SURGE ARRESTERS FOR CABLE SHEATH PREVENTING POWER LOSSES IN M.V. NETWORKS

Transcription

1 SURGE ARRESTERS FOR CABLE SHEATH PREVENTING POWER LOSSES IN M.V. NETWORKS A. Heiß Energie-AG (EAM), Kasse G. Bazer Darmstadt University of Technoogy O. Schmitt ABB Caor Emag Schatanagen, Mannheim B. Richter ABB Hochspannungstechnik, Baden (Switzerand) Cabe sheaths of power cabes in the distribution networks are generay earthed on both terminas. This method avoids any dieectric stress of the sheath isoation by couping votage in the circuit sheath -to -earth. The same appies to the fiedcontro eectrodes of the seaing ends. This is an advantage for a dieectric fied contro system of cabe terminas, considering capacitive, resistive or refractive controed. The disadvantage is an additiona power oss, caused by the current fowing in the circuit sheath- earth. The oss is dependent on the type of cabe, the oad current and the method of cabe aying. For typica medium votage-pastic-insuated cabes the additiona oss in the cabe sheaths are about 2% up to 10 % of the tota power oss of a cabe connection. additiona cabe-power oss for different kinds of cabe aying and the economica advantage of the soution. It aso describes the effects for the grounding arrangement of a whoe network. However, the power oss can be avoided, if a surge arrester is inserted between the screen and earth at one termina and if the other side is directy earthed. Moreover the surge arrester imits the votage between sheath and earth to the residua votage of the arrester. And in this way the arrester protects the cabe-sheath isoation and the isoation of the seaing end against high votages in case of an overvotage. The fiedcontro eectrode of the seaing end and the cabe sheath have aways an defined votage in ike manner, if both ends of the sheath are earthed. This kind of protective circuit is not new. It is aready appied for cabe connections with high current. With modern meta-oxid surge arresters (mo-arresters) this protective circuit can be instaed on each cabe connection to avoid the additiona power oss in the sheaths. The report describes the design of the moarrester. The arresters are standardized for typica ength of cabe-connections and typica vaues of short-circuit currents. Its protective eve shoud be as ow as possibe, because the withstand strength of the sheath during its service ife is not we-defined and is not assured by any standardized test. On the other hand the rated votage of these arresters shoud be higher than the induces sheath-to-earth votage at maximum faut current. Beyond this the paper specifies the

2 SURGE ARRESTERS FOR CABLE SHEATH PREVENTING POWER LOSSES IN M.V. NETWORKS A. Heiß Energie-AG (EAM), Kasse G. Bazer Darmstadt University of Technoogy O. Schmitt ABB Caor Emag Schatanagen, Mannheim B. Richter ABB Hochspannungstechnik, Baden (Switzerand) 1 INTRODUCTION Cabe sheaths of power cabes in the distribution networks are generay earthed on both sides. This method avoids any dieectric stress on the sheath insuation by transferred votages in the circuit sheath to earth. The same appies to the fied-contro eectrodes of the seaing ends. This is an advantage for a dieectric fied contro systems of cabe terminas, whether capacitive, resistive or refractive controed. The disadvantage is an additiona power oss, caused by the current fowing in the circuit sheath to earth. The amount of osses depends on the type of cabe, the oad current and the method of cabe aying. For typica medium votage poymeric insuated cabes the additiona osses in the cabe sheaths are around 2% up to 10 % of the tota power osses of a cabe connection. However, the power osses can be avoided, if ony one side of the cabe sheath is earthed, and a surge arrester is inserted between the sheath and earth at the other side of the cabe. Additiona the surge arrester imits overvotages between sheath and earth, and in this way the arrester protects the cabe-sheath insuation. The fied-contro eectrode of the seaing end and the cabe sheath have aways a simiary defined votage, if both ends of the sheath are earthed. This kind of protective circuit is not new. It is usuay aready appied to cabe connections with high currents in high votage systems. The arresters are standardized for typica engths of cabe-connections, and typica vaues of short-circuit currents. Its protective eve shoud be as ow as possibe, because the votage withstand strength of the sheath insuation is not we defined, and is not assured by any standardized test. On the other hand the arresters shoud withstand the induces sheath-to-earth votage at maximum faut current. Beyond this the paper specifies the additiona cabe-power oss for different kinds of cabe aying and the economica advantage of the soution. It aso describes the effects of the grounding arrangement of the whoe network. 