# Session Four: Practical Insulation Co-ordination for Lightning Induced Overvoltages

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1 Session Four: ractical Insulation Co-ordination Session Four: ractical Insulation Co-ordination for Lightning Induced Overvoltages Jason Mayer Technical Director, Energy Services, Aurecon Introduction This paper provides guidance on how to undertake insulation co-ordination simulations in high voltage systems, using time domain software. There are a number of methods used in industry to complete this task. Some are simplified approximations, others involve statistical analysis. This is just one method, but it is a practical means of assessing the level of protection afforded by surge arresters in high voltage systems, against lightning strokes to exposed conductors. The paper does not consider overvoltages from switching transients or other sources, although some of the principles presented could be used in this area. What is insulation co-ordination? For the purposes of this paper, insulation co-ordination is defined as the process of determining the appropriate placement and selection of surge arresters (or other surge protective devices), such that lightning induced overvoltages do not exceed the impulse withstand level (aka BIL) of installed equipment. When a lightning stroke hits an exposed conductor, large currents flow along the conductors, causing significant overvoltages. If the resulting overvoltages exceed the BIL of installed equipment, it is likely that damage to the insulation will occur. It is only likely because BIL is a statistical parameter. The BIL of insulation is defined as the voltage at which there is a 90% probability that the insulation will not flashover or fail. Insulation may fail at voltages lower than its BIL, or it might pass with voltages above its BIL. However in practical terms, we assess an insulation failure as an exceedence of the BIL value, and apply some statistical analysis to the probability of this failure occurring. This provides a reasonably objective assessment of the risk of failure. Case study system At this point it is useful to introduce a case study network, in order to understand what we are trying to achieve with an insulation co-ordination simulation. 203 Earthing, Lightning & Surge rotection IDC Technologies

2 Session Four: ractical Insulation Co-ordination Figure shows a typical 32 kv double circuit transmission line. Half way long this feeder, a simple 32/33 kv substation is established, with a single circuit tee connection into the substation. Within the substation a 32 kv busbar system connects the transmission line to a 5 MVA 32/33 kv transformer. The busbar is provided to allow future expansion of the substation to a conventional two transformer, two feeder substation. At this point there are only disconnectors, surge arresters, CVT and the busbar. It is assumed the CT s are in the transformer bushings. Transmission line 32 kv 33 kv Figure Case study network line diagram It is proposed that surge arresters be placed at the line entry to the substation, as well as on the transformer high voltage bushings. This is a conventional arrangement. The system is effectively earthed, so for earth faults, the healthy phases will not rise more than.4 pu. The maximum normal phase to phase voltage will not exceed 45 kv. Therefore the maximum phase to earth voltage occurring on a healthy phase during an earth fault is: Therefore 20 kv 3HSRE arresters are selected, with a maximum continuous operating voltage (MCOV) of 98 kv and a temporary overvoltage rating of 45 kv. They will not conduct during a 32 kv system earth fault. The time domain simulation is required to assess whether this particular combination of surge arresters and their location with respect to the transformer, is appropriate. 203 Earthing, Lightning & Surge rotection IDC Technologies 2

5 Surge Voltage (kv) Session Four: ractical Insulation Co-ordination influence the results (ie it is 20km away no reflections come back from the other end in the time frame considered). The surge current travels at slightly less than the speed of light (approx 300m per us). The surge impedance of a typical transmission line conductor is generally between 400 and 500 ohms. A surge current 6 ka in this example, results in a surge voltage of approximately.5 MV shortly after the application of the current. It can be seen that approximately 3.3 us after the initial strike, a surge voltage appears at the end of the line (000m at 300m/us), but because the line is open, the surge reflects back on itself, causing a doubling of the voltage. The reflected surge then travels back to the strike location. Therefore at 6.6 us after the initial strike the voltage at the strike location is still high due to surge current still being injected, however the returning surge from the end arrives and causes almost a doubling in voltage. This assumes a conductor in air, with no flashover of insulators, so the surge current will reflect back and forth until it eventually attenuates. 3.5k 3.0k 2.5k Stroke Main : Surge Voltages - o Tow er Models End 2.0k.5k.0k 0.5k k -.0k Time m 0.020m 0.030m 0.040m 0.050m 0.060m Figure 4 Surge voltages on a conductor In a cable the situation is very similar, except that the surge impedance of a cable is closer to 30 ohms and the travel time is approximately ⅓ of the speed of light. This is because surge impedance is calculated using the inductance (L) and capacitance (C) of the conductor or cable. An overhead line tends to have higher L and lower C, where a cable has lower L and higher C. Transmission line towers The transmission line model needs to include back flashover models of the transmission towers and insulators. It also needs the tower structure and 203 Earthing, Lightning & Surge rotection IDC Technologies 5

