photovoltaic synchronous generator (pvsg)
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Photovoltaic Synchronous Generator (PVSG):
From Grid Following to Grid Forming
Professor Alex Huang, Progress Energy Distinguished Professor
FREEDM Systems Center, NC State University
aqhuang@ncsu.edu
August 7, 2017
Alex Q. Huang, Ph.D. & IEEE Fellow
Dula D. Cockrell Centennial Chair in Engineering
Department of Electrical and Computer Engineering
The University of Texas at Austin
aqhuang@utexas.edu
Prof. Ross Baldick Prof. Surya Santoso Prof. Hao Zhu Prof. Bob Hebner
© 2017 by Alex Huang
3/22
Presentation Outlines
Background and Motivation
Frequency Regulation
Key Benefit 1 Key Benefit 3
Improved RoCoF and Power Intermittence
Key Benefit 2
Photovoltaic Synchronous Generator (PVSG)
Proposed
Voltage Regulation Hybrid Energy Storage System
Hardware
© 2017 by Alex Huang
4/22
Summary: Today’s PV power plant and Synchronous Generators
Following the grid:
Current source (PQ bus)
Follow the grid
Inject active (and reactive
power)
Fast response to the
intermittent irradiation
levels (no buffer)
PV panels PV inverter Utility grid Synchronous
generator
Forming (Supporting) the grid:
Voltage source (PV bus)
Set grid voltage and frequency
Provide active and reactive power
to the load via voltage
Slow response due to large inertia
Islanding and weak grid operation
How about high
PV Penetration?
© 2017 by Alex Huang
5/22
Introduction Major Challenge #1: DRER Intermittence
PV Daily Output Power @ FREEDM Center
Wind Speed with 1-minute Average Output Power from SW Minnesota
Wind Power Plant [2]
© 2017 by Alex Huang
6/22
Introduction Major Challenge #2: Voltage Rise or Sag
Phasor Diagram of Grid and PCC voltage, (a) PF = 1.0, (b) PF = -0.9 (c) PF = 0.9
© 2017 by Alex Huang
7/22
Introduction
One-line diagram of the IEEE 34 node test feeder
Example
[3]
Major Challenge #2: Voltage Rise or Sag
[3] S.A. Pourmousavi, A.S. Cifala and M.H. Nehrir, Impact of High Penetration of PV Generation on Frequency and Voltage in a Distribution Feeder, 2013
© 2017 by Alex Huang
8/22
Voltage Rise Problem
DRER Side Management
Active power control Use only local real power measurement, No MPPT
Buffering excess active power
Independent of PCC voltage, battery capacity and cost, communication and data exchange
Reactive power control
Independent of generation and network operator, higher currents and losses in the feeder, not efficient for high R/X ratio, more expensive oversize inverters, limited by Grid Codes
STATCOM Cheaper than storage devices, limited by Grid Codes
Inserting a series reactor in service line
Raise X/R ratio, higher power losses
Appliances power control
Load management, economic impact
Network Side Management
On load tap changer Lifetime, communication required
Active grid voltage control
Starting and recovery voltage settings
Reducing the primary substation voltage
Not practical for long lines or many distribution transformers involved
Re-conductoring the network
Expensive and not reasonable and economically
Introduction Existing Solutions
From PV Inverter to Smart PV Inverter
9
Parameters 2-Level NPC TNPC
Power device number 6 18 12
