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Protection and Switching in Low Voltage Smart DC Grids Robert Pinnock TRW Conekt Stratford Road Solihull B90 4GW UK Tel: +44 (0)121 627 3548 Email: [email protected] Web: www.conekt.co.uk Conekt - Who Are We? Consultancy in Engineering, Knowledge and Technology • Conekt is the technology and engineering consultancy business of TRW Automotive • The origins of Conekt date back to the mid 1950s as the former Lucas Research Centre • Conekt provides specialist engineering consultancy and testing / validation services to customers in aerospace, automotive, defence and other markets 2 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Contents of Report (and This Presentation) This work considers the feasibility of developing smart DC power grid technology for domestic and small commercial premises, focussing on Issues relating to circuit switching and protection: 1. Executive Summary 2. Introduction 3. Safety Issues 4. Switching Devices 5. Protection Measures 6. Arcing, erosion and Corrosion 7. Technology Cross-over Opportunities from Existing DC Power Grid Systems 8. DC Voltage Level Issues 9. Gallium Nitride (GaN) Technology – State of the Art (see separate presentation) 10. DC Power Grid Design Issues 11. Conclusions and Recommendations 12. Appendices 3 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Aims of the Report The primary aims of the report may be summarized as follows: -To survey the current state of the art in switching and circuit protection technology for Smart DC grid applications, with particular emphasis on issues which must be addressed before the commercial deployment of such systems in domestic or small commercial premises becomes viable -To assess the capability of new developments in GaN semiconductor devices to meet the switching and circuit protection needs of Smart DC grids (separate presentation by GaN Systems Ltd following this one) -To review the means by which switching and circuit protection are implemented in DC power systems for other markets, particularly automotive and aerospace, and to consider opportunities for technology cross-over from and into the Smart DC grid market. 4 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Introduction – The DC Power Grid Concept The main perceived advantages of a smart DC power grid for domestic and small commercial premises include: - Improved efficiency and ease-of-use (through reduction in unnecessary AC-DC power conversion processes and (hence) power adapters, cabling, etc.) - Improved ability to utilize power derived from renewable resources (solar, wind, etc.) which are often DC or “pseudo-DC” in nature Set against these perceived advantages is the basic question: - Are these advantages sufficient to persuade suppliers and consumers to embrace DC grid technology (given the fact that, whatever its shortcomings, the current AC power distribution system to domestic and small commercial premises works pretty well, and everyone is used to it)? 5 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Introduction – The DC Power Grid Concept (cont.) 6 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Safety Issues The main safety issues arising from the use of Smart DC grids in domestic or small commercial premises are: - electric shock and burn hazards - arcing and arc flash hazards - fire hazards arising from over-current conditions The existing AC power distribution network includes switching and protection devices and systems designed to help prevent accidents and the associated impact on people and equipment from arising. Similar switching and protection measures will also need to be implemented in a Smart DC power distribution system, but the inherent differences between AC and DC power delivery mean that these systems will not be exactly the same 7 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Safety Issues – Electric Shock and Burn Hazards In terms of electric shock and burn hazards, the safety issues associated with a LV DC power grid are similar to those encountered with a typical AC power grid, and similar measures are required to help prevent, or minimize the effect of, hazardous events Electric shock in humans (and animals) is a complex issue, influenced by many factors: - voltage level - current level - source frequency (AC causes more ventricular fibrillation than DC) - length of time the person is exposed to a live electrical source Burns associated with electric shocks may be: - surface burns (at points where electrical current enters and exits the body) - internal tissue burns (where current flows through the body – currents > 1.5 A can cause potentially fatal internal tissue burns DC voltages 60 V and AC voltages 42.4 V peak are generally deemed to be “safe” for direct human contact 8 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Safety Issues – Arcing and Arc Flash Hazards Devices which may cause arcing include switches, circuit breakers, relay contacts, fuses and various fault conditions (poor cable terminations, damaged wires, worn insulation, etc.) The issue is of greater concern in DC power networks than in AC systems because the “zerocrossings” (as the current reverses direction) in an AC power network tend to extinguish any arcs, whilst a DC system tends to maintain an arc across an air gap. Switching inductive loads in DC systems is particularly problematic as the electrical current cannot immediately drop to zero and a transient arc tends to occur across the separating switch contacts. Techniques such as the use of snubber circuits can be used to help prevent arcing. The primary hazards are: - Burns (from touching surfaces