Optimization of Corona Onset and Breakdown Voltage of Small Air Gaps Stressed by DC and Impulse Voltages

Transcription

1 Optimization of Coona Onset and Voltage of Small Ai Gaps Stessed by DC and Impulse Voltages Athanasios Maglaas #1, Tifon Kousiouis *, Fangiskos Topalis *3, Leandos A. Maglaas $4, Konstantina Giannakopoulou #5 # Electical Engineeing Depatment, T.E.I. of Laissa, 4111 Laissa, Geece maglaas@teila.g giannakopoulou@teila.g * Electical and Compute Engineeing Depatment, N.T.U.A. 9, Ioon Polytechniou st Athens, Geece tkous@softlab.ntua.g topalis@ieee.og $ Computes, Telecommunications and Netwoks Engineeing Depatment, Univesity of Thessaly 37 Glavani 8th Octobe St Deligiogi Building, 38 1 Volos, Geece leadosmag@gmail.com Abstact The pesent pape aims to the investigation of the methods used to minimize o maximize the values of the Coona onset and in small od-plate ai gaps when stessed by dc o impulse s. The main factos which influence geatly the distibution of the electic field in the gap, and hence the above values ae the geomety and the selection of gounding and chaging of the electodes, (gound effect), the gap length, the existence of baie in the gap (baie effect), and the Coona effects appeaing pio to beakdown. Combining theoetical, simulation and expeimental wok, it is esulted that: a) The electode chosen to be gound, stongly influences the distibution of the field and the Coona effects and hence the values of the Coona onset and beakdown. When the od is gounded and the plate is negatively chaged the value of Coona onset is highe, the coona effects ae less intense and dc beakdown, without Coona, is highe. When the plate is gounded and the od is negatively chaged the value of the Coona onset is lowe, the coona effects ae moe intense and hence the dc beakdown is highe. b) A dielectic baie, when placed in specific positions in the gap, deceases the Coona effects and aises the beakdown dc. c) In ai gaps stessed by impulse s the esults ae vey diffeent. The coona onset and the beakdown ae maximum when the od is gounded and positive leading to moe intense Coona effects. Keywods: ai gap, Coona, beakdown, high, field, FEM I. INTRODUCTION Ai as an insulato is the most used in vaious aangements and pobably the best conventional solution fo the majoity of the high applications. The ai gap thus is consideed as one of the most impotant paametes fo the design and dimensioning of insulating aangements, in almost evey electotechnical application. In designing nealy evey electical aangement, ai gaps ae essential components that aise necessaily in constuctions (switches, gaps between powe lines, o powe lines and eath, gaps between electical and electonic components in most devices, etc.). In the last decades ai gaps ae necessay and opeational components of moden constuctions and devices, while thei dielectic behavio is the basis of thei opeation, [1]-[]. The basic effects which ae efeed to as the dielectic behavio of an ai gap ae the Coona effects and the beakdown [1[-[]. The basic magnitudes which descibe the dielectic behavio of an ai gap ae the Coona o patial dischages onset, the Coona leakage, and the beakdown Voltage, [1-4]. The equations that calculate the values of the coona onset field stength on the tip of a od ae taken fom bibliogaphy, such as the empiical equation [1],,3 E c = (kv/cm) fo ac, (1) as well as the empiical equation : 1 E c =,4 1 + (kv/cm) fo dc, () 3 whee E c (kv/cm) is the coona onset field stength on the tip of a od with adius, which is located sepaately and fa fom the gounded envionment.

