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Voltage stability assessment and coordinated control utilizing PMUs
Dinh Thuc Duong
Kjetil Uhlen
2
Outline
• Background: From VIP to NiP to STRONgrid
• Overall aim: Coordination and optimal utilization of
FACTS (SVCs) using PMUs – Primary and secondary voltage control
– Wide area power oscillation damping
• Voltage stability assessment with PMUs and partial
state estimation
Acknowledgement: The work presented on
coordinated control has been a collaboration
between ABB, Statnett and Sintef.
SINTEF Energy Research
The Voltage Instability Predictor (VIP)
Z app Maximum power transfer
<=> ZNET = (Zapp)*
ZNET
V
Zapp
network equiv. load
E
Relay tracks system strength
based on local V & I
22
11
IZVE
IZVE
NET
NET
Circle of
radius | Z NET |
SINTEF Energy Research 4
NiP – Vision and time schedule
Control Signals
Decision making
for on-line Control
Monitoring
based on VIP & PMU
J
Hasle
Device
level
VIP box
WEB
HMI
Measurements
Phase 0 - 2000/1 NIP Phase 1 - 2001/2
Processed Data
NIP Phase 2 - 2003/4
Close - Loop
PMU box
SINTEF Energy Research 5
Integration with SCADA/EMS Simple and understandable indicators are needed
VIP example
0.5 1 1.5 2 2.5 0
0.2
0.4
0.6
0.8
1
1.2
PLoad (pu)
Vo
lta
ge
(p
u)
Nose Curve
Worst Contingency
Nose Curve
Normal Operation
Actual Operation
Point
Pmargin
SINTEF Energy Research 6
Pilot node voltages
SVC :
+/- 160 Mvar
SVC :
+/- 250 Mvar
SVC :
360 Mvar
SVR
control
Set point
controls
Voltage setpoint
Coordinated Secondary Voltage control
SINTEF Energy Research 7
Coordinated Secondary Voltage control
(SVR scheme)
Overall objective of the SVR scheme is:
To maintain a sufficient and balanced reactive reserve in SVC units and rotating condensers during normal operation.
To keep the pilot node voltages within specified limits.
Benefits:
Desired voltage profiles are maintained automatically.
By having automatic and coordinated control of the SVCs, the operators can concentrate on maintaining the necessary reactive reserves (e.g. by connecting/disconnecting capacitor banks).
Motivation: Transfer limits are normally determined from the ”N-1” security criterion. Sufficient on-line reactive reserves are important in order to reduce the consequences of critical contingencies.
SINTEF Energy Research 8
Operation: Principles and characteristic
Voltage [kV] as function of sum reactive power [MVar]
360
380
400
420
440
-1000 -800 -600 -400 -200 0 200 400 600 800 1000
Voltage [kV]
ReserveNormal operating range
Gain: K = 50 Mvar/kV
[Mvar]
SINTEF Energy Research 9
SVC + SC :
-250/320 Mvar
SVC :
+/- 250 Mvar TCR :
-360 Mvar
SC :
-90/250 Mvar
SINTEF Energy Research 10
First day
operational results
Trend curves of pilot
node voltage (420 kV
Frogner) and total Mvar
setpoint.
