Supercapacitors & Safety
A white paper on the use of supercapacitors in the railway industry
Industry demands for reliability, efficiency and performance improvements have utilized technological advancements made with capacitors. While used in early locomotives, the last few decades have seen capacitors become an increasingly critical piece of technology used for traction control, engine cranking and cranking assist technologies. In these cases, the locomotive may not operate without these components. Engine, generators, compressors and motors are all main locomotive components and now high energy capacitors must be added to the list of components required for a locomotive to work.
With the benefits of these high energy sources also comes the usual and important questions regarding safety. What are the existing practices and how can the industry improve on them?
WHAT ARE CAPACITORS?
Capacitors are an electric device that store electrical energy in an electric field. They typically contain at least two electrical plates (conductors) that are separated by a dielectric. Unlike batteries, there are no chemical reactions and they are capable of operating at high voltages.
Supercapacitors do not have the type of dielectric used by other common capacitors; rather the plates are soaked in an electrolyte and separated by a thin layer. They are typically of high capacitance (ability to store charge) but at much lower voltages (~ 2.8VDC per capacitor).
Common Uses on Locomotives
Early Locomotives – Low Energy Use
Capacitors are often employed to ramp or “smooth out” the response from excitation circuits. As an example, the locomotive crew will throttle up and down for train handling. Common excitation response circuits correlate a voltage for each throttle position, which is fed as a target to the excitation system. Step change voltage targets to the excitation system will result in choppy response and engine overload, so capacitors are employed to delay or ramp the voltage changes, resulting in smoother train handling.
Time Delay Circuits for Relay:
Relays actuate based on the voltage applied to the coil. When the voltage is removed, the coil collapses and the relay (or contactor) deactivates rapidly. In contactor sequencing, it may be important to delay the drop out of a relay so a time delay circuit, utilizing a capacitor, is applied across the coil.
Capacitors are commonly used in wide array of electronics to filter unwanted frequencies often referred to as noise. Examples include filter circuits for speedometers where the desired low frequency signal from the axle alternator is allowed through, but high frequency ripples are blocked.
Modernized Locomotives – High Energy Use
Traction Alternator Crank:
On some locomotive models the traction alternator is used to synchronous motor for diesel engine starting purposes. During a starting operation, a high voltage (~700 V) is applied to a capacitor using the traction alternator stator windings and electrical resonance. The capacitor voltage is then used for forced commutation of the SCRs in the cranking thyristor panels allowing for the generation of rotating magnetic field using a DC power source (batteries).
DC Link Capacitors:
DC Link Capacitors take the rectified output from the Traction Alternator and applies it to the Inverters. The inverters drive AC Traction Motors with each of the phases produced by the inverter. The locomotive control system provides signals to each phase module to determine how much tractive or braking effort is generated.
As opposed to dynamic braking, where the kinetic energy is dissipated in the form of heat, regenerative braking systems convert the energy into a form where it can be used immediately or stored until it is required. Supercapacitors are commonly used in this application.
Cranking Assist Circuits:
Lead Acid Batteries have been the primary energy source to crank the engine. Other battery technologies have been sampled in the industry but the lead acid batteries have the capability to discharge energy at a high rate – a requirement to get the diesel prime mover rotating.
Lead acid batteries also suffer from noticeable drawbacks. Performance is affected by several factors, such as demands of AESS usage and the lack of adequate voltage regulation and charging systems. Any discussion with a locomotive reliability officer will turn to the headaches they have with batteries, specifically reliability with respect to cranking.
Supercapacitors offer advantages over lead acid batteries:
- Faster charge and discharge rates
- Thousands of cycles can be performed without degradation
- Ability to maintain charge for long durations (months)
- Tolerant of temperature extremes
- Able to handle high current discharge without degradation:
- The deep cycling of the batteries during crank (peak current demands) is decreased therefore smaller lower capacity batteries may now be used on locomotives
For these reasons, several industries have turned to supercapacitors as a source of energy to assist with engine cranking. In some cases the capacitors are used as the prime source of energy for the cranking process while the battery is used to power onboard systems when the engine is shutdown.
With another high energy system added to the locomotive, along with the noted benefits, comes the usual (and necessary) question: what do our operators and maintenance personnel need to do in order to ensure safety?