2 The above equations ae not applicable fo the inteelectode ai gaps, like the od-plate gaps. The most known effects which influence the values of the above mentioned magnitudes ae the polaity effect, [1[-[3], and the baie effect, [5]-[8]. Othe lately investigated phenomena which have geat influence on the dielectic behavio of the ai gaps ae: The gound effect, that is the influence of the diffeent electode of the gap chosen to be gounded on the field distibution and hence the dielectic behavio of a gap [8]-[9]. The Coona effect, that is the influence of the Coona on the value of the beakdown of a gap when stessed by dc, [1]-[11]. The Coona effects and the beakdown of ai gaps have been expeimentally investigated by many eseaches fo the most commonly used aangements whee the gap is fomed between two electodes one of which is gounded and the othe is stessed by high s. Especially in od plate ai gaps the used so fa technique of the expeimental wok concens aangements whee the plate electode is gounded and is applied to the electode of the od, [1]-[6]. Aangements symmetically chaged have been just lately investigated, as well as od plate ai gaps with the od gounded and the plate stessed by high dc o ac s, while a suggestion to be efeed to as the gound effect has been poposed, [7]-[9]. It is well known that ai gaps pesent a vey diffeent dielectic behavio when stessed by impulse s, due to the influence of the citical volume in the aea of the od electode and the equied time lag, []. Seveal methods fo contolling the beakdown have been poposed, []-[3]. In the pesent pape a new method is investigated based on the esults given fom the influence of gounding (gound effect) in connection to the baie effect and the polaity effect eithe unde dc s, o impulse s of both polaities. Thee ae no bibliogaphic efeences on the influence of the gounding on the od-plate ai gaps stessed by impulse s. This effect is investigated fo the fist time in the pesent pape. The models used fo the optimization of the dielectic behavio of ai gaps, concening the incease in the values of beakdown combined with a vaiation of the Coona effects, ae essential fo evey application using High Voltages. These models cove economic and safety issues and thei use aises advantages since it helps avoid destuction of the insulated mateials of the devices, o impoves the opeation of othes. field stength in specific positions in the gaps, unde diffeent conditions, and in combination with the effects of gounding and baie have been ecoded and pocessed. All the analyzed models ae axisymetic, with a spheical bounday shield big enough in diamete. Special softwae Quickfield, fom Tea Analysis, Denmak, commecially available, has been used fo the simulation analysis of the ai gap models. It is based on the Finite Element Method with the use of Poisson s equation: V = ρ ε, (3) and the Diichlet bounday conditions V=, whee V (V) is the applied, ρ (C/m 3 ) the space chage density, and ε (F/m) the dielectic constant of the ai o the gas used. A. Rod Plate Ai Gaps Without Baie The initial conditions of a od-plate ai gap ae given by the following equations (Fig. ). Fo the aangements with the od gounded, while the plate is stessed by 1 V (ρ=): =, if ( x, R 1 Γ, (4) = 1, if ( x, P (5) Fo the aangements with the plate gounded, while the od is stessed by 1 V (ρ=): =, if ( x, P Γ, (6) = 1, if ( x, R, (7) 1 whee: Γ = ( x, R : x + y = R, (8) ( x, R : y, R1 = a + G y x G + y G G ( x, R : x + b, P = b b G b b b G b + + pl x y p x l (1), (9) V is the applied, G is the gap length, pl is the adius of the plate, is the adius of the od, R is the adius of the shield, a is the length of the od, and b is the thickness of the plate. The significant diffeences between equations (4), (5) and (6), (7) lead to significant diffeences of the dielectic behavio between the two aangements. Baie y R Plate, II. THE PROCEDURE FOLLOWED Mathematical models of the expeimental aangements have been designed, and the equation of initial conditions concening the analysis of the electic field in the gaps, in connection to the gound and the baie effect has been fomulated and analyzed. Appopiate models have been designed and simulation analysis has been held, with the use of the Finite Element Method. The values of the, the Rod pl c pl a G/ b G/ (a) Without baie (b) With a baie Fig. 1 Theoetical models of od-plate ai gaps x

