Not Shocking at All!
One danger present in traffic signal cabinets has always been the potential for serious injury or death of maintenance and operations staff due to the electrical hazard present in the cabinet. While most connections are covered and protect the user from shock or electrocution, it is still possible for injury due to failure of grounding, improper handling, carelessness and insufficient training. Some agencies have addressed this by requiring that their technicians be certified electricians, others have created custom cabinet configurations that separate the low-voltage (detection and communications) from the high voltage (signals and lighting), while others choose to use terminology in job descriptions to address the issue (qualified personnel). Care has always been required when working inside of the cabinet, but even the most careful technician is not immune to accidental shock or arc flash injury. The spectrum of Personal Protective Equipment (PPE) required to work inside these cabinets also varies widely, from full insulated elbow length gloves and face shields, to none at all. Often those who are tasked with maintaining the traffic signals are thrust into the duty as an afterthought or due to budget constraints, which increases the potential for accidental harm. The low-voltage cabinet was designed to make the cabinet safer to operate, maintain, and reduce the potential of catastrophic damage due to component failure or short-circuiting. The NEC and UL define a low-voltage system in different terms and applications; here we are defining low-voltage as non-ripple DC voltage of less than 50V.
For more information on Intelight's ITSv2 cabinet please see the product page here.
Compact and Green
By designing the cabinet circuitry to utilize lower power signals and more efficient devices, it was also possible to reduce the total power consumption. The typical current draw of the entire system (controller, cabinet, plug in modules, and signals) for an 8-phase intersection with four pedestrian movements is less than 5.5A at 48VDC – which equates to 264 watts total. This greatly reduced load makes many of the renewable and alternate power sources more cost effective and useful. While solar would still require a reasonably large array to completely remove the signal from the grid, it does make solar an effective and applicable option for a backup or supplemental power source, as does the potential for a wind turbine.
One Small Step
While many are still wary of converting to a 48V option, some agencies have embraced the technology and have taken it upon themselves to push the technology into interesting and creative ways. One such agency is Harris County, TX. Encompassing over 800 square miles of the greater Houston area, Harris County is the largest and most diverse county in Texas. Dotted sporadically around the 800 square miles are several hundred of the most advanced traffic signals in the United States. Harris County has been a leader and trailblazer in signal design and implementation of new technology, and has been a test bed for many of the technologies used in the traffic industry today. The ITS cabinet was born here, as was using toll tags to calculate travel times, and most recently – the 48V cabinet. Not one to simply take the technology for what it is currently; they push the envelope by not only installing the 48V cabinet at multiple locations, but also incorporating several different backup power sources. The most recent pilot project proved that not only is the 48V traffic signal possible and sustainable, but the power sources for it are also varied and deployment ready. In this latest project, they chose to incorporate two different fuel cell technologies – Methanol and Compressed Natural Gas (CNG).
Houston has its share of traffic and, as hurricane Rita and Ike showed them, its share of prolonged power outages. First responders are often trapped in the traffic jams caused by the power outage, and the signal crew is often left trapped themselves, unable to reach the signal to restore power or reset the signal. Even the shortest power outages cause severe impact to traffic, one that requires significant time to return to normal. It was with this in mind that Harris County hoped to mitigate the impact of power outages by providing the signal with enhanced extended runtime (in the case of the methanol fuel cell location) or a nearly limitless supply (in the case of the CNG fuel cell location). The thought behind the project was to push the technology as far as it would go and in doing so prove to the rest of the industry that not only was the 48V traffic signal a good idea, but one whose time has come.
Anatomy of Change
The 48V system is comprised of three basic block components: the grid supply, the AC to DC conversion cabinet power supplies, and the signaling system.
The Grid Power Pedestal
Similar to the standard utility service cabinet, the grid supply component houses the utility line breakers and power metering equipment. At some locations, a utility gas meter is also installed to provide the Compressed Natural Gas for the system.
The AC to DC Conversion System
In the Harris County project, it was originally planned to provide a single AC to DC conversion unit that would supply all of the necessary voltages in one module. Due to project time constraints, the single unit design was shelved for a dual conversion system. As the block diagrams in Figure 1 and Figure 2 show, AC power is routed into the cabinet and supplied to a 120VAC to 24VDC conversion module. This module supplies the DC power for both the battery charging/regulation and also the power for the DC to DC conversion, stepping up the 24VDC to the regulated 48.0VDC supply for the signal components. The system also provides facilities for management of a solar array input and solar charging circuits, as well as monitoring and management devices for the overall conversion system. The fuel cell modules are also tied into the system and configured to provide the 24VDC if the utility power is to fail and the battery bank is depleted. It should be known that fuel cell modules today have cycle life of about 10,000 hours under continuous usage before needing to be replenished. The fuel cells have a distinct advantage over generators in that a fuel cell can sit idle for years without any maintenance, while a generator will become unusable after a relatively short time if they are not regularly exercised (started, run, oil changed, fuel reservoir emptied and refilled, fuel lines checked). The fuel cells in this system have a life of 250 full power cycles, but it is projected that in a typical year they may only run three or four times, so the actual life of the fuel cell could be in the range of 25+ years. With the cycle life of the fuel cell in mind, the system is designed that they cycle as little as possible thus prolonging the overall life of the modules. During a prolonged outage where the solar array and battery bank cannot maintain the voltage for the 48.0VDC supply module, the system monitors the 24VDC battery bank and only starts the fuel cells when the voltage drops below 24VDC (typical float voltage of the battery bank is 27.7VDC). At 24VDC, the fuel cells are activated and require 30 minutes to get up to full production readiness. Once producing power, they will run only until the utility power is restored (though there is hysteresis to prevent cycling on/off the fuel cell). The system monitoring unit constantly checks to ensure that utility power is restored and not just temporarily active before switching back to normal operation as the fuel cells also require a cooling down period (about 20 minutes).
