realistic simulation, the inductance of the leads which connect surge capacitors to the terminals of the protected transformer is accounted. An inductor of.µh inductance is added in the simulation to account for stray inductance of the surge capacitance lead. The results of the surge capacitor protection are shown only for the case with dry-type transformers since the impact of the protection is the same for both transformer types, where in the case with the dry-type transformer substantially quicker voltage steps are obtained. In Fig.8 simulation results of the case with a dry-type transformer and a case with surge arrester and a surge capacitor can be observed. It can be seen that the magnitude of the transients decreased significantly. The rise times of some voltage steps are still short due to the stray inductance of the surge capacitor leads. F. RC Protection The RC protection is used for mitigation of the very fast transients and protection of transformers and induction motors [0], []. It is very effective and proved to reduce the number of reignitions [0]. The principle of an RC protection is very simple. The wave reflections which give the highest magnitude of transient overvoltages occur in the systems where the
U TX BIL DT BIL DT 0 0... Fig. 8. Voltage strikes TX - dry-type transformer with surge arrester and surge capacitor surge impedance of the transformer Z tr is much higher than the cable surge impedance Z cab. In order to prevent such reflections, a resistor with a capacitor is connected in parallel to the protected transformer. The resistance of the resistor is chosen so that it matches the surge impedance of the cable Z cab. The surge capacitor is added in series with the resistor in order to increase the impedance for low frequency signals and to increase the rise times of the voltage surges. The simulation is performed only with a dry-type transformer protected with surge arresters. The results obtained are presented in Fig.9. BIL DT BIL DT In Fig.9 it can be seen that the RC protection combined with surge arresters protected the transformer as good as the combination of surge arresters and surge capacitors. Both RC and surge capacitor protection combined with the surge arresters provided good protection of the transformer by lowering the magnitude of the voltage steps and increasing their rise time. With this protection added, the protected transformer is exposed only to transients of a magnitude more than two times lower than the BIL. ACKNOWLEDGEMENT The financial support provided by Vindforsk is gratefully acknowledged. REFERENCES [] IEEE standard test code for dry-type distribution and power transformers, IEEE Std C..9-00, 00. [] IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers, IEEE Std C..-00, March, 008. [] M. Reza, H. Breder, Cable System Transient Study - Vindforsk V- 0. - Experiments with switching transients and their mitigation in a wind power collection grid scale model, Vindforsk Tech. Rep., Sweden, January, 009. [] T. Abdulahovic, Analysis of High-Frequency Electrical Transients in Offshore Wind Parks, Licentiate Thesis, Chalmers University of Technology, Gothenburg, Sweden, April, 009. [] M. Boyra, Transient Overvoltages in Cable Systems Part - Experiments on fast transients in cable systems, Masters Thesis, Chalmers University of Technology, Gothenburg, Sweden, 00. [] J. H. Harlow, Electric Power Transformer Engineering, CRC Press, 00. [] B. K. Gupta, N. E. Nilsson, D. K. Sharma, Protection of Motors Against High Voltage Switching Surges, IEEE Trans. on Energy Conversion, vol., no., March, 99. [8] R. L. Doughty, F. Heredos, Cost effective motor surge capability, in Proc. Petroleum and Chemical Industry Conference, 99. Record of Conference Papers., Industry Applications Society nd Annual, pp. 9-0, - Sep. 99. [9] D. C. Bacvarov, D. W. Jackson, C. L. Lee, Effect of Surge Capacitor Lead Length on Protection of Motors from Steep Switching Surges, IEEE Transactions on Power Apparatus and Systems, vol. PAS-0, no., pp. 89-88, July 98. [0] J.P. Eichenberg, H. Hennenfent, L. Ljiljestrand, Multiple Re-Strikes Phenomenon when Using Vacuum Circuit Breakers to Start Refiner Motors, Pulp & Paper Canada vol. 98, no., pp. -, July 99. [] M. Popov, L. van der Sluis, G.C. Paap, A simplified transformer model for the simulation of fast surges and adequate protection measures, Power Engineering Society Winter Meeting, 00. IEEE, vol., pp. -8, 8 Jan- Feb 00. U TX 0 0... Fig. 9. Voltage strikes at TX - dry-type transformer with surge arrester and RC