Electric Stresses on Surge Arrester Insulation under Standard and

Transcription

1 Chapter 5 Electric Stresses on Surge Arrester Insulation under Standard and Non-standard Impulse Voltages 5.1 Introduction Metal oxide surge arresters are used to protect medium and high voltage systems and insulation of equipment against overvoltages. Surge arresters are often subjected to high voltage stresses due to high earth resistance of the grid grounding system, sealing defects and environmental contamination, resonance and switching and lightning overvoltages, repeated transients of long duration and large magnitude [50, 51]. Metal-oxide surge arresters are designed to be highly resistive under normal operation and conductive under transient overvoltages. The duration of arrester current flow depends on overvoltage duration [12]. Thus, surge arresters have a nonlinear voltage versus current (V-I) characteristic [227],[22]. Assessment of the dynamic characteristics of surge arrester is an important aspect of insulation coordination. The protective performance of surge arrester models are determined by the residual voltage test [198]. The most widely used surge arresters models for accurate simulation of surge arrester behavior under different kinds of stress are the IEEE [54], Pinceti Gianettoni [228] and the Fernandez Diaz model [229]. Computer simulation of residual impulse voltage test on surge arrester models has been done using ATP/PSCAD. In this chapter, frequency-dependent surge arresters from different manufacturers, nominal discharge currents and rated voltage levels are simulated using MATLAB -Simulink. Lightning impulse residual voltage test are performed on the surge arrester and simulated results are compared with manufacturer data. The proposed approach will help to select optimum arrester model which is subjected to standard and non-standard lightning impulse voltages. 5.2 Model Descriptions The IEEE [54], Pinceti Gianettoni [228] and the Fernandez Diaz [229] surge arrester models are considered for investigation in this work. The referred models [54, 228, 229] are described below:

2 I. The IEEE model [54] Fig. 5.1 IEEE model [54] The IEEE model [54] (Fig. 5.1) has two non-linear resistances A 0 and A 1, and five linear elements L 0, R 0, L 1, R 1 and C 0. The function of the inductive elements is to characterize the model behavior with respect to fast surges [54, 229]. The non-linear elements are represented by non-linear resistances (A 0 and A 1 ) whose variations are given by equation (1) and the nonlinear V-I characteristics are shown in Fig. 5.2 [54]. The per unit voltage values are referred to the peak value of the residual voltage measured during a discharge test with a 10 ka, 8/20 µs lightning current impulse. V I characteristics of the surge arresters depends upon the wave shape of the arrester current and is expressed as [230, 231]: i = p (v/ V ref ) q (5.1) where i and v are the arrester discharge current and residual voltage respectively, p and q are constants of device and V ref is an arbitrary reference voltage near to the rated voltage, V r. V ref is calculated using the expression given as [232]: V r = kv ref (5.2) Where value of k varies between 0.9 to 0.98 The constants p and q for the segment between (I 0,V 0 ) to (I 1,V 1 ) in V-I characteristics can be obtained from the following expressions [232]: p = I 0 / (V 0 /V ref ) q (5.3) q = ln (I 1 /I 0 )/ ln (V 1 /V 0 ) (5.4)

3 Fig V-I characteristics for non linear resistors A 0 and A 1 The inductance, L 0 represents the inductance associated with the magnetic fields in the surrounding and the resistor, R 0 is used to obtain numerical stability. The capacitance, C 0 represents the external capacitance due to height of the arrester [22]. This model can give satisfactory results for discharge currents within a range of times to crest for 0.5 µs to 45 µs [54, 228, 229]. The values L 0, R 0, L 1, R 1 and C 0 are determined using the physical parameters related to overall height, diameter of block, number of columns etc. of the arrester. II. Pinceti Gianettoni model [228] Fig. 5.3 Pinceti Gianettoni model [228] The Pinceti Gianettoni model [228] (Fig. 5.3) consists of three linear elements L 0, R 0, and L 1 and two non linear resistors (A 0 and A 1 ). It is basically a simplified IEEE model [54]. Comparing the IEEE model and the Pinceti Gianettoni model, it is seen that in the later model the capacitor, C 0 is eliminated as it has negligible effect on the model behavior. Also, the two resistances R 0 and R 1 connected in parallel with the inductances L 0 and L 1 are replaced by one resistance R 0 (about 1 MΩ) between the input terminals to obtain numerical stability. The electrical data of the arrester are used to determine the model parameters. It does not take into account any physical dimension of the arrester for parameter calculation. This model can