Compressed Natural Gas System Diagram
Methanol System Diagram
CNG Fuel Cell System Cabinet Enclosure
Battery Bank and Solar Array
As mentioned, the 48V system installed by Harris County required additional components to support the fuel cell system while it warmed up to full output. The battery bank was designed so that general short-term grid power outages (duration less than 20 hours) would be supported without needing to activate the fuel cells. The batteries are housed in a single chamber containment module that allows them to be placed in a ground vault near the cabinet. The vault protects the module from the harsh Texas heat and also protects them from damage should a vehicle leave the roadway and collide with the cabinet. There are eight batteries each weighing 70lbs and the total weight of the battery module (pod) was nearly 1000lbs.
Battery Vault installed in ground
The solar array is designed to provide trickle charge current to the batteries and supplement the grid voltage when necessary. The panels were sized to provide 240 watts and take very little space in the overall installation. While not fully capable of support the system long term, the panels were sized to demonstrate the ease of integration of additional alternate power sources. Originally, the system was designed to support a wind turbine source, but it was found impractical and was left out of the final design.
High Density Output Assembly
The Signaling System
While housed in a standard traffic signal cabinet (the ITS 340 cabinet in this application), many of the components have been replaced with low-voltage assemblies. The most significant update is the Output Assembly which replaces the Power Distribution Assembly (PDA) and the 14-pack Output Assembly, enhancing it further by increasing its channels to 16. The 48VDC Output Assembly provides facilities for the Conflict Monitor Unit (CMU), the Serial Interface Unit (SIU), seven High-Density Switch Packs (HDSP), one High-Density Flasher Unit (HDFU), all Flash Transfer Relays (FTR), police panel and technician maintenance switches, and field output terminal blocks. This all in one assembly is equal in size to the previous ITS cabinet PDA assembly (19.25”x5.25”x10”). This reduces the footprint of the Output Assembly by more than two thirds, freeing up valuable rack space for other equipment.
The Output Assembly can support up to 16 channels with three outputs per channel. The typical configuration is for eight vehicle movements, four pedestrian movements, and four overlap movements. The design also incorporates the ability to support complex displays through output and channel mapping through the local control software and monitoring unit. The CMU / HDSP monitoring combination is a brand new concept that amplifies its safety and diagnostic capabilities over the existing ITS monitoring functions, as it now provides real time load current monitoring at levels compatible with new ultra-low power LED signals. This new load current monitoring function increases safety at the intersection since the CMU can detect and respond to a dark approach (Red Fail) immediately. Troubleshooting diagnostics are also elevated to a new level since the CMU is now analyzing actual power delivered to the load and the confusing effects of load switch leakage currents are eliminated. In the future this load current functionality may be used for detection of LED signal head failures in parallel signal displays.
The High-Density Switch Packs (HDSP) are a very interesting and exciting industry innovation. Designed to provide two full channels of outputs per load switch (six outputs instead of three) in a compact detector sized card, they communicate directly with the CMU (eliminating the need for an AMU in each OA) providing their measured output voltages and load currents. Furthering the utilitarian design of the low-voltage system, any load switch can be plugged into a flasher slot or vice-versa, functioning as a flasher if inserted into the flasher slot or a load switch if inserted into a load switch slot. The remaining assemblies and components of the cabinet are relatively the same as those normally found in a conventional ITS cabinet, with the exception of DC-DC isolator cards for the pedestrian input channels and low-voltage lighting and cooling fans. All of the detection cards are standard off the shelf units as they are already configured for DC operation.
The signal displays were designed to the highest standards possible and with longevity in mind. With the removal of the AC-DC power supply in the LED module, the heat generated by this conversion was also removed, allowing for a much longer expected life. Additionally, since the displays are isolated from the utility grid, voltage spikes and surges are removed providing a much more stable operating voltage, further increasing the potential for longer life. The typical operating power levels for the displays are 4 watts each – much lower than their AC powered counterparts.
Only the Beginning
The move to low-voltage DC-based traffic signal systems are well on their way. With agencies embracing LED technology, backup power systems, and the benefits of the ITS/ATC cabinet architecture, the time is right more than ever for a concerted effort to move the low-voltage traffic signal system to the forefront. While efforts have been made in previous years to bring the DC cabinet to market, the technologies that would allow for a deployable and sustainable product to fruition were not cost-effective or even technically possible. That is changing with the advent of innovative high-density load switches, compact cabinet designs, and the sheer willpower of agencies that wish to shake up the business-as-usual world of traffic cabinet technologies. Working within the confines of a sustainable standardization of the DC cabinet and practical application of the technology, Harris County has pushed the 48V cabinet into the limelight. Over the past two years, Harris County, with the cooperation of some of the greatest minds in the traffic industry have succeeded in deploying several of the 48V cabinets. With funding from the Federal Government, Harris County has completed installation of several stand-alone systems and three test case locations utilizing both the low-voltage 48V cabinet and a mixed deployment of fuel cell and solar configurations. While most agencies needs for reserve backup power may not be as severe or exotic as those in Harris County, the advantages of the low-voltage DC cabinet can no longer be ignored. I have had the great fortune and pleasure to be involved in this project and hope that my reporting spurs additional interest and development of this technology.
Article and Pictures by Mike Talyor