Output voltage quality Low high high
Active power capability 25.2kW 67.2kW 28.5kW
Total loss for Pmax 384.8W 533.24W 330W
Loss percentage for Pmax 1.527% 0.7935% 1.158%
Reactive power capability 28.8kW 68.7kW 57.6kW
Loss percentage Qmax 418.8W 476W 597.8W
Loss percentage for Q 1.4546% 0.6931% 1.038%
Reactive power constraint
factor k 1.129 1.02 2.02
Loss per kVar 14.5W 6.9W 10.37W
2 2
2 2
( )1
( * )
pv curl pv
nom nom
P P Q
S k S
*Supported under the DOE Sunlamp program
© 2017 by Alex Huang
10/22
Introduction
Frequency Stability
DRER Variability increases fluctuation of net load
DRER generation decouples from grid frequency by PLL
Lack of Rotational Inertia
No Up and Down Reserve
Swing Equation ROCOF
Major Challenge #3: Frequency Stability
© 2017 by Alex Huang
11/22
Introduction Frequency Response Example
Lack of System Inertia
[4]
© 2017 by Alex Huang
12/22
Grid Forming PV System: Photovoltaic Synchronous Generator (PVSG)
Lf
Cdc
iout
vg Cf
Lg
vdc
iin
+
-
iL
vC
circuit breaker
AC Grid
Inverter bridgePV
arrays Terminal
PWM
PVSG controller
iLd
vCd vdc
dd dq
Power Stage
abc/dq
dq/abc
d
iLq
vCq
δiLvC
iPV
iPV
Energy storages Terminal
line impedance
Based on virtual synchronous generator concepts [1], [2]
Emulate SG behaviors in P and Q
Voltage source (amplitude and frequency) instead of a current source
Auxiliary energy storages are used to support the functions
[1] Qing-Chang Zhong, Phi-Long Nguyen, Zhenyu Ma, and Wanxing Sheng, “Self-Synchronized Synchronverters: Inverters Without a Dedicated
Synchronization Unit,” IEEE Transactions on Power Electronics, vol. 29, no. 2, pp. 617–630, Feb. 2014.
[2] M. Ashabani, and Y.A.-R.I. Mohamed, “Novel Comprehensive Control Framework for Incorporating VSCs to Smart Power Grids Using
Bidirectional Synchronous-VSC,” IEEE Transactions on Power Systems, vol. 29, no. 2, pp. 805–814, March. 2014.
© 2017 by Alex Huang
13/22
PVSG Control Diagram
ddiLd_ref
+-
iLd
+-
+-
+-
+iLq_ref
iLq
++ dq
voltage loop
-
vCq
ωR Kω
vdc_P&O
vdc_ref
ω
vdc_f
+-
-
+-
+ ω
vo
ltag
e r
efe
ren
ce g
en
era
tor
vCd
ER
++QR KE+
-
E
vCd_ref
vCq_ref
δ
vivp
KK
s
vivp
KK
s
cicp
KK
s
cicp
KK
s
RL1
s
1
virJ s
vdc
QfQ
AC frequency and DC voltage
regulation
GLPF1(s)
GLPF2(s)
current loop
V-Q droop control
vdc_ω
P&Ovdc
iPV
ab
c/d
q
2E
RL
+-
++
RC
RC
MPPT control
Kes
+-
vdc_P&O
vdc_fes
GLPF4(s)vdc
Pes_R
++
VDC-P droopPes_ref
+-
Pes
PIies_ref
+-
ies
PI des
PVSG control
DC-DC converter control for energy storages
Energy Storage
MPPT Voltage Control
Dual Loop Control
Frequency Control
© 2017 by Alex Huang
14/22
Battery vs. Ultra Capacitor
Battery: High Energy Density, slow charge and discharge process, low power density, degrade over time, slow and steady energy supplier
Ultracapacitor: Fast charge and discharge, high power density, no storage capability loss, low energy density, no energy sustainment
© 2017 by Alex Huang
15/22
Energy Storage Coordination
Ultracapacitor Battery
© 2017 by Alex Huang
16/22
Irradiation
decreases Irradiation
increases PV
po
wer
(W)
PV
SG
po
wer
(W)
PV
freq
uen
cy
(rad
/s)
DC
vo
ltag
e
(V)
Ult
ra-c
ap
vo
ltag
e
(V)
Time (s)
Simulation results of a 1.5 kW PVSG: Inertia
Autonomously support grid
