Instantaneous Voltage Regulated Seamless Transfer
Control Strategy for Utility-interconnected Fuel cell
Inverters with an LCL-filter
Guoqiao Shen, Dehong Xu
Xiaoming Yuan
College of Electrical Engineering, Zhejiang University,
General Electric Corporate R&D-Shanghai
Yugu Lu, Hangzhou, P. R. China, 310027
200233, Shanghai, China
xdh@cee.zju.edu.cn
xiaoming.yuan@geahk.ge.com
Abstract—This paper proposes a new control strategy for
the transition between grid-tied mode and off-grid mode of
the utility interconnected fuel cell inverters with an LCL
filter. During the transition from grid-tied mode to
stand-alone mode, the inverter control is switched to
voltage-controlled mode from the beginning of the transition,
using inverter instantaneous output voltage regulation to
reduce the output grid current of the inverter quickly, so
that the static transfer switch is turned off immediately and
the inverter maintain a continuous voltage output. It will
result in a short transition time and minimum voltage
fluctuation on the load. The algorithm is described along
with their transition performances. Simulations and
experiments on a 5kW fuel cell inverter prototype are
provided to validate the algorithm.
I. INTRODUCTION
Fuel cell generation now is regarded as one of the
most competitive energy source for the distributed energy
generations due to its advantages of higher efficiency,
lower emissions, and higher power density. Most of the
utility interactive fuel cell inverters have the capability to
operate in two modes, a) utility interactive mode for peak
shaving to reduce the overall cost of power by generating
during peak hours, b) stand-alone mode to provide power
during utility outages until service can be restored.[1]
In the case of sensitive and mission critical loads at
the user end, a continuous uninterrupted AC power is of
utmost importance. So it is very valuable for fuel cell
inverters to be able to operate in both grid-tied and
off-grid modes with seamless transitions between the two
modes. When the inverter is grid-connected, it is operated
in current-controlled mode. In the stand-alone mode, the
inverter is operated in the voltage-controlled mode.
Heretofore, many researches have made on these two
modes of inverters [2], but no enough attention has been
paid to the issue of a seamless transition between the two
modes. The control algorithm reported in [3] switches the
inverter
from
current-controlled
mode
to
voltage-controlled mode after the grid-disconnecting
switch (SCR) current goes to zero free-wheeling, so the
output voltage on the load is uncontrolled from the instant
of utility fault to the time of the first zero crossing of the
1-4244-0449-5/06/$20.00 ©2006 IEEE
grid current. To overcome the turn-off delay and reduce
the voltage uncontrolled time during the transfer, the
voltage amplitude regulation algorithm (VAR) and the
voltage phase regulation algorithm (VPR) are introduced
to reduce the grid current promptly [4]. However, they
both depend on the in-phase running for the grid voltage
and current, which in some case, will be of a problem.
Focusing on the seamless transition between
grid-tied and off-grid modes of operation, this paper
proposes a new control algorithm based on
voltage-controlled method for utility-interactive fuel cell
inverters with an LCL filter, the instantaneous voltage
regulation algorithm (IVR), which is introduced to
implement forced current commutation between the grid
and the inverter. The algorithm and its transition
performances are described and discussed in this paper.
Simulations results on a 5kVA single-phase inverter are
presented using PSPICE. The results show the feasibility
of the proposed approaches and the effectiveness of the
algorithms in minimizing voltage transients across the
inverter and loads, even at the moment of utility fault. A
5kVA DSP controlled inverter prototype has been made,
and the present control strategy is validated.
II.
TOPOLOGY AND RUNNING MODES OF
THE INVERTER
Fig.1 shows the system topology for the utility
interactive fuel cell inverter. The topology comprise a fuel
cell stack, a DC to DC converter, a PWM inverter, a
low-pass LCL filter and a grid-disconnecting transfer
switch (STS). Due to the advantages of short turn-off
time, large current capacity and low cost, SCR is selected
as the static transfer switch to enable the inverter
disconnect from the grid rapidly. The LCL form of
low-pass filter offers the potential for improved harmonic
performance at lower switching frequencies [5], which is
a significant advantage in higher-power applications, (e.g.
several hundreds kilowatt inverters). For sensitive and
mission critical loads at the user end, a continuous
uninterrupted AC power is required when the utility is
abnormal.
