IEEE Electrification Magazine - September 2017 - 64

Magnitude (dB)

Output Impedance of LC-Input Filter-Magnitude

30
20
10
0
-10
-20
102

103
104
Frequency (Hz)
Network Analyzer

Estimation
(a)

Phase (°)

Output Impedance of LC-Input Filter-Phase

45
0
-45
-90
102

103
104
Frequency (Hz)
Network Analyzer

Estimation
(b)

Figure 13. An experimental source impedance measurement of (a) an

Magnitude (dB)

undamped LC input filter and (b) a network analyzer reference impedance.

25
20

Magnitude of Downstream Impedance
Estimation
Network Analyzer

15
10
5
102

103

104
(a)

Phase (°)

Phase of Downstream Impedance
50

Estimation
Network Analyzer

Adaptive Control-A Method to
Increase System Stability

-50
102

103
Frequency (Hz)
(b)

104

Figure 14. A comparison of (a) an identified Zload (resistor) and
(b) a network analyzer measurement.

voltage and current responses during identification,
showing that the perturbations do not exceed 10% of
their nominal values.
First, the method is applied to estimate the source
output impedance. The undamped input filter is connected to the converter input port, and the input voltage and
current to the converter are measured while perturbing the

duty cycle. After postprocessing, an estimation of the filter
output impedance is obtained. The result is compared to
a network analyzer measurement for validation, as
shown in Figure 13. A good estimation of the filter's main
characteristics is accomplished, including the resonant
frequency, resonant peak, inductor parasitic resistance at
low frequencies, and capacitor parasitic series resistance
at high frequencies. The parasitic resistance effects can
be seen particularly in the phase plots.
Second, the load subsystem input impedance is measured for the three different cases of Figure 11. For this
measurement, the load subsystem is connected to the
converter output port, and the output voltage and current are measured while the test signal is injected. An
estimation of the load subsystem input impedance is
obtained after postprocessing the measured data. In the
case of a resistive load, a 3-Ω power wire-wound resistor
was connected; the estimated load impedance is shown
in Figure 14, along with the corresponding network analyzer measurement as a reference. Notice that the wirewound resistor is resistive at low frequency and inductive
at high frequency, as expected. Figure 15 displays the
results obtained for the case of an LCR filter as load subsystem, properly capturing the series-resonant characteristics, as seen when compared to the reference. Lastly,
a CPL, implemented as in Figure 11, is connected to the
converter output. In Figure 16, the estimated load impedance is compared to the analytical model as a reference.
As expected from Figure 2, the small-signal impedance of
this nonlinear load is a negative resistance, having constant amplitude and constant 180° phase.
In each of the previous results, good matching
between the estimated impedance and the corresponding reference is obtained up to approximately half of
the Nyquist frequency (50 kHz). This is adequate for
control purposes, because the closed-loop converter
bandwidth typically does not exceed one-third of the
Nyquist frequency.

I E E E E l e c t r i f i c ati o n M agaz ine / SEPTEMBER 2017

In a power electronic-based distribution system, power
converter interactions may lead to reduced system stability. As a result, the performance of a standalone-stable
system may depart significantly from the desired characteristics when operating in an interconnected system.
Moreover, in dc systems, power converters are interfaced
with source and load subsystems having time-varying
equivalent impedances. An example is the dc distribution
system on the U.S. Navy all-electric ship. In such a system,
different reconfigurations may occur on the source and
load sides of a given converter during a mission (see Figure 3). A solution to improve dc system stability is to have
each converter adaptively tune its own local controller to
perform as desired, adjusting to changes in its operating
environment. This requires a controller capable of

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2017