made hot through arcing) - Fires - Electric Shock (from damaged components or devices) - Arc Flash: An arc flash is an electrical arc having sufficient energy to cause an electrical short circuit through the air to ground or another conductor. Arc flash temperatures can reach in excess of 22,000 °C in less than 1 ms. Arc flash can occur at voltages as low as 50 V Picture from www.arcadvisor.com Used with permission 9 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Safety Issues – Hazards from Over-current Conditions An over-current is a current that exceeds the ampere rating of the conductors (cable) and / or equipment (motors, instruments, electrical devices, etc.) on an electrical circuit. Over-currents include overloads and short circuits Overload current: an over-current that is confined to normal current paths (typically occurs when too many loads are present on a power network so that the rated current limit is exceeded) Short circuit: an over-current flowing outside normal current paths (typically occurs when electrical insulation of a conductor is damaged). Short circuit currents may reach thousands of Amps and require very rapid interruption Fuses and circuit-breakers provide protection against over-current faults in both AC and DC power distribution systems. However, this is a particular challenge in DC networks, due to the difficulty of rapidly switching very high DC short circuit currents (primarily because of lack of “zero-crossings” of the electrical current) 10 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Switching Devices Mechanical Switches: - familiar as the means by which individual electrical loads are switched in a domestic AC ring main - such switches will still be an important feature of a domestic or small commercial DC grid, but will need to be capable of operating appropriately and safely, particularly as regards the management of switching arcs - commercially-available switches for electrical power circuits are generally only rated for use on AC circuits: where a DC rating is given, the DC current rating is considerably less than the AC rating Electromechanical Switches (Relays): - electromagnetic devices that convert a magnetic flux, generated by the application of a low voltage electrical control signal (either AC or DC) across the relay terminals, into a pulling mechanical force which operates the electrical contacts within the relay - contact resistance (primarily due to arcing causing damage to the relay contacts) is worse in DC power systems due to their greater susceptibility to arcing (leads to reduced relay life – but careful choice of silver-based alloy materials for the contact tips can help) - reed relay switches are used for rapidly switching relatively low currents at low voltages – but, smaller contact size leads to greater propensity for arcing damage in DC power systems - device ratings for AC and DC use are different: for example, an electromechanical relay rated at 10 A at 250 V AC may typically be rated at 10 A at only 30 V DC or just 0.15 A at 250 V DC 11 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Switching Devices (cont.) Circuit Breakers: - automatically-operated electrical switches designed to protect an electrical circuit from damage arising from overload conditions or short circuit faults Residual Current Devices (RCDs): - automatically-operated electrical switches designed to switch when an earth leakage current causes an imbalance in the electrical current in the live and neutral arms of a power circuit Both of these are specifically circuit protection devices (discussed further in “Protection Measures” section) Solid State Devices (Switches, Relays, Circuit Breakers, etc.) : - based on a variety of power semiconductor switching technologies (e.g. power diodes, bipolar power transistors, power MOSFETs, insulated gate bipolar transistors (IGBTs), silicon-controlled rectifiers (SCRs), gate turn-off thyristors (GTOs), integrated gate commutated thyristors (IGCTs), etc.) - provide high-speed switching without the problems arising from contact erosion due to arcing (however, may still require snubber circuits for protection when switching inductive loads) - older devices based on Si: newer devices based on wide band-gap semiconductors such as SiC and GaN): these provide enhance performance (switching at GHz rates, high temperature (> 200 °C) operation, etc. – described further in following presentation by GaN Systems Ltd) 12 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Protection Measures As with AC power grids, a well-designed protection system will be necessary to ensure safe and reliable operation of a Smart DC grid. The protection system will comprise protective components (fuses, circuit breakers, etc.) together with suitable grounding arrangements. However, for the Smart DC power grid, the protection system design will be influenced by many factors, including such fundamentals as the DC voltage level used (and whether there is more than one DC voltage present, or a combination of DC and AC) and whether the DC power grid is connected to the AC power distribution system (interconnected operation) or isolated from it (islanded operation) Reliable implementation of DC power grids will require a number of requirements to be met in terms of system protection. Depending on the particular implementation, these may include: - no single point of failure - redundantly-fed electrical zones or buses that can be rapidly separated from the bulk power system - ability rapidly to re-configure the system (e.g. “island” the system from the AC power system) - ability to locate and isolate faults without the need for inter-component