Bus and Truck Pantographs: Up or Down? Roger Bedell, CEO Opbrid 15 Jan, 2016

Updated 23 June, 3016

At Opbrid, in 2010 we started with pantographs on the vehicle, with a stationary infrastructure. It needed large pantographs on the vehicle to raise to meet the fixed height infrastructure. It worked fine. Next, we made a hybrid system, with a lowering infrastructure meeting small pantos on the vehicle that went up. It also worked fine. In 2014 we deployed an inverted pantograph that lowers to a stationary current collector on the vehicle, similar to the system from Siemens, but with a different geometry. It is also working well. So, based on my experience and knowledge, what are the pros and cons of Up (vehicle based) vs Down (Infrastructure based)?

Up Pantographs

The first Up pantographs for buses were off-the-shelf rail pantographs that Opbrid

adapted for use in 2010. Since then, the Schunk group has developed several newly

developed versions of Up pantographs just for buses, culminating in the SLS101

shown below in the photographs. It is a new, relatively lightweight (approx. 70kg)

non-articulated pantograph with a special connection head that mates with a V

shaped receptacle on an overhead pole. The system has four contacts, Positive,

Negative, Earth and Pilot.

Down Pantographs

In 2014, Volvo and Siemens unveiled their “Inverted Pantograph” (Down) style

system in Hamburg, Germany. It consists of an articulated Stemman Technik

pantograph that descends from the overhead pole to meet two lightweight (approx.

15-20kg) parallel current collector rails on the roof of the bus. The rails on the bus

are separated in the center by an insulator, to make a total of 4 contacts, Positive,

Negative, Earth and Pilot.

We at Opbrid, while supporting the “Volvo compatible” system, also have our own 4

in-line contact Down pantograph system that takes up very little room on the bus

roof, an important concern with so much electronic equipment being mounted on bus

roofs today (Figure 6).

In the United States, Proterra has been using their own proprietary Down system for

a number of years with good success.

Both Up and Down systems have been shown to work well in revenue bus systems,

and basically achieve the same purpose with equal ability, namely, fast charging of

the bus batteries at the end station of a bus route. However, they are obviously

completely incompatible, a bus with an Up pantograph cannot charge at a charging

station built for a Down pantograph, and vice versa.

A Bus Industry Standardization organization group has been started to create a

interim industry standard for the mechanical, electrical, and communication for fast

charging. At this time, the two options being considered by this group are the

Volvo/Siemens developed “OppCharge” Down pantograph system, and the

Fraunhofer/Schunk developed SLS101 Up pantograph system.

A bus operator or city needs to choose one or the other, so I’ve tried to summarize

the pros and cons of each in this paper to help in this decision.

Figure 1 Volvo/Siemens OppCharge Down Pantograph Station, Gothenburg, Sweden

Figure 2 Volvo/Siemens “OppCharge” Down Pantograph Detail, Volvo Electric Hybrid

Figure 3 BYD bus at Busworld 2015 with Fraunhofer/Schunk SLS101 Up Pantograph

Figure 4 Detail of Schunk SLS101 Up Pantograph

Other systems on the market:

Figure 5 Proterra Charging Station California, USA

Figure 6 Opbrid Busbaar V3 in-line inverted pantograph system, Hybricon Bus Systems, Umea, Sweden

Pros and Cons Up (on vehicle) Vs Down (on infrastructure) Pantograph Systems:

Weight and cost on the vehicle: Definitely better for Down. There is only a simple, non-moving current collector is on the vehicle. Weight savings for Down are around 50kg, almost one more passenger.

Size on the Bus (roof real estate): Down generally takes up less room than an Up pantograph since the current collectors are relatively small and fixed. In particular, the Opbrid in-line Down collector takes up almost no space on the roof.

Overall system cost: Usually better for Down, since there are typically 5 to 10 vehicles per charging station. A single inverted Down pantograph vs 5 to 10 Up pantographs.

Standardization: Standardization is underway for bus charging systems, CENELEC wishes to have a standard by 2019. An equivalent SAE effort is underway in North America. As mentioned above, an industry group is working to provide an interim standard. Volvo has pledged that their “OppCharge” Down panto system will be "Open Architecture". http://electrichybrid.volvobuses.com/pdf/Opportunity_Charging_System_FS_EN.pdf At this time no Up panto system has made this pledge that I know of, they are so far proprietary.

Reliability and Maintenance: Down is arguably better. A Down panto can be made very strong and robust since weight is not an issue. A Down pantograph also is not subject to the vibration and shock of being on top of the vehicle. Overall maintenance costs are reduced since there are much fewer pantographs overall, and maintenance and repair is shifted to the infrastructure owner/operator (often an electric utility). This reduces the maintenance burden on the bus operator.