14 hours of operation
FROGNER420BU_P
400
405
410
415
420
425
430
00:00:00 02:00:00 04:00:00 06:00:00 08:00:00 10:00:00 12:00:00 14:00:00
Time of day (2003-12-05)
Pilo
t n
od
e v
olt
ag
e [
kV
]
DIV_RCCSSVCBEREGNET_Q
-500
-400
-300
-200
-100
0
100
00:00:00 02:00:00 04:00:00 06:00:00 08:00:00 10:00:00 12:00:00 14:00:00
Time of day (2003-12-05)
Re
ac
tiv
e p
ow
er
se
tpo
int
[Mv
ar]
SINTEF Energy Research 11
410
415
420
425
430
-500 -400 -300 -200 -100 0 100
Pilot node voltage [kV]
Su
m r
ea
cti
ve
po
we
r [M
va
r]
First day operational results
Measured pilot node voltage versus reactive power setpoint
Gain:
K = 33 Mvar/kV
After increasing the
gain to: K = 50 Mvar/kV
SINTEF Energy Research 12
Testing dynamic
response
Step change in voltage
setpoint: 415 420 kV
Gain: K = 50 Mvar / kV
Response time (time
constant): ~1-2 minutes
«step-wise» response
due to SCADA
implementation
Pilot node voltage [kV]
412
414
416
418
420
422
13:15:00 13:20:00 13:25:00 13:30:00 13:35:00 13:40:00 13:45:00
Time (2003-12-05)Sum reactive power [Mvar]
-400
-300
-200
-100
0
100
13:15:00 13:20:00 13:25:00 13:30:00 13:35:00 13:40:00 13:45:00
Time (2003-12-05)
SINTEF Energy Research
Use phasor measurements from several pilot nodes
Optimize the use of the SVCs and SCs
Coordinate with stabilizing controls
13
Pilot nodes
Voltage phasors
SVC :
+/- 160 Mvar
SVC :
+/- 250 Mvar SVC :
360 Mvar
SVR
control Set point
controls
Voltage setpoint
Further developments
14
“Wide-area POD” design
To improve damping of low frequency inter-area modes:
Targeting more than one mode: Two main inter-area modes of interest
(approximately at 0.3 Hz and 0.5 Hz)
Observability: New measurements available by utilizing the PMUs
Controllability: Several SVCs available (see map)
Robust design: Ability to improve damping in a largest possible
range of operating conditions.
Coordinated design: Avoid adverse interactions
Between different SVCs
Between other control objectives (e.g. voltage control)
Field test of WA-POD in Hasle
Field test in Hasle substation 8 November 2011-11-15.
Purpose: Test and verification of Wide Area Power
Oscillation Damper (WA-POD) on a 180 Mvar TCR
Static Var Compensation unit in Hasle.
System damping measured and analysed during
disconnection and re-connection of the 420 kV line
Hasle-Tegneby.
Performance of WA-POD was tested and compared
against ABB’s state of the art Phasor-POD and the case
without any power oscillation damper.
Wide area measurements for
POD Control
PMUs streaming phasors from:
Nedre Røssåga
Kristiansand
SVC Located at Hasle
PDC receiving voltage phasors
Extracts voltage phasor angle
ABB Mach2 Controller
Local control
WAPOD Control
Switch-over logic
Hasle
Nedre
Røssåga
Kristiansand
Hasle SVC (4x90 Mvar TCR)
SVC and WA-POD implementation
Voltage Regulator
POD Control
Switch-Over Logic
WAPODControl
( )V t
( )V t
refV
Voltage Setpoint
( )V t
Slope
Voltage Meas.
From PDC
( )ijP t
From Adjacent Line
( )ttV
( )shB t
( )refTOT
tV
( )SVCtQ
1 1( ), ( )
L Lt Q tP
Line used for Testing
From Norway (Tegneby S/S)
From Norway (Tveiten S/S)
From Sweden (Borgvik S/S)
From Sweden (Halden S/S)
2 2( ), ( )
L Lt Q tP
( )t 1V
2V
2 × 90 Mvar TCR
Test: Switching of 420kV Hasle-Tegneby
20 30 40 50 60 70 80
-55
-50
-45
-40
Time (sec)
Vo
ltag
e A
ng
le D
iffe
ren
ces
(deg
)
No POD, 11:49:30.000
WAPOD, 12:39:30.000
Local POD, 13:04:30.000
20 30 40 50 60 70 80
0.6
0.7
0.8
0.9
1
1.1
1.2
Time (sec)
Act
ive
Po
wer
(p
u)
No POD, 11:49:30.000
WAPOD, 12:39:30.000
Local POD, 13:04:30.000
Disconnection of 420 kV
Hasle-Tegneby
0 5 10 15 20 25 30-50
-48
-46
-44
-42
-40
-38
Time (sec.)
Vo
lta
ge
an
gle
fiff. (
alig
ne
d)
No POD
WAPOD
Local POD
Smart Transmission Grids Operation and Control
A project funded by
– And co-funded by Nordic TSOs and DSOs
• Objectives:
– Address the challenges that the secure and reliable operation of the
power grids will face in the future.
