Applications of Disconnecting Circuit-Breakers

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APPLICATIONS OF DISCONNECTING CIRCUIT-BREAKERS

P-O Andersson H-E Olovsson B Franzén U Lager J Lundquist* ABB Power Technologies Svenska Kraftnät Sydkraft Nät STRI

(Sweden)

1. INTRODUCTION The concept of a disconnecting circuit-breaker (DCB) for air-insulated substations (AIS) has been discussed in several Cigré papers since the middle of the 1990’s [1] [2] [3]. The DCB is aimed at replacing the traditional combination of a circuit-breaker (CB) and its associated conventional disconnectors (DS). The long intervals between maintenance of the DCB, compared with the conventional open-air DS, provide high availability also with simplified substation configurations. In addition, the lower number of switching devices means that space requirements are reduced and that control systems can be simplified. An example of DCB application in a 400 kV transmission substation is shown in Figure 1.

Figure 1. DCB in Hemsjö 400 kV transmission substation.

*[email protected] The main advantage of the DCB compared to a conventional disconnector is that the electrical contacts are enclosed in SF6 gas, thereby protected from the influence of ambient conditions including the effects of pollution. The protected environment provides improved reliability and prolonged intervals between de-energisation for maintenance of the DCB. The possibility of excluding the conventional disconnectors is based on the reduced maintenance demands of modern circuit-breakers; one major traditional function of the disconnector has been to isolate the breaker for maintenance of the breaker itself. With the prolonged maintenance intervals associated with modern circuit-breakers (de-energisation required typically only every 15 years), this isolating function of the disconnector is becoming obsolete. Furthermore, other major functions of the conventional disconnector, e.g. the isolation of lines, transformers, etc., for operational or maintenance purposes, can be equally well handled by the DCB, as will be described in this paper. The development of a policy for application of disconnectors in the Swedish transmission system is reviewed, and the future trends are discussed in Section 2 of the paper. The technical design aspects of the DCB have been presented in [4]. Other aspects, such as using the DCB to achieve simplified substation configurations with high availability and reduced environmental impact, were discussed in [3]. Since 1999, the DCB concept has been applied in several transmission and sub-transmission substations in various countries, from 72 to 420 kV. A recent example is the application of DCB for rehabilitation of a major 400/130 kV substation in the south of Sweden, owned by Svenska Kraftnät and Sydkraft Nät, respectively. The reasons for selecting the DCB concept for both the 400 kV and the 130 kV voltage levels are discussed in Section 3. An important aspect of the DCB is its ability to provide safe working conditions during maintenance and repair work in substations. When the DCB is used as a disconnector for this purpose, it has to be assured that the open contacts are not closed unintentionally, or bridged by a disruptive discharge. This important aspect has been considered in the design and specification of the DCB. In Section 4 of the paper, the safety locking system of the DCB is described, and an analysis of the insulating properties of the DCB in the open position is presented, taking into account the voltage stresses and the risks and consequences of a disruptive discharge. The importance of the safety aspects is reflected in the upcoming IEC standard for combined function disconnecting circuit-breakers. The current results of the standardisation work are briefly outlined in Section 5.

2. SVENSKA KRAFTNÄT’S DISCONNECTOR POLICY Svenska Kraftnät, the Swedish transmission network utility for the 220 kV and 400 kV grids, has developed a strategy for the application of disconnectors in their transmission substations. The policy is based on service experience with conventional open-air disconnectors, the reduced need for maintenance of modern circuit-breakers, and the development of new technologies, e.g. the DCB. Already in the seventies, the open-air disconnector in outdoor substations was identified by Svenska Kraftnät as a component that could jeopardize the availability of the substation, leading to serious problems in the network. A new disconnector policy was introduced and later applied to new or rebuilt 400 kV substations: the circuit-breakers were to be directly connected to the busbars, i.e., no busbar disconnectors were to be used. Instead, manual disconnection facilities were introduced in such a way that the circuit-breaker could quickly be removed from the busbar for maintenance or repair. Conventional disconnectors were still used on the opposite side of the circuit-breakers for disconnection of OH-lines, power transformers etc. The new policy was applied to 400 kV substations beginning in 1981. By the reduction in number of conventional disconnectors, the need for maintenance has decreased and the reliability has improved. The following rules are applied regarding manual disconnection towards the busbar: If the work is expected to last less than 8 hours, the busbar is taken out of service and all other bays are connected to the other busbar. If the work is expected to last more than 8 hours, the connection to the busbar is removed and the busbar is re-energised.

