FP_B.1_ABB _Switchgear with improved maintenance concept IMPROVED MAINTENANCE CONCEPT BY OPTIMIZED SWITCHGEAR DESIGN Josef Hanson, ABB AG, +49 621 381 2033, [email protected]

1

ABSTRACT

Switchgears are used in power distribution networks for control and protection of electrical devices to increase the reliability of the power grid. Within the gas-insulated technology, the current-carrying components are protected by the insulating gas due to the hermetically closed enclosure. Furthermore a gas-insulated switchgear is a preferred solution wherever space is a constraint, e.g. in mega-cities, or in aggressive hostile environmental areas. The gas-insulated switchgear described in this paper is characterized by environmentally friendly features and an improved accessibility and thus provides essential benefits for our environment as well as for operation and maintenance issues. The switchgear is designed for a rated voltage up to 145 kV and a rated current up to 3150 A. It can protect power grids with rated short-circuit currents up to 40 kA. The connection to the station control can be performed with the standardized communication protocol according IEC 61850 standard. The compact and modular design of the switchgear enables a quick and easy installation of the substation. Its new eco-efficient design with an optimized three-phase encapsulation using single-phase insulators reduces the need for the required insulating gas considerably. Another important feature is the improved access to the drives from the operator aisle for any inspection control or manual operation. The small bay width of 800 mm enables additional access corridors without increasing the already low footprint of the complete substation. Consequently lower operating and maintenance costs can be achieved. With the switchgear design described here the use of additional buffer gas compartments can be avoided to the greatest possible extend. The design of the three-phase encapsulated switchgear allows the use of single-phase insulators which enable a considerable higher differential pressure due to its symmetrical design. The separation of the disconnector and the earthing switch lead to the design of volume optimized modules and an additional buffer compartment which is part of each bay layout. This basic design feature enables improved service continuity using the switchgears built-in solution. Thus, e.g. in case of repairs the service technician is enabled to replace the circuit breaker interrupter while the busbars are energized. An additional gas compartment is not required. A major focus during the development of the switchgear was the minimization of the required insulating gas. Due to the comparatively small amount of gas used, the required time for evacuation of the switchgear is significantly reduced. This enables considerable faster commissioning of the switchgear after installation and even after any maintenance work. Consequently the availability of the entire switchgear is increased. The technical paper describes the key design features of the switchgear as certain maintenance improvements and presents the positive impact on the improved environmental performance.

KEYWORDS: MINIMIZED INSULATION GAS, LATE CUSTOMIZING, SERVICE CONTINUITY, IMPROVED ACCESSIBILITY

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INTRODUCTION

Gas-insulated switchgear, in short GIS, needs only a fraction (about 30 %) of the area and space required by conventional air insulated switchgear. For applications in areas of high electrical load density, especially in mega cities and industrial cities, gas-insulated switchgears are an economic solution. Especially in urban centers required space for an expansion of the switchgear is not available. Therefore switchgear expansion mostly is done on the fly due to the high requirement on the availability of the power supply as the switchgear cannot completely be switched off. The replacement of air insulated switchgears by compact, modular gas-insulated switchgears provide ideal solutions for such requirements. However, due to their compact design and the close distance of switching components higher efforts are necessary to fulfill the high requirements on the maintenance and safety concept. In principal those requirements can be realized with a suitable switchgear layout using additional buffer gas compartments between switching devices. The ABB range of high voltage gas-insulated switchgears covers ratings and applications from 72.5 kV to 1200 kV matching current and future requirements for modern switchgears. In all installations the equipment complies with the IEC rules currently in force, thereby meeting nearly all other national or customer own specifications. As the world's largest demand for gas-insulated switchgear is covered by the ratings up to 145 kV, 3150 A and 40 kA short-circuit current ABB has developed a new three-phase encapsulated switchgear for sub-transmission. The resulting design enables the execution of maintenance and repair work, while the substation keeps energized.

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DESIGN PRINCIPLES

The whole design is focused to minimize the SF6 use and therefore the enclosure is “shrinked” to a minimum. Compared to its predecessor, the gas consumption for a complete DBB-layout is reduced nearly in the same way as it is achieved for the bay itself and sum up to 30 %. The in-line arrangement of the conductors results in less complexity of the internal structure and leads to the use of robust single phase insulators which have a superior mechanical performance and a uniform field distribution. Bursting pressures of partitions are even higher than enclosures. Although single phase insulators need a higher number of flange connections, the achieved gas tightness is < 0.1 % / a. 

Optimized for ratings up to 145 kV, 3150 A and 40 kA



Increased load current up to 1.1x rated current



Peak withstand current of 108kA

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Up to 60 % less SF6 than its predecessor product



Reduced bay width of 800 mm

Figure 1: Double busbar cable bay 145kV, 3150A, 40kA

The horizontal circuit breaker arrangement provides some essential benefits: 

Vertical placed current transformers allow its configuration independent from the footprint of the bay, as their height does not impact the bay depth. This feature, together with a high standardization of the functional units, enables late customizing but also an early start of civil work on the customer site.



