Each coil will be independently fed by a dedicated converter, rated for 300 A and 240 V, which has to produce the required current/voltage following in real time the reference provided by the JT-60SA MHD Controller.
The effectiveness of the simulation is confirmed by implementing the system and carrying out grid connection tests, obtaining efficiencies over 98% for values over the 30% of the bidirectional converter rated power.
A large-capacity low-speed flywheel energy storage system (FESS) based on a doubly-fed induction machine (DFIM) consists of a wound-rotor induction machine and a back-to-back converter rated at 30 35% of the machine power rating used for rotor excitation.
As P conv denotes the converter rating, therefore P = P_{conv} + P_{ol} = alpha_{ol} P_{conv} (5b).
In (5a), P is expressed in percentage of the converter rating and the overloading duration Tol,lim is in second.
The worst case is when the dispatched power has already reached the converter rating, i.e. P D = P conv.
Considering the intermittent nature of many renewable energy based DG units, the ancillary services of DGs using the available interfacing converter rating are also discussed in the paper.
This would be possible provided that the sum of the net level of P D and the frequency regulating power component, denoted as P d,f in Fig. 2, does not exceed the discharging converter rating.
Therefore, it can be assumed that the NaS BESS maximum discharging/charging power level is much greater than the converter rating, if the discharge/charge duration of interest is no more than 30 s.
This is particularly true when considering that most renewable energy based DG systems are not always operating at their rated power, and the available converter rating can be used to facilitate grid frequency and voltage regulation, harmonics control, unbalance voltage compensation, etc.
