Report
Title: Insulation Coordination Study on XXXX 380 kV / 115 kV GIS
Part 1:
Lightning and Very Fast Transient
Over-voltage Studies
Manitoba HVDC Research Centre
244 Cree Crescent
Winnipeg, MB R3J 3W1
CANADA
File # 454
Rev:
Date:
Executive Summary
The Manitoba HVDC Research Centre was contracted byxxxxto perform insulation coordination studies to investigate the adequacy of the surge arresters on the 380 kV side of the xxxx
380kV/115 kV GIS . The surge arresters should limit the fast front over voltages at the transformers windings and at other GIS apparatus to levels below the designed ‘Maximum Protection Level’. The ‘Maximum Protection Level’ as defined in the Scope of Work documents is 80% of the equipment BIL (Basic Insulation Level).
a) Lightning-induced over-voltages due to the following were studied:
?Direct lightning strike on a phase conductor due to shielding failure (on a tower adjacent to the GIS station entrance)
?Back flashover induced over voltages due to lightning strikes on the shielding wire (on a tower adjacent to the GIS station entrance)
b) Investigating the very fast front voltage surges, caused by the sparking of disconnect
switches in the GIS station was also investigated.
A PSCAD system model was developed to represent the GIS station equipment, the main transformers and the transmission towers and line spans close to the GIS station.
The impact of the tower footing resistance, the surge magnitude and shape and the system voltage magnitude (power frequency) at the instant of the lightning strike was investigated by considering those as variable study parameters.
Direct Lightning Strike on a Phase Conductor
The maximum lightning current that can directly strike a phase conductor of a tower near the xxx GIS was determined to be 9.8 kA. The study showed that the voltage levels at the transformers and other GIS equipment were within the ‘Maximum Protection Level’ even for direct strikes that are double that magnitude. The maximum surge arrester energy dissipation levels were also well below the maximum allowable ratings.
Back Flashover
Lightning strikes to ground wires cause a build-up of voltage across the insulation between the tower and the phase conductor. This voltage build, if excessive, can cause a secondary flashover (back flashover).
Tower footing resistance influences the occurrence and the severity of back flashover. Footing resistances in the range of 3-20Ω were investigated. In summary:
?Back flashover did not occur for lightning currents lower than 120 kA with tower footing resistances as high as 20Ω.
?Despite the occurrence of back flashover at higher impulse currents, the Maximum Protection Level for the GIS equipment is not violated for lightning impulse currents
up to 180kA.
?Currents in excess of 180kA could violate the maximum protection level for the GIS equipment. This is considered an extreme case and approximately only 1% of
lightning strikes may have a peak magnitude in excess of 200 kA [1].
?Simulations also indicated that the over-voltage levels at both transformers T801 and T802 will be below their respective maximum protection levels.
?The maximum surge arrester energy dissipation levels were also well below the maximum allowable ratings.
Very Fast Transient Over-voltages
Disconnector switching operations at the GIS station can give rise to very fast transient over-voltage. The GIS equipment for the investigation were modeled as outlined in international standards and publications ([3]-[5]). The over-voltage magnitudes and the dominant frequencies at different GIS station locations were under investigation.
The observed over-voltages were below the Maximum Protection Level of equipment given in Table 1. The worst case observed at the transformer terminal Txxx was 1.07p.u (446 kV) and was well below the respective maximum protection level, i.e. 1040kV. The frequencies of the VFT transients were in the 0.9 – 1.1MHz range
Contents
1.Introduction (5)
2.System Component Models (5)
2.1.GIS Components (5)
2.2.Transmission Lines and Towers (8)
3.Lightning Impulse Model (10)
4.Implementation of back flashover (10)
5.Simulation Results (11)
5.1.1.Configuration 1 (13)
5.1.2.Configuration 3 (14)
5.1.3.Concluding Remarks on Direct Stroke Study (15)
5.2.Back Flashover (15)
5.2.1.Volt-Time Method (17)
5.2.1.1.Configuration 1 (17)
5.2.1.2.Configuration 3 (18)
5.2.2.Leader Propagation Method (19)
5.2.2.1.Configuration 1 (19)
5.2.2.2.Configuration 3 (20)
5.2.3.Concluding Remarks on Back Flashover Study (20)
6.GIS Very Fast Front Transient Over-voltages (22)
6.1.Simulation Results (22)
6.1.1.Configuration 1 ........................................................................ 错误！未定义书签。
6.1.2.Configuration 2 (26)
6.1.3.Concluding Remarks on VFT Study (28)
1. Introduction
The first part of the insulation coordination study investigates the over-voltages in the xxx GIS caused by lightning. The simulations focused on such over-voltages inflicted upon the two transformers T801 and T802 in this substation.
