dual dc buses nanogrid with interlink...
TRANSCRIPT
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Dual DC Buses Nanogrid with Interlink
Converter
SEPOC 2018
Santa Maria – RS – October 2018
Graduate students and research collaborators of Luiz A.C. Lopes
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Outline
Introduction: From centralized to distributed generation.
Types of residential nanogrids: AC or DC?
DC bus voltages and number of buses.
Control strategy for the DC nanogrids
• DC bus signaling (DBS)
Control strategies for the interlink (LV-HV) converter
• Isolated and non-isolated high gain DC
• Two-state and tri-state modulation strategies
Fault detection and clearance in DC nanogrids.
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Introduction (1/2)
Conventional power system: Centralized.
Load demand increases, then what?
Environmental impact: Is renewables the solution?
Dealing with the stochastic fluctuating nature of
renewables…
Distributed power generation: DG
Issues of protection and power flow control.
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Introduction (2/2)
Concept of microgrids and intergrids.
Autonomy and coordination of microgrids
Microgrids (neighborhoods) and nanogrids (homes)
RES for co-generation and the net-zero energy home
(NZEH)
Impact of electric vehicles on the distribution systems.
Linking EVs (picogrids) to residential nanogrids (V2H)?
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Residential AC nanogrid (Boroyevich Optim10)
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Residential nanogrids (1/2)
Residential distribution system: Single-phase 3-wire (120:240V).
Three-phase 4-wire (127:220V)…
AC nanogrid: Good choice for a net-zero energy home?
Sources and storage units are mostly DC.
DC-AC converters are used now.
Modern appliances present an AC-DC converter, an unregulated
DC bus followed by another DC-AC or DC-DC converter.
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Residential nanogrids (2/2)
What about a DC distribution system?
Edison vs. Tesla revisited!
Power electronics allow voltage level variations.
DC-DC converters tend to be more efficient than DC-AC.
Modern appliances are DC compatible. Remove AC-DC
converter and PFC elements.
No reactive power flow/demand.
Conductors can be reduced for the some power transfer.
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Residential DC nanogrid (Boroyevich Optim10)
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Residential AC or DC nanogrid (Riccobono IEEE 16)
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Choice of DC bus voltage
Extra Low Voltage (ELV) DC presents a magnitude of
less than 120 V and lower risk of electrical shock.
48V DC is a standard telecom voltage level.
Supply of “kW” loads done with >20 A
Issue with conductor size for reducing voltage drops…
Following category is the Low Voltage (LV): <1500V.
There is an “industry standard” at 380 V.
Option of “dual DC bus system” promoted by EMerge
Alliance: 24V & 380V.
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EMerge Alliance
• Dual DC buses: 380 V + 24V/100W buses for lighting.
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EMerge Alliance
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380 V DC eco-system development:
Present status and future challenges
(INTELEC 2013)
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Advanced LVDC electrical power architect-
tures and microgrids (Dragicevic,2013)
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The dual DC buses nanogrid 48V for “low power” loads and 380 V for “higher power” loads.
Generation and storage placed in both buses.
Bidirectional interlink DC-DC converter.
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Hierarchical control:
DC bus signaling (DBS) is used for coordinating the action of
interfaces. Io* = f(Vo) or Vo
* = f(Io). Variable DC bus voltage…
Low bandwidth communication link for “regulation.”
Control strategy for the DC Nanogrids
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DC bus signaling and droop control
• Droop control. Params: No-load voltage and droop slope
𝐼𝐿𝑜𝑎𝑑 = 𝐼𝑆 + 𝐼𝑔 + 𝐼𝑏IDC_x =VNL_x−VDC
Rd_xRd_x=
∆VDC∆IDC_x
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DC/DC Interlink Converter - Aniel Morais UFU
DA
B
n=7.92
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Control strategies
• Usually the DBS is intended for supporting power balancing
in one bus. Power injected into one bus will come from the
other.
• Option #1: “Averaging” DBS efforts
• Option #2: Equalize, by means of a PI controller, the %
error between the voltages in the LV and HV bus.