3 B. Rod Plate Ai Gaps with Baie In the od -plate ai gaps with a baie, the lineaity of equations is not valid since the esults depend on the position and the chaging of the baie. The suface of the baie is chaged though small patial dischages that occu in the gap. Thus develops on the suface of the baie, esulting to the influence of the electic field distibution in the gap. The latte is influenced diffeently in a od-plate gap accoding to the electode chosen to be gounded and its dielectic behavio is geatly affected. a) Fo the aangements in which the plate s is V pl, the baies vaies linealy, the cente of the baie having V bo, the baies edge V b1, while the od and the bounday shield ae gounded, equations (4), (5) ae valid, as well as equations: V( x, = Vb + y K1, if ( x, S, (11) and V pl V (1) b V( x, = Vb + y K1, if ( x, S, (13) and V pl V, (when the baie is nea the od) (14) b b) Fo the aangements in which the plate is gounded and the od is stessed equations (6), (7) ae valid, in combination with equations: = Vb (1 y K), if ( x, S (15) and (when the baie is nea the od) V 1 b, (16) whee: G ( x, R : x = + a, S =, (17) b y b V b V Vb 1 V K1 = V b b Vb V K = V, (18) whee a is the distance between the od and the baie and b the adius of the baie (the plate and the baie ae in the fom of a disc). Radius R is vey big (Fig. 1). Thee ae significant diffeences between equations (4), (5), (11), (1) and (6), (7), (13), (14), because of the gound effect. III. THE INVESTIGATION ARRANGEMENTS The aangements, which have been modeled, analyzed, and expeimentally studied, ae typical od-plate ai gap aangements of diffeent electode geomety and gap length. The od electode is a hemispheical capped long cylinde with a elatively small diamete (4-1 mm), and the plate electode is a disk of 1 mm in diamete, both made of bass. High dc o lightning impulse of negative o positive polaity is applied to one electode while the othe is at eath potential (gounded). The ate of dc ise is appoximately kv/s. The influence of the suounding is minimized, by keeping elatively big distances between the models and the bounday shielding, as well as between the expeimental aangements and the gounded elements of the laboatoy. All the analyzed models ae axisymetic with a spheical b b b1 b bounday shield big enough in diamete (at least times bigge than the gap s length) at eath potential. The aveage value of the field stength, along the axis of an ai gap is defined by equation: Eav = V G (19) The field facto (o efficiency facto) n is a net numbe, which defines the inhomogeneity of the field in the gap and is expessed by equation: n = E E () max av Fo a sphee-sphee ai gap the field facto is calculated fom equation:,5 n = [( G D + 1) + {( G D + 1) + 8} ]/ 4, (1) o n=g/(d), if G>>D, whee V is the applied, G is the gap length, E max is the maximum value of the field stength (on the od), E av =V/G is the aveage value of the field stength along the axis of the gap, and D is the diamete of the sphee. The values of the field stength at coona onset ae affected by the polaity of the applied (polaity effect), [1], [1]. The coona onset, the coona and the beakdown of the coespondent expeimental hoizontal aangements have been measued. The esults show the influence of the gounding on the coona, the coona onset and the beakdown of small ai gaps. The maximum and minimum values have been ecoded. IV. THE SIMULATION RESULTS The simulation analysis with the Finite Element Method and the use of special softwae has shown a significant influence of the effect of gounding and of the baie on the field distibution in od-plate ai gaps. The diffeences between the aangements, with the od o the plate gounded, o with the existence of a baie between the electodes ae obvious and shown in Figs. and 3. A. Rod-Plate Ai Gaps Without Baie In od plate ai gaps without baie the effect of gounding is intense when the gap length is elatively big (> cm) and the od s diametes is elatively small (<1 mm). Plate-gounded. Symm. Chaged Rod-gounded Fig. Field stength distibution in od-plate ai gap models fo the diffeent aangements fom simulation analysis. The effect of gounding is obvious. The value of the applied is 1V fo each of the thee cases.

4 In the od gounded aangements (-g) the field is less inhomogeneous than in the plate gounded (pl-g) aangements. The quantity of intensification is given by the maximum values of the field stength on the od and the values of the field facto along the axis, which ae lowe in the aangements with the od gounded (-g) and highe in the aangements with the plate gounded (pl-g) (Fig. ). The diffeences depend on the gap s geomety and ise to 7%, fo a gap length up to 1 cm, when the od s diamete is 1 mm, (Figs. and 3). Field stength on the od (V/m) Field facto hoizontal. The minimum value of the field stength on the od appeas when the gap length is big enough (>1 cm), fo evey gounding and chaging condition of the aangement, while the field facto seems to each a maximum value only fo the aangement with the od gounded, when the gap length is appoximately 