The modular and standardized design of the functional units enables a simplified planning, industrial flow production and reduces the delivery time.

Increased quality is achieved, as the outside located current transformers prevent a contamination of the CTinsulation by SF6-decomposition products and a gas-tight feed of the secondary terminals is not needed. All maintenance earthing switches and the fast acting earthing switch provide a removable link to the earthing switch terminals for isolated access to the active parts. The maintenance earthing switches can be used to perform measurements of the voltage drop to check the current path as well as of the opening / closing time and speed after an interrupter replacement. An outage of the bus bars is not required. The constant efforts of the designer to reduce the usage of SF6 result in a small enclosure and in a simple, lean dielectric design for the three phases. This leads to a separation of the disconnector and the earthing switch. The common point maintenance earthing switch is operated in an own gas compartment between the circuit-breaker and the bus bar disconnector. This additional buffer compartment is part of each layout and enables improved service continuity (built-in solution). In case of repairs the service technician is enabled to replace the circuit breaker interrupter while the busbars are energized. Maintenance earthing switch (operated in own gas compartment)

Busbar disconnector

Circuit breaker

Fast acting earthing switch

Exit disconnector

Figure 2: Gas schematic of a 145 kV double busbar cable bay according Figure 1. The maintenance earthing switch is operated in an own gas compartment (built-in solution) between the circuit-breaker and the bus bar disconnector.

The move from the single motion principle for the interrupter chamber to the double motion principle reduces the energy required for switching by more than 50 %. This facilitates the reduction of the drive operating energy as well and enables the use of a compact spring operating mechanism. Torsion springs instead of compression springs favor an optimal use of the spring energy and the low moving mass as well as the spring principle reduces the reaction forces to the GIS and thus the dynamic load of the foundation structure. The closing spring in the operating mechanism generates the required driving force to close the circuit breaker and charge the opening spring. As such, the mechanical energy needed for the opening operation is always stored in the opening spring when the circuit breaker is in the closed position. The power unit is characterized by the following advantages: 

Torsion springs for opening and closing



No overcharge of the torsion springs due to a mechanical stop and torque limiter

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Trip and close latches are fast acting and vibration proofed

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Damping device to retard the motion of the contact system at the end of an opening operation

4 4.1

MAINTENANCE AND REPAIR Buffer gas compartments for improved serviceability

In general sub transmission switchgears use three-phase encapsulated systems. So far, the basic design of the substation layout from all manufacturers follows the same principle (traditional design) and during service and maintenance work usually the three-phase barrier insulators are not stressed with the full differential pressure (ambient pressure vs. SF6 gas pressure). Depending on the required availability and the maintenance concept of the utility, additional buffer gas compartments (A) between the feeders as well as within the feeder between circuit breaker and busbar (B) are used (Figure ).

Figure 3: GIS schematic with additional gas buffer compartments Due to the additional buffer gas compartment (A) between the bays the combined disconnector and earthing switch can be replaced without de-energizing the adjacent bays next to it. The additional buffer gas compartment (B) enables the exchange of the circuit breaker interrupter without de-energizing the bus bar.

Extension

Repair

To enable an extension of the switchgear at a later stage, at both bus bar ends additional gas compartments (C) are installed to avoid a complete shutdown of the bus bar and thus the de-energizing of the complete switchgear. As an example Table 1 summarize service continuity criteria and the corresponding layout requirements for a double bus bar GIS. Component

Service continuity criteria

Requirement

1

Busbar disconnector (Busbar DS)

Module A

2

Circuit breaker

Shutdown only of affected feeder and associated busbar Other bays (including those adjacent to the affected bay) shall be energized Shutdown only of the affected feeder Both busbars shall be energized

3

Earthing switch (next to busbar DS)

Shutdown only of the affected feeder and one busbar (in case of internal fault)

Module A

Module B

Criteria

Requirement

Maintain service continuity of the bays adjacent to the extension point during busbar connection All the existing feeders shall be energized

Gas buffer compartment (C) at the end of each bus bar

Table 1: Service continuity criteria for double-bus bar GIS

The additional buffer gas compartments (A), (B) and (C) facilitates the service continuity criteria according Table 1. For the "traditional" switchgear layout these additional buffer gas compartments have to be considered already during the planning phase. A later integration is difficult or impossible due to space restrictions.

4.2

Improved maintenance concept

The switchgear layout in Figure 3 is essentially determined by the design of the switchgear and the maintenance concept of the facility operator. Keeping the availability of the substation unchanged, a further optimization of the switchgear can be achieved only by a change of the principle bay design. For the new switchgear design this leads to the separation of the disconnector and the earthing switch, forced by the efforts to minimize the gas volume of each module. Within this design the earthing switch is operated in an own gas compartment between the circuit-breaker and the bus bar disconnector. This additional buffer compartment is part of each layout and enables improved service continuity (built-in solution). In case of repairs the service technician is enabled to replace the circuit breaker interrupter while the busbars are energized. An additional gas compartment (B) as indicated in Table 1 is not required.

