DC wiring for LED lighting systems
Modern lighting systems all run on DC, and the use of DC reticulation in LED based lighting systems is becoming common in “smart” lighting systems.
Lighting systems for premises are conventionally powered from an AC source and wiring is based on AC standards. Modern lighting systems, however, whether CFL, HFF, or LED-based, all work off DC. LEDs are specifically DC based and, to use such devices from an AC system, requires a converter in each device.
LED voltage requirements
LED systems are designed to operate on voltages of 12, 24 or 48 V DC. LEDs will be equipped with an internal converter to control both the voltage and the operation. The choice of voltage depends on the room size and other factors. The higher DC voltages will give lower resistive conductor losses. Use of lower voltages over a large area and long cable lengths can lead to higher currents and higher cable losses which may cancel out any gains made by using DC. Voltages in use in DC systems are 24 and 48 V.
Newer systems are moving towards 60 V as a standard [4] to allow larger areas to be served. The most common arrangement is to use AC as a source and to convert to DC in each light fitting. In addition to adding cost, converters have losses and generate additional heat.
Fig. 1 shows a block diagram of a typical LED lighting fixture [1].
Fig. 1: Arrangement of a typical AC driven LED fixture.
The move to CFLs and LED-based systems was driven by two main factors:
- The need to save energy on lighting.
- The need to save costs (LEDs and CFLs have a longer life than incandescent lamps).
Legacy lighting has been able to operate directly from the AC line. But that lighting itself has been inefficient and this is the reason why lighting consumes somewhere around 30% of the total energy produce. DC-driven LEDs promise to slash the energy use attributable to lighting in general and DC power distribution can enhance overall efficiency.
Energy can be saved by reducing losses that exist in the power delivery chain. First, we can minimise the number of lossy power conversions by using a DC grid. Second, renewable sources such as solar panels produce DC power, and despite the relatively low conversion efficiency of solar panels, they can power efficient LED fixtures. Thirdly, a DC grid offers the possibility of storage of energy in battery systems, against power outages or solar variations [2].
Power conversions inherently result in power lost as heat. Conversions happen in AC/DC power supplies that convert voltage level, and in DC/DC power supplies that only convert voltage level. The driver or controller used in LED lighting is simply a constant-current or constant voltage power supply. Generally, more conversions equate to greater energy loss, and larger level changes are less efficient than smaller level changes.
Fig. 2: LED installation using low voltage DC distribution.
The obvious first step is to replace the AC/DC converter in each fitting with a common centralised front end to provide protection and convert from AC to DC, and then to distribute DC to the LED fixtures. A typical example is shown in Fig. 2. The power unit has a low voltage DC output in the range 24 – 60 V DC. The capacity of the unit is limited by the capacity of the cables that will carry the DC to the light fittings and the lighting load distribution. Cable lengths are limited, and this will determine the size of the converter. Typically, office or school buildings will require several converters placed as required.
DC LEDs were originally developed for the solar applications but have now found commercial and other applications. They are particularly suited for applications with rooftop solar, which are becoming more and more popular with end-users and commercial enterprises.
DC wiring systems
The main development areas in DC distribution for buildings are DC distribution for data centres and lighting for commercial buildings. Typical DC wiring systems under development operate at a voltages of 380 – 400 V DC, primarily to serve high-load equipment. The DC lighting wiring is a spin-off of this sector and may be incorporated in such a system. However, most DC lighting systems are designed to operate independently of other systems. An example of an LED lighting system driven from a DC main supply is shown in Fig. 3.
Fig. 3: DC driven LED system.
Development of systems
With the development of LEDs has come the concept of “smart lighting”, which requires individual control of luminaires and light fittings, as well as the use of light fittings as sensors. The application of the smart lighting concept to conventional lighting requires control wiring separate from the power wiring, which increases price and complicates the installation.
The low voltage DC used in LED installations using centralised converters and the low power demand of LEDs make it possible to run LED power and data communications cables in the same enclosure, and combined data and power cables such as Ethernet cabling can, in fact, be used to provide both power and control functions to LED fixtures. This combined LVDC and data cabling forms the basis of most “smart” LED lighting installations available today.
A number of DC distribution systems have emerged over the years. All offer a range of standard components such as standard plug-and-play connections for LED lighting, as well as power sources and controllers allowing simplified design and installation of lighting systems.
EMerge Alliance [3]
DC distribution in office buildings started with the EMerge Alliance system, based on 48 V DC distribution. The system is supported by industry, and a number of major suppliers have compatible products, but there are not many installations. There has been no high-volume commercialisation of the EMerge platform, despite its many technical merits [3]. Fig. 4 shows a typical installation.
Power over Ethernet (PoE) [4]
PoE takes advantage of two LED lighting attributes: low power requirements and the capability for digital connectivity and control. 240 V AC electrical wires and all the cost, regulations, and infrastructure that go with them exceed greatly what LED lights need. LED luminaires contain electronics that require lower voltages, typically 12 – 24 V DC. Ethernet cable, which is already present in offices, safely carries lower DC voltages which do not require special protection and other safety measures.