All simulations in this study were performed using PSCAD?/EMTDC?, according to the guidelines and models introduced in [1].
Effects of the following phenomena were studied in the xxx GIS:
?Direct stroke to the overhead line phase conductor (shielding failure)
?Back flashover caused by stroke to the overhead line shield wires
The BIL and over-voltage protection levels for the transformers and other apparatus in the GIS are given in Table 1. The aim of the study was to verify the adequacy of the surge arresters to maintain the over-voltage levels below the ‘Maximum Protection Level’ and also to verify the capability of the arresters to absorb the energy without exceeding their respective energy ratings.
Table 1: Maximum impulse over-voltage limits
The second part of the study investigates the very fast transient over-voltages inside the GIs station due to the operation (and the resulting sparking) of disconnectors.
A number of different Configuration s were studied to identify conditions under the ‘worst case’. Summary of the methodology and results are presented in the following sections.
2. System Component Models
The GIS components and transmission lines were modeled in accordance with the guidelines in [1] and [2]. The data provided by the client were used in the models whenever the necessary data were provided. Typical values were used for those that were not listed in the documents provided by the client. Due diligence was given to identify the sensitivity of such parameters (where typical values were used) to the overall results that were under observation.
2.1. GIS Components
The GIS substation components must be modeled in accordance with methods recommended in international standards [1]-[2]. A list of the GIS component models implemented in
PSCAD?/EMTDC?, as well as their respective parameters, is given in
Table 2.
Table 2: Equivalent models used for the GIS equipment and transmission line
As an example, the PSCAD? implementation of Cross-bay 03 in xxx GIS can be found in Figure 1.
F i g u r e 1. C r o s s -b a y 3, i m p l e m e n t e d i n P S C A D , w i t h b r e a k e r s /d i s c o n n e c t o r s a s s u m e d t o b e i n c l o s e d s t a t u s .
2.2. Transmission Lines and Towers
The first 5 spans of the xxxx 380 kV double circuit line were modeled using frequency
dependent line parameter model available in PSCAD ?/EMTDC ? []. The span distances are shown in Figure 2.
The magnitude of a current impulse due to a lightning discharge is a probability function. Low
discharge levels between 5 to 20 kA may result in a higher tendency for the lightning strike to pass by any shield wires and directly hit a phase conductor. The larger lightning impulse currents may tend to strike the tower top and lead to a back flashover. This requires that the tower be modeled as a vertical distributed transmission line. The surge impedance of the tower and the propagation velocity down the tower are estimated and applied using a ‘Bergeron distributed line model ’, as shown in Figure 3. The insulators were modeled as capacitors.
Z surge = 150Ω , τ = 3.448 x10-3
μs/m R foot = 3Ω – 20Ω C ins = 0.476pF
Figure 3. Representation of transmission tower for the fast-front studies in this report
The line configuration (xxx) is given in Figure 4. Implementation of towers in PSCAD/EMTDC is illustrated in Figure 4(b), where the tower footing resistance was included in the model as a ‘variable’ resistance for each tower, in order to investigate the impact of different tower footing resistances on the over-voltages caused by back flashover. The footing resistance was varied from 3Ω to 20Ω over multiple simulation runs (PSCAD ‘multiple -run’ component was used to automate such simulations). The over-voltages were then monitored at different locations inside the GIS.
C ins
Figure 2: The last five spans of line
(a)
(b)
Figure 4. a) Transmission line configuration; b) Tower model, implemented in PSCAD?/EMTDC ?
.
In the model, the transmission line was represented by surge impedances for spans beyond the fifth tower.
0.476 [pF] uF
Vline_1Vline_2Vline_3
3. Lightning Impulse Model
Lightning impulse was modelled as a current source with a typical wave shape, as shown in Figure 5.
Figure 5. A typical lightning impulse current wave shape
For each study (i.e. direct stroke and back flashover), the peak current was estimated based on guidelines provided in [1], as described in Appendix 1.
