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A Supervisory Approach towards Cyber-Secure Generator Protection
RAJESH G. KAVASSERI, Y. CUI AND N. R. CHAUDHURI DEPARTMENT OF ELECTRICAL AND COMPUTER
ENGINEERING NORTH DAKOTA STATE UNIVERSITY
CPS WEEK, CPSR Workshop, VIENNA, April 2016
Support from NSF CPS #1544621 gratefully acknowledged (*)
Context • Modern system protection largely served by
microprocessor-based relays; • Multifunctional role for relays – besides
protection - control/automation/metering… • Network/remote access to multiple parties:
relay technicians, protection engineers, control engineers, corporate groups and vendors.
What’s at stake? – A lot! • The “heart” (sanctum sanctorum) of a
relay lies in its “settings”. • Incorrect settings (intentionally or
otherwise) can be severely detrimental to system operation.
• How do we safeguard these settings? In this multi-party, multi-access scenario?
• Current practice: access restrictions, privileges, relay recommissioning,..
Example (from [1])
SETTINGS
Example (from [2], BPA)
SETTINGS
Synchrophasor Vector Processor
Focus: Out of Step (OOS) Events for Synchronous generators
• OOS: generator exhibits undamped power swings with (potential) loss of synchronism
• Protection solutions based on “settings” a) Rate of change of positive sequence
impedance seen at generator terminals (implemented by impedance relays/blinders)
b) More recently, slip and acceleration based relays requiring PMU inputs – used by SEL
Example/Relay characteristics
7
TRIP
Settings: (R,X) center, radius, blinder positions, separation timers
Example/characteristics (from [2])
8
Settings: Discriminating lines
Compromised settings will impact protection – potentially reversing TRIP and BLOCK decisions !
Key Idea – Can we computationally supervise this relay through an independent path?
For OOS, the key variable to monitor is the rotor angular separation
Challenge – The voltage angles can be directly measured, but
not the rotor angle – The apparent impedance is used to approximate
the angular separation between the rotor angle and voltage angle (Device 78)
10
Measurable Immeasurable
Solution: * Estimate the angular separation using Dynamic State Estimation (DSE) * Kalman filter-based methods can be used; but here, we use particle filters for robustness and accuracy
Dynamic State Estimation (DSE)
11
DSE
𝑽 ,∠𝜽 𝑰 ,∠𝝓
• Generator model • Exciter, turbine model • Measurement model
Supervisory Scheme
Local PMU
𝜹,𝝎,𝑬𝒅′ ,𝑬𝒒′,𝑬𝒇𝒅,𝑻𝒎
Phasor information and resultant product (e.g. power output)
Estimated dynamic states
Particle Filter Approach
12
𝑥𝑘 = 𝑓𝑘 𝑥𝑘−1,𝑢𝑘−1,𝑤𝑘−1
𝑦𝑘 = ℎ𝑘 𝑥𝑘 ,𝑢𝑘 , 𝑣𝑘 System Equation
Measurement Equation
𝑝 𝑥𝑘|𝑌𝑘 = pdf of 𝑥𝑘(𝑠𝑠𝑠𝑠𝑠 𝑣𝑠𝑣𝑠𝑣𝑣) given a set of measurements
We have
We solve for
At each time step k: • The a priori particles are computed from the system dynamics (f()) and the
known pdf of the process noise.
• For m measurements, the probability of a priori particles conditioned on the measurement is given by:
• The probabilities are normalized and the a posteriori are resampled.
Case Study: New England System
13
G1
30
2
25
G837
26 28 29
G938
1
G10
39
9
8
7
5 6
4
3
G2
31 11
12
10
13
14
G3
32
18
27
17
16
15
19
20
G5
34G4
33
21 22
G6
35
23
G7
36
24
System summary: • 10 generators • 39 buses • 46 branches Prime-mover, excitation controllers and power system stabilizers are all modeled
Self-clear fault on line 28-29
Sample of Tracking Results
14
Summary of 100 Monte Carlo trials
Sample of Tracking Results
15
Summary of 100 Monte Carlo trials
Detection Method
16
17
Consistency Test: Stable Case
18
Consistency Test: Unstable Case
19
Misinterpretation due to Faulty Setting
Stable swing leads to a mis-trip
Unstable swing goes undetectable
20
Angular separation by DSE for generator mis-trip case
Angular separation = rotor angle – voltage angle of HV side of the transformer
• An alternative pathway is proposed to monitor the swing stability independently
• Simulation results show that the proposed approach complies with conventional relay decision
• The alternative pathway can provide supervisory supplement on detecting anomalous operation since it does not rely on the same set of settings and directly reflect angular separation
Conclusions
Sources
• [1] Technical Report/SEL Publication: Advanced Real-Time Synchrophasor Applications Edmund O. Schweitzer, III, David Whitehead, Armando Guzmán, Yanfeng Gong, and Marcos Donolo, Schweitzer Engineering Laboratories, Inc.
• [2] Schweitzer, E. O., Guzman, A., Altuve, H. J., and Tziou-varas, D. A., “Real-Time Synchrophasor Applications
for Wide-Area Protection, Control, and Monitoring,” Tech. report., Schweitzer Eng. Laboratories, 2009.
Danke!
(*) The views and opinions of authors expressed herein do not necessarily state or reflect those of the NSF or any government agency thereof.