Potential Underfrequency Load Shedding Schemes for the Slovenian Power System
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Transcript of Potential Underfrequency Load Shedding Schemes for the Slovenian Power System
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Potential Underfrequency Load
Shedding Schemes for the Slovenian
Power System
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Introduction
Power system underfrequency protection
High importance
last resort tool Active power imbalance frequency deviation
Power system islands
pui,el,pui,mech,pui,el,id
d2 TT
tH
INERTIA FREQUENCY
CHANGE
ACCELERATION
TORQUE
( ACTIVE POWER
IMBALANCE)
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Target performance
Simple (importance)
Fast (automation) Highly reliable (local Vs. global issue)
Highly effective (automation)
Disconnect as less load as possible
Compromises are required
Frequency is a global system parameter
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Current situation
Traditional scheme
Simplest kind of UFLS protection Fixed amounts at fixed frequency thresholds
Under/over shedding might occur
Frequency Amount of
Threshold[Hz] disconnected load[%] Comment49.0 10 % Disconnection of 10 % of the total system load
48.8 15 % Disconnection of additional 15 % of the total system load
48.4 15 % Disconnection of additional 15 % of the total system load48.0 15 % Disconnection of additional 15 % of the total system load
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Space for improvement
Adaptive approach more sophisticated
Follows seriousness of the disturbance Frequency global parameter
COI calculation required
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Center of Inertia - COI
Average frequency of the system
Avoid oscillations
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a) deficit calculation
Swing equation
Measurements at t = 0+ Load voltage dependence
Voltage characteristics vary
Strongly dependent on a single calculation
Questionable effectiveness
Usefull: increased implementation of
microcontroller based relays
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b) predictive approach
No deficit calculation
Predicting the lowest frequency value How to predict the frequency?
0.00 1.25 2.50 3.75 5.00
Time [sec]
f[Hz]
50.0
51.0
49.0
48.0
47.0
46.0
45.0
frequencyf fMIN,forecast
FREQUENCYFORECAST
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Predictive approach No. 1
Source: COI frequency second time derivative
0 2 4 6 8
49.0
50.0
51.0
-1
0
1
d2f/dt2 [Hz/s2]
f[Hz]
Time [sec]
48.0
PDEF = 20 MW, QDEF = 20 Mvar
PDEF = 40 MW, QDEF = 40 Mvar
PDEF = 60 MW, QDEF = 60 MvarPDEF = 80 MW, QDEF = 80 Mvar
47.0
46.0
45.0
44.0
43.0
42.0
Acceleration of the
frequency
Approximation andnumerical
integration
Iterative procedure!
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Predictive approach No. 2
Upgrade
Improvements: First time derivative
No iterative procedure
Locus diagram f
df/dt Idea:
Part of a spiral similar
to an ellipsex
y
T1(a,0)
T2(0, b)
T4(0, -b)
T3(-a,0)
TC(xTC, yTC)
fCOI [Hz]51
dfCOI/dt[Hz/s]
0.6
1.8
-0.6
-1.8
494745
STEADY
STATE
STEADY STATE AFTER
PRIMARY FREQUENCY
CONTROL ACTIVATION
PERMANENT
DROOP
MAXIMAL (INITIAL)
FREQUENCY FTD
THE LOWEST VALUE,
ACHIEVED BY THE
FREQUENCY
a b
a)
b)
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Test results
IEEE 9 bus test system
10.0
16.0
25.025.0 25.0
40.0
40.0 40.0
46.0
55.055.0 55.0
5.5
11.8
19.5
24.2
30.3
36.0
42.7
47.0
6.4
15.9
25.8
29.9
36.6
41.2
47.353.3
0
10
20
30
40
50
60
70
20 30 40 50 60 70 80 90
Totalloadsheddingamount[%]
Active power deficit [%]
Traditional UFLS scheme
Theoretically optimal UFLS scheme
Predictive UFLS scheme (2)
Predictive UFLS scheme (1)
C l i
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Conclusions
A lot of space available for improvement
Advance in computer and communicationtechnology
Typical adaptive schemes NO!
Predictive adaptive schemes
ACTUALPOSSIBILITY!
COI calculation unavoidable