Additional gas buffer compartment (B) (no shut-down during CB-interrupter exchange)

Separate gas compartment for maintenance earthing switch (built-in solution) keeps busbar energized during CB-interrupter exchange

Figure 4: Line-up of “traditional” (left side) and new (right side) bay design for a 145-kV-GIS

The whole GIS-design is focused to minimize the SF6-use and therefore the enclosure is “shrinked” to a minimum. The in-line arrangement of the conductors results in less complexity of the internal structure and leads to the use of robust single phase insulators which have a superior mechanical performance and a uniform field distribution. For the expansion of the switchgear an additional gas compartment (C) as indicated in Table 1 can be omitted and an uninterrupted extension of the switchgear is always possible. In case of the maintenance strategy permits the switch-off of one neighboring bay during the replacement of a busbar disconnector module or an earthing switch next to the bus bar, the additional buffer gas compartment (A) is not required. Due to their high pressure resistance the single-phase insulators can be stressed with the full differential pressure (ambient pressure vs. SF6 gas pressure).

Table 2: Comparison of GIS requirements for different bay designs vs. service continuity criteria

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EXCHANGE OF GIS-MODULES WITH IMPROVED PARTITIONING SCHEME

The volume-optimized design of the switchgear impacts all necessary work for gas handling. Compared to the "traditional" design the time required for evacuating and filling a double busbar bay is reduced by about 50%. This reduced effort is advantageous during installation and commissioning (I&C), and favors also any necessary maintenance and repair work on the GIS modules during the operating phase of the gas-insulated switchgear. At least, essential time consuming work during I&C and maintenance is optimized to a minimum and the outage time of a bay or even the whole switchgear can reduced to a minimum.

Figure 5: Xiang’an Torch substation in Xiamen, China. The 145 kV / 3150 A substation comprise 18 DBB-bays with cable exits, 2 bus coupler and 2 bus sectionalizer.

5.1

Disconnector exchange

An exchange of a bus bar module can performed more efficient compared to the “traditional” design as the control cubicle keeps untouched and no extensive reconnection work and complex testing of the bay control after module replacement is required. Reason is that the control cubical is independently fixed on the steel frame and not connected to the bus bar. The available space, e.g. in-between the LCC-back side and the bus bar module is sufficient for the module exchange (Figure 4). The integrated buffer gas compartment enable the replacement of main switching elements of a GIS bay with less effort and minimum impact to the neighboring bays. Figure 6 shows which steps are necessary to exchange a busbar disconnector, e.g. after an electrical flashover (1). First step is to remove the insulating gas from the defective module, the associated modules at busbar I and the adjacent buffer gas compartment in the affected bay. After filling with atmospheric air the faulty module is removed. All insulators to the adjacent modules may get replaced as a preventative measure to exclude that any previous damage by electrical arcing remains. After installation of the new module the bay is energized again. Due to the significantly lower volume of the gas modules, the required time for the entire evacuation and filling process is reduced to about 50%. Throughout the whole repair procedure, busbar II can remain in operation; the substation does not need to be switched off.

Figure 6: Removal of disconnector with improved partitioning scheme

5.2

Interrupter revision

Another example to demonstrate the benefit of having a buffer gas compartment as an in-built solution available is the replacement of the interrupter chamber. To perform an exchange, e.g. for revision purposes, there is no need to remove the local control cubicle or the drive box. For that purpose the mechanical spring drive has to be disconnected and moved out of its box. All necessary work to exchange the interrupter chamber is conducted via the opening in the CB drive box. Side-feeded cable bundles in the drive box (e.g. looped wiring system) may reduce the available space but do not prevent the replacement procedure. During the whole revision work, any kind of gas handling is limited to the circuit breaker module (Figure 7). Due to their high pressure resistance the single-phase insulators can be stressed with the full differential pressure (ambient pressure vs. SF6 gas pressure). Both busbars are not affected by the replacement work.

Figure 7: Removal of interrupter with improved partitioning scheme After the replacement of the interrupter, the measurement of the opening and closing time and the opening and closing speed can be performed with a portable test device. The measurement is conducted via the isolated maintenance earthing switches on the exit and busbar side.

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CONCLUSIONS

The whole design of the introduced switchgear is focused on a minimized use of SF6. The “shrinked” enclosure requires significantly less insulating gas compared to other switchgears with the same ratings. The switchgear design is compact, enables late customizing and considers already an integrated buffer gas compartment as part of its basic layout. In the unlikely event of a fault, the operator can react very quickly and restore the functionality of the substation in a few hours. Depending on the service continuity concept during the replacement of active GIS-modules at a double bus bar arrangement (disconnector switch, earthing switch, CBinterrupter) at least one bus bar section can stay in operation, and a safe and reliable power supply is always guaranteed. As the required time for any gas handling of the switchgear is significantly reduced, considerable faster commissioning of the switchgear even after installation or any maintenance work is enabled, which increase the availability of the entire switchgear.

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REFERENCES

1

ABB Switchgear Manual, 10th edition, June 2011

2

IEC 62271-203, Edition 2.0, September 2011