Islanding Operation

8/4/2019 Islanding Operation

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PROJECT : NALCO CPP UNIT 7 AND 8

PROBLEM : ISLANDING OPERATION

Islanding operation of units - 1 to 6 was successfully

demonstrated earlier. However, after addition of units - 7 & 8,

under the grid disturbances, the total plant used to trip causing loss

of power to the smelter plants. During islanding operation trials of

unit 7 and 8 of NALCO CPP, wide fluctuations of load, frequency and

voltages were noticed, which could not be stabilized even after 10

minutes.

WHY ISLANDING CRITICAL?

During the grid disturbances, such as under frequency or under

voltage smelter pot lines control gets disturbed and metal in the pot

lines will get solidified causing production loss and down time of

smelters for a week. Due to this, a circuit has been envisaged such

that in NALCO CPP Generators and Smelters will get isolated from

the grid on under frequency or under voltage to protect the pot lines

and avoid losses.


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DETAILS OF THE PLANT:

NALCO CPP is having 8 units of 1 20 MW each, that are feeding the

captive consumption of 600 MW at their smelter plant. Unit 1- 6steam turbines are having Electro-Hydraulic Governing system

( EHTC) and are of KWU design. Unit 7 and 8 steam turbines are

supplied with Electronic High pressure Governing and are of KN series

of Siemens Design.

OBSERVATIONS DURING TRIALS OF ISLANDING OPERATIONS:

TRIAL - 1 dated 28th October, 2005 (From 16:49:30 hrs to

17:15:40 hrs)

Islanding Operation was tried as per the scheme 1 enclosed. During

the trial wide fluctuations of load, frequency and voltages were

noticed which could not be stabilized and smelter feeders tripped onunder voltage (Trend- 1 enclosed). Further during the trial, it was

observed that in unit - 7 high speed controller was interfering and

causing valves to close & open continuously.


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During trial starting and load limiting device (SLLD) of unit 7 was

reduced such that load limiting devices action superseded the speed

controllers action. It was observed on reduction of SLLD hunting had

reduced and on release of SLLD hunting again started.


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TRIAL- 2 dated 25th April, 2006 (From 11:37:30 hrs to 11:51:05 hrs)

From the above trial , certain logics were modified for avoiding high

speed controller interference and in such a way that a gain of 1 . 5

will get automatically reduced to 0.3 during islanding operation to

eliminate the fluctuations. Islanding trial operation of unit - 7

along with units - 5 & and 6 was carried out under the conditions

of export of 30 MW and with smelter load of 195 MW by opening

the bus coupler (refer scheme 2). The transient performance was

not settling even after 1 3 minutes and wide fluctuations of load,

frequency and voltages were observed as below:

? Load fluctuation of 10 MW in unit 5, 30 MW in unit 6 and 60MW in unit 7

? Frequency fluctuations of 1 . 6 Hz in all units? Generator Voltage fluctuation of 2.5 kV in all units


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During wide fluctuation of load, frequency and voltage , it was decided

to take unit- 5 to load control mode for improvement . When Unit -

5 was taken to load control mode , the machine got unloaded fully and

tripped. The system frequency dropped and smelter feeder tripped on

under frequency and units - 6 & 7 tripped on over speed protection

after tripping of smelter feeder.


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The wide fluctuations observed in the above two trials were attributed

to mismatch in Governor/ Valve response time, AVR response time

between the old units and units - 7 & 8. Hence it was decided to

check the Governor/ Valve response time, AVR response of old units

and new units (7 and 8).

From 15th October 2006 to 18th October 2006, responses of valves

were recorded with high speed recorder, for units - 5, 7 & 8. AVR

responses were also recorded. The following were observed:

Serial No. TEST CONDITIONS OBSERVATIONS

1 UNIT 5: 3V 8V

step up command to

position controller

For 3- 8 V step command HPCV

and IPCV started responding

after 0. 2 seconds and stabilized

in 7 seconds (Refer graph 1 )

2 UNIT 5: 8V 3V

step down command

to position

controller

For 8- 3 V step down command

HPCV and IPCV started

responding after 0. 1 second,

HPCV stabilized in 2.6 secondsand IPCV stabilized in 5 seconds

( Refer graph 2)

3 UNIT 7: 3V 8V

step up command to

position controller

HPCV and IPCV responded

immediately, HPCV stabilized in

1 . 54 seconds and IPCV

stabilized in 5.29 seconds.

4 UNIT 7: 8V 3V

step down commandto position

controller

HPCV and IPCV responded

immediately, HPCV stabilized in2. 2 seconds and IPCV stabilized

in 5.4 seconds.


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5 AVR Step response in

unit 5

Found OK

6 AVR Step response inunit 7

Response was faster andretuned to match unit 5.

7 Unit 8 Valve response was similar to

unit 7 and AVR response was

retuned to match unit 5.

The above responses were further studied and it was decided to

carry out the following.

? To slow down the valve response of units - 7 & 8, PT blocks wereprovided (First order time delay function block) in position

reference circuit of HPCV 1 & 2, IPCV 1 & 2 and Over load

Control Valve (OLCV). This time delay block is in operation only

under islanding conditions and when frequency is less than 52 Hz.

? To Disable Power System Stabilizer (PSS) in units - 7 & 8 as wasin units 1 - 6, where no PSS is provided.

The above modifications were carried out on 23rd April 2008. On 25th

April 2008, islanding operation was carried out under the conditions

of export of 30 MW and smelter load of 1 95 MW by opening bus

coupler (Refer scheme 3). The transient performance was found to

be satisfactory with a settling time of 1 minute of load, frequency

and voltage. Machines were run on islanded mode for around 50

minutes. After satisfactory Speed reference of units - 7, 8 and 5

were varied one at a time and voltages and frequencies were matchedand re- synchronized with the grid.


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During wide fluctuation of load, frequency and voltage , it was decided

to take unit- 5 to load control mode for improvement . When Unit -

5 was taken to load control mode ,the machine got unloaded fully and

tripped. The system frequency dropped and smelter feeder tripped on

under frequency and units - 6 & 7 tripped on over speed protection

after tripping of smelter feeder.


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Conclusions:

After the above demonstration of Island operation at NALCO- CPP,it can be concluded that before carrying out islanding trials, the

following are to be checked / carried out for a successful islanding

operation

1 . Valve response time of each machine is to be measured,compared and matched with other units by suitable modifications

in position reference circuit and or in the servo timing of

valves.2. AVR response time of each machine is to be measured andmatched precisely by retuning AVR PID.

Documents

Islanding Operation