7 cm. The influence of the gounded shield is negligible when the shield adius is moe than 8 times bigge than the gap length (Fig. 4), and deceases when the diamete of the plate inceases. The diamete of the od used in the model is 1 mm, and the simulation 1 V. B. Rod Plate Ai Gaps with Baie Stessed by DC Voltage The baie effect is well known to influence the beakdown of the ai gaps [6]. The models that wee analyzed in the pesent pape wee simila to the expeimental models. The esults of the analysis evealed images of the field distibution in od-plate gaps with baie, stessed with equal to o lowe than the od s, o without additional chage. These esults ae shown in Figs. 5 and 6 fo od-plate gaps with eithe one of the electodes gounded with gounded and a baie placed between them, pependicula to the axis of the gap, and fo diffeent values of the at the cente of the baie. It is clealy shown that the on the baie s suface influences the field distibution in the gap. The electic field becomes less inhomogeneous, and the chage accumulated on the suface of the baie becomes a wall to the movement of the coona chages. The effect of gounding, influences the electic field distibution in the gaps with a baie (Fig. 6), and hence thei dielectic behavio. Fig. 3 The value of the field stength on the od and the field facto along the axis of od-plate ai gaps, fo the diffeent aangements with the od (-g), o the plate gounded (pl-g), eithe with symmetically chaged electodes (symm.). The plate s diamete is 1 mm, and the od s is 1 mm. Field stength on the od V/m E--g E-pl-g E-symm-ch Gounded shield diamete mm Fig. 4 The values of the field stength on the od of od-plate ai gaps, fo the diffeent aangements with the od (-g), o the plate gounded (pl-g), eithe with symmetically chaged electodes (symm-ch), fo diffeent values of the gound shield diamete. The od s diamete is 1 mm and the plate s is 1 mm. The cuve of the values of the field stength as a function of the gap length seems to end asymptotically at the V b= V b=v / V b=v Fig. 5 The field stength distibution in od plate gaps with the plate gounded and a baie, fo thee diffeent values of the on the suface of the baie. V b is the on the suface of the baie (cente) and V is the of the od. V=1 V, [17]. Plate gounded Rod gounded Fig. 6 The field stength distibution in a od-plate gap with a baie in compaison fo the two diffeent aangements with the od o the plate gounded. V=1 V. V. THE EXPERIMENTAL RESULTS The influence of the gound effect and the polaity effect on the values of the coona onset and the beakdown, as well as on the Coona wee expeimentally

5 investigated fo od-plate gaps with o without baie when stessed by dc o impulse s. The expeimental aangements wee od-plate ai gaps with a od diamete fom to 1 mm, and a plate diamete fom 5 to 15 mm. The electodes wee made of bass, and the baie was a plate made of pespan pape,.3 mm thick. A. Rod Plate Ai Gaps without Baie Stessed by DC Voltage It is known that coona is ahead of beakdown, which follows. In elatively small gaps though (<3 cm), beakdown is possible to occu without the appeaance of detectable coona effects, [9]. This in geneal happens when the field is less inhomogeneous, meaning small gaps, when the od s diamete is elatively big (>8 mm) and especially in od-plate gaps with the od gounded, stessed by dc (-). Voltage (KV) Vb, -g Vc, pl-g Fig. 7 The effect of gounding on the coona onset and the beakdown of od-plate ai gaps 1-1 mm, stessed by dc negative. Coona onset (kv) Vb, 1-1, -g, dc(-) Vc, 1-1, -g, dc(+) Vc, 1-1, pl-g, dc(+) Vc, 1-1, pl-g, dc(-) Rod-plate, dc Vc, 4-1, -g, dc(-) Vc, 4-1, -g, dc(+) Vc, 4-1,pl-g, dc(+) Vc, 4-1, pl-g, dc(-) Fig. 8 The Coona onset of od-plate ai gaps stessed by dc, in combination with the gound effect. In od-plate ai gaps with a od s diamete of 1 mm and plate s diamete of 1 mm (1-1) with the od gounded thee ae no coona effects befoe beakdown, and thus the beakdown is taken into consideation, instead of the Coona onset. In Fig. 7 the values of the coona onset of the 1-1 mm aangements with the plate gounded (pl-g) ae compaed to the beakdown of the aangements with the od gounded (-g). The influence of the gound effect is obvious; the beakdown of the aangements with the od gounded is highe than the coona onset of the ai gaps with the plate gounded. 