IPEMC 2006
L1
DC/AC
DC/DC
L2
i1
DC+
STS
ig
i2
Fuelcell
Booster
LCL-Filter
Disconnect
Load
the
static
switch
continuous uninterrupted AC power across the load,
-
quickly turning-off of the SCR and earlier voltage
L2
L1
PCS
Vo
i2
C
PCC
STS
controlling of the inverter output are required.
III. PRINCIPLE AND PERFORMANCE OF
Lg
TRANSFER STRATEGIES
Vg
Local
Load
VS
Grid
A. Control algorithm with grid current free-wheeling
(CFW)[3]
PWM
Gi
i2*+
by
voltage-controlled mode. In order to maintain a
+
i1
I2*
grid
inverter control method from current-controlled mode to
（a）Voltage-controlled stand-alone mode
DCBUS
the
for the SCR current to go down to zero. b) Change
Grid
PID
Vc*
from
control algorithm should ensure appropriate conditions
VS
PWM
Voltage
controlled
Grid
(SCR).Because SCR has no self turn-off capability, the
ig
C
Vg
-
Lg
PCC
i1
Vo
VS
-
System topology for the utility interactive fuel cell inverter
L1
PCS
DCBUS
-
Inverter
Fig.1
Local
Loads
Vo
~
DC-
+
+
+
C
Lg
PCC
For the transfer from utility-interactive mode to
-
stand-alone
Current controller
Sin
（b）Current-controlled utility-interactive mode
Fig.2 Schematic diagram of the converter in different modes
mode,
the
inverter
is
initially
current-controlled and the grid maintains the voltage at
the PCC. When the inverter starts to switch to the
stand-alone mode, the drive of the SCR is removed firstly.
Fig.2a shows the schematic diagram of the
The grid current is freewheeling through the SCR, as it
converter when it is disconnected from the grid. The
has no self turn-off capability. The grid is disconnected
inverter is operated in voltage-controlled mode. Voltage
until the line current goes to zero, and the SCR has been
feedback is used to regulate the voltage across the load.
turned off. At this instant, the PWM inverter is shifted
Fig.2b shows the schematic diagram of the converter
from current controlled mode to the voltage controlled
when it is grid-connected. The Inverter is operated in
mode. Thus there is no current spike during the transition.
current-controlled mode at this time. It is controlled
But in the case of a utility fault, the voltage across the
through current feedback to regulate the current injected
load may be dropped within half a line cycle (in addition
into the grid. The utility is assumed to be relatively strong
to the time required to detect the fault) due to the delay of
and maintains the voltage across the load.
switching inverter to voltage-controlled mode.
For the transition from stand-alone to grid-connected,
B. Control Algorithm with VAR and VPR [4]
it is relatively easy and well known as in the
With VAR transfer control, the grid current is in
uninterruptible power supply (UPS), so no more
phase with the grid voltage before switching, because a
descriptions
is given here. For the transition from
unity power factor is required. The drive of the SCR is
grid-connected mode to stand-alone mode, there are two
removed at the beginning of the operation mode transition.
essential work should be done for the control algorithm: a)
At the same time, the inverter is shifted to the
voltage-controlled mode with the desired output voltage
on the grid side inductance in despite of the waveforms of
Vo being lower or higher than the grid voltage Vs (Fig.3a),
the grid voltage and current. Fig. 4a shows the waveforms
introduces a voltage drop VL across the line inductance
of the grid voltage Vg, the corresponding desired output
and filter inductance L2, just being opposite to the grid
voltage Vo of the inverter for the proposed IVR transition
current Ig . So the grid current is forced to decrease
control, and consequently the voltage drop VL introduced
quickly. Once the grid current goes to zero, the SCR is
on the grid side inductance L2 by the IVR control.
turned off, and the output voltage is changed to the rated
Whenever the transition is carried out, and no matter the
value soon. Consequently, it takes less time to complete
grid voltage is distorted, the voltage across the inductance
the transfer, and the voltage transition across the load is
L2 is same because the inverter output voltage is
minimized (Fig.3).
