Unstable Power Swing ?

[shalhoob109]

Hi,

For the phenomena "out of step" or unstable power swing what shall be used in protection?

1- Just blocking the distance relay as stable power swing?
2- Tripping lines to protect generator?
3- If tripping is required to protect the system. What exactly is needed in the system study? R/X values ? short circuit level ?



shalhoob

 

[shalhoob109]

up up

 

[electricpete]

2- Tripping lines to protect generator?

Hmmm, that could make the situation worse if generator remains on-line and excited. Trip the generator.

3- If tripping is required to protect the system.

The tripping is to protect the generator (not the system) from unstable swings that could cause pole slip of the generator.

1- Just blocking the distance relay as stable power swing?
....
3-..What exactly is needed in the system study? R/X values ? short circuit level ?

I'm not particularly familiar with how it is set. I remember for GE electomechanical relays there are 2 relays (KD something), at least one is a distance relay, and one blocks the other...that's about the extent of my memory. I'm guessing the literature for out of step protection relaying will have some info on how it works and how to set it.

=====================================
(2B)+(2B)' ?

 

[davidbeach]

Basically you set your line out-of-step the way the planners say you should. That determination generally isn't made by the protection engineers.

 

[m3ntosan]

2 and 3 are the same thing, you'll find it active on tie lines (interconnection lines) between countries/states to stop major disturbance spreading, in coordination with 1, you can't have one without the other (at least in Europe ENTSO-E area).
In the UK for instance, which is very strongly meshed HV network and without AC links with neighbours, these options are not used at all.
Check on SEL website, there are some very good papers about out-of-step philosophy.

May you grow up to be righteous, may you grow up to be true...

 

[shalhoob109]

thanks every one.

I found this in SEL website.


Out-of-Step Tripping and Blocking Functions


Out-of-step tripping schemes are designed to protect the power system during unstable
conditions, isolating unstable generators or larger power system areas from each other with the
formation of system islands, in order to maintain stability within each island by balancing the
generation resources with the area load.
To accomplish this, OOS tripping systems must be applied at preselected network locations,
typically near the network electrical center, and network separation must take place at such points
to preserve a close balance between load and generation. However, as discussed earlier, many
relay systems are prone to operate at different locations in the power system during an OOS
condition and cause undesired tripping. Therefore, OST systems must be complemented with outof-
step blocking functions to prevent undesired relay system operations, prevent equipment
damage and shutdown of major portions of the power system, and achieve a controlled system
separation.


In addition, OOS blocking must be used at other locations in the network to prevent system
separation in an indiscriminate manner. Where a load-generation balance cannot be achieved,
some means of shedding nonessential load or generation will have to take place to avoid a
complete shutdown of the area.
Typically, the location of OST relay systems determines the location where system islanding
takes place during loss of synchronism. However, in some systems it may be necessary to
separate the network at a location other than the one where OST is installed. This is accomplished
with the application of a transfer tripping type of scheme. Another important aspect of OOS
tripping is to avoid tripping a line when the angle between systems is near 180 degrees. Tripping
during this condition imposes high stresses on the breaker and can cause restrikes and breaker
damage.

 

[rcw retired EE]

What is needed for an OOS trip setting calculation for a generator? A lot of data on the generator, transmission system and power system is required.

To properly set a 78 function, the behavior and response of the generator during and after a fault or disturbance is required. The simulation will need all the generator characteristics: impedances, inertia, governor response, exciter response, load prior to fault, load after fault, breaker clearing times, relay settings etc. The simulation will determine if the generator will remain stable and provide the loci of the generator operating point on the R-X plane. The simulation will have to cover multiple faults and fault locations.

The OOS relay can be set to allow a short, recoverable excursion into the tripping zone but trip on an actual out-of step.
Most OOS relays set up a tripping zone around the X axis on the R-X diagram. If the generator operating point, as seen by the relay, goes into the zone and out the other side a pole slip has occurred and the unit trips. Or if the generator stays in that zone too long it trips.

It's possible to estimate a setting that protects the generator but may trip it unnecessarily during disturbances using rule of thumb data on generator inertia and reaction times.

Some relays define the zones by the angle between the generator and system voltages. On an R-X diagram this is the angle between vectors drawn from the generator and system infinite bus points to the operating point. If that angle is >90 or 120 degrees, it is unlikely that the system will recover. If it is >180, the pole has already slipped.

 

[shalhoob109]

rcwilson thanks for your participatin. this matter was raised after our consultant has proposed the use of this function but it was new for me as a protection engineer. calculation which you mentioned shall be done by planning division peronnel who have
parameters of the whole syetem and especially data of generators.
this can be set and sent to us at site. Another interest to me is how to simulate unstable power swing using test kits at site after
recieving setting or data required for OOS.

 

[DTR2011]

Testing OOS function is tricky. To my knowledge, Omicron has a dedicated test module for this that works well. Other ways are to use COMTRADE output from some coordination software.

 

[OliverJanke]

We (OMICRON) also offer application notes on how to test the power swing blocking or tripping function with a number of our test modules. You can find them in our customer area on our webpage for download (free of charge). The attached one describes how to test with the OMICRON NetSim module.

Oliver Janke
OMICRON