TRANSMISSION & DISTRIBUTION
Superconducting support for the grid31 May 2005
The first prototype AMSC superconducting SuperVAR synchronous condenser is currently undergoing trials at a Tennessee Valley Authority site in the west of the state. It is characterised by compactness and superior dynamic capabilities.
A letter issued by the North American Electricity Reliability Council based on an analysis of the August 2003 blackout cited the need to ensure appropriate levels of reactive power as the highest priority for transmission owners and operators. This was in the context of preventing major outages, protecting service quality and maximising transmission capacity by ensuring that grid voltage is properly regulated.
AMSC’s SuperVARTM synchronous condensers are designed to serve as reactive power "shock absorbers" for the grid, dynamically generating or absorbing reactive power (VARs), depending on the voltage level of the transmission system. They can respond quickly enough to protect against voltage transients – sags and surges – which can be created by lightning storms, short circuits through nearby tree branches or from animal contact, and other causes.
The first SuperVAR was installed in December of last year at a TVA substation serving a steel mill operated by Hoeganaes Corp in Gallatin, Tennessee. The machine will help stabilise voltage on TVA's grid by injecting reactive power to compensate for the sudden large surges in reactive power drawn by the steel mill during operation of its arc furnace. Developed by AMSC in collaboration with TVA, these condensers are built on the same high temperature superconductor (HTS) rotating machine platform as the advanced HTS ship propulsion motors AMSC is building for the US Navy.
The prototype was first started up operationally in January. Based on the results of the initial shakedown, the machine was dismantled, certain components were tested and an improved rotor design was created. The new rotor is currently being fabricated and factory tests of the advanced prototype were scheduled for June. This prototype is expected to be re-installed in the TVA substation and synchronised with the grid by the end of July 2004. Both parties to the development, TVA and AMSC, are reported to be pleased with the evaluation process. TVA has ordered five of these units, contingent on the successful evaluation and testing of the prototype unit.
A synchronous condenser is a dynamic VAR compensator in the form of a rotating machine that runs synchronised with the grid and has an architecture similar to a synchronous motor or generator (Figures 1, 2). Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed by the system. AMSC’s superconducting VAR condenser departs from standard technology by combining a coventional armature with a new power-dense field winding wound using HTS wire. The result is a synchronous condenser of higher efficiency without the penalty of high rotor maintenance costs usually associated with frequent field current changes. The machine can also provide up to 8 pu current for short periods to support transient VAR requirements during a fault.
The transmission of 'active' AC power is conventionally achieved by controlling the difference in angular phase between voltages at the sending and receiving points, which is feasible within wide limits, while the transmission of reactive power requires a difference in magnitude of these same voltages, which is feasible only within very narrow limits. Reactive losses become greater as the distance along AC lines between generator and load increases, and necessarily therefore so does the requirement for electric equipment capable of supplying reactive power. It is consumed by most of the network elements and by most of the consumer loads. And as reactive power cannot be transmitted very easily, it ought to be generated where is needed.
The provision of reactive power production near utility loads can increase revenues by enabling wheeling, and by allowing generation at more optimum power factors; and will economically stabilise systems in response to transients. In principle at least it should improve on the performance of power electronics, fixed capacitor and alternative rotating machine solutions conventionally deployed for this purpose.
Reactive power, the out-of-phase component of AC power, can be either lagging (current lagging voltage, corresponding to inductive reactance) or leading (current leading voltage, corresponding to capacitive reactance).
A SuperVar condenser is made to generate or absorb large amounts of reactive power ('MegaVARs') at appropriate locations on a power grid: it is designed for continuous, steady-state dynamic VAR support with minimum harmonic content while having multiples of its rated output in reserve for transient events. The first prototype has an 8 MVAR continuous rating and will deliver 8 pu peak current during a fault.
By utilising AMSC's HTS wire, SuperVAR machines incorporate a more efficient system (Figure 3) and should offer significant advantages, such as lower standby losses, higher output, lower cost and higher reliability, over conventional devices. Losses in the field winding are half those in a standard machine. In addition, the HTS rotor design increases the device's over-excitation and under-excitation output limits to its full-rating without loss of critical clearing time following a transient fault. SuperVar machines do not experience thermal fatigue of the field coils as the VAR output is varied from no-load to full-load allowing their use for peaking as well as base load applications without loss of life due to frequent load changes. These machines do not introduce harmonics into the grid, but do retain high operating efficiency (losses versus VARS) down to 20% of rated output.
The HTS field winding is maintained at an essentially constant temperature for all levels of field current and employs redundant refrigeration elements in the stationary reference frame. In a conventional machine the field winding current can vary from
1 to 3 pu, which changes conductor temperature. As a result, conductors expand enough to cause mechanical fatigue of the insulation. This is a major source of failure in contemporary machines
The commercial product
Initially AMSC is introducing the SuperVAR unit as a 10 MVAR product rated for 13.8 kV line to line voltage. The machine employs closed a loop cryogenic system and a brushless exciter. Although the prototype is rated for 8 MVAR it will operate at 13.8 kV; during a short-circuit fault at the machine terminals; it will deliver a short-circuit current of 8 pu (first peak) and an average of 6.5 pu over the first 5 cycles. This machine will also withstand a 2 pu current loading for up to 60 seconds. The machine is designed to operate in an ambient temperature of -30'C to +40'C in an outdoor installation.
It is expected to have a life expectancy
greater than 40 years and would have an efficiency advantage of >1% compared to conventional condensers. The machine has a foot print of 6ft x 2ft x 6ft tall and weighs 20 000 Ib. It can be pad mounted or installed in a trailer, Two cryocoolers with two compressors are employed, each compressor being 2 x 2 x 2ft and weighing about 2401b. Compressors are cooled with a close cycle water circuit while the SC stator and compressor are air cooled. The SC requires about 50 kW of auxiliary power and a suitable dry type transformer to interface with 13.8 kV station bus.
AMSC is offering a machine which it believes has superior transient performance permitting a long fault clearing time sufficient to cover almost all fault situations in a grid. Terminal voltage is recovered in about one second without the necessity for field forcing with a fast exciter. The machine is equipped with a protection system that allows it to withstand a terminal fault without damage to the machine.
It should enable existing transmission assets to be operated at higher capacities, reducing the need to install new power generating facilities or additional transmission lines in areas of increasing demand. It can replace old, polluting inner city Reliability-Must-Run (RMR) generating facilities which are required to operate in order to maintain system reliability and voltage support. Units of larger ratings are planned for the future.
SuperVAR grid performance
Damping of oscillations following a sudden change of load is shown in Figure 4. The machine has no dynamic stability limit within its MVA rating.
Figures 5 to 7 show recovery of the SuperVAR machine from 3-phase faults. Prior to the fault, the machine is assumed to be synchronised with the grid. A 3-phase fault is assumed on the high voltage side of the transformer (on the 0.063 pu line).
Figure 5 shows recovery from a fault on the high voltage side of transformer. The fault duration is 25 cycles, well over a factor of 3 more than the maximum time needed by a typical protection system to clear a fault.
Figure 6 shows the current during the fault and recovery periods. The machine draws about 5 pu current (1st peak) after the fault is cleared and returns to its pre- fault current level in about a second.
Machine terminal voltage during pre-fault, fault and post-fault periods is shown in Figure 7. The machine terminal voltage before the fault is a nominal 1.0 pu. Once the fault is cleared, the terminal voltage starts to recover and achieves its pre-fault level in about a second, without any change in the field excitation.