communications - ability to recover from a fault to a reliable configuration and mode of operation - load prioritization into sensitive versus non-sensitive - minimization of the effect of a faulted load on other loads - redundant feeds to sensitive loads - ability quickly to shed non-sensitive loads - load and power flow management from a centralized control - transient operational capability in degraded modes, such as loss of cooling and loss of power flow control - condition-based maintenance 13 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Protection Measures (cont.) Whilst many protection devices intended for operation in AC power delivery systems will also operate in DC systems, there are certain factors which need to be considered carefully when designing protection measures for Smart DC grids because they may influence the device ratings which need to be applied in the two situations Devices available for protection against short-circuit currents and over-currents arising from overload conditions are fuses and various forms of circuit breaker. In addition, residual current devices (RCDs) can provide protection from possible electric shocks arising from earth leakage conditions Fuses: - characterized by parameters such as response time, current and voltage rating, interrupt capability, AC or DC (or both) operation, etc. - In DC power grid applications, a fuse does not benefit from the current-reversal effect inherent in AC power systems: hence, the fuse must be capable of acting rapidly enough itself to extinguish any arcs. Fuses designed specifically for DC power grid applications are therefore typically more complex (and costly) - If AC fuses are used in DC systems, care must be taken to ensure that the voltage and current ratings are appropriate for DC application, since these will not in most cases be the same as for the AC application 14 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Protection Measures (cont.) Circuit Breakers: - circuit breakers are generally rated both by the normal current that they are expected to carry, and the maximum short circuit current that they can safely interrupt - many commercially-available circuit breakers are optimized on the assumption that the circuit-breaking process will be aided by the natural zero-current condition of the AC system: hence, such devices are at best sub-optimally applied to DC systems - circuit breakers designed specifically for use in DC power grid systems exist – again these tend to be more complex and costly than their AC counterparts - solid-state circuit breakers potentially have considerable advantages for application in DC power grids, in terms both of providing much more rapid switching of fault currents, and in improving the flexibility of the protection system: Current Control Inductance Solid-state Switch Current Sensor Filter Capacitor To other DC Loads To DC Load In this example of a solid-state circuit breaker, the solid-state switch is the main device for current-limiting and fault current interruption. The inductor limits the rate of rise of the fault current, and the freewheeling diode limits di/dt on the inductor when the solid-state switch is turned off 15 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Protection Measures (cont.) Grounding and Ground Fault Detection Using RCDs: - grounding is a complex issue and there are many different approaches to designing grounding in an electrical power distribution system: depending on factors such as the actual voltages involved and the general system lay-out, DC power grids may be ungrounded, high-resistance grounded, or low-resistance grounded. In addition, the ground in such systems could be connected either to one of the poles or to the mid-rail point of the DC supply - in general, grounding arrangements used for AC power distribution grids can be translated into equivalent arrangements for DC power grids: these arrangements include: - TN grounding (consumer appliance grounded through power distribution system’s ground connection) - TT grounding (consumer appliance having ground connection independent of the ground connection of the power distribution system) - IT grounding (the power distribution system itself has no ground connection) - for various reasons (e.g. the problem of corrosion of grounding cables in DC power systems), IT grounding is often advocated for DC power distribution systems - RCD devices will be necessary in Smart DC grids as they are for AC systems: however, the operating principle of RCDs used for AC systems is unsuitable for use in DC systems. Hence, new RCD technology will need to be developed for use in DC power grids: such devices do not seem to be available at present 16 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Arcing, Erosion and Corrosion Any DC power distribution system will need to deal with the problems of arcing (and sparking), erosion and corrosion in system components such as switches, relays, plugs, cables, etc. Arcing (and Sparking) and Erosion The arc phenomenon is an electrical breakdown of a gas (e.g. air) which produces an on-going plasma discharge, allowing current to continue flowing across broken electrical contacts - in a typical Smart DC grid installation, arcing may occur, for example, when power to a load is removed, either by means of a mechanical switch or electromechanical relay, or when a power plug is removed - arcing may also occur in fault conditions (e.g. worn sheathing allowing arcing between exposed conductors - arcing issues are exacerbated in DC power grids because DC systems do not have the self-extinguishing tendency associated with AC power systems (due to the current “zero-crossing”) - arcing issues can be mitigated in DC systems through the use of snubber circuits, “free-wheeling” diodes, or solidstate switching devices Spark phenomena may occur in situations where there exists a strong electric field able to create an ionized conductive channel through the air. This effect may occur, for example, during plugging in of loads to the DC power grid, particularly in the case of capacitive loads Arcing and sparking events are hazardous primarily because they can lead to burn and fire hazards, Also, they can be indicative of wiring faults which could represent an electric shock hazard. They also can lead to excessive wear and shortened life of switches and plugs due to erosion of the electrical contacts Picture from Wikipedia Commons Author: ArcsuppressionT 17 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Arcing, Erosion and Corrosion (cont.) Corrosion Corrosion (both electrolytic corrosion and galvanic corrosion) is a typical problem of DC power systems in the open air. Corrosion can also be a problem inside domestic dwellings, for example in humid areas, or where moisture is allowed to penetrate through the insulation layers of electrical cabling - the problem of corrosion is considerably larger in DC systems than in AC systems: in particular, grounding electrodes are much more susceptible to corrosion in DC systems because of the continuous presence of a reduction (or “redox”) potential between the active and return conductor (thereby continually fulfilling one of the conditions needed for corrosion to occur) - the other condition - the presence of an electrolyte - is also fulfilled whenever moisture is allowed to come into contact with the conductors - corrosion is also potentially a serious issue at the electronic board and component level (e.g. a study with a major public service transport organisation in the 1990s showed that corrosion of electronic components and boards was one of the biggest causes of maintenance issues) Prevention of corrosion is best achieved by ensuring good separation of the electrolyte and the conductors or contacts by insulation. For a DC power grid system for a domestic or small commercial building, this may require the installation of electrical cabling with improved insulation properties. 18 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Existing DC Power Grid Systems and Technology Cross-over Opportunities Automotive The traditional electrical system in vehicles (12 V / 14 V in cars, 24 V / 28 V in trucks and larger commercial vehicles) is a good example of a DC power grid. For reasons of improving fuel economy, reducing emissions, and providing greater levels of vehicle occupant comfort and safety, many functions which previously were done mechanically are now done through electrical and electronic means, through motors, sensors, ECUs , and associated solid-state electronic components Fuses Rectifier ECU 1 Control Signals Electrical Loads Ignition Switch A Alternator M Starter Motor Battery ECU 2 ECU n Control Signals Electrical Loads Control Signals Electrical Loads 19 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Existing DC Power Grid Systems and Technology Cross-over Opportunities (cont.) Automotive - Switches: mechanical switches in automotive systems are increasingly being replaced by semiconductor power switches which enable more efficient use of vehicle power, and better control of vehicle electrical loads - Fuses: whilst traditional “blade-type” automotive fuses are still used, newer types of “re-settable” fuse (e.g. polymeric positive temperature coefficient (PTC) fuses and semiconductor fuses) are being increasingly used - Hybrid and Electric Vehicles: these use batteries operating at much higher voltage levels (hundreds of volts) than “traditional” vehicles, and require specific features to manage issues associated with high power DC grids (e.g. special measures to avoid arcing erosion in relays switching large DC currents) - All sectors of the automotive market are driving the development of power semiconductor components such as IGBTs for high-speed, high-power switching. New wide band-gap semiconductor materials such as SiC and GaN offer even greater performance enhancements The development and use of power semiconductors in automotive applications has been derived from their development and use in the power distribution industry. Continued investment amongst automotive suppliers in developing these new semiconductor components depends on the availability of suitable markets for them: however, if the automotive market (particularly the HEV / EV market) does grow as predicted, then the associated development of these new power semiconductor switches may well feed back into other markets such as the power distribution sector 20 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Existing DC Power Grid Systems and Technology Cross-over Opportunities (cont.) Automotive 42 Volts: Parallels with Domestic Smart DC Grids In the 1990s, and into the new millennium, there was considerable concern amongst automotive vehicle manufacturers and system suppliers that higher future vehicle electrical loads would render the 12 / 14 V vehicle electrical system inadequate. For this reason, the 42 V standard for vehicle electrical systems was proposed. The potential benefits included: - lower load currents - better fuel economy - increased system flexibility - opportunities for new vehicle systems The potential transition from the 12 V / 14 V vehicle electrical system to a dual 14 V / 42 V system also meant that a number of technical issues had to be tackled by the automotive supply chain in order to allow