• Option #3*: Address the needs of the LV bus, until the HV
bus is “stressed.”
𝐼𝑖𝑛𝑗_𝐿𝑉 = 𝐼𝐷𝐵𝑆_𝐿𝑉−𝐼𝐷𝐵𝑆_𝐻𝑉 =𝑉𝑁𝐿_𝐿𝑉 − 𝑉𝐷𝐶_𝐿𝑉
𝑅𝑑_𝐿𝑉−𝑉𝑁𝐿_𝐻𝑉 − 𝑉𝐷𝐶_𝐻𝑉
𝑅𝑑_𝐻𝑉
𝐼𝑖𝑛𝑗_𝐿𝑉 =𝑉𝑁𝐿_𝐿𝑉 − 𝑉𝐷𝐶_𝐿𝑉
𝑅𝑑_𝐿𝑉
𝐼𝑖𝑛𝑗_𝐿𝑉 ≤ 0, if 𝑉𝐷𝐶𝐻𝑉 > 390 V
𝐼𝑖𝑛𝑗_𝐿𝑉 ≥ 0, if 𝑉𝐷𝐶𝐻𝑉 < 370 V
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Interlink Converter control strategies
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Interlink Converter control strategies
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Interlink Converter control strategies
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Interlink Converter control strategies
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Simulation
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SimulationLoad 1
Operate from
30 ms to 130 ms
Load 2
Operate from
80 ms to 180 ms
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Simulated Converter
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Load 1
11.5 kW
No Load
0 kW
Loads 1 & 2
17.3 kW
Load 2
5.8 kW
No Load
0 kW
Zone I Zone II Zone IVZone III
30 ms 80 ms 130 ms 180 ms
Averaged droop control
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Load 1
11.5 kW
No Load
0 kW
Loads 1 & 2
17.3 kW
Load 2
5.8 kW
No Load
0 kW
Zone I Zone II Zone IVZone III
30 ms 80 ms 130 ms 180 ms
Constant voltage ratio
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Load 1
11.5 kW
No Load
0 kW
Loads 1 & 2
17.3 kW
Load 2
5.8 kW
No Load
0 kW
Zone I Zone II Zone IVZone III
DC-Bus Voltage
30 ms 80 ms 130 ms 180 ms
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Alternative control strategy (recall…)
• The interlink converter is controlled with DBS based
(solely) on the voltage level of the LV DC bus (48V).
• Assuming that the HV DC bus (380 V) is connected
to the AC utility grid, a strong system, it can
provide/absorb power for balancing the HV DC bus.
• What if it is not active?
• The interlink converter is prevented from:
– injecting power in the HV DC bus, if its voltage is
higher than 390 V
– Drawing power from the HV DC bus, if its voltage
is lower than 370 V.
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Loads vary 370 V ≤ VHV ≤ 390 VDifferent load variations (time and value)
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Loads vary with power shortage at HV bus
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Loads vary with power surplus at HV bus
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Alternative non-isolated high gain topology:
3-switch bidirectional DC-DC converter
V1 V2C1 C2
+
-
+
+ +S3
S1 S2
D3
D2D1LT
-
V1 V2
+ +
S3
S1 S2
D3
D2
+
+
+ + -
+
- -
-
-
- +
LM
VLT1 VLT2
- -
IM
I1 I2D1
: 1nILT1 ILT2
V1 V2
+ +
S3
S1 S2
D3
D2
+
+
+ + -- -
-
- +
LM
VLT1 VLT2
- -
IM
I1 I2D1
: 1nILT1 ILT2
+
-
Various possibilities of modulation schemes… tri-state!