2 CALCULATION OF VOLTAGES /CURRENTS 2.1 Genera The votages and currents of the cabe sheath is infuenced by two different type of currents regarding ampitude and time duration: - short-circuit current with a time duration of t = 3 s(singe and three phase) - oad current (infinite time). For these two current stresses the votages have to be considered. Two types of aying of the cabes are in use according to figure 2.1: trianguar and singe pane configuration. Figure 2.1: Laying of m.v. cabes The different types of aying produce different votage stresses, and different oad currents depending on the geometry of the cabe, and the earthing. The subcauses describe the genera equations to cacuate the votage and current distribution due to the earthing condition. 2.2 Votages Votages caused by three phase currents In genera the votage sheath U S can be cacuated depending on the ine current. In the case of singe pane configuration the votage stress of the sheath depends on the position of the ine considered: the center or the outer position. Due to this the maximum votage wi be the induced votage of the outer cabe sheath Votages caused by singe phase currents If a singe phase current is assumed the votage of the cabe sheath wi not be infuenced by the other ines, which do not carry the short circuit current. The votage U S can be cacuated depending on the return path through earth Capacitive votage transfer If the sheath is ony earthed at one side according to figure 2.2 the votage U 0 between the ine conductor and earth wi be transferred to the votage U S and U L at the remote end of the cabe.

3 Figure 2.2: Capacitive votage transfer U L ine-to-sheath U S sheath-to-earth Without consideration of the oad fow condition, the votage depends on the capacitance C S and C E and the cabe ength. Therefore the maximum votage U Smax at the end of the cabe can be estimated due to the capacitive votage divider if both sides are isoated. The votage U Smax is ony possibe if the cabe ength is infinite, in the opposite of this the votage wi be reduced due to the short-circuit at one side of the cabe. 2.3 Sheath currents and power osses A current of the sheath is caused, if the sheath is earthed at both sides. The consequence is that additiona power osses are generated. According to the above mentioned instaations, the currents wi be cacuated with the worst-case assumption, that there is no return current through earth. This wi ead to the maximum power osses. According to the time duration the power osses are ony reevant for norma operation conditions (no short-circuit currents). And for this reason three phase currents are ony considered. 2.4 Exampe Genera The most common cabe in Germany is a poymeric type (VPE) with auminum conductors (NA2XS2Y). If in the case of a triange instaation the distance between the three ines is twice the outer radius of the cabe (d = 36 mm), the distance is increased to fourth times the vaue, if a pane eve instaation is considered. According to the above mentioned circumstances the votages and currents are cacuated using the equations, which are reported in /1/ Sheath votages Inductive votage transfer Reated to a current of I = 1000 A, the sheath votage to earth can be cacuated in dependence on the ength according to the above mentioned types of instaations and the current: Three phase: Trianguar: U S = [ V km] Singe pane: U S = [ V km] singe phase: U S = [ V km] Capacitive votage transfer The capacitance C S and C E can be cacuated to C S = 254 nf/km and C E = 760 nf/km. This eads to the votage U Smax = 0.25 U 0. That means, that 25 % of the votage ine-to-earth wi be transferred. In reaity this votages wi be smaer due to the earthing at one side. In dependence of the cabe ength the foowing vaues are cacuated assuming a ine-to-sheath votage of 24 kv/ 3 and a frequency of 50 Hz: U S = 0.3 V/5.1 V cabe ength 500 m/2000 m The votages sheath-to-earth generay can be negected considering the energy consumption of the instaed surge arresters in the case of the steady state condition. If in the opposite of this the conductor of the cabe (ength 2000 m) is stressed by a ramp-function with time-to-peak of 4 µs and the same ampitude of 24 kv/ 3, the votages U S and U L (figure 2.2) at the remote side are U S = kv U L = kv These vaues are generated incuding the transient phenomena and refection of the surge at the open end of the cabe sheath. Therefore it shoud mandatory to insta a surge arrester between the isoated sheath and earth Sheath currents and power osses Considering the cabe type NA2XS2Y, as mentioned in subcause 2.4.1, the foowing sheath currents I S can be cacuated according to the configuration depending on the symmetrica ine current I L. Trianguar: Singe pane: I S = I L I S = I L In the case of a singe pane configuration the sheath current of the center ine wi be reduced, according to the strong couping of the two outer ines. Due to the current I S and the resistance of the sheath, the power osses of the sheaths can be cacuated depending on the configuration and the maximum ine current. Trianguar: singe pane: P S P S = 740 W/km = 5300 W/km 2.5 Seection of surge arresters In this subcause the votages were considered caused by three phase short-circuit currents in resonant earthed networks. In principe singe phase short-circuit currents wi ead to higher vaues, but the current depends on the network earthing and may differ in a wide range, so that actua cacuations have to be performed.