2 Today, the policy of Svenska Kraftnät is to integrate the breaking and disconnecting functions, which can be obtained by using the DCB. Reliability studies carried out by Svenska Kraftnät have shown that the expected outage rate, and the resulting unavailability of a substation, is lower when DCBs are used to replace conventional circuit-breakers and disconnectors.

3. THE DCB IN SIMPLIFIED SUBSTATION CONFIGURATIONS The availability properties of simplified substation configurations utilising the DCB were analysed in [3] for three common substation types: sectionalised single busbar and double busbar schemes for HV/MV substations, and a 1½-breaker scheme for EHV/HV substations. The studies included time- based maintenance and sustained active failures of equipment, taking into account the switching sequences associated with the failures. It was shown that the simplified substation configurations using DCB have a better availability compared with conventional solutions. In line with the concepts of manual disconnection facilities introduced by Svenska Kraftnät as discussed in Section 2, the availability of simplified substations may be further improved if the DCB is equipped with manual disconnecting devices as shown in Figure 2. The devices are composed of standard substation hardware. They provide convenient and safe disconnection and reconnection of the DCB if the need arises for maintenance or repair of the DCB itself. When the DCB is disconnected in this way, and proper safety distances are ensured, the other parts of the substation may be re-energised. To illustrate the concept of the DCB in combination with manual disconnection possibilities, the recent applications in the Hemsjö 400/130 kV transmission substation are described in the following section.

Figure 2. Typical manual disconnection devices during installation.

3.1. DCB application in a 400/130 kV transmission substation

3.1.1. 400 kV double-breaker substation configuration The 400 kV part of the Hemsjö substation is owned and operated by Svenska Kraftnät. The substation was completely rebuilt during 2002-2003, i.e., all primary and secondary equipment was replaced. Hemsjö is an important substation in the southern part of the Swedish 400 kV transmission network. Outage of the complete substation due to a busbar fault would endanger the operation of the entire grid in this part of Sweden. Therefore, the busbar configuration was changed from a main and transfer busbar scheme, to a double-breaker configuration when the substation was rebuilt. Thus, a busbar fault will no longer disturb the power flow through the substation, since the busbars are completely redundant with one breaker towards each of the busbars in each bay. As seen from the single line diagram in Figure 3, the 400 kV substation includes four OH-line bays, three capacitor banks and one 750 MVA power transformer feeding the regional 130 kV network. The concept of manual disconnection possibilities is utilised towards the busbars, the OH-line connections,

3 and the capacitor bank with two breakers. This will enable the “second” DCB to sustain service to these objects in case the “first” DCB needs to be taken out of service. In the rehabilitation of the 400 kV substation, the existing main busbar could be retained and, after extension, be used as one of the busbars in the rebuilt substation.

OH- OH- OH- line line line

400/130 kV OH- 150 MVAr 150 MVAr 100 MVAr 750 MVA line

Figure 3. Single line diagram of Hemsjö 400 kV substation. (Legend: See Figure 4.)