4 give satisfactory results for discharge currents within a range of times to crest for 1 µs to 30 µs [228]. III. Fernandez Diaz model [229] Fig. 5.4 Fernandez Diaz model [229] Fernandez-Diaz model [229] (Fig. 5.4) consists of three linear elements L 0, R 0, and C and two non linear resistors (A 0 and A 1 ). It is also a simplified IEEE model [54]. R 1 -L 1 filter circuit between the non linear resistors A 0 and A 1 (as shown in Fig. 5.1) is neglected in Fernandez- Diaz model [229] and only the inductance, L 0 is taken into account. Capacitance, C is used to represent the terminal-to-terminal capacitance of the arrester. The resistance, R 0 is connected in parallel to A 0 to avoid numerical instability and oscillations. This model can give satisfactory results for discharge currents within a range of times to crest from 8 µs. 5.3 Modelling and Simulations The simulations are performed in MATLAB -Simulink using 8/20 µs standard lightning current impulse waveform having amplitude of 5, and 10 ka on surge arresters models of 15 kv, 120 kv and 150 kv rated voltages. The reference voltage of the surge arrester models are adjusted with respect to the rated voltage and manufacturer s design data. To select the optimum surge arrester model, relative errors between observed residual voltages and manufacturer data are calculated and compared. Formulas to calculate parameters of the circuits shown in Fig. 5.1, 5.3, and 5.4 were initially suggested in [54, 228, 229]. Recently, many different procedures based on numerical optimization techniques have been presented in literature for estimating the parameters in less time and more accurately. In this work, optimized values are obtained using Genetic Algorithm [233, 231], Powell s Optimization method [234] and Modified Particle Swarm

5 Optimization (MPSO) algorithm [230]. The electrical data of the tested surge arresters are given in Table 5.1 [230, 233, 234]. The model parameters are given in Table 5.2, 5.3 and 5.4 for the three surge arresters under consideration. Table 5.1 Surge arrester design data [230, 233, 234] Reference [233] Reference [230] Reference [234] Rated voltage MCOV Max.Residual voltages with 8/20 µs Height Insulation material Rated voltage MCOV Max.Residual voltages with 8/20 µs Height Rated voltage MCOV Max.Residual voltages with 8/20 µs Height 15 kv 12 kv 5 ka kv 10 ka kv 20 ka kv 183 mm Silicon rubber 120 kv 94 kv 5 ka 275 kv 10 ka 294 kv 20 ka 319 kv 1140 mm 150 kv 120 kv 5 ka 367 kv 10 ka 396 kv 20 ka 449 kv 1330 mm * MCOV: Maximum Continuous Operating Voltage Table 5.2 Model parameters for 15 kv surge arresters [54, 228, 229, 233] Initial values [54] IEEE Pinceti Gianettoni Fernandez Diaz Optimized Initial Optimized Initial Optimized values values values values values [233] [228] [233] [229] [233] R 0 Ω M 1.24 M 1 M 0.89M R 1 Ω L 0 µh L 1 µh C pf

6 Table 5.3 Model parameters for 120 kv surge arresters [54, ] Initial values [54] IEEE Pinceti Gianettoni Fernandez Diaz Optimiz Initial Optimize Initial ed values d values values values [228] [230] [229] [230] Optimized values [230] R 0 Ω M M R 1 Ω L 0 µh L 1 µh C nf Table 5.4 Model parameters for 150 kv surge arresters [54, 228, 229, 234] Initial values [54] IEEE Pinceti Gianettoni Fernandez Diaz Optimized Initial Optimized Initial values values values values [234] [228] [234] [229] Optimized values [234] R 0 Ω M M 1 M R 1 Ω L 0 µh L 1 µh C pf Observations and Validation of Model Discharge currents of 5 ka, and 10 ka of 8/20 µs is applied to the models. The residual voltages obtained are shown in Fig The maximum value of the voltage at the arrester terminals (residual voltages) is recorded and compared with the manufacturer data to validate the model. The observations are tabulated in Table 5.5 and 5.6. (a) Residual voltage obtained with initial model parameters for 15 kv surge arrester model against 5 ka, 8/20 µs current impulse