4 The reevant temporary overvotage U TOV, for the seection of the surge arresters, are cacuated in chapter and are summarized in tabe 2.1 depending on the cabe ength of 0.5 km and 2 km, which are typica for the instaation in m.v.-networks. The maximum three phase short-circuit current can be assumed to either k = I 10 ka or I k = 20 ka in distribution networks, whereas the oad current of the cabe is I = 350 A. /m U TOV /V I k = 10 ka I k = 20 ka I = Tabe 2.1: Maximum cabe sheath votages U TOV I k short-circuit current I oad current The isted votages infuence the continuos operating votages of the meta-oxide surge arresters depending on the time-votage curve. Due to the vaues of tabe 2.1 the foowing time durations have to be considered: Ik s.c. current t = 3 sec I oad current t According to tabe 2.1 the continuos operating votage U c of meta-oxide arresters can be seected under the consideration of the curve T with respect to the temporary overvotage U TOV versus the time duration t. Considering the two different time durations 3 s and infinite, T is for the considered ABB surge arresters POLIM-C: T = 1.0 or for t or 3 s Therefore the votage U TOV, caused by the short-circuit current, wi ead to the highest vaues U c. Due to these data the votages U c are equa or higher than isted in tabe 2.2. /m U c /V I k = 10 ka I k = 20 ka Tabe 2.2: Min. continuos operating votage U c In addition to the U c -seection the energy capabiity of the surge arrester has to be controed and the cabe sheath arrester has ony to discharge the reated cabe section /1/. A If the maximum votage U TOV is assumed to U TOV = 0.75 U 0max of the ine-to-earth votage (subcause 2.4.2), the energy can be estimated for a cabe ength of = 1 km to E max = 1.37 kj/km, if the maximum votage ine-toearth U 0max (figure 2.2) is considered to 80 kv, which shoud be the protective eve of the instaed surge arrester. 3 RESULTS OF MEASUREMENTS Today usuay singe-conductor VPE-cabes are used in a networks up to the system votage of 123 kv. The basis of these cabes is a braided shied of copper wires. This shied is covered by a transverse copper spira. With this construction the eectromagnetic behavior of the cabe sheath is ceary defined, different from a meta encosure /2/. In a cabe section with ength of 1027 m the sheath currents I S in three phases were measured with the resut depending on the ine current I L of a 20 kv cabe system (NA2XS(F)2Y): - outer ine I S = 0,159 I L center ine I S = 0,1 I L Originay the cabes are instaed in a trianguar configuration. But the asymmetrica vaues of the sheath current measured demonstrate that after a onger service time the singe conductors are in a asymmetrica configuration, caused by the earth pressure. In this context the harmonic components in the sheath current are reevant. Figure 3.1 shows the harmonic content of the sheath current reated to the power frequency current. The content of the 5th and 7th harmonic is marked in this case. 25 % Harmonics Figure 3.1: Harmonic content of the cabe sheath current I S Figure 3.2 shows the power oss in the sheaths for a 24- kv-vpe-cabe type NA2XS(F)2Y 150 mm 2 with a copper sheath of 25 mm 2 in dependent on the oad current of the cabe connection in a network. The power osses were cacuated using the measured sheath currents and the resistance of the sheath R S = 0,73 Ω / km with a service temperature of the cabe at 20 C. The deviation between the theoreticay cacuated vaues (subcause 2.5.3) and the vaues of figure 3.2 deas with the actua distance of the cabe instaations, which obviousy differs from the cacuated ones, considering mainy the triange instaation.

