Overview of the Existing Under Frequency Load Shedding

Scheme

Stage Criteria Load Shed

I 48.75 Hz + 100 ms 5.0 %

II 48.50 Hz + 500 ms 5.0 %

III 48.25 Hz + 1s 48.25 Hz + 500 ms OR 49 Hz AND df/dt > -0.85 Hz/s + 100 ms

7.0 %3.0 %

IV 48.00 Hz + 1.5 s48.00 Hz + 500 ms OR 49 Hz AND df/dt > -0.85 Hz/s+ 100 ms

7.0 %3.0 %

V 47.5 Hz instantaneous 47.5 Hz instantaneous OR 49 Hz AND df/dt > -0.85 Hz/s + 100 ms

6.0 %4.0 %

VI 49 Hz AND df/dt > -0.85 s/Hz + 100 ms 10.0 % + (3%+3%+4%) embedded with stage III,

IV and V

Present status of the UFLS scheme

• Under frequency relays on following feeders yet to be installed 11 kV feeders of Sub E – stage IV 11 kV feeders of Sub A – stage IV 11 kV feeders of Sub I – stage V 11 kV feeders of Sub F – stage IV Habarana Valachchane line from Habarana end –

stage VI

Present status of the UFLS scheme ctdStage Expected amount Actual amount

I 5.0 % 5.3 %

II 5.0 % 5.2 %

III 7.0 %3.0 %

5.16 %2.65 %

IV 7.0 %3.0 %

6.6 %2.4 %

V 6.0 %4.0 %

4.5 %3.6 %

VI 10.0 % + (3%+3%+4%) embedded with stage

III, IV and V

9.7 % + (2.65%+2.4%+3.6%) embedded with

stage III, IV and V

System Frequency Response• PCPP tripped rejecting 300 MW on 29.01.13

ppt

0 7.79999999999998 15.6000000000001 23.400000000000247.75

48

48.25

48.5

48.75

49

49.25

49.5

49.75

50

50.25

50.5

df/dt = 0.678 Hz/sMin fre = 47.945 Hz

df/dt = 0.60 Hz/sMin fre = 47.967 Hz

Free Governor Operation

• Only Victoria and Kothmale machines operated

Frequency Response without Kothmale m/c and Victoria one m/c

With Free Governor

Without Free Governor

Critical Generator Under Frequency Trip Settings

Power Station Under Frequency Trip Setting

O/Lax 48 Hz with 8 sec delay

KCCP ST 48 Hz with 3 sec delay

Barge 48 Hz with 10 sec delay

Initial Frequency Response• It depends on the characteristics of the

synchronous loads and synchronous generators.

• df/dt t=0+= ∆P/2Hsystem

• Hsystem includes the inertia contribution from generators and synchronous loads.

(MW-s/MVA)

System Inertia from committed generators

n

nnn

ii

n

iii

systemMVAMVAMVA

xMVAHxMVAHxMVAH

MVA

MVAHH

............

21

2211

1

1

Operation of Under frequency • df/dt setting was not operated – initial df/dt = 0.6 Hz/s

• Stage I – 62 MW (4.35 %) – 7 MW of load not tripped• Stage II – 40 MW (2.8 %) – 25 MW of load not tripped• Stage III – 92 MW (6.4 %) – 20 MW of load not tripped• Stage IV – 20 MW (1.4 %) – lack of load shedding in

stage IV since frequency remains appro: 1.6 seconds below 48 Hz

• Total load shedding – 214 MW

Frequency response if all feeders operate at stage I,II and III

Frequency response with reduced time setting in stage IV

• Reduce time delay to 1 sec from 1.5 sec

More load shedding In stage IV – 72 MW

THANK YOU