LED Lighting Systems in Smart Buildings with DC Microgrids

The conditions of energy generation and its use in the context of modern buildings have been facing major change in recent years.

This development is being driven by increasingly decentralised energy generation and storage. Photovoltaic systems and batteries, as well as storage systems play an important role in this, as do fuel cells, all of which generate direct current (DC). On the demand side, the importance of DC appliances is increasing. While electric motors for ventilation and air-conditioning systems were DC-based in the past, powerful IT systems, LED lighting systems and, last but not least, charging systems for electromobility are now increasingly being used; these are all demands that are best supplied directly with direct current (DC).

Therefore, it is an obvious step to equip modern buildings with direct current grids and to save energy losses during the conversion from direct current to alternating current (AC) or vice versa (DC-AC-DC). In recent years there have been various investigations [1], field trials and successfully realised projects in Europe, North America, Japan and China. The reduction of conversion losses and reactive   power as well as a more efficient transmission have resulted in energetic savings of 5 - 10 % [2], and even up to 30 % depending on the application. In addition, direct current grids contribute to improvements in network quality (backlash due to harmonics) and enable genuine uninterrupted operation when switching to UPS systems.

In the course of these projects, however, a number of unresolved questions have emerged, in particular with regard to safety and protection concepts, not least as a result of the fact that over the past 100 years, empirical experience has been gained in dealing with alternating current grids in buildings, but less in dealing with direct current grids. The DKE German Commission for Electrical, Electronic & Information Technologies, together with VDE and experts, has addressed these questions and summarized the corresponding recommendations for action and use cases for various applications in the German standardization roadmap for direct current in low-voltage grids (LVDC) [3].

Smart Microgrids

Especially, in the self-use of renewable energies, there is a necessity to improve the coordination of energy generation and use of time- and intensity-variable consumers (Demand Side Management DSM). This is achieved by so-called Smart Microgrids, in which a central or decentralized control takes over load and storage management by means of IT-based communication. Such an infrastructure can also be used across the board for the Smart Building. With sensors and actuators, the Smart Building is able to react to changing environmental conditions and user needs.

The users increasingly arrive at the building by means of electric vehicles. Accumulators in these vehicles are also originally charged with DC. The acceptance of electromobility will largely depend on the charging infrastructure and charging times. Today, modern charging systems such as CCS (IEC 62196-3), CHAdeMo or Supercharger support fast charging with DC (so-called charge mode 4). The pertinent IEC standards provide for a current flow in both directions and corresponding bidirectional communication, enabling batteries in connected vehicles to help compensate for load fluctuations in the future (in the case of the Combined Charging System, this extended communication is mandatory according to ISO 15118).

What exactly could such a DC grid (DC microgrid) look like, for example, for commercial buildings? (Figure 1) Instead of equipping many devices with individual rectifiers, as is the case today, the alternating current grid is connected via a central rectifier with mains filter. The PV system, the battery storage system and the fast-charging system for E-mobility are then also connected to this DC grid. Different topologies can be used for this. Control can be provided via a DC grid manager.

Established Voltage Standards

Since data centers have a large demand for direct current and the highest demands on network stability, they play a leading role in the development of standards (see also EMerge Alliance). For the first time, ETSI EN 300 132 3-2 defines a voltage range of 260 to 400 V DC for high power ranges. The standardization bodies and Europe, North America, Japan and China finally agreed on 380 V as the reference voltage for the distribution network in commercial buildings. The 380 V DC enable small line cross-sections and are compatible for the connection of regenerative energy sources, storage systems and charging systems for E-mobility. This standard has helped the industry to develop a wide range of components for the 380 V DC ecosystem.

The various loads can be connected directly to the 380 V DC distribution network or via a step-down DC-DC converter (see also IEC 61204-6) via a reduced voltage of 12, 24 or 48 V DC, for example. Such voltages are common in various industries such as aviation, railways, shipbuilding and vehicle construction where they have been tried and tested and standardized for many years. Since 48 V DC is still in the range of safety extra-low voltage and, compared to 12 or 24 V, enables smaller line cross-sections and lower line losses, this standard is not only establishing itself in the lighting sector and has been defined by IEC/SEG 4 as the preferred voltage level for low power ranges.

This article therefore further focuses on the application possibilities of the two preferred voltages of 380 V DC for the high and 48 V DC for the low power range in the lighting of commercially used buildings and does not address other voltages, such as 216 V DC for emergency power systems for switchable operation of 230 V AC (cf. DIN EN 50171).

