By Phil Kreveld
Learnings from the Australian Protection Symposium, 17-18 September, presentation by Maja Knezev, DEL Engineering.
The recent Australian Protection Symposium shed some light on the challenge the renewable transition is providing in the operation of transmission lines, the more so as they stretch to 300km or more.
The engineering for voltage control for transmission lines were part of DEL Engineering’s task in which they had to meet requirements of Transgrid. In outline, the presentation was about a logically controlled switching scheme for reactors and capacitors providing VAr compensation in conjunction with a synchronous condenser.
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The capacitor and reactor banks are integrated with the excitation control of the syncon to provide reactive power under both slowly varying and highly variable load conditions brought about by the participation of renewables in the energy mix.
In the parlance of energy summits, political discussions and disputation, to which add the mantra ‘no transition without transmission’, transmission lines are presented as extension cords. No matter how distant the renewable energy zone, the transmission line connecting it to the energy consumption point will perform its task—or so is the assumption.
But things are not that convenient.
By the time energy is consumed at distribution network zone substations, power factors (i.e., ratio of megawatts/megavars) is close to unity, but even at a high ratio of 0.95 the reactive power is approx. 30% of the power delivered. In terms if the future in which synchronous generation will have largely disappeared, the supply of additional reactive power will fall to inverter-based resources (IBR).
Generally, the semiconductor switches of IBR can handle currents somewhere between 50-100% over their rating, although for limited times. Transmission lines also require reactive support in order to provide voltage control. This is almost conveniently, it seems, lost track of.
Short lines of less than 100km are mainly inductive as far as VAr compensation is concerned, whereas longer lines have a charging current component due to the capacitance of the line becoming predominant.
Terminated, loaded lines have both charging and inductive current components adding to the current associated with power transfer. Consequently, voltage control at substation level has to become more complex.
The DEL Engineering solution is, in essence, one that meets both rapid responses required by very fast load changes and longer-term reactive power variation requirements. The synchronous condenser provides the rapid response factor and the reactor and capacitor banks provide VAr components between a lower and upper threshold.
The control scheme utilising SCADA provides for the synchronous condenser to minimise its energy consumption. To prevent hunting, there is a lower and upper reactive level, in between, which the synchronous condenser operates. The capacitor and inductor banks operate outside the two levels of leading and lagging VAr.
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In the break following the DEL Engineering presentation, there were comments made by delegates on the likely shortfall of reactive support as synchronous generation disappears with the shutdown of coal fired generators. It leaves an unanswered question; should there be a market for voltage support and, in general, reactive power?
As matters stand, costs associated with system strength and voltage control are the responsibility of transmission operators—but not always. At times costs have to be shared between connecting generators and the transmission operator. There are views in the industry that generators should not be held responsible and that reactive power provision should be the responsibility of transmission networks.