3.1.2. 130 kV sectionalised single busbar configuration The 130 kV sub-transmission part of the Hemsjö substation is owned and operated by Sydkraft Nät. The switchgear was rehabilitated during 2002-2003 using a simplified busbar scheme based on the DCB. The 130 kV switchgear includes bays for ten 130 kV lines, two capacitor banks, one 130/50 kV transformer, and one incoming bay from the 400/130 kV transformer. Before the rehabilitation, the 130 kV switchgear had a conventional double-busbar configuration, comprising 14 circuit-breakers and 40 disconnectors. When planning the new switchgear, the pervading demand was to minimise the number of switching devices. By this, it would be possible to reduce the maintenance costs and improve the substation availability by reducing both the total failure rate and the need for outages due to maintenance activities. The new switchgear is built in a sectionalised single busbar configuration comprising only 15 DCBs, as shown in the single line diagram in Figure 4. The simplified busbar configuration will limit the switching flexibility compared to a double busbar scheme, however, this aspect is overridden by the improved availability. Manual disconnection devices are utilised towards the busbar, as indicated in Figure 4. In summary, the simplified configuration of the 130 kV substation resulted in higher availability, substantially reduced need for maintenance, space saving, less auxiliary cabling, and a shorter design and installation time. Furthermore, the new switchgear is an improvement from safety point of view, since maintenance personnel will spend less time in the substation.

130/50 kV 400/130 kV 25 MVA 750 MVA Legend OH- OH- OH- OH- OH- line line line line line Disconnecting Circuit Breaker

Disconnector

Earthing switch Current Transformer Voltage Transformer

Power Transformer

Auto Transformer

Capacitor Bank Manual disconnection point

OH- OH- OH- OH- OH- line line line line line

Figure 4. Single line diagram of Hemsjö 130 kV substation.

4. SAFETY ASPECTS OF THE DCB

4.1. Description of blocking and interlocking systems When the DCB is used to isolate other equipment for maintenance or repair work, it needs to be locked in the open position in a failsafe way. The locking of the DCB consists of electrical and mechanical locking of the operating mechanism, as well as mechanical locking of the main linkage system for the breaker pole. As an example, the interlocking sequence for the 400 kV DCB is described below: In open position, the operating mechanism of the DCB is blocked by de-energising the coil controlling the latch shown in Figure 5. This prevents the DCB from closing even if an electrical closing signal should be given. Subsequently, mechanical blocking of the operating rod of the DCB is made, see Figure 6 (detail of locking mechanism is shown in Figure 7). When the mechanical blockings have been carried out, the output signal “DCB in disconnected position” is given to the interlocking system. As soon as all the DCB feeding the point to be earthed are in the disconnected position and the primary voltage of this point is low (checked to ensure that the remote line end is open), the interlocking system will give a release signal making it possible to close the earthing switch. Normal safety actions, e.g. padlocking and labelling of the closed earthing switch, are carried out in the usual way. Indication information is also communicated to the control centre.

Figure 5. Latch for blocking the Figure 6. Operating rod of DCB Figure 7. Detail of locking operating mechanism in the open locked in open position by mechanism for DCB operating position. separate operating device. rod (locked position).

