Capacitative transfer promises significant reduction in losses

Transmission losses in the UK equate to £1.1 bn annually and could account for as much as 7 425 000 tonnes of carbon dioxide. A project to create a practical transmission cable using capacitative transfer is currently being carried out by the cable’s inventor Enertechnos and cable maker Eland. If successful it promises to significantly reduce power loss and cut carbon emissions in the UK transmission and distribution sectors. With over 60 million km of power distribution cable in use in the UK, of which 75% is over 25 years old, this technology could, potentially, deliver huge efficiency gains as the network is updated.

The development effort is supported by a consortium of partners, including Eland Cables, Brunel University and The Welding Institute, in order to bring the concept to a practical application. It is also supported by Innovate UK, which has granted £1m towards the development costs.

Brunel University’s Institute of Energy Futures will develop a software simulation programme to test CTS on a virtual grid to demonstrate the energy savings across extended cable runs. The Welding Institute is providing materials handling, jointing and other practical installation assistance to ensure that the in-field deployment of this cable can be achieved in the same manner as existing products.

CTS exhibits several properties that it is hoped will allow it to transfer active and reactive power across distance with losses significantly lower than those of the current prevalent technology, that is, resistive cable and overhead lines.

Prototypes

Eland Cables engineers at its cable test laboratory (rated ISO17025 UKAS) are currently developing prototypes, blending the CTS technology with existing cable manufacturing processes to create a product that will deliver the performance while meeting the standards and long-term requirements of a buried power distribution cable. Following the prototype phase, the next stage will be the commissioning of a full manufacturing run.

The technology is already installed at a test site running 15 km of ‘standard’ cable at 33 kV alongside the existing installation to offer direct comparison.

The technology

CTS is presented as a linear capacitor, formed of two electrodes comprising a separate supply circuit and a load circuit (Figures 1 and 2). It employs a unique and patented geometry based on capacitor technology to produce a much lower voltage drop than conventional cable and wire owing to its extremely low reactance, and if the successful development work to date should continue on a similar trajectory it will be able to address most of the loss issues associated with current transmission practice.

Test data from tests of three different configurations of CTS indicate it should deliver electric power with lower losses and reduced reactive impedance over much greater distances than conventional cables, eliminating the need for many voltage booster transformers. Tests have strongly indicated that CTS can be undergrounded over longer distances than conventional AC cable with a lower loss rate than usually caused by capacitive coupling to earth. CTS may also be suitable in its unsheathed form for overhead transmission line installation. It may have lower electro-magnetic emissions than conventional cable as well as lower losses; future testing to determine this is planned.

Practical aspects

Undergrounding of conventional AC cables and running subsea cables is complicated owing to the capacitive charging currents, and significant investment in reactive compensation by capacitor stations is required, especially for long links and high-voltage links. With CTS, the capacitive coupling to earth can be engineered by controlling the geometry of the structure and the properties of the dielectric. In other words, the multi-layer structures of CTS provide an additional degree of control over the capacitive properties of the cable, and hence the capacitive coupling to earth. For this reason, CTS can overcome problems with line charging effects without requiring ancillary reactive control components. This makes CTS a potential alternative to HVDC for subsea links.

Current design concepts for a commercialised CTS suggest that it will behave like HVDC cable but without the inverters, rectifiers and filters that are required at each end of an HVDC line. These typically can account for as much as 40% of the total project cost. CTS therefore should, based on an extrapolation using the current understanding of the electrical effects being observed in research and development, combine the virtues of muchimprovedtransmissionefficiencywitha lowercapitalcost.

Based on analysis and observations to date, Enertechnos expects the cable to demonstrate other potentially valuable properties and the “platform” technology and patents are applicable to a number of other promising applications, although less research has taken place to date in these areas. Enertechnos is optimistic that CTS will continue to perform satisfactorily during the extensive further trials which are planned, including in-the-ground testing of a 15 km length of CTS carrying three phases at 33 kV, which in turn should help to corroborate the encouraging data collected so far.

Testing

To date, testing has been carried out on 130 m of CTS wound on to spools, 580 m of linear CTS in planar form (Type 1) and 1000 m of linear CTS in annular form (Type II). A testing programme on the newest configuration

(Type III), shown in Figure 2, is now under way. The experimental data derived to date suggest that although the conventional expectation with a capacitor is that in AC it will transmit only reactive power owing to a phase shift of 90° between voltage and current, using CTS a power factor in excess of 0.99 (where 1 is perfect) can be achieved. Active power has been measured when voltage is applied and a resistive load is attached. Inductive loads, that use reactive power, can also be driven using CTS.

Power increase with capacitance

As the capacitance of CTS increases, so does its capacity to carry power. The capacitance of CTS is a function of the number of layers in each electrode and the length of the cable. As the CTS increases in length, its capacitance increases. The capacity of CTS to carry power is directly proportional to its capacitance, so the longer the cable the more power it can carry. This is significantly at variance with conventional ohmic cable.

Voltage drop and capacitance

As the capacitance of CTS increases, the voltage drop percentage across CTS decreases. Because the capacitance value of a CTS line is a function of the number of layers in each individual wire and the cable’s length, as the CTS increases in length its capacitance value increases and its voltage drop percentage reduces. Mitigation of voltage drop is the reason for intermediate booster transformers in longer T & D lines. The ability to transmit power over longer distances at lower voltages could make planning and routing easier.