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• Conventional (two-state) forward Buck-Boost:
Static Analysis
VS1
t0 t1 t2
V1+nV2
TS
VS2
t0 t1 t2
V1+nV2
n
TS
IS1
t0 t1 t2
TS
IS2
t0 t1 t2
TS
-nIM2
-nIM1
IM2
IM1
IS3
t0 t1 t2
TS
-nIM1
-nIM2
IM2
IM1
t0 t1 t2
TS
IM
t0 t1 t2
TS
VLM
IM2
IM1V1
-nV2
2 1
1 11
V D
V n D
Fig. 6 Forward Buck-Boost:
Voltage waveforms across switches S1 and S2
Fig. 7 Forward Buck-Boost: Current waveforms in the switches
Fig. 8 Forward Buck-Boost:
Magnetizing inductance waveforms Fig. 9 Forward Buck-Boost:
Voltage Conversion Characteristic
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Tri-state 3-switch bidir. DC-DC converter
Research question: How can
one benefit (and simplify)
from this multivariable
control scheme?
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Tri-state 3-switch bidir. DC-DC converter
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5-switch bidirectional DC-DC converter(Power flow direction changed without changing direction of ILM)
V1 V2
S1
S2
S3
S4
S5
D1
D2
D3
D4
LMn
1
I1 I2
ISD1
ISD2
ISD3
ISD4
IL2
IL1 ILM
L1
L2
IS5
+
+
+
+
-
-
-
-
+
+
+
+
-
-
-
-
+
-
+
-
+
-
+
-
V1 V2
S1
S2
S3
S4
S5
D1
D2
D3
D4
LMn
1
I1 I2
ISD1
ISD2
ISD3
ISD4
IL2
IL1 ILM
L1
L2
IS5
+
+
+
+
-
-
-
-
+
+
+
+
-
-
-
-
+
-
+
-
+
-
+
-
2 2
1 2
V D
V n 1 D
Buck-boost forward two-state
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5-switch bidirectional DC-DC converter(Buck-boost reverse two-state)
V1 V2
S1
S2
S3
S4
S5
D1
D2
D3
D4
LMn
1
I1 I2
ISD1
ISD2
ISD3
ISD4
IL2
IL1 ILM
L1
L2
IS5
+
+
+
+
-
-
-
-
+
+
+
+
-
-
-
-
+
-
+
-
+
-
+
-
V1 V2
S1
S2
S3
S4
S5
D1
D2
D3
D4
LMn
1
I1 I2
ISD1
ISD2
ISD3
ISD4
IL2
IL1 ILM
L1
L2
IS5
+
+
+
+
-
-
-
-
+
+
+
+
-
-
-
-
+
-
+
-
+
-
+
-
Various possibilities of modulation schemes: Tri-state, quad-state…
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Performance of the two-state current
control scheme of the 5-switch converter
0
-2
-4
-6
2
4
6
I1_Ref I1_1stOrder_Filter
0 0.1 0.2 0.3 0.4Time (s)
0
-5
5
10
15
20
25
30
ILm_Ref I(Lm)
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Comparison of the two-state current control
scheme of the 3- and 5-switch converter
0.148 0.15 0.152 0.154 0.156Time (s)
0
-10
10
I1_3 Switches Converter I1_Novel 5 Switches Converter I1_Ref
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Fault protection in DC nanogrids
• Fault detection and selective fault clearance.
• How to identify where the fault occur and which circuits
breakers should open.
• Established technology for AC distribution systems.
• What about for DC nanogrids?
• Issue #1: Very small impedances between branches
and nodes.
• Issue #2: Devices to open high DC currents. Arc
extinction… Much more expensive, if available, than AC
• However, the power electronics interfaces provide some
sort of control of the current injected into the DC bus…
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Traditional (tripping by overcurrent) vs. fault current limitation and
interruption with coordination of interfaces and contactors. [3]
Fault Current Limitation
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DC Ring-bus Microgrid Fault Protection
and Identification of Fault Location (2013)
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Configuration of a typical DC Nanogrid
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Prospective topology and control logic
• Normal operation: S1 ON and S3-S4 controlled as a
classical current controlled class C converter.
• Fault current detected? Current control through S1
(PWM).
S1
S2
S3
S4
Cin Cout
L
VinVdc
il
iout
LV
side
HV
side
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Control scheme
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Experimental set-up
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Experimental set-up
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Experimental set-up
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Droop, current limit and reduced current
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Additional modulation schemes: Tri-state
• Conventional has 2 states, Don and Doff
• 3rd freewheeling state, Df . Here S2 and S4 are On.
