Predictive Control of Three Phase AC/DC Converters

UDC Filter
20 CHAPTER 3. CONTROL STRATEGIES FOR VSC
(a)
(b)
1.5
Discretized Step Response From:I dref
to I d
1.5
Discretized Step Response From:I dref
to I d
1
1
Amplitude
Amplitude
0.5
0.5
0
1 1.0005 1.001 1.0015 1.002 1.0025 1.003 1.0035
Time (sec)
0
1 1.0005 1.001 1.0015 1.002 1.0025 1.003 1.0035
Time (sec)
Figure 3.5: Step response of discretized active current control loop in VOC:
(a) without prefilter, (b) with prefilter
Figures 3.4 and 3.5 show step response of active current control loop without
and with prefilter.
For DC-link voltage controller design, inner current control loop can be represented
as a first order transfer function where Voltage Source Converter time
constant T I depends on inner current control loop.
T I = 4τ t (3.5)
Practical control implementation requires additional low pass filter (LPF) T fUDC
on measured DC-link voltage, which reduces voltage pulsations caused by transistors
switching.
T fUDC = 0.003[s] (3.6)
Control loop can be modeled as shown in Fig. 3.6.
1
+1 sTfU
-
KPU(sTIU+1)
UDCref UDCreff UDCerr Idref iload UDC
sTIU
1
sTIC+1
-
1
sC
Prefilter
PI VSC & Filter DC-link
Capacitor
1
sTfUDC+1
IDC ic
Figure 3.6: Block diagram of DC-link voltage control loop in VOC
UDCf
Symmetry optimum (SO) design method has been used for controller parameters
calculation, which gives:
K P U =
C
2(T I + T fUDC )
(3.7)