In PV generators there is the PV array, the DC/DC converters controlled by a maximum power point controller which contains inductive elements. The controller has a settling time for the output power. There is the inverter which contains contains also controller to control the output power and a law pass filter.

So, it contains also delay components.

The same can be applied on the wind energy that have initially mechanically moving parts with real inertia. This is in addition to power electronic components

which has delay times.

I think the matter is not easy and one can find a simple answer for this question.

So i think one has to develop a physical model considering the real behavior of the system components.

Best wishes

Needed for synchronized operation of rotating machines together and their stable operations for transient surges for bulk power switching, though with solar and wind power the important of inertia, and interconnection with tieline have reduced, working power factor have improved from 0.8lag to higher near unity, even for stable operations, or inertia consideration have reduced for stability. Though it cannot be avoided completely till rotational machines are there in systems as load and source, but completely blackout is now just a Dream Story. Development for EHVAC and HVDC generations and distribution have changed to development of bigger and bigger solar and wind power stations 🚉 interest have changed and would be 440KV...

I agree with what Bhupendra Desai is saying. But consider a scenario of future power systems which you are pointing to. In the near future residential microgrids will be the new norm for the utilities to handle and that too as autonomous systems. You can expect local roof-top PV systems and storage devices in such microgrids which have no inertia of their own. I have worked on such systems and they present some interesting issues and problems of their own which require unconventional means to solve them. For more information you can look at my papers in my profile.

The inertia of today's power system decreases as more and more converter connected generation units and load are integrated into the power system. This results in a power system that behaves differently from before which causes concerns for many grid operators.

Term " inertia will and has to exist as long as Synchronous machines are in use in power systems. Whether they are the major contributors or not it is essential for ensuring stability from the point of view synchronous machines.

The concept of "inertia" is being broadened to "Resilience", which includes the impact of rotating plant but adds the stiffening from power electronics and other factors. Inertia is still a valid concept but it is within a broader framework when a large proportion of inverter based load is included in the network.

It is fundamentally an issue of rotatiing devices versus static devices for generation, load, and any other associated complexities. So long the former ones are present inertia would be a factor to be reckoned with.

I think the general term "inertia" is still valid : "Inertia of a power system is the ability of the system to oppose changes in frequency".

This will always be needed for AC power transmission.

Today people use the term "synthetic inertia" for power inverters with batteries or other sort of energy storage. I would use the term "mechanical inertia" to refer to the inertia provided by kinetic energy of rotating masses

If we think of a system where there is no rotating conventional generator, there will be no inertia, it's true. But for frequency stabilization, inertia is needed. That's why researchers are modeling new ways that can act like inertia of rotating machine. Just think of an energy storage which provides active power in case of a load increase, thus reducing frequency excursion. This is exactly what inertia of rotating machine would have done. So, inertia concept will be valid even if there is no conventional rotating generator.

As the power of storage batteries of solar power station via inverters would stabilize the frequency too....

Inertia causes time delay in happening / action with somewhat smooth transition from a state to other. Yes, energy storing and releasing from battery with static-converter mechanism in force both ways would be rendering time-delays, whatever for small time it may be. Inertia is one of the forms to cause time-delay, but every time-delay in the future grid may not be due to inertia.

Well, what I want to truly express is, in a 100% inverter-based system, if we generate a "digital phase" (2pi*f*t) to control the inverter output, then the system frequency has no relationship with (rotational) energy. Then is "inertia" still meaningful?

This issue has already been covered under my previous response, namely, inertia causes time-delay in changing from one state to other, and so in controlling any action through circuitry. But definitely the latter is not due to inertia, but on account of inherent (unintentional) or intentional time-delay.

The basis of existing and future electrical networks are electrical machines, hydraulic and thermal systems, etc. Therefore, the term "inertia" to such networks is applicable-as to any Electromechanical system.

Circuit considered to be inductive, T=L/R where L=do/di, the variation of flux with variation of time....

I recommend to see the following article. It would be helpful.

Teimourzadeh, S., Aminifar, F., Davarpanah, M., & Shahidehpour, M. (2018). Adaptive control of microgrid security. IEEE Transactions on Smart Grid, 9(4), 3909-3910.

Any electric network of alternating voltage even in the presence of a large number of static elements (solar generation and power converters) have inertia: electromagnetic (active - reactive circuits) and Electromechanical (generators, motor load: primarily industrial).

Thanks for asking. I think the concept of inertia is still meaningful with near 100% penetration level of renewable power, considering the synthetic inertia from renewables in discussion now.

The inertia provides the quickest response for contingencies in the large power system. In a conventional power plant like coal, gas and even nuclear, electricity is generated by a turbine, basically a large spinning mass of metal. The inertia stored in these rotating turbines provides an energy store which automatically stabilises the system and insulates it from sudden shocks. In an event of a generation outage or surge in demand, inertial energy is released which prevents the frequency from falling. Equally the inverse happens in the case of a sharp increase in electricity supply or decrease in demand.