– Support the development of better tools for planning, operation and
control of power grids
– We seek to establish an interdisciplinary theoretical and
experimental foundation for research and development
Project partners
Institute of Physical
Energetics
Ind
ust
ry
Aca
de
mia
Stage 1
• PMUs are connected at the LV networks
LV PMU Network in STRONg2rid
On-line PDC
Planned PMU
On-line PMU
NI ABB SEL
SEL PDC Open PDC
PMUs
PDCs
PRL
Research platform
PMU
PDC-KTH
PMU
PDC-NTNU
PMU PMU
PMU
PDC-DTU
PMU
PDC-Aalto
PMU PMU
KTH NTNU
AaltoDTU
Each University (PMUs, PDC)
• Output stream for each other university (only
their PMUs)
Laboratory activities: – Wind power conversion systems and grid connection – Converter controls for Multi-Terminal HVDC – PMU placement
Renewable Energy / Smart Grids Laboratory
26
Statnett PRL
Synchrophasor Software Development Kit • Real-Time Data Mediator for LabView
Sample Application WAMS Visualization Tool - Mobile
• Portable Monitoring Applications
”On-line voltage stability assessment”
28
PMU
PMU
«VIP» PMU
Objective:
To develop a robust
and simple voltage
stability assessment
tool for on-line use
Topology
Power
flow data
SCADA/EMS
State estimator
29
Boundary selection and system reduction
B1
study area
B2
B3B4 B5
G1 G2
B1
study area
B2
G1 G2
S1S2
study area
Sub 1
G1
Sub 2
132kV
400kVPMU
#1
Example of subsystem boundary in sub-transmission system
B1 and B2 are strong buses
30
1. Voltage stability assessment based on impedance matching
ZTh
ZL
IL
VL
ETh
𝑚𝑎𝑟𝑔𝑖𝑛 =𝑆𝑚𝑎𝑥 − 𝑆𝐿
𝑆𝐿× 100% 𝐸𝑇ℎ = 𝑉𝐿 + 𝐼𝐿𝑍𝑇ℎ
𝑍𝐿 = 𝑍𝑇ℎ
𝑆𝑚𝑎𝑥 =|𝐸𝑇ℎ
2 | |𝑍𝑇ℎ| − 𝑖𝑚𝑎𝑔 𝑍𝑇ℎ sin𝛿 + 𝑟𝑒𝑎𝑙 𝑍𝑇ℎ cos𝛿
2 𝑖𝑚𝑎𝑔 𝑍𝑇ℎ cos𝛿 − 𝑟𝑒𝑎𝑙 𝑍𝑇ℎ sin𝛿 2
Smax is prefered as main indicator because of small error
𝐼𝑆𝐼 =𝑍𝑇ℎ𝑍𝐿
31
2. Estimation of Thevenin equivalent
L2
Line 1
G1L1
T1 B2B1
ZL1
ZL
ZL2
ZT
EThEG
B1 B2
Fig. 1: A simple two-node grid.
Fig. 2: Equivalent circuit of the two-node grid
32
2. Estimation of Thevenin equivalence (cont.)
• Thevenin impedance
• Admittance matrix
• Modified admittance matrix, including load L1 but
excluding critical load, L2
𝑌 =
1
𝑍𝐿+
1
𝑍𝑇−
1
𝑍𝐿
−1
𝑍𝐿
1
𝑍𝐿
𝑌𝑒𝑞 =
1
𝑍𝐿+
1
𝑍𝑇+
1
𝑍𝐿1−1
𝑍𝐿
−1
𝑍𝐿
1
𝑍𝐿
𝑍𝑇ℎ = 𝑍𝐿 +𝑍𝐿1𝑍𝑇
𝑍𝐿1 + 𝑍𝑇
33
2. Estimation of Thevenin equivalence (cont.)
• “Full” admittance matrix, including critical load L2
Z’Th
B2
E’Th
B1ZTh
ZL
IL
VL
ETh
1 2
34
3. Effect of limited active power on Thevenin impedance
ZLoad
ZL2ZL1
V1 V2
Simulation:
• V1: unlimited source.