1 cm cm 3 cm 4 cm 5 cm 6 cm In ai gaps stessed by dc s, Coona effects appea in the fom of electical pulses of significant value (up to some Ampees) and vey small duation (a few ns) with high epetition fequency (evey few μs). The phenomenon is detected by dc electic, which epesents the aveage value of the s, as well as by the Coona sound (squeak), o the Coona light and ultaviolet adiation. The fequency of the Coona sound depends on the ai gap aangement and the polaity of the applied. In the pesent expeimental wok, it was obseved that in some cases thee is no Coona sound, and it is esulted that this happens because the fequency of the s belongs in the egion of the ultasonic sounds. The influence of the gound effect in connection with the gap s geomety on the Coona onset is shown in Fig. 8. It is obvious that the Coona onset is lowe in the pl-g aangements with the smalle od s diamete whee the maximum value of the field stength is highe and the field is moe inhomogeneous, while it is highe in the -g aangements whee the field is less inhomogeneous. It can be concluded that the Coona onset is maximized when the od is gounded fo both polaities, while it is minimized when the plate is gounded, and the od s diamete is smalle, fo both polaities as well. (kv), Coona (μα) , pl-g, dc(-) 4-1, -g, dc(+) I-1, pl-g, dc(-) 1-1, -g, dc(+) Vb, 1-1, -g, dc(-) 4-1, pl-g, dc(+) 1-1, pl-g, dc(+) 4-1, -g, dc(-) 1 cm, Ic 1 cm, Vb cm, Ic cm, Vb 3 cm, Ic 3 cm, Vb 4 cm, Ic 4 cm, Vb 5 cm, Ic 5 cm, Vb 6 cm, Ic 6 cm, Vb Fig. 9. The beakdown of od-plate ai gaps stessed by dc, in combination with the gound effect and the coona effect. Fig. 9 shows the values of the beakdown of od plate aangements in combination with the gound effect and the polaity effect. It is esulted that the values of the beakdown ae highe in the aangements with the negative and thinne od, (-), and maximum in the aangements with the plate gounded when stessed by dc(-), in which the Coona is also maximum. Minimum values of the beakdown appea in the gaps with the od gounded when stessed by dc(-), that is in the aangements with the minimum Coona. The beakdown is maximized in the aangements with the plate gounded, stessed by dc(-) and minimized in the aangements with the od gounded and dc(-). The diffeences depend on the coona in combination with the gound effect, the polaity effect and the gap s geomety (od s diamete). It is also esulted fom Fig. 8 that the maximum values of the Coona ae obseved in the ai gaps with smalle

6 od s diamete and the plate gounded, and ae minimized in small ai gaps (< cm) with the od gounded, while thee ae negligible Coona effects befoe beakdown when the od with diamete of 1 mm is gounded. B. Rod Plate Ai Gaps with Baie Stessed by DC Voltage (KV) Coona (μα) Vb (-), -g Ib(-), -g Wb Wb Distance fom od to baie (cm) Fig. 1 The values of the beakdown and the Coona of odplate ai gaps, 1-1 mm, 5 cm long, with a baie, in compaison to ai gaps without baie (Wb). Applied dc (-). (KV (kv dc(+), Wb dc(-), Wb dc(-) dc(+) Wb Wb Distance fom od to baie (cm) a) Aangements with the od gounded dc(-), Wb dc(+), Wb dc(-) dc(+) Wb Wb Distance fom od to baie (cm) b) Aangements with the plate gounded Fig. 11 The values of the beakdown of od-plate ai gaps, 1-1 mm, 5 cm long, with a baie, in compaison to ai gaps without baie (Wb). Applied is dc (-). The baie effect influences the value of the beakdown of ai gaps [5-9]. This is common mostly fo aveage and big od plate gounded, and od-od gaps. Fo the aangements though of od-plate gaps whee the od is gounded and the plate is stessed with high, leading to a less inhomogeneous field, the baie effect hasn t been investigated and as appeas fom the pesent pape the esults of its influence on the beakdown of the gap ae diffeent compaed to the ones fom the aangement with the plate gounded, (Fig. 1), ceating phenomena that maximize o minimize the Coona onset and the coona. The values of the coona in od-plate gaps with od s diamete of 1 mm and plate s diamete of 1 mm espectively with o without baie wee ecoded and the esults ae shown in the cuves of Fig. 1. It is esulted fom Figs. 1 and 11 that the beakdown is highe in the aangements with a baie and the od gounded, with the diffeences depending on the diamete and the polaity of the od, as well as the distance between the od and the baie. Maximum values of the beakdown appea when the od is gounded and the baie is nea the od. B. Rod Plate Ai Gaps Stessed by Impulse Voltage When the od-plate ai gaps ae stessed by lightning impulse s the esults ae diffeent fom the esults ecoded with dc [13]-[14]. The Coona effects appea in a fom of one o aely two patial dischage pulses of vey shot duation that occu befoe beakdown, and they geatly influence the values of the V5% of the beakdown of the gap. (kv) (kv pl-g (-) -g (-) -g (+) pl-g (+) (a) The diamete of the od is 1 mm and of the plate 1 mm g (-), 4 pl-g (-), 4 -g (+), 4 pl-g (+), 4 (b) The diamete of the od is 4 mm and of the plate 1 mm Fig. 1 The values of the beakdown (V5%) of ai gaps stessed by impulse 1,/5 μs in combination with the gound effect and the polaity effect.