4. Implementation of back flashover
Line insulators from tower to conductor can be represented as a capacitor. To model the back flashover phenomenon, a parallel switch is applied where a back flashover might occur (e.g. across each insulator). If the voltage across the insulator exceeds the insulator voltage withstand capability, the back flashover occurs and is simulated by closing the parallel switch. The arc can form in around 20 nanoseconds.
Breakdown of air as an insulator is mostly a function of environmental conditions, in addition to the fast-front voltage build-up. A simplified expression for the insulator voltage withstand capability is proposed in [6]:
where:
V fo = Flashover voltage (kV)
K1 = 400L
K2 = 710L
L = Insulator length (m)
t= Elapsed time after the lightning strike (μs)
The above method, referred to as the ‘volt-time curve’, is commonly used in back flashover studies. Another way to model the back flashover across insulators is the leader progression method [1]. According to this method, the flashover occurs when the leader propagating from one of the electrodes meets the other electrode (or the leaders propagating from both electrodes meet half way through the gap).
In PSCAD, flashover is simulated by closing the breaker parallel to the insulator string once the length of the leader becomes greater than, or equal to, the gap length. The leader progression
model (length of the leader) for standard lightning impulse wave shapes is given by the differential equation
where:
= leader length [m]
d G = length of th
e gap between electrodes [m]
v(t) = voltage across the gap [kV]
E0 = a constant depending on the gap and insulator type [kV/m]
k = a constant depending on the gap and insulator type [m2/kV2s]
CIGRE WG 33.01 [1] provides a table for constants E0 and k for different gap configurations. Values of k = 1.2 [m2/kV2s] and E0 = 490 [kV/m] were used for cap and pin insulators. A lower value than the one provided in the table was used for E0. The lower value was chosen to have consistency with the volt-time method. Note that chosen value E0 = 490 [kV/m] is more likely to lead to flashover than the one provided in the table E0 = 520 [kV/m].
In this study, both methods (volt-time and leader progression) were used and the results of the two methods are presented in sections 5.2.1 and 5.2.2, respectively.
5. Simulation Results
In order to model the worst-case configurations, three GIS configurations were considered in simulations, referred to as Configuration 1-3 (Figure 6):
?Configuration 1: Transformer T801 was energized from Bay 34 (line xxx 1) through Bays 9 and10, Buses 2A and 2B, and Bays 26, 24 and 23. All other cross-bays were
assumed to be disconnected.
?Configuration 2: Transformer T801 was energized from Bay 34 (line xxx 1) through Bays 9, 8 and 6, Buses 1A and 1B, and Bays 22 and 23. All other cross-bays were
disconnected
?Configuration 3: Transformer T802 was energized from Bay 35 (line xxx 2) through Bays 12, 13 and 14. All other cross-bays were assumed to be disconnected. Simulations indicated that Configurations 1 and 2 were almost identical. Therefore, only results for Configurations 1 and 3 are presented in this report.
Effect of the Instantaneous ac power frequency voltage:
The effect of the AC system voltage (power frequency) was modeled by a 3-phase 60-Hz voltage source behind the transmission line surge impedance (?300Ω). To model extreme cases, the AC source line-to-line magnitude was set to 380kV x 1.1 = 420kV rms. Also, three different initial phase angles were applied in the simulations (90o, 0o and -90o) in order to find the worst case.
Figure 6: Three GIS bay configurations selected in this study
Multiple lightning flashes to the same location:
Multiple lightning flashes to the same location will have an impact on the energy duty of the surge arrester. In this study, 5 successive strokes were assumed to estimate the maximum surge arrester energy. Hence, in the results presented in the following subsections, the maximum arrester energies obtained from simulations are multiplied by 5 to reflect the effect of multiple strokes. Surge arresters specifications are given in Appendix 2.
5.1 Direct strike study results
A few selected waveforms from the simulation study are listed in figures 7-10.
5.1.1. Configuration 1
Figure 7. Top: Lightning current (1.2/50μs at 9.8kA);
Bottom: Three-phase voltages at T802 380kV terminals
Figure 8. Three-phase voltages at the incoming line bushing (line xxx 1) 5.1.2. Configuration 3
Figure 9. Top: Lightning current (1.2/50μs at 9.8kA);
Bottom: Three-phase voltages at T801 380kV terminals
Figure 10. Three-phase voltages at the incoming line bushing (Abu Hadriyah 2)
Table 3: Direct stroke results – Configurations 1 and 3
5.1.3. Concluding Remarks on Direct Stroke Study
As presented above, simulations indicated that:
?When proper arresters are installed, direct lightning stroke applied to the transmission line conductors does not cause unacceptable over-voltage levels.