4.2. Longitudinal insulation There is a minute risk that a flashover may occur across any high-voltage insulator in a substation during a pollution event or due to an overvoltage caused by lightning or switching. A flashover may constitute hazard to substation personnel since the power arc will cause UV and heat radiation. The personal risks associated with the earth fault current and the corresponding step and touch voltages are controlled by keeping these voltages down to a secure level, by sufficient earthing grid in the same way as for substations with conventional circuit breakers and disconnectors. Specifically for a DCB in the open position, the longitudinal insulation may be subject to dielectric stresses in the form of power frequency voltages and/or transient overvoltages. One terminal may be energised at operating voltage or grounded, while the voltage on the other terminal may be the operating voltage (with arbitrary phase shift) or a transient overvoltage (caused by lightning or switching). The basic requirements on the longitudinal insulation of switching equipment in general are discussed in [5]. The power frequency voltage across the longitudinal insulation may be up to twice the nominal operating voltage in an out-of-phase condition, e.g. during synchronising of generators. However, such conditions are of short duration, typically seconds or minutes, while lower stresses may exist for longer periods. The transient overvoltages are mainly caused by lightning strokes to the lines, or by switching events. It should be noted that phase-to-ground overvoltage levels are limited by flashovers occurring across line insulators, or by the surge arresters. The risk of a disruptive discharge across the longitudinal insulation of a DCB depends on the voltage stresses and the insulating properties. The air gap of a conventional DS is nearly unaffected by time or climatic conditions, while for the CB and the DCB, the risks of flashovers are related to the internal and external insulating properties of the breaker chamber insulator. These properties may change with time; the internal insulation withstand may be reduced by conductive deposits and the external insulation may be temporarily affected by pollution or ice. For the conventional CB, these aspects are to a certain degree covered by international standards. For the DCB, however, the upcoming IEC standard is considerable more stringent with regard to the long-term performance of the insulation, as discussed in Section 5. The combined function of the DCB is achieved by higher longitudinal insulation strength across open terminals compared with a CB, in order to fulfil also the higher insulation requirements on a DS. In addition, the DCB breaker chamber insulator is made of composite material with hydrophobic silicone-rubber weathersheds providing superior long-term insulating properties. When compared with a conventional CB, the risk of disruptive discharges is therefore lower for the DCB due to higher longitudinal insulation strength in terms of the higher insulation level and the hydrophobic external insulation surface. This is independent of the type of voltage stress, whether it is transient or has power

6 frequency. Compared with a conventional DS, the risk of disruptive discharges across the DCB is affected by the presence of the breaker chamber insulator. The overall risk of disruptive discharges is lower when one side of the DCB is grounded, due to the lower maximum voltage stress on the longitudinal insulation. The risk of external flashovers due to power frequency voltages are minimised by appropriate co- ordination of the external insulation with regard to pollution and ice. The risk of internal or external disruptive discharges due to overvoltages are minimised by proper co-ordination between insulation withstand levels and overvoltage levels. In a situation with closed earthing switch, the power arc current resulting from a disruptive discharge across a DCB will be equal to the earth fault current. The earthing switch will handle the fault current, and the ordinary protection system of the substation will clear the fault. If the earthing switch is open, the power arc current will be equal to the current of a closed DCB in the same position. This means that the current level may be high or low, depending on the prevailing load conditions in the network. In the very unlikely event of an internal sparkover, the interrupting chamber is dimensioned to withstand the stresses without explosion.

4.3. Insulation co-ordination

4.3.1. Insulation co-ordination with regard to pollution and ice The risk of flashover across the DCB in open position due to pollution or ice is determined by the actual environmental conditions and the performance of the breaker chamber insulator under these conditions. The flashover performance is verified by withstand tests in the laboratory under the specified pollution or ice conditions. For polluted environments, the service conditions are preferably described in terms of the expected maximum equivalent salt deposit density (ESDD). Relevant laboratory test methods are the Dry Salt Layer (DSL) test [6] or the Solid Layer method included in IEC 60507 (Artificial pollution tests on high-voltage insulators to be used on a.c. systems). For icing environments, the service conditions are often described in terms of the expected maximum ice conductivity. In addition, the ice thickness may be prescribed. Relevant icing test methods for high-voltage insulators are presented in [7].