Flexibility and working range: - Both Up and Down pantos can generally handle almost any height vehicle, from single deck buses to double deckers. However, it would be very hard to mount an Up panto on a double decker because the maximum bus height would likely be over limit, and likewise an Up panto would be somewhat difficult on a midi or mini bus because of the large size pantograph needed to reach the fixed infrastructure at over 4.7 meters from the road. This is not a problem for down pantographs, they can have a large working range with no penalty. Overall System Reliability: This is where you hear the argument for Up pantos. The reasoning is that a pantograph failure on a Down panto will affect the entire bus line, whereas a single Up panto failure will only affect a single vehicle. This has some merit, but isn't as definitive as you might think. First, the Down panto (infrastructure side) can be made more robust than an Up panto, with very low failure probability, as they are based on very robust rail pantograph designs. Secondly, an Up failure might

very well occur during charging, also bringing down the entire line due to the vehicle being stuck at the station.

My guess is that overall system availability will be similar between Up and Down, especially since the power electronics and grid connection can also fail, and are the same in both cases. Finally, a new Down panto can be replaced reasonably quickly by the charging station operator in case of pantograph failure.

Parking tolerances: This is similar for both systems with some advantage for the Up pantograph in the front to rear direction. However it should be noted that the sideways effect of kneeling is more pronounced for an up pantograph. The head of the up pantograph will move considerably to the side if the bus kneels, but not nearly so much for the down pantograph.

Aerodynamics: Much better for the Down panto, since there is just the fixed current

collector on the vehicle. While not much of an issue for slow speed urban buses, this

can become an issue with medium and long distance buses, or urban buses that

travel a lot on highways.

Safety: Safety is similar for both, everyone uses a 4 contact system based on the

CCS (Combo 2) standard, in general it is impossible to connect incorrectly due to the

geometry of the contacts.

Contact Order: The IEC61851 specifies that the protective earth contacts first,

followed by the power contacts, and the pilot last. This can be assured in both

systems.

Power outage or malfunction at charging station: Up pantographs can get stuck

in the up position, but the vehicle can still drive away from the station and park

nearby to wait for service. If a Down pantograph is stuck in the down position, it

could pose a hazard for tall trucks and other buses. Therefore, Down pantographs

are built with spring loaded automatic retraction in case of a power outage or other

malfunction. If the power goes out, the pantograph will raise automatically.

Fast operation - this is part of the "secret sauce" for each charging station vender

that they compete on, so this will end up being similar in both up and down.

Noise: A quiet connection is highly prized by customers, and is similar in both up

and down.

Power handling: It is easier to construct a very high power down panto than an up

panto, since the heavy cables and construction are on the infrastructure rather than

the vehicle. For example, the Opbrid Busbaar 650kW Down panto system in Umea,

Sweden handles over 900A, with a relatively small current collector on the bus.

Infrastructure size and appearance: The up pantograph infrastructure is generally

smaller and more compact, since the pantograph mechanism is on the vehicle. This

can be disguised somewhat for the down panto using clever design, but is clearly an

advantage for the up pantograph at this time.

Communication: A point in favor of Up pantos is that a wireless data connection is

not required which makes things a little less complicated when connecting, however

this is not a big issue, and has been demonstrated to work properly in a number of

pilots. Security of the wireless connection has been addressed.

Fouling by Dirt and Debris: Some say that the down pantograph has an

inconvenience, if the roof is dirty because of falling leaves, for example, it could be

difficult to have a clean contact. Our experience has not shown this to be an issue so

far. The non-moving contact surfaces for down pantos are generally mounted at

least 15cm above the roof, and are 50mm wide - very similar to the contact surfaces

on the up pantographs we have used since 2010. We have never had problems with

fouling. However, the converse is true, that the mechanism of a roof mounted

pantograph (up) is not as easy to clean as the non-moving contacts of a down panto

system, either manually, or in a bus wash.

Ice and Snow: The fixed contacts on the Down system can be heated to eliminate

this problem completely. It is possible that heavy ice and snow build up could be a

problem with Up pantographs since the raising mechanism is a spring, and too much

ice could possibly weigh it down from raising properly.

Automated Depot Charging: Both systems can be used for an automated depot

charge system. However, in this case the economic advantage of the down panto

over the up panto evaporates, since you will generally need one pantograph per bus

in either case.

In conclusion, both Up and Down pantographs have their pros and cons, and either

can be successfully used in opportunity charged bus systems. Indeed, it may not

matter that much either way, since buses generally run within a local transit system,

so that one city can use one system, while its neighbor uses another. Re-sale value

may be affected if the charging connection needs to be changed to sell a bus to

another city.

Schunk actually makes both up and down pantographs, and have pledged to make

the mounting bracket the same for both, so that a city can switch from one to the

other relatively easily. Note that some bus OEMs mount the pantograph over the

front tires, and others over the rear – this can make it difficult to switch if the charging

bus stop is built to accommodate one position or the other.

While this may look like a critical decision, please don’t let it hold up your bus

electrification project! Both the infrastructure part and the bus part can be changed if

needed in the future. The most important thing is to get more electric buses on the

road!