frequency at high solar
penetration level:
Introduce virtual inertia
for dynamic response
Slow down intermittent
PV output
Ultra Capacitor Based Energy Storage is a good choice the PVSG
© 2017 by Alex Huang
17/22
Preliminary experimental results of PVSG: Inertia
PV shading
CH1: 2A/div CH2: 300W/div CH3: 0.04Hz/div CH4: 10V/div X-axis: time 400ms/div
slow down intermittent PV output
Autonomously support grid frequency and voltage stability
PV unshading
PVSG current
PVSG power
PVSG frequency
DC bus voltage
CH1:4A
CH2:600W
CH3:60Hz
CH3:180V
© 2017 by Alex Huang
18/22
CH1: 4 A
CH2: 300W
CH3:120π rad/s
Pout
ω
vdc
iPV
CH4: 180V
CH1: iPV (2A/div); CH2: Pout (300W/div); CH3: ω (0.08π
rad/s /div); CH4: vdc (10V/div); X-axis: time t (2s/div)
Preliminary experimental results of PVSG: Inertia
Ramp rate:
Significantly reduce the power
ramp rate: 200X from our
simulation model
Small energy requirement
For Virtual inertia: about
1/3*Ppu*1second of energy
is needed
© 2017 by Alex Huang
19/22
PV current
PVSG power
PVSG frequency
Battery current
CH1:6A
CH2:900W
CH3:60Hz
CH4:0A
CH1: 2A/div CH2: 300W/div CH3: 0.04Hz/div
CH4: 2A/div X-axis: time 400ms/div
Preliminary experimental results of PVSG
Primary & secondary Frequency response
Accurate frequency measurement that can be used in wide area monitoring and control
© 2017 by Alex Huang
20/22
How to Apply PVSG to existing PV System?
vg
AC Grid277/480Vac 3ph
iPV
Line impedance
40kW PV inverter system
from AEG
40kW PV arrays
samePCC
1200V/100A IGBT inverter
module
DC Bus
Circuit breaker
Lf
Cf
iLvC
Synchronous generator emulation
control
iPV
vC
iL
vdc
Inner fast PE control
AC voltage reference2E
vC
iL
additional current
sensors for PV part
vdc
bi-directional DC-DC
converter
Energy storage inverter
bi-directional DC-DC
converter
bi-directional DC-DC
converter
bi-directional DC-DC
converter
droop control with distinguish
algorithmvdc
local DC voltage for each unitPVSG
system
CAPER Project focus 1) Upgrade existing PV System
2) System integration, data collection and analysis the impact
Conventional PV
Virtual Inertia + Primary f response
Secondary f response + economic dispatch
UCAP
Battery
© 2017 by Alex Huang
21/22
Simulation results of a 40 kW PVSG:
0.01 Hz
3.5 second
Inertia & Primary Response
Primary & Secondary Response
f(PVSG)
Power
(kW)
0.5 1 1.5 2 2.5 3 3.5 4
Time (s)
0
-5
-10
UCAP (Primary response)
Battery (Secondary response)
© 2017 by Alex Huang
22/22 0 200 400 600 800 1000 1200 1400 1600-0.5
0
0.5
1
1.5
psc
0 200 400 600 800 1000 1200 1400 160037.5
38
38.5
39
39.5
40
pg
pPV
T = 5 s
0 200 400 600 800 1000 1200 1400 1600-0.5
0
0.5
1
psc
Po
wer
(kW
)
Time (s)
Simulation results of a 40 kW PVSG: inertia
Po
wer
(kW
)
Po
wer
(kW
)
Po
wer
(kW
)
0 200 400 600 800 1000 1200 1400 160037.5
38
38.5
39
39.5
40
pg
pPV
T = 1 s Integral of the power
of the supercapacitor is the energy needed
When inertia time = 1
s, E = -8 kJ, voltage of
SC increase from 300
V to 305V with 5F
capacitance
When inertia time = 5 s,
E = -44 kJ, voltage of SC
increase from 300 V to
328V with 5F capacitance
Time (s)
© 2017 by Alex Huang
23/22
Thank you
Questions?
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