7 (b) Residual voltage obtained with optimized model parameters for 15 kv surge arrester model against 5 ka, 8/20 µs current impulse (c) Residual voltage obtained with initial model parameters for 120 kv surge arrester model against 5 ka, 8/20 µs current impulse (d) Residual voltage obtained with optimized model parameters for 120 kv surge arrester model against 5 ka, 8/20 µs current impulse

8 (e) Residual voltage obtained with initial model parameters for 150 kv surge arrester model against 5 ka, 8/20 µs current impulse (f) Residual voltage obtained with optimized model parameters for 150 kv surge arrester model against 5 ka, 8/20 µs current impulse (g) Residual voltage obtained with initial model parameters for 15 kv surge arrester model against 10 ka, 8/20 µs current impulse

9 (h) Residual voltage obtained with optimized model parameters for 15 kv surge arrester model against 10 ka, 8/20 µs current impulse (i) Residual voltage obtained with initial model parameters for 120 kv surge arrester model against 10 ka, 8/20 µs current impulse (j) Residual voltage obtained with optimized model parameters for 120 kv surge arrester model against 10 ka, 8/20 µs current impulse

10 (k) Residual voltage obtained with initial model parameters for 150 kv surge arrester model against 10 ka, 8/20 µs current impulse (l) Residual voltage obtained with optimized model parameters for 150 kv surge arrester model against 10 ka, 8/20 µs current impulse Fig. 5.5 Residual voltages obtained against discharge currents of 5 ka, and 10 ka of 8/20 µs impulse applied to surge arrester models of 15 kv, 120 kv and 150 kv rated voltage. Table 5.5 Error percentage obtained using the initial parameter values Surge arrester rated voltage 15 kv 120 kv 150 kv Input current 8/20 µs Manfacturer's data IEEE model Residual Voltage (kv) Error (%) Pinceti Gianett oni model Error (%) Fernandez Diaz model Error (%) 5 ka ka ka ka ka ka

11 Table 5.6 Error percentage obtained using the optimized parameter values Surge arrester rated voltage 15 kv 120 kv 150 kv Input current 8/20 µs Manufactu rer's data IEEE model Residual Voltage (kv) Error (%) Pinceti Gianettoni model Error (%) Fernand ez Diaz model Error (%) 5 ka ka ka ka ka ka The accuracy of the IEEE W.G [54], the Pinceti Gianettoni [228] and the Fernandez Diaz [229] models developed in MATLAB -Simulink is ascertained from the relative error values of the residual voltages which do not exceed 1.02 %. Based on model accuracy and as recommended by previous studies [ ] further study of effects of standard and nonstandard lightning impulse voltages is performed on the IEEE model. The applied nonstandard impulse voltages are generated based on actual impulse voltage waveforms observed in field as reported in [26, 27, 238]. 5.5 Impulse Voltage Test using Standard and Non-standard Impulse Voltages The IEEE W.G [54] surge arrester model has been developed in MATLAB -Simulink as shown in Fig 5.6. The IEEE W.G [54] model is subjected to impulse waveforms comprising of standard and non-standard lightning impulse voltages. Fig 5.6 IEEE MATLAB -Simulink surge arrester model