the implementation of reliable switching and protection functions. These issues included: - increased switch / connector arcing potential - steering column safety (specific safety concerns over having 42 V in the steering column) - reverse battery and jump starting concerns - 42 V / 14 V short circuits - shock hazard and other safety issues (e.g. load partitioning of critical systems) But… 21 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Existing DC Power Grid Systems and Technology Cross-over Opportunities (cont.) … in the end, 42 V in vehicles didn’t happen, despite appearing to be an “obvious” solution to a pressing need at the time. Some of the reasons for the failure of 42 V are: - improved alternator designs meant that the 14 V system was, in fact, able to cope with the higher electrical demands of new vehicle systems - because not all electrical loads in vehicles are actually better operated at 42 V (and also because there would in any case need to be a transition period towards a 42 V standard), the need for a dual 14 V / 42 V electrical system was realized. Such a dual system had disadvantages in terms both of complexity (e.g. need for careful power management, more complex circuit protection requirements, etc.) and cost In the automotive world, where cost is critical, the need for 42 V turned out to be insufficiently pressing to force its introduction, despite potential advantages. This point is worth bearing in mind when considering the potential introduction of Smart DC grids in domestic and small commercial premises. The potential advantages of such a shift may be clear, but unless the need really is sufficiently pressing, the status quo may be difficult to overcome. 22 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Existing DC Power Grid Systems and Technology Cross-over Opportunities (cont.) Other Existing DC Power Grids: Aerospace: The trend in the aircraft industry is towards “more electric” operation. to optimize aircraft performance, decrease operation and maintenance costs, and reduce emissions of air pollutant gases. To achieve the goals of MEA, a number of different power generation and electrical system topologies are under investigation by the aircraft industry, including means for providing adequate system protection critical for an aircraft system. Picture from Wikipedia Commons: Author: All_Nippon_Airways_Boeing_787-8_Dreamliner_JA801A_OKJ.jpg: Spaceaero2) There is a trend for using only high voltage DC systems for power distribution and management in MEA. The potential advantages of using a high voltage DC distribution system are seen as enabling weight, size and power loss reduction, whilst increasing the levels of transmitted power. Another area where the development of semiconductor power electronics and switches is impacting on aircraft electrical power distribution system design is that of protective solid-state circuit breakers 23 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Existing DC Power Grid Systems and Technology Cross-over Opportunities (cont.) Other Existing DC Power Grids: Rail: Modern DC railway power supplies chiefly operate at traction voltages of 600 - 750 V DC via third-rail collection or 1.5 kV or 3kV from overhead lines Ships: The primary power distribution system installed in ships is AC; however, DC distribution systems are used for combat systems, auxiliary systems, and emergency systems in submarines. Small DC networks, supplied by rectifiers or AC / DC motor-generator sets, supply outlets for battery charging. Some submarines have high voltage DC generators that supply static frequency converters for producing 450 V, three-phase, 60 Hz power for the ship’s user equipment, and DC power for main battery charging. Circuits are protected using both fuses and various types of circuit breaker Photovoltaic Power Systems: The shift towards “green” sources of renewable power has seen the corresponding development of supply chains for providing the system components required for both large-scale and small-scale implementation of these power systems. This is particularly so with photovoltaics (PV) for small- and medium-scale applications. Several companies are now supplying all the components (including DC-rated switches and fuses) required for both off-grid and grid-connected PV systems 24 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Existing DC Power Grid Systems and Technology Cross-over Opportunities (cont.) Other Existing DC Power Grids: Telecommunication and Data Systems: Traditional analogue telephone systems operate from a -48 V DC supply situated in the telephone exchange. Due to this legacy requirement, -48 V is still a common standard for current telecommunication data centres which have to handle data as well as voice systems. Related to the issue of DC power supply in telecommunication data centres are Power Over Ethernet (PoE) and Powered USB technologies. PoE (operating at up to 57 V DC) and Powered USB (operating at up to 24 V DC) are systems designed to transport electrical power, along with data, to telephones, wireless LAN access points, cameras, remote switches, computers, peripherals, and other electronic devices. E-merge Alliance: The E-merge Alliance is open industry association championing the adoption of safe (24 V) DC power distribution in commercial buildings through the development of a family of E-Merge Alliance standards. The work of the E-Merge Alliance is based on the concept of DC micro-grids operating alongside traditional AC networks throughout commercial buildings. This hybrid AC and DC platform is being designed as an open architecture, with the focus on reducing or eliminating