controlled to follow the grid voltage with a constant
voltage difference. Fig.4b shows the waveforms of the
Vo
Ig
Vs
grid voltage and current and the inverter output voltage
before and after the transition. Before the transition, the
inverter was running in current-controlled mode as shown
in Fig.4b, so the inverter output voltage was nearly the
VL
same as the grid voltage. When the transition began at t0,
the inverter turned to voltage-controlled mode, and the
inverter output voltage was controlled to follow the grid
(a) Current and voltage waveforms during transition
L
INV
+
Vo
C
SCR
Lg
Ig
+ VL -
voltage with a fixed difference, as shown in Fig.4a, so
that the grid current fell down to zero at t1, after a delay,
at the time of t2, the inverter output voltage turned to
trace the standard waveform.
Load
Vs
Vg
Vg
Vo
(b) Topology of the utility-interactive inverter
Vo
Ig
Ig
Fig.3 Control algorithm with voltage amplitude regulation (VAR)
VL = Vo - Vg
For the VPR control algorithm, instead of the
voltage amplitude, the voltage phase of the inverter
output voltage is regulated according to the current
waveform, so a opposite voltage is introduced upon the
(a) Grid voltage and the desired inverter output voltage proposed for
grid side inductance to transfer the grid current to the
IVR control transition
inverter.
Vo
It can be seen that both VPR and VAR method are
depend upon the waveform of the grid voltage and grid
current, in-phase running are required, so they are limited
to be used. For the moment of utility fault, the waveforms
of the grid voltage and current are always distorted.
C. Proposed Control Algorithm with the Instantaneous
Voltage Regulation (IVR)
The proposed control algorithm is an improvement
of VAR. Here, the instantaneous voltage of the inverter
output is controlled according to the direction of the grid
current with a fixed voltage difference to the grid voltage,
so that a voltage opposite to the grid current is introduced
VS
Ig
t 0 t1 t2
V L = Vo - Vg
(b)Waveforms during the IVR control transition, the inverter output
voltage from t0 to t2 is controlled to decrease , as shown in (a)
Fig.4 Control algorithm with instantaneous voltage regulation
(IVR)
In this control algorithm, the grid current falling
time can be expressed as ∆t,
∆ t = t1 − t 0 = L
inverter output voltage will result in 1ms less falling
time of the grid current and a short transition period.
ig
VL
For a fixed VL value, the grid current falling time is
proportional to the instant current value. When the grid
current is given, then the grid current falling time under
different control voltage difference VL can be calculated,
as shown in fig.5a. For normal utility condition transition,
the maximum grid current falling time is occur at the
current peak, so it can be derived as:
∆t =
SIMULATION AND EXPERIMENT RESULTS
IV.
The simulation is realized by PSPICE on a
single-phase half-bridge inverter and a six-switch 3-phase
inverter, as shown in Fig.6a and Fig.6b. The rated power
of each phase is around 5kVA. The voltage of DC bus is
800V, the current injected into grid is 0~30A, the
switching frequency is 20 kHz. The parameters of the
filter are: L1=1.6mH, C=12µF, the grid side inductance is
assumed to be 10µH~2.0mH, including the additional
1
ωβ (1 − α )
(1)
inductance.
where β is the radio of grid voltage amplitude to the
voltage amplitude across the grid side inductance L2
L1
DC+
VS
VSM
=
VL ω L2 I SM
+ V L
i1
caused by the grid current in normal operating condition,
β=
L2
Vo
+
-
DC-
C
Local
Loads
STS
Lg
+ ig
Vg
-
+
VS
-
PCC
α=Vo/Vs is the amplitude radio of the desired inverter
output voltage to the grid voltage during the transition.
a) Schematic diagram of a single-phase half-bridge inverter
Current Falling Time DT(mS)
6
5
i= 2 4 A
i= 1 6 A
4
i= 8 A
3
2
1
0
10
20
30
40
50
60
V o lta g e o n th e G rid -s id e In d u c ta n c e
V L(V )
Grid current falling time
a) Grid current falling time under different voltage difference (VL)
b) Schematic diagram of a six-switch 3-phase inverter
Fig.6 Schematic diagram of grid-tied inverters for simulation
5
4
The voltage and current waves of the inverter during
3
the transition from grid-connected to stand-alone in the
2
case of a utility fault are shown in Fig.7. Where, Fig.7a is
1
under the traditional grid current free-wheeling control.
