Alternative cable technology for shore connection charging

A cable is not just a cable. Also this area has technological development that can benefit future charging solutions. This literature review is written by Jeewan Khadka, research assistant with UiT in Narvik, summer 2019.

Introduction

The concept of electric vessels is emerging and several studies on energy savings and emissions has recently been carried out.  There are numbers of vessels available operating on hybrid power, combining multiples source of power such as diesel with electric battery power to achieve reduction in fossil fuel consumption, carbon emissions and other pollutants. However, the complete minimization of the emissions is only possible with vessels operating fully on electric power. For an electric drive system to be “worthwhile” on a ship, batteries must become considerably more efficient. This could be main challenge to date, as the energy density of battery is still too low. That means the batteries cannot store enough energy in relation to their size and weight. Large oceangoing ships have to travel vast distances with one battery charge – the batteries for this are generally still too large and too heavy [1]. Furthermore, port needs a suitable charging infrastructure to supply the power to the ships.

Time available for charging of the batteries are dependent upon the operation routes and types of vessel. It could vary from a few minutes to several hours. For example, the world’s first electric vehicle ferry in Norway has been operating a regular scheduled service since 2015. The 80-meter-long “Ampere” sails back and forth over Sognefjorden between Lavik and Oppedal 34 times per day. Each time, it sails six kilometers, almost completely silent and with no exhaust gases. At each 10-minute stop, the ferry’s batteries are charged briefly and are charged completely overnight [1]. So, an automated and efficient charging mechanism could be beneficial for brief charging of the batteries during the short port callings.

From the beginning of the power supply systems, electrical cables have been used as means for transferring of electrical energy and are widely available on different sizes and types depending upon the purpose of use. The ships with electric drive system could be connected to the inland power system, which could be used for both charging of batteries and supplying ships loads provided necessary arrangements. This require suitable connection arrangements conveniently located on both the ship and the shore for power transmissions. The transmission cables from the source of the electrical power to the vessel must be designed to transmit the required power without over-heating the insulation that surrounds the conductor. The heat generated by losses in the conductor must be removed by conduction and possibly convection to an outside surface for radiation into space. If the temperature of the insulation exceeds a critical value the useful life of the material is sharply reduced [2].

Current trends

Shore Power System

During berthing of ships, electrical loads of the ship are supplied by the shore side power system. This phenomenon is also known as cold ironing or alternate marine power (AMP).  It helps in minimization of emission from the ship by deactivating the use of diesel generator sets and replacing them with the supply from the in-land electricity network. The article in [3] presents the current recommendations, standards and regulations for the design of, “shore to ship” systems. Figure 1 shows the schematic diagram of a shore power system, where a cable management system is used.

Figure 1 – Schematic of shore power system (Source: Cavotec)

The current trends of shore power system usually consist of Plug and receptacle systems for connections. Most of the leading global engineering groups working in the field provides connection solutions for shore power system for ships and ports consisting cable management systems with plugs and receptacles. Some of the shore power system offered by Cavotec is shown in figure 2.

Figure 2 – Cavotec interface equipment for shore power system

Automated system for charging of E-vessel

The automated plug in system (APS) used for charging of E-Vessel makes the connection and disconnection faster because of its automatic operation. This kind of system increases the charging time and are beneficial for the vessel with short port calls. Also, automatic connections eliminate the need for manual interfaces and minimizes the risk for personnel. Some of the current trend of APS system are described below.

ABB Robotic solution

This Robotic Arm is used to charge the HH Ferries Group’s two ferries, The Tycho Brahe and Aurora operating a 4-km ferry route between Helsinborg (Sweden) and Helsingør (Denmark). The robotic arm is installed on the port and a pre-docking procedure are based on 3D laser scanning and wireless communication between ship and shore. During the last 400 mm of the ferry’s approach the robot will reach out and pull the shore cable from the ship as shown in Figure 3. The cable reel releases the cable and the robot moves the connectors to the corresponding connectors below the robot. After the connection is made, the robot moves back to the home-position and the roll-up doors closes. The robot will reside inside its own building when not in use [4].

Figure 3 – ABB robotic solution for HH ferries (Source: Deif, “Tycho Brahe Case story”)

Cavotec APS-Vertical for E-Vessel

This Automated Plug in system is currently used for charging MF Ampere, world first Electric powered car ferry in Norway and Finland’s first electric car Ferry, ’Elektra’. Towered structure of the APS system allows for the descent of the plugs in to a shipboard receptacle system. The height of the plug is controlled by the cable winch that automatically adjusts for tide levels. It has a linearly actuating arms at the top of the tower that controls the plug’s distance from the ship. The receptacle is housed within a custom hatch affixed to the side of the ship’s hull that, when moored, opens to allow the plug to drop inside and connect [5] Figure 4 & 5 shows the details of the APS vertical.

