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236 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
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9.5 DCE-SG
So far, DCE-SGs have been applied in large wind energy conversion units mainly as direct-driven
generators. However, their usage in 1G or 3(4)G transmission drives may also show merits such as
• Elimination of PMs (or reduction of PM weight/kW by hybrid excitation [8])
• Opportunity to use only a diode rectifier in the machine side, where the DC overexcitation
current controller alone provides the power flow to a common DC voltage bus of a wind park
Although the superconducting SC–SGs have been proposed for up to 10 MW (direct driven) [8],
they may be more suitable for 1G transmission drives in order to allow a reasonable frequency in the
generator and the power converter. Superconducting SC–SCGs have been built with iron-back core
(operating frequency 2.2 Hz) and with air-back core (operating frequency 0.833 Hz), giving an effi-
ciency of 96.4%. The SC–SG does not allow any no-load voltage control. It may thus be controlled
as a PMSG with very low internal inductance.
In what follows, we will treat, in short, the DCE-SG:
• The phase circuit model for steady state
• Optimal design for unity power factor for 8 MW, 3.6 kV generator
• The dq model for transient simulation and analysis
• Vector control of the machine-side converter

The steady-state phase circuit model of a DCE-SG is rather standard:

*
iR s + V s = − jωψ s ; ψ s = Li + Li + Li (9.31)
r
dm F
q q
s
d d
d (
pL i F +( L d − ) ) ,,
d
q
T e = 3 1 Li i q
with a phasor diagram as shown in Figure 9.23.
For simplicity, the additional PMs between rotor poles, as shown in Figure 9.23a, are not con-
sidered in (9.32). As the equations are written for the generator mode, the active power and torque
are positive:
1 ω 3
d (
2
2
P e ≈ 3 T e > 0; Q e =−3ω 1 L ii d − ω 1 Li d + Li q) (9.32)
dmF
q
1 p 2
f
Additional
magnet
N –R I cos =1
jω ψ s s
I f 1 s
pm θ V —power angle
V s
NS
θ V
ψ
I s I q s L q I q
L I
d d
I
(a) S (b) I d L dm F
FIGURE 9.23 DCE-SG: (a) topology layout and (b) phasor diagram.