6 Session Four: ractical Insulation Co-ordination earthing of the overhead earthwire included. The purpose of an overhead earthwire is to minimise the number of lightning strokes that terminate on the phase conductors. When a lightning stroke hits an overhead earthwire, the voltages on the struck conductor and tower top build up very quickly and if the insulation strength of the air gap between conductors or of the insulator/crossarm combination is exceeded, a flashover will occur and the surge current will propagate into the phase conductors. This event is called a back flashover. This effect is included in the model by creating flashover switches which measure the phase to tower voltages and determine the strength of the air gap. The insulation strength of the air is modelled using the time dependent characteristics described in [2]. The time dependent characteristics are shown in Figure 5. The air gap strength drops with respect to time in microseconds after the surge arrives at the insulator. The lower value of insulation strength reflects a higher probability of back flashover to the phase conductors. Figure 5 Insulator strength characteristics [2] Figure 6 shows the tower model from the SCAD file for the case study. ote that the mast footing resistance is modelled using a variable resistance that takes the soil ionisation into account when the surge current dissipates into the soil via the mast, as described in []. The masts are modelled as travelling wave models with a surge impedance and a propagation velocity as described in [] and some other related references. 203 Earthing, Lightning & Surge rotection IDC Technologies 6

7 Rt + BRKC + StringCap BRKB + StringCap BRKA + StringCap Session Four: ractical Insulation Co-ordination G2 G Overhead earthwire connections Th + StringCap TH2 A2 B2 C2 BRKA2 BRKB2 BRKC2 Th3 Th4 A B C StringCap StringCap + + Conductor connections Travelling wave tower segment models Footing resistance Tower Footing Resistance Figure 6 Typical 32 kv transmission tower geometry 203 Earthing, Lightning & Surge rotection IDC Technologies 7

8 Surge Voltage (kv) Session Four: ractical Insulation Co-ordination The tower models are added to the transmission line conductor models. Comparing Figure 4 to that of Figure 7 below, the effect of the tower model can be seen. The same current is injected into the earthwire of a transmission line km from the open end. The surge voltages are significantly lower because the current can split and travel down the towers to earth. otice that the waveforms are heavily distorted. This is because of the numerous reflections that occur between the conductor, towers, earth, and the end of the line. In this particular graph there is no insulator flashover, because the surge current to the earthwire is only 6 ka. 300 Stroke Main : Surge Voltages - With Tow er Models End Time m 0.020m 0.030m 0.040m 0.050m 0.060m Figure 7 Surge voltages on a conductor with towers modelled Surge arresters Surge arresters are modelled as non-linear elements using SCAD s metal oxide surge arrester component along with R, L and C components in line with the IEEE fast front arrester model described in [3]. Refer to Figure 8 for details. The surge arrester models are tested with various stroke currents and matched to the residual voltage values in the manufacturers datasheets. The surge arresters are modelled with downleads to the earth grid. The downleads are modelled as travelling wave models with a surge impedance and a propagation velocity as described in []. 203 Earthing, Lightning & Surge rotection IDC Technologies 8

