Download - Potential Underfrequency Load Shedding Schemes for the Slovenian Power System

7/27/2019 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