Fig. 4: EMerge Alliance DC system [2].
The heart of the PoE system is the Ethernet switch, which provides both power and data connectivity to the Ethernet cables. The power rating of Ethernet cables and the power delivered by each cable connection point has steadily increased to accommodate operation of PoE devices. The power rating is now in the region of 60 W per connection.
PoE forms the basis of the so called “smart lighting” system, which combines data connectivity with lighting power. Network companies such as Cisco are active in developing the concept in partnership with lighting companies such as Philips. Many PoE LED installations are already operative, but are limited to sites with extensive
Ethernet cabling in place, or where data connections are prevalent. The PoE LED system finds an ideal application as part of an integrated building management system where all devices in a building are controlled from a central point.
The Eaton DLVP system [5]
The Eaton distributed low voltage power (DLVP) architecture includes centralised power conversion for efficiency and a DC distribution scheme. The power cables and connectors can also carry control data. The company uses the DLVP platform as an option in its solid-state lighting (SSL) portfolio while also seeking to standardise the approach and allow other lighting manufacturers to leverage DLVP [5]. The DLVP system is a simplified version of the PoE which offers all the facilities required for lighting power and control, without the high level of integration of PoE. DLVP can also be used in areas where there is not a great need for connection of other devices on the internet, such as warehouses, halls or indoor sports venues.
Fig. 5: Typical PoE lighting system.
The DVLP is designed as a stand-alone system for lighting control, and features a simpler range of controls than that provided by PoE. It also simplifies wiring where IT facilities are not general. DVLP uses the maximum voltage allowed on Ethernet cables, i.e. 60 V, to achieve efficiency. The DVLP uses pre-terminated low-voltage lighting cables for safe, flexible LED lighting installations. Low-voltage lighting cables provide for the fast, error-free installation of LED light fixtures. Cables are available in pre-terminated lengths typical to application/fixture type, for error-free connectivity of circuits in plenum rated applications.
The heart of the DVLP system is the low voltage power module. Each cable connection can deliver 100 W at
60 V DC. Up to six cable connections can be provided per module, providing for approximately 200 m2 of lighting and controls coverage per module. So, a single power module should be adequate for the average home or small office [5].
LED fixtures used with the DLVP system are equipped with controllers which provide DC/DC conversion and control features.
Additional advantages of DC wiring for LEDs
- Low voltage: The voltage used in DC lighting systems falls within the extra-low voltage (ELV) classification of SANS-0142 and is therefore not subject to the same protection requirements as low voltage. This allows cables to be run in accessible spaces such as plenums, without the need for conduit or trunking.
- Renewable energy or solar: Because DC may be connected directly to the output of a solar power system without going through inverters.
- Backup: DC makes direct use of battery backup without conversion problems. Some luminaires have an integral battery back-up system.
Protection for DC reticulation systems [1]
Fig. 6: Details of the Eaton DLVP system [6].
The primary protection against earth faults and short circuits is disconnection by an overcurrent device, either circuit breaker or fuses.
DC has a disadvantage in that there is no zero crossing in the waveform and therefore conventional AC circuit breakers that rely or operate on zero crossing of the current to operate cannot be used. The DC breaker has to break the full fault current.
DC breakers are available and will become more common as more DC circuits are installed.
Fuses can also be used as protection but do not offer the same convenience as circuit breakers do, and there is always the problem of a blown fuse being replaced with a fuse of different rating.
Protection for manufacturer-supplied systems is usually incorporated in the power modules and does not need to be added externally.
References
[1] K Casey: “LED lighting circuit protection overview” www.mouser.co.za/applications/lighting-circuit-protection/
[2] M Wright: “Lighting systems leverage DC distribution for maximum efficiency ” LEDs Magazine, April 1 2014, www.ledsmagazine.com/articles/iif/print/volume-3/issue-1/features/dc-grids/lighting-systems-leverage-dc-distribution-for-maximum-efficiency.html
[3] M Wright “Low-voltage scheme trivialises installation of LED lighting and supports controls” LEDs Magazine , October 21, 2016 www.ledsmagazine.com/articles/print/volume-13/issue-8/features/dc-power/low-voltage-scheme-trivializes-installation-of-led-lighting-and-supports-controls.html
[4] Genesis “PoE Lighting systems” www.innovativelight.com/commercial-industrial-led-lighting/poe-led-lighting/
[5] Eaton: “Low-voltage power module: Distributed low-voltage power system – technical data” www.cooperindustries.com/content/dam/public/lighting/products/documents/control_systems/spec_sheets/TD503076EN-DLVP-Low-Voltage-Power-Module-sss.pdf
Send your comments to vector@ee.co.za
Related Articles
BROWSER-BASED E-ZINES
BACK ISSUES
SERVICES
DAILY CARTOON
-