Figure 4 – Cavotec APS vertical for E-Vessel (Source: Cavotec)

Figure 5 – Cavotec APS Vertical used for charging MF Ampere (Photo by Tore Stensvold, Teknisk Ukeblad)

Stemmann-technik FerryCharger

The Stemmann-Technik FerryCharger is installed to charge ‘MF-Ampere’ in combination with Cavotec APS Vertical. Unlike Cavotec APS, the FerryCharger uses a Pantograph charging mechanism as shown in Figure 6. The existing FerryCharger has a vertical operation range of 15 ft and horizontal range of 1.3 ft [5]. For the control of dynamic motions during charging it is equipped sensors.

Figure 6 – Stemmann FerryCharger installed with pantograph charging mechanism (Photo by Tore Stensvold, Teknisk Ukeblad)

Wärtsilä inductive charging

The concept of e-vessel charging is taken up a step further by eliminating the use of cable connection between the vessel and shore with the implementation of inductive charging also known as wireless charging. The world’s first wireless technology to charge an electric vessel sucessfully completed its test run on Sep 2017 [6]. This technology makes use of electric coils for transfer of power through electromagnetic inductions. The two coils are placed close enough such that electromagnetic field generated in one coil is converted into electrical energy through another coil. The Wärtsilä inductive charging solution can deliver up to 2.5 MW of power with an efficiency of at least 95% across about half a meter (50 cm) of open space. The unit requires an area of approximately 2 square meters or 1.25 MW/m2 [5].

The Robotic Arm carrying a charging inductive coil is installed on shore. Another coil, also known as receiving coil is mounted on the vessel to the side of the hull, over which the charging coil is placed. After the vessel is secured at the dock, the system activates, lines up the coils and commences charging in seconds. The existing technology operates at 690 V AC [5]. Figure 7 shows the Wärtsilä inductive charging solution.

Figure 7 – Wärtsilä inductive charging employed on MF Folgefonn

Selection of cables

From the technologies mentioned for shore power system and APS system for charging of E-Vessel, it can be seen that there is requirement of cables having exceptional flexibility and high tensile strength. The use of cables for harsh marine environment also requires the cables to be sheathed with special sheathing compound that provides high moisture, chemical and weather resistance.

The thickness of the cable is governed by current ratings and voltage applied. The energy is transferred either by use of high or low voltage. Also, for constant power level, the increase in current results decrease in voltage and vice versa. In AC systems the limit between low voltage (LV) and high voltage (HV) is set at 1 kV [7]. Low voltage connections and procedures are less regulated than high voltage solutions. Crews operating with HV systems are required to have high voltage permits. Furthermore, safety systems, precautions and procedures are more complicated, meaning more time is needed to establish a connection. Also, Low voltage high amperage cables are heavy and difficult to handle, due to the amount of copper needed. In many cases a bundle of lighter cables must be used to make them easier to pull and connect. The availability of flexible cables in high cross sections is also limited. In high voltage solutions the current is much less, whereas the voltage level requires more insulation. This in turn makes the cables less flexible [7].

Depending upon the power capacity typical system specs for different power requirements are given in Table 1, while a comprehensive list of shore power level and cost can be found in [8]

Table 1 – Typical system specs for different power requirements [8]

Power CapacityTypical specs
< 100 kW230/400/440 V - 50/60 Hz
100 - 500 kW400/440/690 V - 50/60 Hz
500 - 1000 kW0.69/6.6/11 kV - 50/60 Hz
> 1 MW6.6/11 kV - 50/60 Hz

The selection of cable for shore power is thus based upon the power requirements and system specs. Furthermore, as a general criterion the rated voltage of the cable must not be lower than the nominal voltage of the circuit in which it is used, and shall be rated for short circuit current that may appear at their location in the installations. IEEE recommended practice for marine cable for use on shipboard and fixed or floating facilities in [9] gives the requirements for single or multi-conductor, with or without metal armor and/or jacket. Rated voltages are from 300V to 35 KV, intended to be installed aboard marine vessel, fixed and floating offshore facilities and in accordance with industry installation standards and regulation of authorities having jurisdictions (AHJ).

