Structure and Analysis

of the Z-Source MVDC Breaker

Keith A. Corzine and Robert W. Ashton
Creative Energy Solutions
95 Mill Road
Thornton, PA 19373

Abstract - The medium-voltage dc (MVDC) architecture shows as a disadvantage in systems where a common ground is
potential for future ship power systems. Components of the preferred. To address this, a completely new topology of the
MVDC system are fairly well established; with the exception of z-source breaker is presented herein. Analysis of this new
circuit breakers. The main problem with the breakers in dc breaker is presented along with simulation examples.
systems is the absence of a zero crossing in the current needed to
extinguish an interrupting arc. Options for breakers in MVDC II. THE CLASSIC Z-SOURCE DC CIRCUIT BREAKER
systems include over-sizing traditional ac breakers, hybrid
mechanical/semiconductor breakers, and solid-state breakers. The Figure 1 shows the schematic diagram of the traditional z-
recently introduced z-source breaker is most similar to the solid- source MVDC circuit breaker [1] which consists of an SCR, a
state breaker, but has the additional feature of automatically crossed L-C connection, diodes, and resistors as indicated in
responding to faults quickly and without the need for fault the dashed box. The system load is represented by the R-C
detection. Furthermore, the z-source breaker isolates the circuit consisting of Rl and Cl . A fault is depicted by the
generation source from the fault current. This paper presents conductance G f . The z-source L-C connection was initially
detailed analysis of the z-source breaker as well as variations on
the topological structure. The various breaker options are suggested as a novel type of inverter input circuit [2] that can
validated through detailed simulation. cause the inverter to operate in boost, as well as the standard
buck, mode. The reason for this is that the z-source allows
I. INTRODUCTION another state wherein the inverter can short-circuit its dc bus.
Herein, this feature is adopted for fault handling in MVDC
This paper explores the recently invented z-source dc power systems. When the fault occurs in this system, there is
circuit breaker [1] which utilizes a principle defined by the no direct short of the capacitor voltages, because of the
introduction of the z-source inverter [2]. A number of inductors in the z-source circuit. The breaker components act
researchers have studied the z-source inverter and documented together to quickly mitigate faults in a dc system. When a
results in recent publications pertaining to comparison with fault occurs at the output of a z-source breaker, current sources
existing topologies [3], operating mode analysis [4], and into the fault from the downstream system capacitance Cl as
variations on the topology [5]. By using a crossed L-C circuit
well as from the z-source capacitances as shown by the fault
arrangement, the z-source inverter introduced an additional
conduction path in Figure 2.
shoot-through state where all inverter transistors could be
gated on. The z-source breaker operates in the shoot-through
state during a fault; thereby avoiding excessive upstream
currents. Furthermore, the z-source breaker can automatically
respond to faults, providing fast isolation without fault
detection [1]. Forced commutation of the z-source breaker,
where a load can be switched off, is also possible so as to
allow coordination with other breaker systems. The
advantages of this new breaker make it an ideal candidate for
future naval ship power systems which employ the medium-
voltage dc (MVDC) architecture [6].
This paper first presents an overview of the traditional z-
source breaker. One of the characteristics of the z-source
circuit is that one of the inductors in the crossed connection Figure 1. The classic z-source MVDC breaker.
ends up in the return path of the dc source. This can be seen

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 Figure 2. Fault conducting path of the z-source breaker.
The full set of z-source waveforms during a fault are
shown in Figure 3. Therein, the variables are as defined in
Figure 1. The parameters and operating conditions for this
example are given in Table I with being the resistance in
series with the diodes. The system is operating in the steady-
state and the fault conductance is ramped from zero to 1 / R fo
in a time interval of Δt . A portion of the fault current will
come from the z-source breaker capacitances. Initially, in the
transient state, the inductor keeps the current iL constant as
seen in Figure 3. The conduction path is then through the z-
source capacitors and back to the source as shown in Figure 2.
Therefore, the capacitor current ic is seen in Figure 3 to Figure 3. Waveforms of the z-source breaker.
increase until it matches iL . At this point, iSCR will go to zero
III. THE NEW Z-SOURCE BREAKER
causing the SCR to commutate off. In this way, the z-source
breaker creates the zero crossing, typically absent in dc power Figure 4 shows a new structure of the z-source circuit
systems; but needed to properly isolate the fault current. A breaker. With this implementation, the z-source circuitry has
simple circuit can then detect that a SCR has switched off and been located entirely in-line with the power transmission. The
remove the gate current from the SCR. After the SCR advantage of this is that the ground connection is the same for
switches off, the z-source components are configured as two the source and all loads. For steady-state and low-frequency
series L-C branches connected to the load and fault. These operation, the load current takes the path through the z-source
circuits start a resonance where they are supplying the fault, inductors. When a fault occurs, the current takes the high-
but since the source has been disconnected and the fault frequency path through the capacitors as indicated by the
impedance is low, the output voltage collapses to zero. By conduction path illustrated in Figure 5. As can be seen, this is
KVL, with the output voltage at zero, vL must be equal to vC . in anti-series to the SCR forward current and, under the right
conditions, results in commutation.
In Figure 3, it can be seen that the inductor and capacitor
voltages become equal when the output voltage goes to zero. Figure 6 shows waveforms of the new z-source MVDC
Also, by KVL, it can be shown that when these voltages reach breaker with parameters and operating conditions shown in
half of the source voltage vs , the SCR will become forward Table I. The fault response is similar to the classic z-source
biased. Therefore, the time when vSCR is positive is the breaker with the exception of the resonance that follows fault
isolation. The next sections present analysis of the breaker
amount of time available for the control circuit to remove the operation.
gate pulse and the SCR to undergo its reverse recovery
transient. The resonance continues until the inductor voltages
attempts to go negative. At this point, the diodes will turn on.
The current in the capacitor will go to zero and the current will
continue in the inductor/diode/resistor loop until it decays to
zero. It can also be shown by KVL that since the inductor
voltage does not go negative, the SCR voltage will not go
above the source voltage. Figure 3 also shows the source
current iSCR immediately going to zero when the fault occurs,
as desired. After the SCR goes off, the fault has successfully
been isolated.
TABLE I. SYSTEM PARAMETERS. Figure 4. The new z-source breaker.
vs = 6 kV R fo = 20 mΩ Δt = 0.1 ms
L = 200 μH C = 125 μH Rd = 0.1 Ω
Rl = 6 Ω Cl = 1 mF