9 Session Four: ractical Insulation Co-ordination Surge Arrester: 20 kv 3HSRE (D is length in metres) D =.428 metres R0(ohms)=00.D L0(uH)=0.2*D 42.8 [ohm].285e-6 [H] R(ohms)=65.D L(uH)=5*D 92.8 [ohm] 2.4E-6 [H] 70E-6 [uf] A0 A C(pF)=00/D Figure 8 Fast front surge arrester model Transformers Under lightning conditions the transformers act as surge capacitances. Hence for modelling purposes, transformers are represented by the winding surge capacitance between each phase and earth. The transformer surge capacitances are determined by the transformer size in MVA and the transformer BIL rating as described in []. The transformer BIL rating for the case study is assumed to be 650 kv, which is typical for 32 kv systems. Voltage transformers and other equipment Under lightning conditions the other equipment in the substation such as disconnector insulators, CVT s station post insulators etc, act as surge capacitances. Hence for modelling purposes, they are represented by surge capacitance between each phase and earth. Typical surge capacitances are described in [5]. Lightning surges Extensive study of lightning surge behaviour has been undertaken over many years by various international bodies, such as the IEEE and CIGRE. Lightning strokes are most basically described in terms of two values, crest current and front steepness. The crest current is the maximum current value that the stroke achieves (measured in ka). Front steepness is essentially how quickly the stroke current reaches the crest current (measured in ka/us). Strictly speaking front steepness is the time taken for the stroke current to rise from 0% to 90% of the crest current. The stroke current and front steepness values can vary and follow a probability function. According to [] the CIGRE probability data is considered to be superior to other data. 203 Earthing, Lightning & Surge rotection IDC Technologies 9

10 robability Session Four: ractical Insulation Co-ordination The probability functions are of the form: I is the stroke current in ka or front steepness in ka/us, M is the median value of stroke current or front steepness and B is the log standard deviation of the stroke current or front steepness The CIGRE data suggests that the median value of stroke current crest is 34 ka and the log standard deviation is The median front steepness value is considered to be 24.3 ka/us with a log standard deviation of 0.6. The following graphs detail the crest current and front steepness probability functions based on the above data. robability of Lightning Stroke Current Crest Current (ka) Figure 9 robability of a particular stroke current occurrence 203 Earthing, Lightning & Surge rotection IDC Technologies 0

11 robability robability of Exceeding Abscissa (%) Session Four: ractical Insulation Co-ordination robability of Lightning Stroke Current Crest Current (ka) Figure 0 robability of a stroke current exceeding a particular value robability of Front Steepness Steepness (ka/us) Figure robability of a particular front steepness occurrence 203 Earthing, Lightning & Surge rotection IDC Technologies

12 robability of Exceeding Abscissa (%) Session Four: ractical Insulation Co-ordination robability of Front Steepness Steepness (ka/us) Figure 2 robability of front steepness exceeding a particular value Therefore a range of stroke current combinations needs to be selected to enable software simulation of case scenarios. Based on this data and various other sources [4], it can be established that larger crest currents tend to have slower front times than lower crest currents. The commonly used values that Aurecon uses for insulation coordination are as follows: 30 ka.2/50 us (high probability crest value with fast front time) 70 ka 4.5/50 us (% probability of exceedence crest value with slower front time) These two stroke current combinations represent both a high probability stroke event and a worst possible case (hence low probability) stroke event. This is considered an acceptable methodology to assess scenarios. Both of these values are used to simulate strokes to overhead earthwires and unshielded phase conductors. Strokes to shielded phase conductors are discussed in the following section. Shielding failures Some shielding failures are expected to occur as it is not possible to prevent all lightning strokes from reaching the phase conductors, unless the phase conductors are completely surrounded by earthwires. Shielding failure currents are calculated based on Eriksson s modified model as stated in []. Figure 3 shows the input to Eriksson s method graphically. 203 Earthing, Lightning & Surge rotection IDC Technologies 2

13 Session Four: ractical Insulation Co-ordination Earthwire b c Rs h a Rc hase conductor y Figure 3 Shielding failure calculation inputs Depending on the transmission line design, the shielding failure current is typically between 6 ka and 0 ka. This means a stroke of 0 ka or less can terminate on the phase conductor as the earthwire fails to provide protection. Scenarios and results The following simulations were undertaken on the case study model: 70 ka stroke to the earthwire at the nearest tower 30 ka stroke to the earthwire at the nearest tower 6 ka shielding failure to the A phase conductor at the nearest tower The results show the overvoltages at the line entry to the substation and at the transformer terminals. It would be beneficial to check other locations within the substation also, such as locations distant from the surge arresters. In order to keep this paper as short as possible, only the above results are shown. It is necessary to add the peak of the 50 Hz voltage waveform to these values to ensure that if the stroke occurs at the peak of the voltage waveform, the BIL is still not exceeded. This is 08 kv for a 32 kv system. This is not an exhaustive list of scenarios and others should be considered, such as mid span strokes, strokes at more distant towers, strokes to the busbar due to shielding failure. The intention is to determine the worst case scenarios to determine if they cause an exceedence of the BIL. 203 Earthing, Lightning & Surge rotection IDC Technologies 3