• V2:
- constant terminal voltage
- P = 10 MW (constant)
Thevenin impedance:
𝑍𝑇ℎ = 𝑍𝐿1//𝑍𝐿2
Not equal
0 10 20 30 40 5040
60
80
100
120
140
160
Impedance (
Ohm
)
Time (s)
0 10 20 30 40 500
50
100
150
200
250
300
Load p
ow
er
MV
A
Load pow er
Load impedance
Thevenin impedance
35
3. Effect of limited active power on Thevenin impedance (cont.)
#1
#2 #3 #4 #5
#6
#7#8
G1 G2
#1
#2
#8
G1
G2
Z18
Z28 Z88
#1 #8
G1
Z18
Z88 Z82new
#1
#2
#8
G1
G2
Z18
Z28new
Z88
36
3. Effect of limited active power on Thevenin impedance (cont.)
0 10 20 30 40 5010
20
30
40
50
60
Time (s)
Appare
nt
pow
er
(MV
A)
With modification
Load
Unmodified Thevenin equivalence
37
Application example: Analysis of voltage stability in the Lofoten peninsula
https://maps.google.no/maps?ll=68.532256,15.320435&spn=1.536177,5.559082&t=m&z=8
38
Boundary selection and system reduction
B1
study area
B2
B3B4 B5
G1 G2
B1
study area
B2
G1 G2
S1S2
study area
Sub 1
G1
Sub 2
132kV
400kVPMU
#1
Example of subsystem boundary in sub-transmission system
B1 and B2 are strong buses
39
Grid model in Lofoten area
Kvandal-Sør
58043
Skjomen
58113
Kanstadbotn
58173
Kvitfors
58183Fygle
58187
Kvitnes
58293
Hinnøy
58213
Melbu
58193
Stokmarknes
58168
Sortland
58203
Risøyhamn
58223
Kilbotn
58253
Medkila
58333
Gråheia
58233
Gåsvatn
58283
Heggen
58263Møkkland
58243
Ma
in tra
nsm
issio
n g
rid
40
Results from simulation on Norwegian transmission system
0 10 20 30 40 500
5
10
15
20
25
Time (s)
Impedance (
pu)
Thevenin impedance
Load impedance
0 10 20 30 40 500.04
0.05
0.06
0.07
0.08
Time (s)
Appare
nt pow
er
(pu)
Estimated max loadability
Load
Increase all loads in study area
0 5 10 150
5
10
15
20
25
Time (s)
Impedance (
pu)
Thevenin impedance
Load impedance
0 5 10 150.04
0.05
0.06
0.07
0.08
Time (s)
Appare
nt pow
er
(pu)
Estimated max loadability
Load
• Boundary: one 420/132kV substation and one large power plant.
• Voltage: 132kV with 30 buses.
• Only 30 bus are taken into the algorithm. Simulation is run on the whole
system.
Increase the weakest load in study area
41
Results from simulation on Norwegian transmission system
Trip of one critical line and Voltage collapse due to load restoration
0 1 2 3 4 50
5
10
15
20
25
Time (s)
Impedance (
pu)
Thevenin impedance
Load impedance
0 1 2 3 4 50
0.02
0.04
0.06
0.08
0.1
Time (s)
Appare
nt pow
er
(pu)
Estimated max loadability
Load
42
Resume
• Background: From VIP to NiP to STRONgrid
• Overall aim: Coordination and optimal utilization of
FACTS (SVCs) using PMUs – Coordinated secondary voltage control
– Wide area power oscillation damping
• Voltage stability assessment with PMUs and partial
state estimation – focusing on the area of concern and using all available information
• Focus on practical applications and tool
43
3. Effect of limited active power on Thevenin impedance (cont.)
2
4
Th
Th
n
ER
P
RTh
RL
IL
VL
G1
ETh
- Fictitious impedance
Modelling limited power source:
- Rated active power of G1: Pn
2
4
Th
Th
n
EZ jX
P
0 5 10 15 20 25 30 35 40 45 5040
60
80
100
120
140
160
Impedance (
Ohm
)
Time (s)
0 5 10 15 20 25 30 35 40 45 500
50
100
150
200
250
300
Load p
ow
er
MV
A
Load pow er
Load impedance
Thevenin impedance
0 10 20 30 40 5040
60
80
100
120
140
160
Time (s)
Pow
er
(MV
A)
Estimated
Load
j
44
3. Effect of limited active power on Thevenin impedance (cont.)
An example of a small meshed system:
#1
#2 #3 #4 #5
#6
#7#8
G1 G2
small
E EB
eq
EB I
Y YY
Y Y
• There is no direct connection from
generators to considered load.
• Keep all generators and the
considered load. Simplify the rest by
Gaussian elimination technique
IeqYeqY
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