12 Six different impulse voltage waveforms including standard (full and tail-chopped) and nonstandard (tail-chopped, non-oscillating and oscillating) are applied to the 150 kv surge arrester model. The applied impulse waveforms are generated in MATLAB and simulations are performed using Simulink. Brief descriptions of the applied waveforms are given below: (i) Standard full lightning impulse voltage waveform As per IEC , a full standard lightning impulse voltage rises to its peak value in 1.2 μs and the tail of the wave decays to a level of 50 percent of the peak in 50 μs. The waveform is mathematically modeled by superposition of two exponential functions with different time constants as given in equation (5.5). The waveshape of a standard lightning impulse is shown in Fig U = 1.04 [exp (-αt) exp (-βt)] (5.5) Where α = , β = Fig.5.7 Standard lightning impulse voltage waveform (ii) Chopped lightning impulse voltage waveform A chopped wave is developed during flashover or puncture. In chopped lightning impulse, the voltage collapse on the tail is rapid compared to the rise time. The standard lightning impulse voltage waveform can be chopped on the tail, peak or front. Such impulses have a rapid voltage collapse on the tail with a small portion of negative overshoot. The standard tail chopped wave having time to chop at 2-5 µs [73]. In this work, tail chopped impulses at 8 µs and 15 µs has been used to study the effect of non-standard tail chopped impulses. The extreme value of negative overshoot of the chopped impulse is normally very small and so has been ignored in this work. In Fig. 5.8, the dotted lines shows the full standard lightning impulse voltage waveform and solid lines shows the tail chopped impulse voltage waveforms

13 a) Chopped lightning impulse voltage waveform at 3 μs b) Chopped lightning impulse voltage waveform at 8 μs c) Chopped lightning impulse voltage waveform at 15 μs Fig. 5.8 Chopped lightning impulse voltage waveform at a) 3 μs, b) 8 μs, and c) 15 μs (iii) Non-standard single pulse impulse voltage waveform Single pulsed impulse waveform is a non-oscillatory, non-standard type of impulse. Compared to the standard lightning impulse waveform, it has a steeper wavefront and short tail [27, 238]. The waveshape of a typical single pulsed non-standard impulse waveform impulse voltage waveform is shown in Fig The wavefront time is 0.8 μs and the tail of the wave decays to a level of 50 percent of the peak in 2.8 μs

14 Fig. 5.9 Single pulse non-standard impulse voltage waveform (iv) Non-standard damped oscillating impulse voltage waveform Oscillations can occur in impulse waveforms due to series and parallel resonance or disagreement in circuit parameters of impulse generator in the laboratory [26, 27, 238]. The waveshape of a typical damped oscillating impulse waveform is shown in Fig The rise time of the first wavefront is 1.9 μs and frequency of oscillation is 0.25 MHz. Fig Damped oscillating impulse voltage waveform The characteristic curves showing the simulated values of variation of maximum voltage to ground against time of occurrence are shown in Fig and Table 5.7. It has been found the maximum voltage to ground is obtained for the non-standard and non-oscillating single pulsed impulse voltage waveform. The maximum residual voltage is obtained when the time of occurrence is low

15 a) Voltage to ground observed using standard full lightning voltage impulse b) Voltage to ground observed using standard lightning voltage impulse chopped at 3 μs (a) Voltage to ground observed using lightning impulse voltage chopped at 8 μs (b) Voltage to ground observed using lightning impulse voltage chopped at 15 μs (c) Voltage to ground observed using single pulse non-standard waveform (d) Voltage to ground observed using non-standard damped oscillating waveform Fig Transient response of surge arrester model against the application of standard and non-standard impulse waveforms

16 Table 5.7 Maximum voltage to ground obtained against the application of standard and nonstandard impulse waveforms Applied Impulse Voltage Waveform Maximum voltage to ground observed (kv) Time of occurrence of maximum voltage to ground (μs) Full standard lightning impulse Impulse chopped at 3 μs Impulse chopped at 8 μs Impulse chopped at 15 μs Single pulsed non-standard impulse Damped oscillating impulse Conclusions In this chapter, the effects of standard and non-standard impulse voltage waveforms on surge arresters were investigated. Surge arrester models were developed in MATLAB -Simulink. The accuracy of the developed models was tested by comparing simulated residual voltage test results with the manufacturer s data. The maximum relative error value obtained for the residual voltages of the models was 1.02%. This confirmed that the response of MATLAB - Simulink based model is reasonably in good conformity with the manufacturer data and therefore, validates the accuracy of the model. Lightning impulse voltage test was done on 150 kv IEEE model using full and tail chopped standard lightning impulse voltage waveforms, non-standard tail chopped impulse voltage waveforms with different chopping times, non-standard as well as non-oscillating single pulsed impulse waveform and nonstandard waveform with damping oscillations. Test results showed that, non-standard impulse voltage waveforms develop high voltage stresses posing higher risk to the equipment s insulation. However, results of this study are meant to be used as the basis for further investigation of the impact of standard and non-standard impulse voltage waveforms representing the real lightning overvoltages on actual surge arresters