14 (kv) (kv) Session Four: ractical Insulation Co-ordination 300 Main : Disconnector and Transformer Voltages VdiscA VdiscB VdiscC VtrfA VtrfB VtrfC m 0.020m 0.030m 0.040m 0.050m 0.060m 0.070m 0.080m Figure 4 70 ka stroke to earthwire In Figure 4 it can be seen that a back flashover occurs on A phase, but the overvoltage stays under 300 kv. Allowing for 08 kv power frequency voltage on top of this still gives quite some margin to the 650 kv BIL of the transformer insulation. 300 Main : Disconnector and Transformer Voltages VdiscA VdiscB VdiscC VtrfA VtrfB VtrfC m 0.020m 0.030m 0.040m 0.050m 0.060m 0.070m 0.080m 0.090m 0.00m 0.0m Figure 5 30 ka stroke to earthwire In Figure 5, there is no back flashover of the insulators on the tower and therefore the earthwire is very effective at providing protection of the phase conductors. There is some minor surge overvoltage noticeable due to induction and capacitive coupling between the conductors. 203 Earthing, Lightning & Surge rotection IDC Technologies 4

15 (kv) Session Four: ractical Insulation Co-ordination 350 Main : Disconnector and Transformer Voltages VdiscA VdiscB VdiscC VtrfA VtrfB VtrfC m 0.020m 0.030m 0.040m 0.050m 0.060m 0.070m 0.080m Figure 6 6 ka shielding failure stroke to the A phase conductor In Figure 6 a direct stroke to the phase conductor causes the highest overvoltage. However even allowing the 08 kv power frequency voltage on top of this, gives a margin to the 650 kv BIL. Therefore, the surge protection is effective at protecting against the scenarios considered. In this particular case study, lightning strokes to the transmission line twin earthwires can split five ways and disperse readily without causing back flashovers except in the worst case stroke currents. Other designs can perform quite differently, such as single circuit dedicated radial lines to a substation. Generally at high voltage substations (32 kv and above) surge protection against lightning strokes is quite effective. At lower voltages (33 kv and below) surge protection can be quite challenging because of the much lower equipment BIL ratings. Lightning is probabilistic in nature and therefore it should be noted that the probability of seeing the calculated overvoltages is actually very small. It is outside the scope of this paper to describe the calculation process, but there are references [4] which show statistically what the ground flash density is at a particular location each year. The probability of a critical stroke scenario occurring can be calculated. The probability of this stroke current being above, the critical value within a particular distance of the substation can be estimated. This can then give an indicative probability of insulation failure per year, or a time between failures. This gives an indicative risk that can be assessed, in order to determine if additional surge protection is required. One other final point to note is that software packages such as SCAD can determine the arrester energy dissipation and this needs to be checked. If an arrester exceeds its energy dissipation rating during a lightning stroke it may fail. 203 Earthing, Lightning & Surge rotection IDC Technologies 5

16 Session Four: ractical Insulation Co-ordination References [] A. J. Hileman, Insulation Co-ordination for ower Systems, Marcel Dekker Inc, 999 [2] IEEE Std 998 Guide for Direct Stroke Shielding of Substations [3] IEEE Working Group 3.4., Modelling of Metal Oxide Surge Arresters IEEE Transactions on ower Delivery, Vol 7 o., January 992 [4] AS/ZS Lightning rotection, Standards Australia, 2003 [5] IEEE Std 37.0 IEEE Guide for Transient Recovery Voltage for AC High Voltage Circuit Breakers 203 Earthing, Lightning & Surge rotection IDC Technologies 6

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