Alternative cable technology

The cables used for shore power supply or charging of electric vessels at present are power cables comprising of number of conducting cores of copper and insulation that adhere marine standards. However, with wide use and advancement of E-Vessel, there could be requirement for fast charging technology. The case is similar for present day electric vehicle, where the advancement of electric vehicle has created need for the fast charging equipment, which are designed to work with higher continuous charging current. The higher current flow through the conductors results in generation of more heat. As a result, conductors between the charging equipment and E-vehicles are to be sized larger to match the higher currents draws. The case could be similar for coming generation of E-vessel.

Larger size of conductors would make the cable less economical and hard to handle. The amperage of the conductor could be increased by use of certain heat dissipation techniques embedded within cables and there have been several patents published that uses cooling mechanism within the cable in an effort to increase its current carrying capacity. So alternative cable technology to increase amperage of the conductor keeping the cable size smaller could be an interesting topic to discuss for its use in charging of E-Vessel. Some of these technologies are discussed in the following sections.

Fluid filled cables

The fluid filled cables were first introduced in 1920s, and development has been continuous ever since to meet the progressive demands for increase in voltage and current carrying capacity. The fluid filled cables were introduced to be used for very high voltage transmission system. During early 1970s first high voltage fluid filled cables of 525 kV were installed at Grand Coulee Dam. Originally known as oil-filled cables, the name was changed to fluid filled to consider the fact that most widely used fluids today are synthetic fluids [10] .

The majority of cooling system currently being used for thermal management of power transmission are based on active cooling methods, where in cooling fluid is circulated through the cables absorbing the heat generated by the conductor and then passed through a heat exchanger to dissipate the absorbed heat [11] . In [11] the authors present and discuss several patents covering cooling methods which are fluid-based, radox and phase change-based cooling.

Water-cooled and air-cooled cables in welding

The patent US20050006116 A1 describes about the power cable assembly of water-cooled and air cooled TIG (Tungsten Inert Gas), MIG (Metallic inert Gas) and plasma torches. In water cooled torch application, the cable is comprised of a flexible outer tube of hose for carrying cooling water from the torch head back to the circulatory reservoir, a copper cable of smaller diameter disposed within the conduit for electrically communicating the torch head with the wielding machine and a thin coating of plastic material encapsulating the copper cable. The smaller diameter of the copper conductor than that of surrounding insulation tube allow for coolant to follow in the channel formed in between the two components [11] [12]. Figure 8 shows a cross-section view of the water-cooled welding cable.

Figure 8 – Water cooled welding power cable (Redrawn from [12])

The water is pumped in the region (42) between conductor (34) and insulation (40) the heat from the conductor will be dissipated into the water. The conductive cable 34 is encased in a thin coating 36 of a flexible plastic material such as PVC and has a thickness of only about 0.008-0.015 of a inch to avoid insulating the conductive cable (34) from the cooling effects of water flow through the power cable. Projecting from coating 36 are a plurality of radial projections 38, preferably integrally formed with coating 36 [12].

Liquid cooled cables (LCC)

In patent US9,701,210 B2 describes about the assembly of charging cable for Electric vehicles (EV), comprising of charging conductor and cooling conduit. The first end of the charging cable is attached to the power supply. The cable comprising the charging conductor and a cooling conduit, each of which extends through the first end to the second end. A connector attached to the second end of the cable, the connector having a form factor corresponding to the charge port of the electric vehicle, where in connector include fluid hub that has inlet opening and outlet opening. The cooling conduit is adapted to convey the fluid that cools the charging conductor and the connectors [13]. A simplified overview are shown in Figure 9.

Figure 9 – Electric vehicle system with liquid cooled cable, patent US9,701,210B2 [13]

Applicant of the patent is Tesla motors and has recently launched its V3 Supercharging station with liquid cooled cables to the public at its Fremont factory with eight 250 kW stalls. The charger is capable to deliver 250 KW peak power as per the announcement of the company.

Similarly, in the race to develop faster charging system for electric vehicles, ITT Canon has introduced a new liquid cooled DC High power charging system as shown in Figure 10. It is a liquid-cooled connector, cable and cooling unit that enables charging at a level of 500 A at 1000V. The cooled HPC charging head uses Novec engineered fluid from 3M to cool in the IP protected design. These fluids are dielectric and non-flammable, making them advanced alternative to traditional cooling fluids such as mineral oils and water glycol, that can be used to cool EV charging cables. The cooling takes place in connector itself, as opposed to cooling only to cable. The coolant flows through the cable enabling very small diameter and then cools the contacts to control the temperature at the connection. This allows the cooled DC HPC charging station to maintain a unique temperature profile. The coolant goes from the cooling unit to the cable to the connector and back again. The charging system were supposed to be delivered from early 2019 [15].