𝐷𝑜𝑛 + 𝐷𝑜𝑓𝑓 + 𝐷𝑓 = 1
• Inductor is short-circuited. No transfer of energy.
• Doff is kept constant to make Don the only control variable.
• Is there a “best sequence of states?”
DonDoff
S1
S2
S3
S4
Cin Cout
L
Vin
Req
Vmeqil
iout
Conventional Boost
Don Doff
S1
S2
S3
S4
Cin Cout
L
Vin
Req
Vmeqil
iout
Df
Tri-State Boost
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Switch RMS Current Comparison
Forward power flow Reverse power flow
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Thank you for your attention!
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Issues under investigation Control strategy for a single and a dual DC buses DC nanogrid.
Hierarchical control with DC bus signaling (DBS) at the primary
level.
Control scheme for a solar PV converter operating in three
modes: Droop, MPPT and current limiting. (AHMAD)
Control of a hybrid energy storage system (HESS). Goal is to
regulate a DC bus voltage, considering the power and energy
density characteristics of batteries and supercapacitors. The
attenuation of the voltage distortion caused by a single-phase AC
grid interface is also considered. (ZAID)
Control strategy for a bidirectional interlink DC-DC converter.
(ANIEL-AHMAD).
Fault protection. Choice of converter and logic for clearing
faulted segments (SAROOSH).
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Load 1
11.5 kW
No Load
0 kW
Loads 1 & 2
17.3 kW
Load 2
5.8 kW
No Load
0 kW
Zone I Zone
II
Zone
IVZone
III
30 ms
Averaged droop control
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Load 1
11.5 kW
No Load
0 kW
Loads 1 & 2
17.3 kW
Load 2
5.8 kW
No Load
0 kW
Zone I Zone II Zone
IVZone
III
30 ms 80 ms
Averaged droop control
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Load 1
11.5 kW
No Load
0 kW
Loads 1 & 2
17.3 kW
Load 2
5.8 kW
No Load
0 kW
Zone I Zone II Zone
IVZone III
30 ms 80 ms 130 ms
Averaged droop control
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Load 1
11.5 kW
No Load
0 kW
Loads 1 & 2
17.3 kW
Load 2
5.8 kW
No Load
0 kW
Zone I Zone II Zone IVZone III
30 ms 80 ms 130 ms 180 ms
Averaged droop control
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Load 1
11.5 kW
No Load
0 kW
Loads 1 & 2
17.3 kW
Load 2
5.8 kW
No Load
0 kW
Zone I Zone
II
Zone
IVZone
III
30 ms
Constant voltage ratio
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Load 1
11.5 kW
No Load
0 kW
Loads 1 & 2
17.3 kW
Load 2
5.8 kW
No Load
0 kW
Zone I Zone II Zone
IVZone
III
30 ms 80 ms
Constant voltage ratio
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Load 1
11.5 kW
No Load
0 kW
Loads 1 & 2
17.3 kW
Load 2
5.8 kW
No Load
0 kW
Zone I Zone II Zone
IVZone III
30 ms 80 ms 130 ms
Constant voltage ratio
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Zaid
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Configuration of a Basic DC Nanogrid
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Hybrid Energy Storage System (HESS)
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Conventional HESS control scheme
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Inner current loop
Inner current loop
Outer voltage loop
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The MPPT Scheme
• The MPPT scheme is based on linearity (KP) between the
optimization current and the maximum output power of the PV.
• The maximum power prediction line is used to estimate the
maximum power the PV can generate for a measured
(instantaneous) current.
• The solar converter operates at MPPT by comparing this estimated
maximum power with the power generated from the PV.
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Case Study
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Simulation and Results
MATLAB/Simulink
A 5 kW PV array (VMPPT = 232 V and IMPPT = 22 A) was
modeled in MATLAB.
Solar converter supplying the load in stand-alone
Solar converter and grid interface converter together
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Solar converter supplying the load in stand-alone
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Solar converter and grid interface converter together