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 supplied by the capacitances as displayed in Figure 5. Since
the fault current is related to the output voltage by
i f = G f vo = K vo t (3)

The portion of fault current supplied by the breaker

capacitance and the load capacitance can be determined
using the current division rule. Then, the change in output
voltage can be found using linearization of the load
capacitance equation. From there, the currents in the system
can be computed [1] resulting in
Figure 5. Conduction path of the new z-source breaker.
vs C R K 2 3
i f = vs K t − t (4)
2Cl

vs C S C R K 2 3
ic = vs CS K t − t (5)
2Cl

vs C R K 3
iL = I SCR + t (6)
12 LCl

vs C R K
iSCR = I SCR − vs CS K t + ( 6 LCS K + 1) t 3 (7)
12 LCl

where
C
CS = (8)
C + 2Cl

2Cl
CR = (9)
C + 2Cl

Figure 6. Waveforms of the new z-source breaker. Using (7), the exact commutation point can be computed
in response to a specific fault. For example, using the
parameters and operating conditions of Table I, the time at
which commutation occurs can be calculated as 7.08 μs and is
A. Fault Interval
found to be 7.03 μs from detailed simulation. Equation (7)
The fault interval is defined as the time in which the SCR
current goes to zero immediately after a fault. This is depicted can also be used to determine the conditions under which
as interval A in Figure 6. Neglecting the SCR voltage drop commutation will occur and the amount of z-source
and the inductor resistance, the steady-state SCR current is capacitance required for commutation [1].

vs B. Z-Source Breaker Resonance Interval

I SCR = (1) Once the SCR has commutated off, the z-source circuit
Rl
appears as shown in Figure 7. At this stage, a resonance
At this stage of the analysis, the source inductance will occurs between the z-source inductors and capacitors. This is
also be neglected. The initial transient is based on the fault seen in Figure 6 and labeled as interval B. At the beginning of
conductance. For the purpose of this analysis, the this interval, each L-C branch has an initial current determined
conductance is assumed to ramp from zero to a final value by (1). The inductor and capacitor voltages in this interval
with a ramp rate of illustrate the resonance. As seen, the inductor voltage
resonates to a peak and then to zero while the capacitor
1 voltage rises from zero to a peak value. The resonance is
K= (2)
Δt ⋅ R fo distorted by the fact that the output voltage collapses at the
beginning of this interval. When the inductor voltage starts to
where is the time for the conductance to ramp to its final go negative, the diodes conduct the inductor current. Since
value which is the reciprocal of . For the first part of the the resistance in series with the diode is relatively small, the
analysis, it is assumed that the inductor current remains inductor voltage is practically held at zero.
constant. Then, the transient fault current takes a path

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 Figure 7. Breaker circuit during z-source resonance.

C. Source Resonance Interval

In the final interval, defined as interval C in Figure 6, the
inductor voltages are clamped at zero, the output voltage has
collapsed, and the capacitors have charged to a value equal to
the source voltage. At this point, the source inductance has a
significant effect on the circuit operation. Figure 8 shows the
equivalent circuit for this mode of operation where the source
inductance is denoted by Ls .
With the example shown in Figure 6, the source Figure 9. Z-source breaker waveforms with Ls = 10 μH .
inductance was negligible and therefore, the voltages are
relatively flat in this interval. However, if the source
inductance is increased, a resonance will occur in interval C.
Figures 9 and 10 show the z-source breaker waveforms for
cases where the source inductance is Ls = 10 μH and
Ls = 50 μH respectively. As can be seen, an increase in
source inductance leads to an increase in capacitor voltage
oscillations. This is significant in that the SCR voltage will
reach a higher peak value and this must be taken into account
when selecting semiconductor devices. For systems with a
high amount of source inductance, an L-C filter with damping
[7] can be placed at the input to the z-source breaker to
counteract this effect.

Figure 8. Resonant circuit due to source inductance. Figure 10. Z-source breaker waveforms with Ls = 50 μH .

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 IV. CONCLUSION REFERENCES
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