inefficient AC to DC conversions between power sources and digital devices by converting and distributing power in DC form. 25 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Voltage Levels An issue which has particular relevance to switching and protection strategies in DC power grids is that of the actual DC voltage level (or levels) used in the power distribution system. Also related to this is the question of whether or not there is a co-existent AC power grid. Some of the issues to take into account are: Voltage Drop and Power Loss in Cables: Voltage drops in cables can be quite considerable.. For example, a current of 20 A and a cable length of 40 m will give a voltage drop of approximately 3 V for cable cross-section of 10 mm and V to 12 V for cable cross-section of 2.5 mm. These are absolute voltage drop values: hence, with respect to a very low DC voltage level (say 24 V), they represent a considerable percentage of the total available voltage Variable DC Ratings of Appliances and the Need for DC-DC Conversion: Suitable DC-DC converters are likely still to be required to enable different domestic appliances to operate from a DC power grid, whatever its nominal voltage level. Whilst “intelligent” and efficient DC-DC converters can be envisaged, making use of fast-acting semiconductor switching technologies and communication techniques to enable “automatic” selection of the required voltage for a particular appliance, a DC power grid does not in itself eliminate the need for voltage conversion. Safety Issues – Rail to Rail Short Circuits: The possibility of electrical short circuits occurring between different sub-grids of a multi-level DC power grid, or between a DC power grid and an AC power grid which are co-existent in a particular property, means that suitable protection against the occurrence of such circumstances will be required, adding significant complication to the protection system for the power grid. 26 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Conclusions - Lessons learned from other DC power distribution users such a automotive and aerospace can be applied to Smart DC grids - In terms of switching and circuit protection, the main problems arising from the deployment of DC (rather than AC) power grids are: - How to deal safely with arcs which occur at switch contacts or other points where a DC circuit is broken - How to deal with over current protection, especially the provision of resettable devices rather than fuses - How to deal with detection of earth faults and other stray leakage paths - How to deal with corrosion of cables, circuits and other system components - How to optimize the DC power grid architecture to provide an appropriate level of safety - Recent developments in silicon power semiconductor technology, and emerging SiC and GaN technologies are making available new, fast, solid-state switching devices which could be employed to produce high efficiency, very low-cost converters, switches and circuit protection devices - The UK has a strong capability in device, packaging and circuit design which could shorten design cycles and drive both performance and cost. The SIGs, KTN’s, NMI, TSB, EPSRC, UKTI and BIS are already having a strong influence and should further encourage (and fund) the necessary co-operations to build a coherent supply chain - There is clearly a dichotomy between, on the one hand, the question “Who is going to invest in creating new electrical appliances compatible with the DC grid concept if the infrastructure does not exist?” and on the other hand, the question “Who is going to create the infrastructure if there is no demand?” Strong, clearly understood, tangible benefits to be gained from the adoption of the new DC power grid technology will be needed in order for both questions to be answered. 27 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Recommendations - Innovation is urgently needed to develop new circuit switching and protection devices and systems for Smart DC grids. These will need to be both functional and cost effective. UK SMEs and universities have much potential to address this with the appropriate support, e.g. from TSB or EPSRC - More work is required to identify in detail the requirements and specifications for Smart DC grids (at both system and component level) along with the most promising market sectors. Focusing on the best commercial opportunities will be essential if the emerging Smart DC sector is to reach critical mass and take advantage of devices and techniques already in use in other markets such as automotive. This will be best achieved through the Smart DC SIG as it can engage with all the key stakeholders - Development of standards will be essential if the Smart DC market is to gain confidence and grow - Emerging semiconductor technologies such as GaN offer an opportunity for the UK to develop a world leading supply chain which could bring long-term benefits to UK plc as Smart DC systems are deployed worldwide. Further rapid investment is essential to maintain and grow the UK’s position in the face of strong international competition - The Smart DC SIG is ideally placed to bring together the appropriate technology developers, energy utilities and end users. Further investment in the Smart DC SIG is strongly recommended as a very cost-effective way to promote UK capability in this emerging technology area. 28 This copyrighted document is the property of TRW Limited. No license is granted by TRW to anyone to use, reproduce or exploit any content. Any authorized copies made by any method must also include this legend. Copyright TRW Limited 2012. Email: [email protected] Web: www.conekt.co.uk