Page 69 - Energize October 2022

TECHNICAL

It is clear from the equation that the voltage rise factor is costs, primarily converter stations. The insulation requirements
proportional to the square of the line length. Like the skin for DC cables are lower than for AC and hence cable costs are
effect and the proximity effect, the Ferranti effect also has lower.
undesirable effects on the power system. Since the receiving- DC transmission systems are normally associated with
end voltage becomes greater than the sending-end voltage, the overhead lines, but cable-based DC systems have been
loads at receiving-end side, which are designed based on the developed. Overhead medium voltage and low voltage DC
nominal value of voltage, i.e., the sending-end voltage, could be systems are in use in distribution networks and could be
damaged. Similarly, the rise in voltage would cause unnecessary extended to underground cable networks in cases where AC
losses which affect the performance of the cable. From the cable systems are being considered. DC cable systems have been
protection point of view, circuit breakers which are designed for installed in a number of countries, and low capacity DC, such as
the sending-end voltage level, could operate when sensing the DC light, has been developed to serve the distribution sector.
rise in voltage.
The Ferranti effect occurs on lightly loaded cables, a situation Optimising cable voltage
that can arise with a high penetration of distributed energy (DER) An innovative solution is proposed by Verana [4]. Since the
systems in distribution cable systems. The impact of this on charging current is dependent upon the voltage, operating
voltage regulation along the length of the cable does not appear the cable below the rated voltage would reduce the charging
to have been studied as yet. Excessive voltage rise could also current. Using the method proposed, an optimum operating
affect connected inverter based systems. Under reverse current voltage could be determined. Distribution voltage is considered
flow, the Ferranti effect could affect the voltage at the cable optimal, when it enables maximum power transfer capability.
connection point. This approach takes the point of view that the rated voltage
is not the operating voltage but is the upper boundary for the
Mitigation operating voltage.
There are a number of ways of mitigating the effect of charging
current, some practical and others a little more creative. References
1. NEI: “Underground vs. Overhead Transmission and
Shunt reactive compensation Distribution”, https://www.puc.nh.gov/2008IceStorm/
Inductance may be added at the ends of the cable or at ST&E%20Presentations/NEI%20Underground%20
intermediate points, to counteract the effects of capacitance. Presentation%2006-09-09.pdf
Where transformers are involved at the ends, additional 2. K Daware: “Capacitance of underground cables”,
windings may be provided to insert the necessary reactance. Electricaleasy.com
The effect of shunt reactance is to decrease the reactive current 3. G Deb: “Ferranti effect in transmission lines”, International
flowing in the circuit and thus allow a higher load current to flow. Journal of Electrical and Computer Engineering (IJECE), Vol.2,
Ideally, full compensation would be beneficial, but this is avoided No.4, August 2012, pp. 447~451
because of the possibility of resonance. 4. T Verana: “Optimal transmission voltage for very long HVAC
cables”, SINTEF, https://www.sintef.no/en/
DC distribution systems 5. S Bilgic and C Bardak: “The Exploration of Capacitive Effects
DC distribution is gaining popularity, and one of the advantages is of XLPE Cables in Medium Voltage Distribution Systems”, JST
the mitigation of the effects of cable reactive components. Long 2021 (14)
underground DC systems are possible. DC distribution does not
suffer from the same problems as AC, but has its own additional Send your comments to rogerl@nowmedia.co.za

SUBSCRIBE TO THE ENERGIZE NEWSLETTER:

e/
sub
scrib
https://
https://www.energize.co.za/home/subscribe/
www.energize.co.
za/home/

energize | October 2022 | 69