Renewable generation such as solar PV array and wind turbines does not have such mechanism to provide the inertia. Therefore the system needs more reserve provided by part-loaded thermal power plants. Alternatively, special control strategy can be applied on wind turbines to emulate the dynamic behavior of "inertia". In the way the power system can save fuel costs and reduce the emission due to reserve provision.

If you are interested in, I calculated the system inertia for UK power system over a year course as an example, in https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8905596

Hope it helps,

Yifu

It's a good explanation indeed.

The issue is correctly reflected dragging spinning reserve from rotating machine based generation under partial loading. But what is next, when the system is with near 100% penetration level of renewable power, predominantly with Solar Generation? That time with Battery Energy Storage System developed and spinning reserve likely to come from that can there be any effect of inertia?

The comment of Preston very rightly quote on weak ac grid in terms of low SCR as well as inertia. One affects voltage regulation whereas the other the frequency excursion vis-à-vis stability. Both are virtually absent in dc system, although with ever-increasing load, voltage at load end is an issue, as there is nothing like, synchronism. Therefore, with a mixture of ac-dc system with all forms of renewable, in particular predominantly solar generation and then wind-based, effect of inertia may still continue, though to a restricted extent.

In PV generators there is the PV array, the DC/DC converters controlled by a maximum power point controller which contains inductive elements. The controller has a settling time for the output power. There is the inverter which contains contains also controller to control the output power and a law pass filter.

So, it contains also delay components.

The same can be applied on the wind energy that have initially mechanically moving parts with real inertia. This is in addition to power electronic components

which has delay times.

I think the matter is not easy and one can find a simple answer for this question.

So i think one has to develop a physical model considering the real behavior of the system components.

Best wishes

The concept of inertia is helpful practical concept especially when it comes to mechanics which cannot be altogether disposed off from the equation when it seems to play a role, but it still can be limited to an extent that it might not play a major role (but it still plays a role).

Dear Zhaoshun Deng

You raised a good issue.

Today, the modeling trend of renewable energy power plants is such that they should perform the tasks of conventional power plants. Therefore, if these types of power plants reach a high penetration level, they will be able to easily control the power system frequency as well as conventional power plants.

The problem now is that these types of power plants have a very low penetration level, and on the other hand, the issues of frequency control and LFO damping have not been a priority for their modeling.

Note that although many studies have been done on renewable energy power plants, they are still in their infancy.

Therefore, in my opinion, with the complete exit of synchronous generators in the future, not only will we not have a problem in terms of reducing inertia, but the speed of the system's response to system power fluctuations will be faster than the current situation.

please refer to papers as below:

Article Optimal fractional-order PID controller of inverter-based po...

Article Damping of low-frequency oscillation in power systems using ...

For more descriptions, Please refer to link as below:

https://www.researchgate.net/project/Damping-of-Low-Frequency-Oscillation-Using-PV-Farms

I am not sure if you are still looking for an answer, but let's try to feed this interesting discussion...

From a dynamic perspective, zero inertia is not possible. Very low inertia may be technically possible, but then we will touch an economical problem.

In ac power systems , deviations of the average system frequency from its rated value are proportional to the mismatch between power generation and demand. In other words, if power generation exceeds the demand, frequency increases beyond its rated value, until energy balance is achieved. Conversely, the frequency decreases if there is power generation deficit. This concept is better understood if one considers all generators in a power system as an equivalent mass rotating around a shaft with damping at a rated angular speed ws.

If Newton's second law of motion is applied to the system, you will end up with the following equation:

J dw/dt = TG - TL - B (w - ws) (1)

where J, B represents the equivalent inertia and damping coefficient of the system; w is the average angular speed of the system; TG and TL are the equivalent torques from generators and loads.

Now, if you multiply eq(1) by w and rearrange, you get the state-space representation of the system with w as state:

dw/dt = - B/J (w - ws) + (PG - PL) / (w J) (2)

where PG and PL are the equivalent active power from generators and loads. By the way, this is the famous Swing Equation (without the simplifications you see in most books).

As you see, if J = 0, dw/dt goes to infinity. If you have a very small J, you will need a very big B to compensate that and make your system stable when power imbalances occur, i.e. when PG is different from PL. Very big B means a very big reserve capacity, also called frequency containment reserves (FCR) in Europe. Very big FCR means you need a lot of power units connected to the system without producing any power, only waiting to compensate the next power imbalance.

So, small inertia is not only a technical, but an economical problem. You don't want to have a lot of units connected to grid only to compensate power imbalances for a very short period and then operate in no-load for most part of the time.

More on that on these reports:

https://eta-publications.lbl.gov/sites/default/files/frequency_control_requirements_lbnl-2001103.pdf

https://www.entsoe.eu/Documents/Publications/SOC/Nordic/2018/System-inertia.zip

https://www.entsoe.eu/Documents/Publications/SOC/Nordic/2018/System-inertia.zip

Most of the industrial loads are still motors (medium voltage, and low voltage).

So, the power consumption side still has inertia despite the generation side is becoming with low inertia with 100% renewable. By the way, hydropower, concentrated solar power (with steam turbines) are renewables and their rotating generators still have inertia.