17 Chapter 6. Conclusions and Scope for Future Work Chapter 6 Conclusions and Scope for Future Work 6.1 Conclusions Assessment of the electrical stresses in and within the power equipment is an important aspect of insulation coordination. The voltages appearing across the insulation (as a function of time) were examined for three test equipment namely power transformer, underground cables and surge arresters against the application of wide variety of standard and nonstandard impulse voltages. High frequency models of power transformer, underground cable and surge arrester has been developed based on MATLAB -Simulink. Test results show that typical non-standard impulse waveforms have developed high voltage stresses posing highest risk to the equipment s insulation for all cases. The peak value, steepness, front and tail of wave plays a dominant role in determining the insulation performance of power equipment. In comparison to full lightning impulse, the increase in voltage observed for non-standard impulse waveforms are given as follows (Table 6.1):

18 Chapter 6. Conclusions and Scope for Future Work Table 6.1 Comparison of maximum voltage against application of standard and non-standard impulse voltage to power equipment Type of equipment Power Transformer Cable Surge Arrester Type of non-standard impulse voltage Impulse waveform with rise time of 1.9 μs and damped oscillation frequency of 0.5 MHz. Non-standard single pulsed waveform with wavefront time of 0.8 μs and a time to half-value of 2.8 μs Non-standard single pulsed waveform with wavefront time of 0.8 μs and a time to half-value of 2.8 μs Percentage increase in voltage in the equipment compared to standard full lightning impulse Voltage between the coils and ground (increase by 6.25%) and across the coils (increase by 160%) Increase by 17.53% for 35 m length of the cable Increase by 2.6% Related Standard which shall be affected IEC ed.1 (2002) IEC ed. 3 (2010) IEC ed.1.0 (2010) IEEE Std. 4 (2013) IEC ed.1 (1966) IEC ed.3 (2014) IEEE Std. 82 (2002) IEEE Std. C62.11 (2012) IEEE Std. C62.22 (2013) IEC ed. 2 (2004) However, results of this study are meant to be used as the basis for further investigation of the impact of standard and non-standard impulse voltage waveforms representing the real lightning overvoltages on power equipment. In future work, laboratory based detail examination will be done. The experimental test results would confirm the need for introducing non-standard impulse voltages or make necessary correction in the existing impulse testing standards like the IEEE Std. 4 (2013), IEC (2010), IEC (2014) etc Scope for Future Work The transient analysis results provide useful decision support for insulation coordination, developing economic design and increase reliability of the power equipment. The work in this thesis has provided many ideas for further explorations. Most important applications of this study would be:

19 Chapter 6. Conclusions and Scope for Future Work 1. Design and development of power equipment: Improvement can be made in the present designs for engineering optimization. The life span of power equipment can be increased with better design of the insulation system and insulation failures can be minimized effectively from better design and manufacturing of power equipment. 2. Identification and detection of impulse faults: Faults may be simulated deliberately within the computational models to study the fault characteristics. This is essential to know the behaviour of the power equipment so that the testing methods used in different equipment may be modified accordingly to take care of the stringent effect of standard as well as non-standard lightning impulse waveforms on the power equipment. 3. Investigate the relevance of the reference standard waveforms used for lightning impulse tests. Standardization of the voltage waveform would facilitate apparatus testing and insulation coordination. 4. Develop novel methods of prognostics for high voltage equipment: The transient response of power equipment can be used for developing online insulation diagnostics methods. An understanding of insulation degradation under the effect of different transient surges can be helpful to create predictive models of insulation breakdown and design intelligent monitoring systems. 5. Developing next generation protection systems: The smart grid assumes prominence in the wake of permeation of information technology and communication technologies (ICT) in the operation of power systems. It would be of interest to identify the possible vulnerabilities in the combined ICT and power systems and propose models for evaluating reliability of the power equipment taking into account the interactions and interdependencies and hence develop better protection systems for the smart grids