5 5000 W/km trianguar singe pane A Figure 3.2: Power oss of the cabe sheaths (25 mm² Cu) in depending on the ine current 4 ECONOMIC ASPECTS In the dereguated market the expense of erection and maintenance of capita assets becomes more and more interesting. This appies to the cost of oss of a m.v. cabe too, with a ong service time of around 30 years. The additiona power oss in the cabe sheaths is an unnecessary part of the tota costs for power oss. In the dereguated market the power-system management aims to increase the utiization of a components in the network. Therefore it is worth to ook into the power oss of a cabe section. In this paragraph the present vaues of costs of the power osses in the cabe sheaths, according to figure 3.2, are cacuated and compared with costs of the connection of the surge arresters with sheaths to prevent the additiona oss. The present vaues of costs of osses (DM/ km) are cacuated over a service time of 20 years with the foowing parameters: - power oss time in the network T PL = 3000 h/a - rate for eectrica energy K E = 0,09 DM/kWh - interest rate p = 6 % - factor for present vaue of costs r = 11,47 Nowadays German utiities evauate the power oss of network components with the hep of these parameters. The costs for provision of eectrica power (DM/ kw) are not considered. The present vaues of costs of the power osses in the cabe sheaths in dependency of the oad current of the cabes are given in tabe 4.1 for underground aying, both in a trianguar instaation and in a singe pane. In German m.v.-distribution networks the utiization of cabe sections between the stations is usuay around 30% of the rated power. This vaue corresponds to a oad current I L = 100 A for the cabe type in figure 3.2. The present vaues and the annua costs of the power osses in the cabe sheaths for this oad is between 670 and 1370 DM/km. In many cases measurements of the sheath votage in a m.v.-network show asymmetrica vaues in the three phases, corresponding to a singe pane underground instaation, athough the cabes were instaed in a trianguar formation. This must be considered when evauating of the power oss in the cabe sheaths. In reaity the data given in tabe 4.1 woud be higher, as the rise of the capacity utiization during the service time of the cabes is negected. The cost-benefit reation between the arresters and the additiona power osses of the cabes is the reason for choosing arresters, particuar for cabe sections of more than 500 m. E.g. for cabe ength of 1000 m the present vaue of the power osses is two times higher than the arrester set, cacuated for 30 % of the rated cabe current. trianguar singe pane I L /A C P C P Tabe 4.1: Present vaues (P/DM/km) and annua costs (C/DM/km a) of the power osses in the cabe sheaths 5 APPLICATION OF CABLE SHEATH ARRESTERS According to figure 5.1 the transient votages U L and U S were cacuated. The cabe is connected to the overhead ine and a ightning stroke hits a three ine conductors in the vicinity of the cabe conjunction ( = 0.2 km). The ampitude of the votage surge is U 0 = 3000 kv and this vaue is possibe, if a m.v. overhead ine with wooden towers is assumed. In this case the votage shape is ony imited by the fashover to earth, so that the highest votage strength may occur. Therefore the resuts of the cacuations shoud be on the conservative side. Figure 5.1: Equivaent circuit for cacuation of the transient votages Due to figure 5.1 surge arresters at the cabe conjunction (A1) and at the isoated cabe sheath (A2) are instaed.