Figure 10 – ITT Canon liquid cooled high power charging solution (Source: ITT-Canon-EVC-DC Liquid cooled brochure)

Similarly, RADOX® High power charging system from HUBER+SUHNER also installed charging system with integrated cooling system for super-fast charging of electric vehicles and trucks [16]. However, it has currently shut down its charging station due to a short circuit fault occurred in the plug-in test site Germany. The manufacturer reports that the incident involved a prototype cable and connector that differs in design from the final product that is installed at public charging locations.

The liquid cooled cables for fast charging of EV offers several benefits over conventional cables, where some of the major advantages are mentioned below:

  • Enhancement of cable amperage capacity
  • Liquid cooling allows cable to be more flexible while carrying the same amount of current
  • Reduction of cable weight by about 40 %
  • Fast charging technology

The use of LCC for charging at present is limited to fast charging of electric vehicles only. It is a very new concept and could have potential to be developed for the use in fast charging of electric vessels as well.

Super conducting cables

These are cables comprising superconductors as energy transferring medium. When the super conductors are cooled below their transitional temperature, they have zero electrical resistance. Super conductors has no power losses in the cables and possess higher current densities compared to that of conventional cables. However, the cost of construction and refrigeration of the materials to superconductive temperature is higher and often requires cryogens such as liquid helium or liquid nitrogen for cooling of the superconducting materials. Depending on the materials, they require cooling to temperature near -2690 C. While for High temperature superconductor (HTS), the cooling temperature are higher. A rare earth barium oxide (REBCO) as super conductive material needs- 1960C. The cable with REBCO is made by Karlsruhe Institute of Technology (KIT) and known as High-temperature Superconductor Cross Conductor (HTS CroCo) [17] . The HTS cables use tapes or wires made of superconducting materials as current carrying elements. Bi2Sr2Ca2Cu3O10 (BSCCO) with a critical temperature of 110K and YBa2Cu3o7(YBCO) with critical temperature of 92K are commercially available superconductors used in HTS cables [18]. The HTS were initially discovered in 1980s for which the German researchers Karl Alexander Müller and Johannes Georg Bednorz were awarded Nobel prize in physics for their work. The different cable types of superconducting cables are shown in figure below.

Figure 11 – Super conductive cables (Source: KIT)

Super conducting cables offer the potential to transmit currents of several thousand amps at voltage low enough to eliminate transformation stages. A paper in [19] presented the result of a study replacing 110 kV conventional cables with a 10 kV super conducting cable leading to simplified 110/10 kV substations in German city of Essen. The project called Ampacity including the superconductor cable in Essen now has been operational for almost 5 years, which was first commissioned a trial operation back in 2014. Since then, the world’s longest superconductor cable, measuring one kilometres in length, has been used to connect two transformer substations in the northwest of the city, being an integral part of Essen distribution network. The cables used ceramic superconductive material and reach the transitional temperature at -2000 C. In order to reach this temperature, the cables are sheathed in an insulating sleeve that carries liquid nitrogen [20] .

In [22] authors also present discussion on the possibility of using super conducting cables for Railway applications, space craft charging application and naval applications.  Where in naval application, they present the ideas of using HTS as de-gaussing cables instead of conventional copper cables. Here authors also summarize the worldwide developments of HTS cable projects. Which results shows existing use of HTS cables in the range of 1.3 to 138 KV and up to 1 Km of length.

The superconducting cables currently in use are found to be suitable for underground power applications and generally for transmission and distribution purposes. However, several researches are ongoing in the field to make the economic and flexible use of HTS cables in different applications.

Conclusion

The available trends of charging E-Vessel & including shore power system are discussed where conventional cables are used. Further developments on alternative cable technology available are presented. The study on several patents for use of liquid cooled cables along with current fast charging system based upon of liquid cooled cables for Electric vehicles are presented. It is found that for the fast charging of Electric vehicle, the use of liquid cooled cables is proven to be beneficial and similar technology for use in electric vessels can also be studied. As of now the fast charging for E-vessel are is just a concept therefore use of similar technology of cable can be beneficial.  Furthermore, it is found that the development on superconducting cables as of now is applicable for the underground transmission and distribution application but have potential to be used in other applications in coming future.

References

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