PowerApps software product is a Proprietary Property of Dr.Raghunatha Ramaswamy. No company with whom Dr.Raghunatha Ramaswamy was associated in the past either as consultant or as an employee do not own the proprietary property for this software.
 
Home
PowerApps Software Modules
PowerApps Calculation Engine
Software Services
Power System Studies
Engineering Services
Power System Training
Downloads
Profile
Contact Us
Partners and Associates
Site Map
Send an Enquiry
Sample Study Cases of the Services Rendered
Real Time Security Assessment and Remedial Action Solution
The current time is ?

 

1         Introduction

 

In power system studies/analysis we often come across a situation, where a significant portion of the external grid network is not represented for the studies. This external grid network however needs to be represented as an equivalent generator for purpose of the studies to reflect the influence of the external grid network on the portion of the network for which studies are being carried out. This document explains the factors to be considered for such equivalent generator representation.

2         Representation of the External Grid for the Load Flow Studies

 

In this case the representation is the simplest form. All we need to specify the load/generation at the bus representing the equivalent bus. This load/generation will represent the power exchange between the retained portion of the network and the omitted portion of the network. The equivalent bus may be voltage controlled or simple load bus.  A voltage controlled bus may be used, when we know that the equivalent bus voltage of the external grid lies within a specified range. Else a load bus can be adequate. However, if the model has to be used for stability studies , it will need to be represented as a generator bus, with suitable inertia constant and voltage controlled bus. This will be elaborated in section 4.

3         Representation of the External Grid for the Short Circuit Studies

 

In this case the omitted portion of the network should be represented by Thevenin’s equivalent Generator/source, such that the fault current contribution from the Thevenin’s equivalent source will be same as that from the omitted portion of the network.

Thus if a bus has a fault level of Iflt, with fault current contributions I1, I2, I3, ……… , In, and if Ij. ….. In are the branches omitted from network representation and I1…. Ij-1 are the fault currents from the branches represented in the network model, then the Thevenin’s equivalent generation must be such that it contributes the fault currents matching the omitted branches in the network. i.e. Total fault current contribution from Thevenin’s equivalent source should be equal to sum of [Ij+….. +In].

4         Representation of the External Grid for Stability Studies

 

For Stability studies, apart from the load flow and Thevenin’s equivalent source representation of the sections 2 and 3, we need to consider suitable “inertia” constant for the generator, other electrical parameters of the generator and also suitable governor/excitation system control. These choices of parameters and representation of the equivalent generator, depend on factors that influence the power swings between the omitted portion of the network and the retained portion of the network, such that the equivalent generator produces similar power swings and other dynamics as the complete system representation used for power system studies. This requires knowledge of how the system will behave under complete system representation.

5         Thevenin’s Source Impedance Calculations

 

1.     Let MVA3Ph be the 3 phase fault level at the equivalent external grid bus due to omitted portion of the external network and I3Ph, be the corresponding 3 phase fault kA at the bus. Let I3Ph = 35.41 kA

2.     Let MVA1Ph be the single phase to ground fault level at the equivalent external grid bus due to omitted portion of the external network and I1Ph, be the corresponding single phase to ground fault kA at the bus. Let I1Ph = 31.57 kA

3.     Let the equivalent generator needs to be generated at the 220 kV, bus at which the external network representation is omitted. The fault levels in MVA are computed as = Sqrt(3.0)*220*Fault KA , for the 3 phase and single line to ground fault levels and these values are 13493.0222 MVa and          12029.78568 MVA respectively.

4.     The per unit value of fault levels on 100 MVA base are 134.93 per unit and 120.29 per unit respectively.

5.     The positive sequence fault current for 3 phase to ground fault level is the same as the 3 phase fault level MVA , i.e 134.93 per unit. Since I1, the positive sequence fault current is [1/Z1], where Z1 is the positive sequence per unit source impedance, we get Z1 = 1/I1 = 1/134.93 = 0.007411238 per unit.

6.     We assume that negative sequence source impedance is same as positive sequence impedance, which is a reasonable assumption for the external grid source representation. Consequently Z2 = Z1 = 0.007411238 per unit.

7.     For the single line to ground fault, because the Phase current Ia = I1+I2+I0, and I1=I2=I0, I1 the positive sequence current will be 1/3rd of the single line to ground fault level , i.e = [120.29/3] = 40.0992856 per unit.

8.     For single line to ground fault current, since I1 = 1.0 /(Z1+Z2+Z0) and Z1 = Z2, using values of Z1 and Z2 from step 5 and I1 from 7, we get Z0 = 0.010065242 per unit.

6         Note

 

This web page was prepared as part of explanation to the practicing engineers working under guidance of this author. It is hoped to be useful to the practicing engineers who would like to know the method of representation of the equivalent external generators for the power system studies.


http://www.google.com
http://www.activesearchresults.com
Mail your questions, enquiries to raghunatha@powerapps.org