6 The U c rating of the surge arresters is according to the ength of the ength = 2 km and short-circuit current of I k = 10 ka: A1: U c = 24 kv (ABB POLIM-D) A2: U c = 2 kv (ABB POLIM-C) Figure 5.2 shows the votages U L and U S at the end of the cabe section. Furthermore the infuence of the cabe sheath arrester is isted. Whereas the votage U L is ony infuenced by the additiona arrester A2 from U L = 109 kv to U LA2 = 70 kv, the votage U S = 88 kv is reduced significanty to U SA2 = 6.3 kv. The energy consumption of the arrester A2 is about E = 0.8 kj. The protection eve of the sheath arresters is substantiay ower than the impuse votage strength of the cabe sheath with a thickness of about 2 mm. This is vaid for a types of modern m.v.-cabes (VPE). kv U L U LA2 U SA2 U S Figure 5.2: Transient votages at the end of the cabe section U L, U LA2 votage ine-to-sheath without/with arrester A2 U S, U SA2 votage sheath-to-earth without/with arrester A2 The specia requirements of the network shoud be considered in the seection of surge arresters (tabe 5.1). The maximum faut current in the network is the main design vaue for the arresters U c. The maximum faut currents of the network with different methods of neutra point connections are isted in tabe 5.1, first ine. The continuos operating votage of these arresters shoud be higher than the induced votage between sheath and earth at the maximum faut current. The sheath arresters infuence the earth-eectrode potentia U E in the station at the same side of the cabe. The earth faut current I E is the design vaue for the votage U E, and for the corresponding touch votage U T. The data for I E are isted in the second ine. If one side of the sheath is isoated, the screening factor of the cabe wi be r K = 1 instead of r K = This modification is not important because normay the reducing infuence of the cabe sheaths does not have to be considered for the design of the votages U E and U T in isoated or resonant earthed systems. The permissibe µs vaues for the earth faut current in the 24 kv networks with isoated neutra is I c = 35 A and for resonant earthed systems is I E = 60 A. With these currents usuay the imiting vaues for U E und U T are kept in the networks. vaue isoated resonant earthed U s, U c I k3 I k3 I k1 = 0 I ; < k2e k2 I k1 = 0 k3 I k2e = Ik2 earthed via impedance I k1 (eq. 2.4) U E ; U T I c I E I k1 I kmin I k2 I k2e < I k3 I k2 I k2 (Z 0 < Z 1 ) I k1 (Z 0 > Z 1 ) Tabe 5.1: Vaues for the seection of the cabe sheath surge arresters (U s, U c ), earth eectrode potentia (U E ), and the protection ( I kmin ) I c earth faut current (capacitive) I E earthing current /3/ I k1, I k2, I k3 short-circuit current Ony in impedance earthed systems the screening factor of the cabes r K = 1 has an important infuence for the earth-eectrode potentia U E and for touch votage U T in the station. In this case there is no connection of the faut current I k1 to the grounding systems of neighboring stations in the network. If the votage U E and U T wi extend the permissibe vaues, the soution shoud be, that ony two sheaths of the system are earthed via a surge arrester, whereas the third one is soidy connected to ground. The cabe sheaths of a singe phase connect in parae a the grounding resistances of the stations and in this way a reduction of the effective grounding resistance is given. The therma short-circuit current carrying capacity of cabe sheath in a singe ine for VPE-cabes is (the vaues for paper insuated cabes are ower): 16 mm 2 copper I th = 3.2 ka (t=1s) and I th = 2.1 ka (t= 3s) 25 mm 2 copper I th = 5.0 ka (t=1s) and I th = 3.1 ka (t= 3s) Generay these vaues of the therma short-circuit current carrying capacity are sufficient for a impedance earthed network with I k1 = 2000 A. It must be considered that in addition the grounding resistance of the ow votage systems reduces the effective ground resistance of a station. 6 CONCLUSION

7 To summarized it can be said that cabe-sheath arresters can be instaed at one termina of a cabe section between two ring main units or switchgears in a kinds of m.v.-networks. The cost benefit reation between arresters and the additiona power osses of the cabes is a reason for choosing arresters, particuar for cabe sections of more than 500 m and oad currents of more than 30 % of the rated current. Ony in the case of impedance earthing systems specia requirements must be considered to keep the imiting vaues for the eartheectrode potentia U E and the touch votage U T. In a cases the possibiity is, that at two cabe sheaths arresters are instaed, whereas the third sheath is soidy grounded. Reference: /1/ ABB-Report: Seection of Surge Arresters for Cabe Sheaths. Baden/Mannheim /2/ Braun, A.: Schirmspannungen und Schirmveruste bei Mittespannungs-VPE-Kaben, E-Wirtschaft, Jg. 88(1989), H. 26, S /3/ EN 50179: Erection of eectrica power instaations in systems with nomina votages above 1 kv AC