Technology

It may be helpful to provide some brief explanations of terms used in this section.

Note this is NOT intended to be a technical explanation for engineers. If you are interested in a deeper technical discussion please contact us.

  • A capacitor is generally understood to be a passive two-terminal electrical component that stores electrical energy in an electrical field[1].
  • Reactance is the opposition of a circuit element in an AC circuit to a change in current or voltage. It has a similar effect to that of electrical resistance and is a form of impedance (that is, it impedes the transfer of power through a cable).
  • The power factor of an AC electrical power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit and it is a dimensionless number in a closed-loop between 1 and -1 [2] & [3]. A power factor of 1 is desirable as it delivers more power to the load.
  • Capacitive coupling to earth can be described as a leakage of AC electrical energy through a solid object such as insulation. It is a major cause of losses in undergrounded cable.

CTS is a newly invented form of electrical transmission cable, designed to transport electrical energy within T & D grids to consumers.  It uses a unique and patented geometry which produces much lower voltage drop than conventional cable and wire.

CTS, as a new concept in low-loss cable based on capacitor technology that is expected to enable electric power to be transmitted with extremely low reactance, and with a power factor near unity (i.e. high proportion of useful “active” power required by most electrical applications), could address all of these issues should the successful development work to date continue on a similar trajectory. 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 lower losses 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 is planned to determine this.

Data from testing using high frequency to emulate greater cable length

Using high frequency testing, the performance characteristic of a much longer length of cable can be emulated. Thus, for each 50Hz increment in the graph above, the effective length of the cable increased by 1,000 metres.

Voltage drop over distance is unhelpful, since it means that less power can be transferred from the generation side to the consumption side leading to poor performance of motors, dimming of lights and potential “brown-out”. With reduced voltage a given load will draw greater current which in turn increases losses due to heating.

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 expensive ancillary reactive control components. This makes CTS a potential alternative to HVDC for subsea links.

Current design concepts for a commercialised CTS expect it to behave similarly to HVDC cable but without the costly 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 much improved transmission efficiency with a lower capital cost.

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

Design

CTS is presented as a linear capacitor, formed of two electrodes comprising a separate supply circuit and a load circuit.

 

A simplified circuit diagram showing a supply, the CTS and a load

  

A cross sectional representation of the Type III cable. In this case, each of the individual bundles comprises 6 upstream electrodes (connect to supply) and 6 downstream electrodes (connected to load. Each bundle is composed of twisted, insulated wires and then the bundles are twisted as a group to form a twisted cable which is then sheathed according to applicable international standards.

The illustration above shows the electrodes of a length of CTS. The electrodes are enclosed by insulation (known as a dielectric layer) and in production the cable is sheathed externally for protection and reinforced as required for mechanical strength.

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, i.e. resistive cable or overhead lines. These are discussed below.

Experimentally Demonstrated Properties of CTS

The Company has so far tested 130m of CTS that was wound on to spools at the contract manufacturer’s factory, 580 metres of linear CTS in planar form [Type 1] (made in the Company’s own facility) and 1,000 metres of linear CTS in annular form [Type II] (made by a commercial cable manufacturer).  The Company has just commenced a testing programme on a new cable geometry [Type III] shown in Figure 7 above. The Enertechnos Board believes the experimental data derived to date support the statements below though it intends that significant further testing will be conducted to achieve a greater level of confidence.

Transmits active power

The conventional expectation with a capacitor is that in AC it will transmit only reactive power due to a phase shift of 90° between voltage and current. However, using CTS to transfer power a power factor in excess of 0.99 (where 1 is perfect) can be achieved and 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. Active power is used to generate work (for instance, of an engine) or heat. Reactive power has a real impact on the network as it leads to heat loss, overload in distribution transformers, overheating of power cables and voltage drops.

Power increases with voltage

As the voltage at which CTS is used increases, so does its capacity to carry power.

Power increases 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; thus the longer the cable the more power it can carry. This is significantly at variance to conventional ohmic cable.

Voltage drop reduces with increased capacitance

As the capacitance of CTS increases, the voltage drop percentage across CTS decreases. The capacitance of CTS is a function of the number of layers in each electrode and the length of the cable. Thus, as the CTS increases in length,  its capacitance 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 may make planning and routing easier.

Current limiting

When power is supplied via a conventional conductor and a downstream short to earth occurs, the conductor will overheat and unless protection equipment intervenes, the conductor will rapidly melt. This is the mechanism of a conventional fuse whereby excessive current causes the fuse to melt, thus disconnecting the circuit and protecting the infrastructure. By contrast, the CTS continues to carry current up to its maximum current carrying capacity based on the equation: , where I is current, f is frequency, C is capacitance of the CTS and V is the supplied voltage.  This behaviour has been observed in experiments at Brunel University and in the Company’s test sites. Circuit protection equipment must respond in milliseconds because the existing lines and other components lack resilience. CTS could extend temporary current limitation to the system for several seconds, in a situation where protection equipment today must respond in milliseconds. Further testing is planned to explore this.

[1] Alexander, Charles; Sadiku, Matthew. Fundamentals of Electric Circuits (3 ed.). McGraw-Hill.

[2] Authoritative Dictionary of Standards Terms (7th ed.), IEEE, ISBN 0-7381-2601-2, Std. 100.p. 206.

[3] Trial-Use Standard Definitions for the Measurement of Electric Power Quantities Under Sinusoidal, Nonsinusoidal, Balanced, or Unbalanced Conditions, IEEE, 2000, ISBN 0-7381-1963-6, Std. 1459-2000. Note 1, section 3.1.1.1, when defining the quantities for power factor, asserts that real power only flows to the load and can never be negative. As of 2013, one of the authors acknowledged that this note was incorrect, and is being revised for the next edition. See https://powerstandards.com/Shymanski/draft.pd