According to the data provided by the client, the surge arresters have a Maximum Continuous Operating Voltage (MCOV) of 288kV and an energy absorption capability of 16.2 kJ/kV x MCOV, i.e.
16.2 x 288kV = 46565 kJ. From Table 3, one can observe that the energy of all arresters remain well below the rated energy absorption level of 46565kJ.
Note: Even though the estimated maximum direct strike current peak is 9.8 kA, we performed simulation studies for current peaks up to 20 kA. Voltage limit nor arrester energy rating violations were not observed even for such levels of (direct) stroke currents.
5.2. Back Flashover
The lightning stroke to the overhead line shield wire can cause a flashover from the tower to the phase conductor(s). This phenomenon is called back flashover. Breakdown of air as an insulator is mostly a function of environmental conditions, in addition to the fast-front voltage build-up. In this study, both methods described in section 4 were used to model back flashover. In all simulations, the lightning stroke was applied to the tower next to the GIS.
A summary of observations for each method is presented below.
?Volt-Time Method:
o Back flashover would not occur for lightning impulse currents below 180kA, and the over-voltage protection levels of Table 1 are not violated at any point
in the GIS.
o With a tower footing resistance of 20Ω, back flashover could take place for lightning impulse currents higher than 180kA. Currents higher than 190kA
can also cause the over-voltage protection level to be violated at the
incoming line bushings and could reach 1191kV at Bay 34 when I impulse =
200kA (see Table 4). Lightning currents of this magnitude, however, are
considered extreme conditions and the probability of lightning currents
reaching such levels is less than 1% [1].
o Even at such high impulse current levels, the overvoltage at the transformer terminals were within the ‘maximum protection level’.
o For the design footing resistance of 3-5Ω, back flashover would not occur (even for I impulse = 200kA) and the over-voltage levels remain below the limits
in Table 1.
Leader Propagation Method:
o Back flashover would not occur for lightning impulse currents below 120kA, and the over-voltage protection levels of Table 1 are not violated at any point
in the GIS.
o With a tower footing resistance of 20Ω, back flashover could take place for lightning impulse currents higher than 120 kA. However, over-voltage
protection levels are not violated for lightning impulse currents below 180 kA.
An extreme impulse current of 200kA could also result in an over-
voltage as high as 1240kV at the incoming line bushings (see
Table 5).
Nevertheless, simulations suggest that with appropriately sized arresters, the over-voltages observed by transformers T801 and T802 will remain well below the protection level of 1040 kV (Table 1), even during back flashover transients.
Some typical waveforms are shown in Figure 11 to Figure 18, for I impulse = 200kA and
R foot= 20Ω. Results of this part of the study are summarized in Table 4and Table 5.
5.2.1. Volt-Time Method
5.2.1.1. Configuration 1
Figure 11. Top: Lightning current (1.2/50μs at 200kA);
Bottom: Three-phase voltages at Txxx 380kV terminals
Figure 12. Three-phase voltages at the incoming line bushings.
Top: Abuxxx 1; Bottom: Abu xxx 2.
5.2.1.2. Configuration 3
Figure 13. Top: Lightning current (1.2/50μs at 200kA);
Bottom: Three-phase voltages at Txxx 380kV terminals
Figure 14. Three-phase voltages at the incoming line bushings.
Top: xxx 1; Bottom: xxx 2.
Table 4: Back flashover study results: Volt-time method – Configurations 1 and 3
5.2.2. Leader Propagation Method
5.2.2.1. Configuration 1
Bottom: Three-phase voltages at T802 380kV terminals
Top: xxx 1; Bottom: xxx 2.
5.2.2.2. Configuration 3
Figure 17. Top: Lightning current (1.2/50μs at 200kA);
Bottom: Three-phase voltages at Txxx 380kV terminals
Figure 18. Three-phase voltages at the incoming line bushings.
Top: xxx 1; Bottom: xxxx 2.
Table 5: Back flashover study results: Leader Propagation method – Configurations 1 and 3
5.2.3. Concluding Remarks on Back Flashover Study
As presented above, simulations indicated that: