What will MV switchgear look like in the future? by Jean-Marc Biasse

Transcription

1 What will MV switchgear look like in the future? by Jean-Marc Biasse

2 Table of contents Introduction... 2 Brief history of the technologies used in medium voltage switchgear and control gear... 4 Evolution of the single-line diagrams... 8 Future switchgear for MV consumer sites and switching substations Conclusion... 14

3 Introduction The electricity industry is conservative. Among the reasons for this is the fact that the lifetime of medium voltage and high voltage switchgear is around 40 years. Transmission system operators (TSOs) and distribution network operators (DNOs) need stability. Maintenance and repair of such long-life devices needs to be ensured. And of course, work is easier for service crews if there is no change in technology. However, some drastic evolutions appear about every 20 years. MV switchgear white paper 02

4

5 Brief history of the technologies used in medium voltage switchgear and controlgear All elements of a medium voltage installation are subject to evolution In a substation are found all three categories of components of protection chains: sensors, protection relays and circuit breakers (CBs). Traditionally, the design of these components has evolved independently, but with some constraints at interfaces to ensure interoperability. Protection relays are particularly sensitive to the type of signal coming from current transformers. Some association are possible; others are not. For example, you may connect old technology 5A CTs to most modern protection relays, but the opposite connecting an LPCT to an old electromechanical relay is impossible. Available technologies for electrical switchgear Electrical switchgear need an insulation medium for two different functions: current breaking and isolation between conductors or between conductors and earth. For current breaking, the available technologies are air, oil, SF6, and vacuum. To isolate conductors, the same technologies may be used plus solid insulation. Voltage level Switching media Insulation medium Circuit- breaking Load- breaking High voltage SF6, vacuum NA SF6, air Medium voltage SF6, vacuum Air, oil, SF6, vacuum Air, oil, Air, SF6, solids, oil, Table 1: Insulation media MV switchgear white paper 04

6 Evolution of circuit-breaker technologies The first technology used for breaking in CBs was air. These CBs were big because the principle of breaking was a large expansion of the arc and noisy because of the breaking in the air. They needed much maintenance and, for that reason, were withdrawable (Fig 1). 1 2 In an effort to reduce the footprint, oil CBs came next (Fig 2). However, they also needed much maintenance, for example to change oil after some operations. Additionally, oil breakers are not safe to operate because of the fire risk. Oil CB failures can easily result in a fatal accident among operators and the public. 3 In the late sixties came SF6 and vacuum circuit-breakers. Both technologies brought many similar advantages. They are compact thanks to vacuum or SF6 insulation. They are much safer, drastically reducing fire risk. They became more and more reliable. Electrical endurance has been increased, thus CBs were able to perform a much higher number of fault and load breakings. As a consequence of the improved reliability, maintenance is less and less required and we can consider that state-of-the-art CBs are now almost maintenance-free. Often, they remain withdrawable because of installation in traditional metalenclosed panels Merlin Gerin circuit breaker DST 2. Drawout oil circuit breaker with arc control 3. Withdrawable vacuum CB 4. Withdrawable SF6 puffer CB Evolution of primary distribution switchboard technologies From 1930 to 1950, most of the MV switchboards were in fact an assembly of fixed components in an electrical room connected to visible busbars. Only simple wire fencing prevented to access the live parts. 5 Then, because of more safety awareness, switching components and busbars were integrated in metal-enclosed cubicles. Doors and sheet plates and frames were earthed to avoid any accident from direct or indirect contact with live parts. Busbars and connections were air insulated. There were several generations of metal-enclosed air-insulated switchgear (AIS) cubicles. The first generation, from 1950 to 1970, integrated withdrawable air or oil CBs. The second generation, from 1970 to 1990, integrated withdrawable SF6 and vacuum CBs. Another step in safety was introduced in the current, third generation of metal-enclosed cubicles, which began in This new generation introduced internal arc withstand capability to protect people standing in front of the switchboard in case of an extremely rare internal fault. Generally, CBs are withdrawable and installed in cassette to allow wall mounting and front access cables. But more recently, in the 1990s, fixed CBs were also used. This change was possible with modern highly reliable CBs and new testing facilities of the protection relays Air-insulated masonery cubicles 6. Metal-enclosed AIS panel with CB cassette 7. Metal-enclosed AIS panel with fixed CB 05 MV switchgear white paper

7 There are some recent variants in metal-enclosed cubicles with fixed CBs, where the insulation of busbars and all components, including CBs and connections, are made with epoxy or some other resin. These panels are generally called a solid insulation system (SIS). 8 However, always looking for better electricity availability, utilities started to require more and more insensitivity to ambient environmental conditions. And, all AIS and SIS panels are still sensitive to environmental conditions if not properly installed in protected rooms. 8. Metal-enclosed GIS switchboard with fixed CBs That was the reason for the arrival of metal-enclosed gas-insulated switchgear (GIS) in the 1990s. All components, busbars, and connections are fitted in one or several hermetically sealed tanks filled with SF6. Thanks to SF6 insulation, this type of equipment is very compact. Both AIS and GIS panels coexist today. The final choice may differ for each application, depending on the importance given to many criteria such as compactness, insensitivity to the environment, the availability of high performance, criticality of the application, power restoration mode in case of failure, ergonomy of operation, and/or ergonomy of cable testing. Evolution of secondary distribution switchboard technologies Secondary distribution switchgear also followed a similar evolution, but with some differences. Rated currents at the distribution level are lower and the number of substations is higher. Then, looking for money saving, only simple switches with fuse protection were used. A typical switchboard includes three functions, two switches and a switch fuse to protect the MV/LV transformer The same evolution as for primary distribution appeared from masonery cubicles to modular metal-enclosed AIS cubicles. But, due to the typical three-function repetitive arrangement, a special ring main unit (RMU) configuration appeared in the 1950s For more compactness, the three functions have been fitted in one metallic tank. The first RMUs of this type were oil RMUs with the same inconvenient fire risk. Modern RMUs now use SF6 as it provides compactness and insensitivity to ambient environmental conditions. Moreover, both with the need to protect more powerful MV/LV transformers and to bring more precise features in the protection scheme, modern RMUs are now equipped with CBs for MV/LV transformer protection. The same evolution in safety concerns resulted in new designs having internal withstand capabilities. Sometimes the advantage of a compact and repetitive RMU solution becomes inconvenient when extension is needed or if more than four- or five- function switchboard are needed. O/C E/F 9. Typical RMU arrangement with switch fuse 10. Oil switch unit 11. Typical RMU arrangement with CB transformer protection 12. SF6 RMU with CB MV switchgear white paper 06

8 Evolution of the technology of sensors and protection relays Technologies of sensors and protection relays evolved in parallel because both types of components are closely linked. Sensors, like current transformers, shall permanently give an image of the current and this image is transmitted to the protection relay. We can consider the relay to be the brain, as it is able to receive the signal and analyse it to decide whether the signal is normal or represents a fault. In case of a fault, the protection relay sends a tripping message to the circuit-breaker mechanism. Up until the 1970s, protection relays were made using electromechanical technology. Coils and disks were parts of these relays that needed high auxiliary power to operate. Consequently, the current transformers had to supply high burden. 5A on secondary output was necessary to operate these protection relays In the 1980s, electronic protection relays occurred with less need of auxiliary power from the CTs. They could be operated by current transformers having 1A rating on secondary winding. But the high voltage sector is very conservative and many user specifications were still asking for 5A CTs even if no longer needed. 13. Typical line distance electromechanical relay 14. Typical overcurrent electronic relay Statimax type 15. Digital relays VIP 400 (left) and Sepam 20 (right) Later in the 1990s came the first digital relays. With this technology, the need of signal power from the CTs becomes very low. A new category of CTs was developed: the low power current transformers (LPCT). They deliver a voltage signal representing the primary current. In spite of the advantages in space and flexibility, their deployment has been very slow because of users conservatism, still asking for 1A or even 5A CTs to feed digital relays. This overpower in input needs adapter transformers in the protection relay to lower the input power. Now, the situation is finally going to change. Digital relays are very common and advantages of LPCT are recognized. Moreover, clear IEC standards have been published, making interchangeability of LPCTs or protection relays easier. 07 MV switchgear white paper

9 Evolution of the single-line diagrams Even if sometimes conservative, customers tend to look after reduced dimensions, lower cost, better reliability, and better ability to withstand harsh environments. To meet these needs, there is a progressive move from withdrawable to fixed equipment. Together with the evolution of the technology of medium voltage switchgear, single-line diagrams of incomers and feeders were regularly challenged. It is possible to make some comparisons between the most typical single-line diagrams, just highlighting some points of importance. Diagram with withdrawable technology The diagram with withdrawable CBs is the oldest one. It is still in use and not obsolete in some primary distribution applications. Disconnection is made by racking out the CB truck, providing visible disconnection and usually an earthing switch is directly acting on cable ends. 16 Maintenance of the CB is very easy and this was necessary for old CBs. In addition, access to terminals for cable testing is quite easy. However, some points have to be carefully considered. Remote control of the disconnector is not really practical because of the truck to be racked out. Earthing the busbar needs a dedicated earthing truck, which is heavy to handle. Testing the cables needs a direct access to cables, opening the cable compartment. And finally, the equipment should be installed in clean air rooms as it is sensitive to environmental conditions because of the AIS technology. 16. Single-line diagram and typical panel for withdrawable technology Typical diagram for GIS technology To drastically eliminate the sensitivity to environment, gas-insulated switchgear (GIS) were developed. First derived from HV GIS technology, these equipment are fitted with fixed CBs and separate disconnectors. 17 This technology was made possible thanks to the design improvements of CBs that now need very little maintenance. Gas insulation and plug-type cable connectors ensure the highest degree of insensitivity to harsh environments. Among the points to be aware of is that operation is not so intuitive because of a five-position scheme. Particularly, cable earthing is made through CB closing that must remain closed to ensure end-user safety when working. 17. Single-line diagram and typical panel for primary GIS technology MV switchgear white paper 08

10 Simplified diagram with upstream two-position selector In an attempt to simplify the five-position single-line diagram, it is possible to design an upstream two-position selector. This arrangement reduces the number of positions thanks to the two-position selector. As the cost is also reduced, it has been possible to use this arrangement in secondary distribution. 18 However, there are still four positions that make the operation not so intuitive, especially for secondary distribution. And, earthing the cable remains made through CB closing. When the cable is earthed, the CB must stay closed to ensure safety. The positive earthing indication depends on the status of the combination of two devices. 18. Single-line diagram with upstream two-position selector Reverse diagram for GIS technology Trying to improve ergonomy, moving to a direct cable earthing, some equipment uses a reverse diagram with GIS technology. Now earthing the cables is made directly via an earthing switch having making capacity. This also gives the possibility to design a dedicated device for cable testing via a removable link. 19 But there are still four positions and a need of keys for safety interlocks. Cost is increasing because of separate earthing switch having making capacity. 19. Reverse single-line diagram with GIS technology All-in-one arrangement diagram for GIS RMU 20 For secondary applications, simplicity, insensitivity, and cost effectiveness often are a must. These criteria were the drivers to move to an all-in-one arrangement for GIS RMU. The main device is an SF6 disconnecting load-break switch or circuit breaker allowing for a very simple three-position diagram. Breaking and disconnection are performed in a single operation, leading to the three-position scheme (line, open and disconnected, earthed). Local or remote operations are very simple. The mimic diagram is very easy to interpret. Earthing of the cables is made directly. Interlocking safety is inherent between the different positions. It is very easy to implement a cable testing device, allowing access to cable without opening the cable box nor interfering with the cable terminations Typical three-position GIS RMU diagram 21. Examples of GIS RMU 09 MV switchgear white paper

11 New three-position diagram The three-position arrangement for GIS RMU has experienced great success for around 30 years now and is still well appreciated. Nowadays, even if the technology is not much changing, there is a trend to use vacuum breakers in secondary applications. The question was whether it was still possible to keep the same simplicity of the three-position diagram using another technology. Recent developments brought an original solution, keeping the same advantages of the three-position arrangement of GIS RMUs, but using vacuum breaking. The new proposed arrangement includes an upstream vacuum disconnector load-break switch or CB and a downstream earthing switch providing a doublegap isolation between cables and busbars. All previous advantages are kept with this real three-position scheme (line, open and disconnected, earthed). 1st position: CB or load-break switch closed 2nd position: CB or load-break switch opened and disconnected in a single operation 3rd position: cable earthing in one single operation New three-position diagram including vacuum breaking and typical unit. 23 Breaking and disconnection are made in one single operation of a vacuum interrupter. Earthing the cables is done directly, using an earthing switch having making capability. This diagram facilitates the implementation of clear mimic indications, making operations very intuitive and thus safer. Safety interlocks are built-in, short, key free, and positively driven. This diagram also allows the use of a dedicated cable testing device, increasing the safety of people and switchgear. As it is well known that MV cables are generally much older than switchgear, they will need more and more testing and conditional replacement. 23. Mimic diagram of three-position scheme using 24 vacuum interrupter Prior to the cable test, opening the switch or CB disconnector and closing the earthing switch provides a double gap between cable and busbar. Then a safe and fully interlocked earth link switch may be opened to give direct access to the cable conductor. During testing, the cable box remains closed, the cable connections remain intact, and the main contacts of the earthing switch remain in the same position. This recommended test procedure ensures the highest safety for people doing the tests and also avoids any damaging of the main circuit or cable connections. 24. Dedicated cable testing device. To meet the same advantages of GIS RMUs, the new arrangement shall be insensitive to harsh environment. This is ensured by a complete Shielded and Solid Insulation System (2SIS) solution. Busbars and a vacuum interrupter encapsulation and earthing switch enclosure are made of solid insulation that is covered by a conductive layer connected to the earth. The equipment can support any kind of harsh environment as well as GIS RMUs. 25 Compared to GIS RMUs, this 2SIS technology associated with this new threeposition diagram arrangement offers much better modularity as the general architecture is based on single units. Thus, it is easy to build switchboards for many kind of applications requiring a large number of units While it is obvious that this modular architecture, based on 2SIS technology using vacuum breaking, has many advantages, it is necessary to analyse whether it is completely adapted to the smart-grid deployment of today. 25. Example of switchboard made with modular 2SIS units MV switchgear white paper 10

12 Future switchgear for MV consumer sites and switching substations The challenge of smart grids Smart grids have two main objectives. One is to optimise the relation between the demand and the supplying of energy. The second is to provide the necessary conditions to integrate more distributed and renewable energies. Comparing the two-way flow that is needed for these objectives with the simple one-way flow still valid with centralised energy production, the challenge is big. As for each other link in the chain, one question arises: are MV switchgear ready for this challenge, or is an evolution necessary? Looking at existing grids and at some experimentations, it is possible to highlight some switchgear values that will help to meet this challenge. Smart grids will use more CBs than in the past For some years, experimentations have proven that adding CBs in distribution network loops is an efficient way to decrease the number of customers affected by an outage and to reduce power restoration time. The distribution network is generally operated in an open loop, allowing a backup solution in case of fault. It is historically equipped with manual switches, with only one protection device per feeder, located in the HV/MV substation. The increasing demand for quality of supply led to the deployment of remote controlled substations, bringing lower shortage duration. Nevertheless, in case of fault, all the customers supplied by the faulty feeder are disconnected. But in fact, the customers upstream of the fault could have been unaffected. The use of CBs instead of switches in the loop allows disconnecting only the customers connected to the faulty part, a significant benefit regarding the number of affected customers compared to the traditional solutions. On an ideal point of view, solutions including low cost CBs, low cost sensors, no communication, without specific network architecture and easy possible upgrade could reduce the outage at a cost-effective level. Today, adequate and economically viable answers to the needs of MV/LV substations do exist in both following areas: optimised integrated CBs for network applications, including LPCTs, adaptation of existing protection systems by the reduction of time discrimination interval or the use of logic discrimination in substations between incoming and outgoing feeders. In a similar way, it is more efficient and precise to use CBs to protect MV/LV transformers. Traditionally, MV/LV transformers have been protected by switch-fuses because of the significant cost differential compared to withdrawable CBs and relays. The main advantage of using an RMU with a fixed integral low cost CB is that 11 MV switchgear white paper

13 it allows to have improved transformer protection at an equivalent lifetime cost, thus making transformer CB protection affordable. An MV/LV transformer generally has a very low failure rate. All faults are starting interturn faults or earth-phase faults and are located inside the primary or secondary windings or on the LV zone. Only CBs can quickly and surely detect the faults at early stage when they are of low or very low magnitude. At the same time, fuses are sometimes not able to break or have to wait until the fault has degenerated into a two-phase or three-phase fault of high magnitude to operate properly. The main advantages of the CB solution are: better discrimination with other MV and LV protection devices; improved protection performance for inrush current, overloads, low magnitude phase-faults and earth faults; greater harsh climate withstand; reduced maintenance and spare parts. Migration of withdrawable CBs towards fixed CBs and the use of vacuum breaking make them cost-effective. Dissemination of modern highly reliable CBs was a key factor for the acceptance of fixed CBs. In this respect, the modular architecture of 2SIS, based on highly reliable vacuum interrupters, is very flexible and allows for an infinite number of combinations. Remote control will be mandatory for smart grids To be more compact and efficient, switchgear with integrated control and monitoring features provide better optimisation. Remote control of the switchgear becomes essential and must be very easy. End users will no longer accept long power outages. Feeder automation, self healing using remote control is the only way to shorten the time of loss of power. Optimising the loads in some parts of the distribution network will also be possible using remote control to operate the switchgear and change the protection settings. Of course, manual operation mode will also be very easy. For that, no matter the technology,, the three-position operation mode (line, open/disconnected and earthed) is the simplest one, also increasing safety. One big advantage of this three-operation mode is that it is the same for remote control as for local manual operation. MV switchgear white paper 12

14 LPCTs and LPVTs will be essential for the huge development of power management and metering Control and monitoring will increase to properly manage the real-time connections to the grid. For that purpose, more and more sensors will be used. Thanks to modern control & monitoring devices and digital protection relays, compact low power current transformers (LPCT) and low power voltage transformers (LPVT) can replace heavy traditional CTs and VTs. 26 The introduction of digital technology for measurement and protection (Figure 26) has modified the requirements of current transformer burden. The manufacturers have developed protection devices based on low power microprocessor technology with wide range of use, low consumption and innovative current sensors that allow constituting a consistent protection chain. 27 Perfectly adapted to these small burdens, the LPCT consists of a current transformer having a small core secondary winding connected to a integrated shunt resistor (Figure 27). The shunt resistor converts the secondary current output into a low-voltage signal. The iron core LPCT is based on the well known CT technology. LPCT technology is an optimised technology with several advantages: Simpler choice: engineering is simplified due to the wide operating range. One type LPCT can cover applications from 5A to 1250A where the traditional CTs require a range of five sizes. A single sensor is performing both measurement and protection purposes; Easy and safe installation: LPCT output is plugged directly into the protection relay with no risk of over voltage when disconnecting; Flexibility of use: easy adaptation to the power consumption changes and/or protection setting during the MV system design or operating life. High accuracy up to the short-time circuit current with low saturation; Compactness: the reduced size and weight allows for an easy integration and therefore MV switchgear dimension reduction. Figure 28 shows the size comparison between CTs 24 kv and 36 kv and LPCT, meeting the same MV network protection and measuring technical requirements One phase analog ammeter with a 1.1VA consumption and numerical power meter with a 0.15VA consumption of the current input 27. LPCT sensor principle 28. Size comparison between LPCT (left) and CTs (right) Power management will increase as it will be very important to have a real-time view of the available power. Metering equipment will need to be cost-effective, compact and integrated. As a great advantage of 2SIS architecture, it is now possible to have 2SIS LPCTs and LPVTs, making metering equipment insensitive to harsh environments. 13 MV switchgear white paper

15 Modularity is a must to meet the infinite number of different applications The variety of electrical installations resulting in an infinite combination of switchboard sizes and configurations will increase with the integration of renewable energies and with the need of energy efficiency to save energy. Modularity of switchgear is a key to answer the need of flexibility. MV switchgear also will be more distributed in the network. With this respect, the 2SIS system brings the highest flexibility. As each part of the busbar and each part of cable connection are 2SIS technology, there is no external influence, no matter the arrangement of the switchboard. As a result, many possibilities of cable entries are provided and extension of a switchboard is very easy. Moreover, high insensitivity to harsh environmental conditions and less maintenance will be very appreciable. Conclusion The development of smart grids will result in the inclusion of more intelligence in MV equipment. This network evolution may be the opportunity to introduce new criteria for the choice of products, such as flexibility, insensitivity to harsh environments, compactness, optimisation of remote control, etc. In conclusion, the physics are the same but some technological points are changing as well as the way to optimise them. For all these reasons, there is a great confidence that the 2SIS modular architecture using the three-position scheme and vacuum interrupters is very well adapted for the coming deployment of smart grids. This architecture can address a large number of applications in secondary distribution but, thanks to its modularity, can also challenge some low-end applications where, traditionally, primary equipment is used. In this respect, this architecture is able to bridge the gap between secondary and primary specialised equipment. MV switchgear white paper 14

16 2013 Schneider Electric. All Rights Reserved. All trademarks are owned by Schneider Electric Industries SAS or its affiliated companies _GMA-GB