LVDC for Lighting Systems

First of all, it is obvious to supply the LED lighting systems used today directly with 380 V DC and then operate them with a step-down DC-DC converter, which reduces the maximum output voltage and generates the usually required constant current. The advantage of this approach is that it can transmit quite high powers with small line cross-sections to any light spot. The drawbacks are the high voltage up to each light spot and higher component costs at each light spot compared with converters for smaller input voltages. As the required power ratings of LED lighting systems become lower and lower compared to conventional light sources due to their high efficiency, the reference voltage of 48 V DC for lower loads is becoming increasingly important.

One approach for LVDC transmission is the Power over Ethernet (PoE) method based on IEEE 802.3. Here the data cable designed for Ethernet communication is used to simultaneously supply the loads (so-called Power Devices PD) with direct current via free or signal-carrying line pairs, as in this case the LED lighting system. PoE operates with a voltage range of 37 - 57 V DC depending on standard version and wire length, which means that it is not a 48 V DC network in the narrow sense. Feed-in takes place via so-called Power Sourcing Equipment (PSE), which, in turn, can be supplied via the 380 V DC distribution network.

The appeal of the PoE lies in its ability to map data communication with direct IP addressing and power supply in the same infrastructure. The limitations of this approach derive from the fact that the transmittable power is extremely limited due to the small cable cross-sections and that relatively high power losses are incurred. In recent years, manufacturers and committees have increased the maximum power output of the PSE from 15.4 watts to up to 40 watts over two line pairs (Class 5) and from 60 to 100 watts over four line pairs (IEEE 802.3bt-2018). Nevertheless, the high power losses involved contradict the basic idea of more efficient energy transmission through direct current networks.

A highly promising third approach tries to compensate the disadvantages of the direct use of 380 V DC and PoE. The 380 V DC is transformed to 48 V DC at central points (Point of Load Conversion) and routed to the light points via sufficiently dimensioned lines, low-voltage tracks or energy rails. The constant current control of the LED luminaires can then be carried out via very compact DC-DC converters in the 48 V DC input. With sufficiently dimensioned line cross-sections, powers of up to 1 KW can be transmitted. This dimension fits quite well to a practical number of LED luminaires with typical power ratings. With regard to installation, it should be noted that the 48 V DC side operates within the safety extra-low voltage range.

At the same time, the extremely compact 48 V DC-DC converter meets the trend of miniaturization of LED luminaires. These converters are available as small independent boards that can be easily integrated into luminaires or adapters, or are already integrated on LED boards, e.g. for standardized ZHAGA formats (see also Vossloh-Schwabe or Tyco Electronics). Such converters can be designed without lifetime-critical electrolytic capacitors (drying out) of conventional switching power supplies, which significantly extend the maintenance cycles of the distributed light spots. In addition, the first COB concepts are being developed that can be operated directly with 48 V DC, for example via integrated linear regulators without additional DC-DC converters (see Osram ConVoLED). Due to the growing use of 48 V DC also in other applications, such as the on-board power supply of modern automobiles and electric vehicles (see VDA 320), a large number of corresponding components are available at low cost [4].

Practically, concepts can be used in which LED lighting systems with 380 V DC power supply, PoE and 48 V DC can be optionally combined. Figure 2 shows a project for shop lighting with a mix of linear luminaires for 380 V DC (blue) and track-mounted luminaires for 48 V DC.

Figure 2: A distribution and cabling example for a shop lighting project with a mix of linear luminaires for 380 V DC (blue) and track-mounted luminaires for 48 V DC  

Smart Solutions for LVDC

However, this does not yet answer the question of how to control luminaires and communication, for example in combination with sensors, as represented by PoE (see above). There are basically three possible concepts for control and communication: communication via live DC lines (Power Line Communication (PLC)), communication via additional control lines (e.g. DALI) or radio-based communication (e.g. WiFi, ZigBee, Bluetooth, EnOcean).