SAFETY PRACTICES TODAY
Supercapacitors in Cranking Circuits
Functionally speaking, there are typically two common methods in which they are applied:
- The capacitors are charged from the battery prior to the engine crank, typically during the prime cycle. The energy is then discharged into the starting circuit during crank. At all other times there is a mechanism in place to ensure the capacitors are discharged.
- The capacitors are charged when the engine is running and maintain their charge when the engine is off, always ready to provide cranking energy. One could climb aboard a locomotive that has been inactive for months and find the capacitors with enough charge to turn the engine. This may the preferred option since, as opposed to option one, the energy for the capacitors comes from the auxiliary generator instead of the battery, reducing overall Ah draw on the batteries.
Electrically speaking, there are two ways the capacitors will be integrated into the starting circuit, on the battery side of the knife switch or on the load side.
Technicians tend to open the battery knife switch, thinking the circuits are dead – With the above, this is not always the case.
There is something to keep in mind: getting people to do something different than they do today is one of the most challenging things in our industry. So perhaps the best approach is to have the supercapacitors integrated in ways that will minimize altering existing practices.
With that in mind, one of the simpler things is to integrate the supercapacitors on the battery side of the knife switch. Technicians are well aware of the batteries on the battery side of the switch and treat it as “live”. In this case, the smartest and easiest thing to do is to treat them like batteries during operation, maintenance and troubleshooting. By adding some safety labels around the capacitors and electrical cubical, we reinforce something the technicians already know.
What if the cranking technology exists on the load side of the knife switch? It was stated earlier that technicians generally treat these circuits as dead once the knife switch is open. Reinforcing the “treat them like batteries approach” may not be enough. Instruments as simple as voltmeters should be used to verify absence of voltage before diving into troubleshooting but time constraints and years of familiarity may lead to human error – people don’t always check everything they are supposed to.
In this case, the best approach is to provide isolation circuits around the supercapacitors themselves. For example, the capacitors are connected across the starting circuit during engine cranking or across the auxiliary generator when charging. In all other instances, circuits isolate the capacitors from exposed bus bars and connections. This will be in line with systems they are used to dealing with i.e. if power is pulled, the terminals should be dead.
In the event the supercapacitors need to be removed or replaced, the safest practice is to ensure they are properly discharged prior to removal. As with DC Link Capacitors, there are different ways to safely discharge capacitors including automated devices, down to using a resistor to discharge the device.
Special attention should be paid to the OEM or manufacturer’s guidelines. As previously mentioned, some supercapacitor-based products will provide isolation circuits such that when the system is not in use or powered down, the capacitors are isolated in the enclosure. Even then, special attention must be paid to ensure there isn’t a malfunction with the isolation circuits. Again, absence of voltage tests are recommended before beginning any work.
Once the device has been safely discharged, a good practice is to place and leave a grounding strap across the positive and negative terminals. This indicates to the handlers that the device is in discharged state.
Per guidelines from manufacturers of supercapacitors, these devices should be shipped in a discharged state with a ground strap across the terminals.
If the device is not enclosed in a control housing and being shipped on its own, there should be a strong outer packing for shipment. If the device is enclosed as part of a control system, consult the manufacturer of the system to obtain and follow recommended shipping & packing practices.
Everybody Calm Down
So now that we have put enough fear into everyone over these new monstrous energy sources, let’s go over some simple facts:
Supercapacitors are similar to batteries - both should be treated with care and handled to avoid contact with the main terminals.
Supercapacitors and batteries serve a common purpose in that both provide energy – the battery stores energy in a chemical reaction and is designed to provide steady power for long term loads, whereas the supercapacitor stores its energy in an electrical field and is designed for rapid high power releases.
Although the supercapacitor provides a quick high power release and energy for the crank, the battery itself actually stores much more energy. Most supercapacitor-based systems are designed to only assist at crank, so while they are capable of greater power delivery, they store much less energy than the batteries which need to provide long term energy for locomotive subcomponents.
The voltage out of the capacitors will not be higher than the source voltage, in this case a nominal 72VDC. Further to this, if a supercapacitor crank assist system is designed correctly there should be protection circuits in place to prevent exposure to overvoltage. This will extend the life of the capacitors and ensure charge voltages are maintained at expected levels.
Supercapacitors are maintenance free.
They are more environmentally friendly, as they do not contain lead and other potentially harmful substances.
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