7 V5%, (kv) , -g, (+) 1-1, pl-g, (-) 1-1, pl-g, (+) 1-1,-g, (-) 4-1, -g, (+) 4-1, pl-g, (-) Aangement 4-1, pl-g, (+) 4-1,-g, (-) (μα) highe, but since the field is moe inhomogeneous the Coona effects ae moe intense. V5% Ic max Fig. 13 The values of the beakdown of ai gaps 3 cm long stessed by impulse 1,/5 μs of both polaities, in compaison to the peak values of the s occued befoe beakdown. Fom the compaison of Fig. 1 with Fig. 7 it can be concluded that the V5% values of the beakdown of small ai gaps ae elatively highe than the beakdown of the gaps stessed by dc. Highe values of beakdown appea in the aangements with the od gounded stessed by positive impulse, o the plate gounded stessed by negative (aangements with negative od), while lowe values appea in the od gounded gaps stessed by negative, o the plate gounded stessed by positive (aangements with positive od). These esults ae connected to the Coona effects (patial dischage pulses) as shown in the oscillogams of Figs. 14, 15, and 16. It is esulted that in the plate gounded aangements, stessed by impulse (-), o in the od gounded aangements stessed by impulse (+), in which the od is negative ( (-)), the magnitude of the coona pulses occued befoe beakdown is highe and hence the value of the beakdown is lowe. In the od gounded aangements stessed by impulse (-), o in the plate gounded aangements stessed by impulse (+), in which the od is positive ( (+)), the coona pulses, when occued befoe beakdown, ae much smalle, and hence the value of the beakdown is highe. The maximum values of beakdown impulse occu in the plate gounded aangement stessed by impulse (-), in which the minimum values of coona pulses ae ecoded, while the minimum values of the beakdown occu in the plate gounded aangements stessed by impulse (-) in which the maximum values of the coona pulses ae ecoded (Fig. 13). The diffeences depend on the intensity of the coona effects in combination with the gound effect and the gap s geomety and ae bigge in the aangements with the od gounded, with smalle od s diamete. In the od-gounded aangements the field is less inhomogeneous and hence the values of the Coona onset ae highe the s smalle and the beakdown highe. In the 4-1 mm ai gaps, the citical volume is smalle than in the 1-1 mm aangements, and hence the values of the Coona onset and the beakdown ae (a) Plate gounded ai gap, lage coona pulse. kv/div,.8 A/div, 5μs/div (b) Rod gounded ai gap, vey small coona pulse. kv/div,.8 A/div, 1μs/div Fig. 14 Oscillogamms showing the beakdown positive impulse and the occuing befoe beakdown fo the 1-1 mm od-plate ai gaps, with a length of 3 cm. (a) Plate gounded ai gap, lage. kv/div,.8 A/div, μs/div (b) Rod gounded ai gap, vey small coona pulse. kv/div,.8 A/div, 1μs/div Fig. 15 Oscillogamms showing the beakdown positive impulse and the occuing befoe beakdown fo the 4-1 mm od-plate ai gaps, with a length of 3 cm.

