Electrical, Electronic and Computer Engineering ENEL4HB - High Voltage 2

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Electrical, Electronic and Computer Engineering ENEL4HB - High Voltage 2 Main Examination October 2015 Instructions Answer all questions and show all working.

conductor separation of 20 m. Each pole has a 6 conductor bundle with a sub-conductor radius of 1.9 cm and a geometrical radius R = 30 cm. Determine: The equivalent radius of the conductor bundle.

1 Electrical, Electronic and Computer Engineering ENEL4HB - High Voltage 2 Main Examination October 2015 Instructions Answer all questions and show all working. Time allowed = 2 hours Full marks = 100 Question 1 A ±533 kv DC high voltage transmission line at sea level has a horizontal pole conductor geometry with a pole conductor height of 30 m and pole conductor separation of 20 m. Each pole has a 6 conductor bundle with a sub-conductor radius of 1.9 cm and a geometrical radius R = 30 cm. Determine: The equivalent radius of the conductor bundle. [2 Marks] The potential coefficients, the total charge on the conductor bundle and the average and average maximum bundle gradient on the pole conductor. [15 Marks] If corona occurs on the pole conductor through the use of Peek s empirical expressions when the temperature is 30 o C, the pressure is 640 torr and the conductor surface roughness is m = 0.7. [8 Marks] [Total Marks: 25] Question 2 Determine the creepage values for a toughened glass cap and pin insulator string at 88 kv under medium pollution given that the highest rated voltage is 100 kv. The unified specific creepage distance for medium pollution is 34.7 mm/kv. [5 Marks] Determine the number of insulator discs required if the creepage distance of a single disc is 330 mm and the spacing distance 127 mm. What is the the equivalent creepage value (in mm/kv) if two discs fail? [5 Marks] Discuss the differences between ceramic and non-ceramic insulators and propose a more suitable insulator for the medium polluted area. [10 Marks] [Total Marks: 20] 1

2 Question 3 An overvoltage travels on a 132 kv overhead line with a propagation velocity of v = 0.8c and steepness of S = 1800kV/µs. A metal oxide surge arrester is to be installed in front of a transformer with a U BIL = 550 kv. The distance between the arrester and transformer is 20 m. Determine the voltage at the transformer if the surge arrester has a residual voltage of 315 kv. [2 Marks] Comment on why there is an increase in voltage across the terminals of the transformer and whether the transformer is protected or not. [4 Marks] Given that there is one incoming line with a double conductor bundle and a span of 400 m, determine the protection distance that should be used and whether the transformer is currently protected or not. [4 Marks] Table 1: Performance factor for incoming overhead line Line Type Factor A (kv) Transmission lines (1φ to earth flashover) Single conductor 4500 Double conductor bundle 7000 Four conductor bundle Six and eight conductor bundle [Total Marks: 10] Question 4 A single phase system has a normal voltage of 25 kv phase to earth and an installed transformer is to be protected from lightning overvoltages. The contractors have proposed the following equipment with the specifications in the tables below. There is 1 m between the surge arrester and the transformer and the contractor has installed separate earth electrodes for the equipment each with 20 Ω resistance. Discuss what the contractor has done wrong. Table 2: Transformer Specifications U n U m BIL 33 kv 36 kv 165 kv [Total Marks: 15] Table 3: Surge Arrester Specifications U c U r I n 25 kv 45 kv 10 ka 2

3 L V 1 V 2 L L 20Ω 20Ω Figure 1: Installed System Question 5 A transmission line is situated in an area with ground flash density of 5 strikes/km 2 /year and has 2 shielding wires at a height of 30 m spaced 10 m apart and phase conductors at a height of 25 m. The phase conductor has an insulator flashover voltage (U 50 ) of 880 kv and a surge impedance of 260 Ω. Determine the number of strikes per 100 km per year to the line. [3 Marks] Determine I crit. [3 Marks] Using the equation for attractive radius, describe a shielding failure and the concept of perfect shielding. [6 Marks] Determine the optimal separation distance for perfect shielding. [3 Marks] Determine the probability of a lightning strike causing a shielding failure. [5 Marks] [Total Marks: 20] Question 6 Using any relevant sketches describe what partial discharge is and how it is measured. Include in your answer why it is necessary to measure and quantify partial discharge just after manufacturing and over a long time.. [Total Marks: 10] 3

4 Useful Formulae Constants Standard temperature: t 0 = 20 o C Standard pressure: P 0 = 760 torr Permittivity of free space: ε 0 = F/m E negative = 31mδ( ) δrc E acpeak = 29.8mδ( ) δrc Formulae Relative air density: Electric field in relation to a line charge q: E = q r The equivalent bundle radius: θ r eq = ( nrr n 1) 1 n Relationship between Q and V : [P ][Q] = [V ] [Q] = [P ] 1 [V ] δ = P P t t Voltage across cap and pin insulator string: V n = V sinh sinh Cp C s n Cp C s N Efficiency of cap and pin insulator: η = V NV N Resultant electric field: E R = 1 [P ] 1 [V ][ 1 r θ + 1 r θ ] Method of determinants: [ ] a11 a = det 12 = a 21 a 22 a 11 a 12 a 21 a 22 = a 11a 22 a 21 a 12 Self potential coefficient: P ii = 1 ln 2h i r i Mutual potential coefficient: P ij = 1 ln D ij d ij D ij = d ij = ((x i x j ) 2 + (y i + y j ) 2 ((x i x j ) 2 + (y i y j ) 2 Average bundle gradient: E a = q 1 n 2πɛ 0 r Average maximum bundle gradient: [ E am = E a 1 + (n 1) r ] R Peek s empirical expressions: E positive = 33.7mδ( ) δrc = det a 11 a 12 a 13 a 21 a 22 a 23 a 31 a 32 a 33 = = a 11 a 22 a 33 + a 12 a 23 a 31 + a 13 a 21 a 32 a 31 a 22 a 13 a 32 a 23 a 11 a 33 a 21 a 12 Probability of negative lightning stroke: P = ( Ip 31 )2.6 Average number of strikes to line: ( 28h 0.6 ) + b N s = N g 10 Attractive radius: R a = 0.67h 0.67 t I 0.74 Flashover current: I crit = 2U 50 Z 0 Shielding separation: X sp = R s Rp 2 (Y s Y p ) 2 a 11 a 12 a 13 a 21 a 22 a 23 a 31 a 32 a 33 4

5 Reflection co-efficient: ρ r = Z 2 Z 1 Z 1 + Z 2 Insulator flashover volt-time curve: ( U(t) = ) t 0.75 W Switching impulse stength of airgaps: U 50 = k d Voltage at protected equipment: U equip = U a + 2 (d + a) S v Protective zone: L p = N ( ) UBIL A 1.15 U a L sp Paschen s eqn: V b = 6.72 pd (pd) 5

Electrical, Electronic and Computer Engineering ENEL4HB - High Voltage 2

Electrical, Electronic and Computer Engineering ENEL4HB - High oltage 2 Main Examination October 2016 Instructions Answer all questions, show all working and include all necessary comments (it is your