Hi Soudipan, the answer to this depends on your application type and on the load type that you are aggregating. For example if your application is power flow simulations and all the 66 kV substations have constant power loads, then you can simply add them up with some percentage added for transformer (66kV) losses and attach the sum directly to the high voltage station. If the loads are a mix of constant current, voltage and load type and/ or your application is dynamic simulations of power systems, then aggregation requires a proper modeling and simulation effort

Srivats Shukla : Thank you for your answer. My requirement is indeed running dynamic simulations in the power system and I am currently using only constant impedance type loads. It would be great if you could suggest some relevant literature on how to do the aggregation of loads for this very purpose. The main challenge is that the network I am using is a highly meshed one and any 66 kV S/S can be fed by multiple 220 kV or 132 kV S/S. Is it possible the segregate the loads among each and every 220 kV or 132 kV S/S in the system in such a way that it closely replicates the behaviour of the original system? Are there any available literature on this?

Hi there, I think you can use a ZIP model load with the values of Z, I and P each as 33%.

You may be thinking that the load variation is huge at low voltage levels and it may affect the modeling if you consider it as a simple load for your system. However, in practice the operators at transmission level sees the load almost as constant. This is because when you aggregate lot of varying loads, all the variations add up and the total load is almost seen as constant.

Have a look at the attached paper too for more information.

Soudipan Maity , apart from the reference that Samundra Gurung has suggested, you might also take a look at Dr. Davy and Dr. Hisken's work attached herein.

Search for "load aggregation for dynamic simulation"

I am guessing it all depends on how accurate you want your results to be.

Two papers shared provides very good source material to understand load model. However these papers are dated Aug 1995 and Jan 1997. They predate prolification of power electronics based loads such as Voltage Source Converter, switched mode power supplies (TV, modern fridge/freezers, LED lighting, variable speed drives for motors - generally all loads which are energy efficient) etc. How do you model VSC or SMPS as a load? Does it fit well with constant impedance (Z), constant current (I) or constant power (P) load models?

The other load model I have seen is voltage dependent load model. Having said that, I do not have any papers describing this model. However it was implemented in DIgSILENT Powerfactory - network modelling software package I used to model distirbution network models in Auckland and Wellington, New Zealand. You can use voltage dependent load model to model Z, I or P by changing the power of the volatge function. Please Google search "voltage dependent load model".

One suggestion I can offer for your modelling is that if you have access to a complete network model, then use it to perform load flow keeping note of 220kV and 132kV bus loads and voltage results. After that, replace the lower voltage network with equivalent "loads". Make educated guess of your equaivalent loads to see how well the results compares with your previous load flow results. If they are within acceptable region, then you have found your equivalent loads representing 66kV and lower voltage networks and connecetd loads.

Elimination of lower voltage network would reduce your network model considerbly thus making your dynamic simulation faster. Other than this advantage, I do not see any other reasons why you need to reduce your network model. Or are you limited by number of nodes that your software can accomodate to model networks?

I can think of a simple solution. You can get a load flow solution for the lower level networks assuming the higher voltage feeding buses as generator buses. This will provide you with an information the kind of load being served by higher voltage entry points. Depending the conditions at which you are trying to find out the loads at the higher voltage buses you will be getting different loadings.

changing the level of supply voltage from Major station,sub transmission system to primary distribution system depends on switches regulators and load flow studies such as given below.

Circuit breakers

Disconnecting switches

Station buses and insulators

Current-limiting reactors

Shunt reactors

Current transformers

Potential transformers

Capacitor voltage transformers

some how it also depends on the level of converters from one stage of level to second stage of levels.

Hi, Using a load model is very difficult, since it depends on what the physical characteristic of your load really is for the accuracy of the results. The other important aspect that is vital is the type of study you want to perform. These two aspects are what tell you what is the approach you can take. When you are referring to a dynamic analysis, what type of study do you want to carry out:

-Rotor Angle Stability

-Voltage Stability

-Frequency Stability

The type of study is important because they have different analysis time and from there to know which load model to use.

You can consult the book Power System Stability and Control of P. Kundur and the paper Definitions and Classification of Power Systems Stability that can clarify you.

The subject of the load is a really beautiful and controversial subject in the studies of electrical power systems. In my personal experience carrying out transient stability studies, not taking the proper model of the load has generated incorrect results, such as generators that are motorized when this does not happen.