ROCOF settings used for islanding 1

[SA07]

Hi
For a small steam turbo-alternator 950 kW 400 V connected to the utility 22 kV network through a transformer, can we use a setting of 2.5 Hz/s delay 0.5 s? Or else what setting for such a plant is usually used? We want to use this ROCOF setting to open the tie breaker so that the plant can island in case of disturbance on the network.

 

[charlierod]

Hi SA07,

A 2,5 Hz/s setting is a rule of thumb that should work fine for low inertia small units if there enough power imbalance after loss of network inteconnection. You can still estimate the rate of change of frequency from the motion equiation of rotating masses (Newton's second law) based on power imbalance and generator/ turbine/ exciter WK2.

A delay of 500 ms seems to high in my opinion taking into account that the -2,5 Hz/s setting is secure enough so as to prevent false operations during interconnected system normal and abnormal frequency excursions. Also a maximum frequency for operation should be set and you have to consider steam turbine limits. Load shedding blocks of interconnected systems typically have more sensitive frequency setings and lower df/dt as equivalent inertia is much higher.

Regards,

Carlos

 

[crshears]

Further to what charlierod wrote:

"Maximum frequency for operation" is commonly part of the turbine speed governor; the steam turbine units I worked with some years ago had a separate overspeed governor with a fixed setting such that it would bleed pilot [ sometimes called "sensitive" ] oil from the governor hydraulic control system piping [ thus closing throttles, reducing steam flow, and checking rise in speed ] whenever the turbine had any degree of overspeed > 1%.

Minimum speed controls might be much more crucial, given how close to a critical speed steam turbines often operate at, often not very far below synchronous.

In my experience working with larger grids, I would highly doubt that you will ever see a grid ROCOF that even approaches 2.5 Hz/s, which you indicate you want to use to operate the grid tie breaker to island the plant from the grid. If the overall size and inertia characteristics of your grid are not that extensive, perhaps possibly you might see that ROCOF depending upon that system's most severe single contingency, but at first blush it seems unlikely to me.

I therefore submit it would make better sense to have a discrete frequency trip relay to separate the plant from the grid when the grid frequency deviates > +/- 1% from nominal, using experience moving forward to warrant any tightening or relaxing of this setting.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[RRaghunath]

Agree with suggestion of Carlos.
Time delay is not desirable and if adopted, the islanding may fail as the STG may not be able to sustain the disturbance. If the islanding is critical from process point of view, I would go for more sensitive ROCOF setting so as to increase chances of successful islanding, especially if disturbances frequent in 22kV system.

 

[crshears]

If Carlos' suggestion is better than mine, and I'm not saying it isn't, what is the sample rate of ROCOF relaying, and how many successive samples of sufficient departure rate must be registered before the ROCOF portion of the relaying operates? I ask because I've seen numerous cases where equipment trips upon frequency falling to 57.5 Hz [on a 60 Hz system ] with no intentional time delay, and in such cases the ROCOF protection would have to operate considerably faster than that to provide for some margin.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[redlinej]

Crshears,
I think OP is talking about anti islanding protection and if he is ROCOF is use to disconnect the genset from the grid.

 

[mgtrp]

More information about what the OP is actually trying to achieve might be useful.

A ROCOF scheme would typically be for detecting when the generation and some of the nearby distribution network have already become islanded from the wider grid. They're only useful/effective if there is a significant difference between the generation and the amount of load on the island. If there isn't, then something else is needed, e.g. an intertripping scheme. As an example of a typical setting for these (from memory!), the UK requires ROCOF for embedded generation to be set at 1 Hz/s for 0.5 seconds. The idea is that the rate and time delay are set above the largest frequency change that might happen to the national grid as a whole if the national grid were having a *really* bad day, but below the change that would happen if the embedded generation and the nearby distribution network became islanded from the wider grid.

If the OP is trying to isolate their plant from disturbances on the wider grid that aren't related to islanding (e.g., nearby faults that temporarily depress the voltage at the generation site but don't result in islanding) then ROCOF probably isn't the answer.

A situation I've not encountered but would be interesting would be if the OP were operating in a wider grid that was particularly unreliable and looking to isolate themselves from grid collapses or major frequency swings. Setting a ROCOF for that would probably be dependent on the grid and the types of frequency swings seen/how quickly the grid collapses/how well the particular STG can recover the frequency once islanded. Note that in some jurisdictions, there may be grid code requirements about ride-through for voltage and frequency disturbances.

Lastly, with regard to ROCOF operating times--this can vary between relay manufacturers (with documentation often hard to come by) and can be dependent on how fast the frequency is changing (will typically be faster for higher rates of change), but typically starts in the low hundreds of ms, up to about 0.5 seconds or so (plus any additional intentional time delay added).

Cheers,
mgtrp

 

[SA07]

The utility company has given this setting of ROCOF 2.5 Hz/s delay 0.5 s. It is for a small isolated island. The total demand of the island is around 400-500 MW.
We will use this setting among other protections to island our turbo-alternator.

 

[crshears]

Hello SA07, please, once you've gained some experience with this setting, let us know how it works out.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[SA07]

Thanks for your advice. I will give a feedback.