0
0.6
β = 20
0.7
0.8
0.9
1
Voltage ratio
β = 50
1.1
β = 100
α
1.2
1.3
1.4
β = 1 000
Fig.7b is under the voltage amplitude regulation control.
The utility fault takes place at the time of t0, and grid
voltage has fallen to 50% of the normal value. The
b) Maximum grid current falling time for normal grid condition
detecting time of fault is 1ms. The result shows that the
Fig.5 Grid current falling time vs voltage difference
duration of voltage drop under the traditional grid current
Fig.5b shows the maximum grid current falling time
free-wheeling control is comparative longer (7ms),
vs. the voltage phase difference between the inverter
whereas the duration of voltage drop under the IVR
output voltage and the grid voltage for different β values.
control is only 1.2ms.
The grid side inductance is 1.6mH. The grid current
falling time is decreasing as enlarges. For β=20, 15%
voltage difference between the grid voltage and the
Delta Power Electronic Technology and Education Fund,
and the support of GE (China) Research & Development
Center Co., Ltd.
a) Under traditional grid current free-wheeling control
(3_ Grid current, 20A/div. 4_Voltage on local load )
Fig.8 Experimental results for transition from grid-tied mode to
off-grid mode under IVR near current zero-cross
b) Under IVR control
Fig.7 Transition in the case of a fault on the grid
A 5kVA DSP controlled inverter prototype has been
made. The experimental results are shown as Fig.8 to
Fig.9. Fig.8 shows the grid current (curve1#) and the load
voltage (curve2#) under IVR when the transitions start
near
current
zero-cross
points.
Fig.9
shows
the
waveforms when the transitions start at current peak
points.
Whenever the transition is carried out, the
transition time is very short, and the voltage on load is
(1_ Grid current, 20A/div. 2_Voltage on local load )
Fig.9 Experimental results for transition from grid-tied mode to
off-grid mode under IVR at current peak point.
REFERENCES
[1] Coles, L.R.; Chapel, S.W.; Iamucci, J.J.;
continuous.
“Valuation of Modular
Generation, Storage, and Targeted Demand-side Management”,
V.
CONCLUSIONS
A novel control algorithm for the transition between
Energy Conversion, IEEE Transactions on , Volume: 10 Issue: 1 ,
Mar 1995 ,Page(s): 182 –187
grid-tied and off-grid modes of the utility-interactive inverters is
[2] O'Sullivan, G.A.;“Fuel Cell Inverters for Utility Applications”，
proposed. The principle and performances have been discussed.
Power Electronics Specialists Conference, 2000 , Volume: 3 ,
The simulation and experimental results show that the output
Page(s): 1191 -1194 vol.3
instantaneous voltage regulation algorithm can provide seamless
[3] Tirumala, R.; Mohan, N.; Henze, C., “Seamless Transfer of
transfers between the two modes for the inverter, avoiding the
Grid-connected PWM Inverters between Utility-interactive and
temporarily uncontrolled output voltage of the inverter，which
Stand-alone modes”, IEEE APEC2002. Volume: 2 , pp.1081 -1086.
occurs during the time of grid current free-wheeling under the
[4] Guoqiao Shen, Dehong Xu, Danji Xi, “Novel seamless transfer
traditional control algorithm. The algorithm presented in this
strategies for fuel cell inverters from grid-tied mode to off-grid
paper is valid even in the event of a fault on the grid.
mode”, APEC2005, March 2005, Pages:109-113
[5] Guoqiao Shen, Dehong Xu, Danji Xi, “An Improved Control
ACKNOWLEDGEMENGT
The authors would like to acknowledge the support of
Strategy for Grid-Connected Voltage Source Inverters with a LCL
Filter”, APEC2006, Session 29.1, March 2006