Rotor angle stability literally refers to the stability of conventional synchronous generators no matter it is large-disturbance/transient stability due to lack of synchronizing torque or small-disturbance stability due to lack of damping. If there is no SG in the system, technically there should not be any rotor angle stability problem or it should not be called rotor angle stability. But in this case, does the synchronism/damping problem still exist? The answer is Yes. You must have seen a lot of papers talking about PLL stability right? (if you would like to name it rotor angle stability, you can define youself!) The 'new' synchronism/damping problem for power electronic converter-interfaced equipment mostly manifest in the form of undamped oscillations due to poor tuning of controller parameters in the lower voltage systems. When a grid has both SG and new power electronic converter-connected generation, the dynamics might be much more complex considering their interactions across a range of frequency.

Recently, there is a paper entitled 'Microgrid Stability Defintions, Analysis and Examples' provided by IEEE PES Task Force on Microgrid Stability, which might help you understand this issue better.

Charitha Buddhika Heendeniya : "Rotor angle stability" is a type of "transient stability" problem related to rotating machines. Traditionally, "rotor angle stability" has been studied for rotor angles of the synchronously rotating machines (generators) in the transmission system. Also, traditionally, in the distribution systems, the "transient stability" problem has been studied in the context of induction motors. In the paradigm of high penetration of inertia-less distributed generation (for example PV) in the distribution system, one might:

(i) study rotor angle stability problems in small microgrid systems equipped with small diesel generators, along with non-rotational PV generation.

(ii) study the common problem of transient stability of motors, with or without microgrid systems.

It would be interesting to see how the system stability is effected with different levels of PV penetration. A caveat is that this problem is hard to simulate.

I think unless the distribution system has multiple synchronous generators in it, the classical rotor angle stability (specifically transient stability) problem may not be seen.

However, if the test system has a mixture of PV and synchronous machines; The PV inverter and especially their controllers can interact with each other or with other inverter based technologies as well as with controllers of synchronous generators (AVR and PSS) which can lead to rotor angle stability problem.

Hope it helps

Samundra

Srivats Shukla & Samundra Gurung Thank you very much for your answers. As you said, for the direct coupled generators, the rotor angle stability is still defined. However, do you know how the same rotor-angle stability problem can be applied (or modified) to suit for inverter coupled PV generators acting under largely fluctuating frequency regime?

The idea is to understand what are the limitations for inverter coupled PV units (or battery units) to stay connected with the grid without falling out under extreme frequency fluctuations, which is similar to rotor-angle stability for a rotating machine.

For example, in a voltage stability scenario where the DGs are expected to provide large reactive power compensation, the capabilities are limited by the inverter power rating. Are there any such conditions for frequency stability?

Samundra Gurung The interactions that you mentioned between controllers of PV units: Do you mind elaborating a little on what kind of interactions you mean? Maybe if possible, it would really help if you could provide an example. Thank you.

Charitha Buddhika Heendeniya : Just for my clarification, are you discussing the frequency stability or the transient/rotor angle stability problem? Frequency stability and its regulation is very well discussed in the literature for both transmission and distribution systems (with and without PV, eg. https://ieeexplore.ieee.org/abstract/document/4696040). Rotor angle instability cannot occur if there is no rotating part. However, as Samundra Gurung pointed out, interactions between different control systems can give rise to other types of transient instabilities, which should not be denominated as "rotor angle stability".

I agree with the answers given earlier. Rotor angle stability can be investigated if load is modelled in detail for the study purposes and the external system is represented either with two or more equivalent generators at the points of supply to the distribution system.

It at also be possible with one source if there is only one point of supply.

With all these, final outcome will also depend on the converter inverter capacities and the controllers used.

Srivats Shukla : Thank you very much. I think I had a slight misunderstanding there. Your input was invaluable.

Charitha Buddhika Heendeniya : Happy to hear that we can help each other. Just to be consistent with the power system community, our group rigorously follows the definitions and classification of different types of stability laid out by the IEEE/CIGRE joint task force. You could follow this publication (if not already done so):

Article Definition and Classification of Power System Stability IEEE...

You have the same issue, no matters is there is a low voltage or high voltage system. In principal with large amount of PV you will significantly decrease the inertia in system. That means less time to loose stability and you have additional issue with lot of electronics, like problems for relay protection.

I believe the deliverables of two Horizon 2020 projects could be of interest to you: RE-SERVE and MIGRATE.

Both of those projects deal with scenarios of 100% inverter-based systems and with dynamic stability of such systems.

Defining dynamic stability of inverter-based systems and interactions between synchronous machines and inverters is a hot topic in research right now.

https://www.re-serve.eu/deliverables.html

https://www.h2020-migrate.eu/downloads.html

Matej Krpan : Thank you. This looks very useful.

Rotor angle stability is basically associated with rotating devices like generators. It indicates whether generators are in synchronization or they have lost it and we all know if synchronization between different generators is lost then what would be its impact on the whole system. In case of PV base sources (non rotating), there is no moving parts involved but it might go towards de-synchronization (due to its control strategy) in different transient conditions, I think it would be the equivalent case to rotor angle stability (in case of rotating devices).

Charitha Buddhika Heendeniya

Rotor angle stability literally refers to the stability of conventional synchronous generators no matter it is large-disturbance/transient stability due to lack of synchronizing torque or small-disturbance stability due to lack of damping. If there is no SG in the system, technically there should not be any rotor angle stability problem or it should not be called rotor angle stability. But in this case, does the synchronism/damping problem still exist? The answer is Yes. You must have seen a lot of papers talking about PLL stability right? (if you would like to name it rotor angle stability, you can define youself!) The 'new' synchronism/damping problem for power electronic converter-interfaced equipment mostly manifest in the form of undamped oscillations due to poor tuning of controller parameters in the lower voltage systems. When a grid has both SG and new power electronic converter-connected generation, the dynamics might be much more complex considering their interactions across a range of frequency.

Recently, there is a paper entitled 'Microgrid Stability Defintions, Analysis and Examples' provided by IEEE PES Task Force on Microgrid Stability, which might help you understand this issue better.