PLC is only regulated in the frequency range from 3 to 148.5 KHz by the EN 50061-1 standard to protect against interference and ensure electromagnetic compatibility. Further standardization of this technology is lacking. Inevitably, as in other industries, only proprietary solutions such as Bits2Power or DC-String (Tridonic) have emerged in the lighting industry in conjunction with 48 V DC supply. In principle, these solutions are structured in such a way that a protocol such as DALI, which is received at the central converter for 48 V DC, is converted to a PLC signal and then communicates with the compact DC-DC converters of the individual luminaires. Currently these solutions do not support the supply of the central 48 V DC converter with 380 V DC input voltage but only with 230 V AC.

DALI has proven to be a digital standard (EN 62386) for line-based lighting control based on the Manchester protocol. Special features of the DALI bus are that the two DALI control lines require basic insulation and can therefore be laid together with the current-carrying 230 V AC cores in one cable, the DALI control line can supply power to sensors and actuators (max. 20 mA per consumer) and the system is protected against polarity reversal. The DALI bus itself operates with 250 mA and max. 22.5 V DC, but the DALI inputs must be designed for 230 V due to the risk of reverse polarity with conventional AC inputs. For the 48 V DC supply concept, there are, for example, 4-pole busbars (see e.g. Eutrac Low Voltage) that permit DALI or other line-based communication such as 1-10 V or DMX. Corresponding compact DC-DC converters with additional DALI inputs are also available on the market. When implementing the solutions in combination with 48 V DC, it is important to note that the input of the compact DC-DC converters is generally not designed for higher voltages, which could principally be looped through via the DALI bus in the event of reverse polarity.

In recent years, radio-based communication standards for lighting management systems (LMS) and the Internet of Things (IoT) have become increasingly popular. In addition to WiFi (IEEE 802.11), these are mainly low-power standards such as Thread or ZigBee or ZigBee Green (IEEE 802.15.4), Bluetooth (IEEE 802.15.1) and EnOcean (IEC 14543-3-10). Wireless solutions of this kind complement a basic 48 V DC supply very easily with lighting control and sensors.

EnOcean and ZigBee Green even allow Micro Energy Harvesting (MEH), eliminating the need for switch actuators to be powered and allowing them to be placed wirelessly.

Since radio-based solutions are already finding their way into Smart Buildings, they can be used for comprehensive event-based building automation at the field level. Typically, the different systems at the field level are then merged via an IP backbone (e.g. BACnet-IP) at the automation level (BACnet explicitly stipulates the ZigBee protocol as the communication layer at the field level). The management level can be mapped via BACnet (see DIN EN ISO 16484-5), via other building management standards such as oBIX or via the general automation standard OPC-UA (IEC 62541).

Challenges

Despite their many advantages, DC systems also present challenges that should not remain unmentioned. These include, in particular, technical challenges and still underdeveloped standardization. Both topics are closely linked, since the industry can only reliably address some technical challenges once certain gaps in standardization have been closed or contradictions eliminated.

The greatest technical challenge is the danger of arcing due to the lack of zero crossing of the DC voltage. This danger occurs in particular when pulling a plug under load, during switching and in the event of insulation faults. From a voltage of around 20 volts, the arc is no longer self-extinguishing. As a result, circuit breakers, cables, switches and plugs must meet special requirements. For this reason, connections without special precautions or conditions may only be disconnected without load resp. currentlessly plugged. The cables and conductors should be separately color-coded. For example, the new IEC 60445:2018 stipulates red for the positive outer conductor, white for the negative outer conductor and pink for the functional grounding conductor.

Studies on corresponding faults show that the vast majority of arc faults are due to faults in the grounding concept (cf. IEEE Std 493-2007). Therefore, the grounding concept is of particular importance in the context of direct current systems. ETSI EN 301 605 describes two permissible grounding concepts for direct current systems: the IT system and the TN-S system.

At LVDC, the TN-S system operates with an earthed minus to the central grounding point (CGP) and protection is normally provided by the switch-off conditions of the overcurrent protection devices, unless there is a comparatively complex residual current monitoring RCM (according to DIN EN 62020). Therefore, the TN-S system without residual current monitoring is only used in applications where system availability requirements are not particularly high.

Conversely, in the IT system, the first error is only reported by the Insulation Monitoring Device IMD (DIN EN 61557-8). Only then is grounding produced as in the TN-S system, whereby the so-called High Resistance Midpoint Grounding HRMG is preferred for direct current systems. This means that the IT system can continue to operate safely after the first fault has occurred, which is particularly favorable, for example, in IT, the process industry or railways. Electro-mechanical all-pole switches have proven highly effective for the IT system in conjunction with LVDC.