4.3.2. Insulation co-ordination with regard to transient overvoltages The risk of disruptive discharge across the DCB in open position due to transient overvoltages is determined by the statistical distributions of voltage strengths and stresses of the longitudinal insulation. The statistical insulation co-ordination procedures, based on the rate and distribution of overvoltage levels and withstand levels, are described in IEC 60071 (Insulation co-ordination). The rate of lightning overvoltages appearing across the longitudinal insulation depends mainly on the ground flash density, the lightning performance of the transmission lines, and the number and length of transmission lines connected to the energised terminal of the DCB. The rate of switching overvoltages depends primarily on the same parameters; the majority of switching overvoltages are due to high-speed auto re-closing of lines after earth faults caused by lightning. The statistical distributions of overvoltage levels depend on the number of transmission lines connected to the energised terminal of the DCB, and on the flashover voltage of the line insulators. In addition, the overvoltage distributions may be heavily affected by surge arresters in the substation or on the lines. When surge arresters are applied, the distance between the surge arresters and the DCB is important to the lightning overvoltage levels at the DCB terminal. (It should be noted that surge arresters could be mounted closer to the DCB in comparison with conventional switching devices.) The risk of disruptive discharge across the longitudinal insulation can be reduced to an arbitrary low level by selecting the withstand level of the DCB with regard to the rates and distributions of the lightning and switching overvoltages, in accordance with IEC 60071.

7 5. INTERNATIONAL STANDARDISATION WORK The disconnecting circuit-breaker is a combined function switching device, which means it has to comply with both circuit-breaker and disconnector standards. The IEC has initiated the work on a new standard for this novel type of equipment: IEC 62271-108, “Combined function disconnecting circuit- breakers”, expected to be published in 2005. The main difference in the standard’s requirements on the “combined function disconnecting circuit- breaker” compared to a conventional circuit-breaker are the repeated dielectric tests following the mechanical and short-circuit tests. The aim of repeating the dielectric tests (with requirements according to the disconnector standard) is to prove that the disconnecting properties of the DCB are fulfilled during its service life, despite contact wear and any decomposition by-product generated by arc interruption.

6. CONCLUSIONS The concept of disconnecting circuit-breakers has been illustrated by presenting the application of the DCB in a major 400/130 kV transmission substation. It was shown that the substation configuration could be simplified in a considerable way by using the DCB instead of conventional switching devices, at the same time improving the overall availability of the substation. The safety aspects of the DCB have been discussed with reference to the blocking system (ensuring that the DCB is not closed unintentionally when used as a disconnector), and with reference to the longitudinal insulation in open position. The risk of a disruptive discharge across the open DCB is always lower than with a conventional CB. Furthermore, the risk is reduced to an arbitrarily low level by adequate insulation co-ordination with respect to overvoltage levels and environmental conditions. A new international standard, taking into account the special requirements on “Combined function disconnecting circuit-breakers”, is soon to be published.

7. REFERENCES [1] B Wahlström, Y Aoshima, Y Mino, C Lajoie-Mazenc, D R Torgerson, A N Zomers, “The Future Substation: a reflective approach, “ Report 23-207, Cigré Session, Paris, 1996. [2] P Norberg, M Tapper, W Lord, A Engqvist, “The Future Substation - Reflection About Design,” Report 23-105, Cigré Session, Paris, 1998. [3] C-E Sölver, H-E Olovsson, W Lord, P Norberg, J Lundquist, “Innovative Substations with High Availability using Switching Modules and Disconnecting Circuit-breakers,” Report 23- 102, Cigré Session, Paris, 2000. [4] H-E Olovsson, C-E Sölver, “Innovative Solutions for Increasing Reliability and Availability in AIS Substations,” Cigré SC23 Colloquium, Venezuela, 2001. [5] K Markussen et al., ”The Insulation Between Terminals of Circuit-breakers and Disconnectors,” Electra No. 26, 1973. [6] C S Engelbrecht, R Hartings, H Tunell, B Engström, H Janssen, R Hennings, “Pollution Tests for Coastal Conditions on an 800-kV Composite Bushing,” IEEE Transactions on Power Delivery, Vol. 18, No. 3, July 2003, pp. 953-959. [7] M. Farzaneh et al., “Insulator Icing Test Methods and Procedures – A Position Paper Prepared By the IEEE Task Force on Insulator Icing Test Methods,” IEEE Transactions on Power Delivery, Vol. 18, No. 4, October 2003, pp. 1503-1515.