8 (od negative). The diffeences ae bigge in the od gounded than in the plate gounded aangements. The values of the Coona onset, the coona pulse, and the beakdown ae elatively highe in the aangements with smalle od s diamete. a) Rod s diamete 4 mm, lage. kv/div,.8 A/div, 1μs/div b) Rod s diamete 1 mm, smalle. kv/div,.8 A/div, 1μs/div Fig. 16 Oscillogamms showing the beakdown negative impulse and the occuing befoe beakdown fo the od-plate ai gaps, with the od gounded and a length of 3 cm. VI. CONCLUSION Simulation analysis with the FEM eveals that the gound effect and the baie effect influence the electic field distibution in the ai gaps. The field is less inhomogeneous in od-gounded aangements, and in the ai gaps with a chaged baie. Expeimental esults show that minimum values of the dc Coona onset ae ecoded in the plate gounded ai gaps without baie, depending on the od s diamete. Expeimental esults show that maximum values of dc beakdown ae ecoded in the plate gounded ai gaps without baie, depending on the polaity of the applied and the od s diamete, and mainly on the intensity of the Coona effects occuing befoe beakdown. The lage the coona is, the highe the beakdown becomes. In the ai gaps with a baie the values of the beakdown ae maximum in the od gounded aangements, stessed by dc (-), when the baie is placed nea the od. When lightning impulse is applied on small ai gaps without baie, the esults ae diffeent, due to the influence of the magnitude of the citical volume, in connection to the gound effect and the polaity effect. Maximum values of the s and minimum values of the beakdown s ae ecoded in the plate gounded aangements, when stessed by positive (od positive). On the contay minimum values of the s and maximum values of the beakdown s ae ecoded in the od gounded aangements when the plate is positively chaged ACKNOWLEDGMENT This eseach has been co-financed by the Euopean Union (Euopean Social Fund - ESF) and Geek national funds though the Opeational Pogam "Education and Lifelong Leaning" of the National Stategic Refeence Famewok (NSRF) - Reseach Funding Pogam: ARCHIMEDES III. Investing in knowledge society though the Euopean Social Fund. Special thanks to Ass. Pof. John Paassidis, who offeed consideable help in mathematical analysis. REFERENCES [1] Kuffel E., Zaengl W.S., Kuffel J., High Voltage Engineeing, Fundamentals, Newnes Oxfod,.S. [] Phillips D. B., Olsen R. G., and Pedov P.D., Coona Onset as a Design Optimization Citeion fo High Voltage Hadwae, IEEE Tans. Dielect. Elect. Insul. Vol. 7 No 6, pp , Decembe.J. Beckling, Ed., [3] Khalifa M., High engineeing, theoy and pactice, Macel Dekke inc., New Yok, USA, 199. [4] Li J., Hua W.and Zhonggan Y., The influence of ions on electical beakdown in ai, IEEE Poc. Dielect. Mateials, Vol. 1, pp , 1988 S. Zhang, C. Zhu, J. K. O. Sin, and P. K. T. Mok, A novel ultathin elevated channel low-tempeatue poly-si TFT, IEEE Electon Device Lett., vol., pp , Nov [5] Li J., Hua W.and Zhonggan Y., The influence of ions on electical beakdown in ai, IEEE Poc. Dielect. Mateials, Vol. 1, pp , [6] Hidaka K., Chiba M., and Kouno T. (1988), Baie effect on electical beakdown in ai, Poc. of 9th Int. Conf. Gas Dischages and thei Applications, pp , Venezia, [7] F. V. Topalis, I. F. Gonos, I. A. Stathopulos, Insulation popeties of composite dielectic aangements, Intenational Jounal of Powe and Enegy Systems, Vol., No. 3, pp ,. [8] Maglaas A., Maglaas L., Numeical Modeling and Analysis of electic field distibution in od plate ai gaps, with o without baie, stessed by beakdown s, 1st IC-EpsMsO, Athens, 5. [9] Maglaas L, Maglaas A., J. Digojias, Investigation of the Gound Effect in connection to the Baie Effect in od-plate ai gaps, WSEAS TRANSACTIONS on POWER SYSTEMS, Issue, Volume 1, Febuay 6. [1] Maglaas A., Topalis F. V., Influence of Gound and Coona Cuents on Dielectic Behaviou of Small Ai Gaps, IEEE Tans. Dielect. Elect. Insul, Vol. 16, No 1, pp. 3-41, 9. [11] Maglaas A., Topalis F. V., The Influence of the Gounding of the Electodes and the Coona Cuent to the Dielectic Behavio of the Ai Gaps, Int. Conf. Med Powe 8, Thessalonica, 8. [1] Leandos A. Maglaas, Athanasios L. Maglaas, Fangiskos V. Topalis, The influence of the Effect of Gounding and Coona Cuent on the Field Stength the Coona Onset and the Voltage of Small Ai Gaps, WSEAS TRANSACTIONS on POWER SYSTEMS, Issue 1, Volume 3, Januay 8, ISSN: [13] Topalis F.V., Danikas M. G., in ai gaps with solid insulating baie unde impulse stess, Facta Univesitatis (NIS), Se:Elec. Eneg. Vol. 18, pp.87-14, 5. [14] Lazaidis L. A., Mikopoulos P. N., and Stasinopoulos C. A., in ai and along a pocelain insulato unde positive switching impulse s, ISH 15th Inten. Sympos. High Voltage Engineeing, Ljubljana, Slovenia, T4-561, pp. 154, 7.