As far as standardization is concerned, it should be pointed out that there are specific standards for a large number of DC applications, which, however, are not designed for building installation. In principle, the construction of DC networks is covered by DIN EN 60364, but there is a lack of concrete LVDC standards for insulation measurement, connectors and switches. In some cases, contradictions among individual standards, e.g. for the assessment of clearance distances between the basic standard DIN EN 60664 and the cable, connector and device standards, must be harmonized.

Until these standards are revised, special attention must be paid to the selection of suitable components for LVDC. With the increasing number of ambitious projects implemented with renowned partners, best-practice solutions have developed which can be used as a basis for orientation. Finally, due to their limited experience with LVDC, electrical specialists should receive special training. For example, NFPA 70E already included special requirements for handling DC systems > 100 V in 2012.

Outlook

The European Union has recognized both the limited availability of fossil fuel resources and the dangers posed to our civilization from the CO² induced greenhouse effect. Since renewable energy sources are comparatively expensive, the silver bullet lies in increasing energy efficiency.  It meets this requirement with the 2010/31/EU Directive (Energy Performance of Buildings EPBD) in the area of standards for new buildings and thus prescribes Nearly Zero Energy Building (NZEB) from 2021.

In the course of the amendment EPBD 2018 (2018/844/EU) further objectives were also set for the energetic restoration of existing buildings, the intelligence of technical building systems to adapt to user requirements and optimize overall efficiency and the provision of charging infrastructure for electro mobility.

LED lighting systems already contribute to saving energy and improving the CO² balance. Further potential savings can be exploited in the future primarily through systemic optimization of commercially used buildings.

In this article it has been shown that direct current systems in the low voltage range (LVDC) offer a considerable savings potential in the context of systemic optimization and have a large number of additional advantages. Semiconductor-based LED lighting systems operate with direct current by definition - as do an increasing number of other loads - which makes them ideal for being supplied with LVDC.

In addition, LVDC can be used to better emphasize the actual advantages of LEDs, such as miniaturization and long service life, by eliminating some life-critical components for AC-DC conversion. This opens up degrees of freedom in the design of LED luminaires and extends their maintenance cycles.

With the integration of smart IoT solutions, the LED luminaires can be used to determine real-time location-based data using a wide variety of sensors. LED luminaires can be ideally combined with sensors, as they are typically placed in locations suitable for sensors while the LEDs are DC-powered anyway. The data obtained can be used for a variety of purposes such as energy optimization, simplification of facility management, individualization of lighting portfolio, management of resources or improvement of the user experience in a building (Indoor Location Services).    

In addition to the integration of renewable energies and storage systems as well as charging systems for electric mobility, LVDC offers another advantage: the solution contributes to improving grid stability. This aspect should not be underestimated, considering the increasing problems with the contamination of AC grids.    

Given this background, an increased use of LVDC systems together with LED lighting solutions in commercial buildings should be observed in the coming years, both as pure LVDC solutions and in the form of hybrid DC/AC solutions. Data centers will only likely to have played a pioneering role in this.

This development is likely to be promoted both by the current achievement of the economic efficiency of internal consumption of electricity from new PV systems without subsidies and by the internal consumption of electricity from existing systems where EEG subsidies in Germany expire for the first time in 2020.

LVDC not only promises potential savings when it comes to supplying buildings. Since similar savings potentials can be exploited in industrial applications (70% of industrial electricity consumption is accounted for by electric motors) [5], this technology is expected to be used in numerous industries. Together with progress in standardization it will stimulate a broader range of respective components.

Last but not least, DC microgrids not only benefit the highly developed industrialized countries, but also enable the autonomous electrification of underdeveloped rural regions, for example in conjunction with cheaper batteries and PV systems.

References:
[1]    B. Wunder, 380 V DC in Commercial Buildings and Offices, Fraunhofer, 2013
[2]    U. Boeke, M. Wendt, DC-Power Grids for Buildings, IEEE, 2015
[3]    Deutsche Normungs-Roadmap Gleichstrom im
        Niederspannungsbereich, DKE/VDE, 2016
[4]    48-Volt-Bordnetz - Schlüsseltechnologie auf dem Weg
        zur Elektromobilität, ZVEI, 2015
[5]    Forschungsprojekt DC-INDUSTRIE Gleichspannungsnetze in
        der industriellen Produktion, ZVEI, 2017