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Tuesday, 4 August 2015

MATLAB coding for Y Bus partition

Last Updated: Feb 26, 2017

Voltage Stability:

A power system is said to be voltage stable if it is able to maintain steady voltages at all its buses after a disturbance. In other words, one can say that voltage stability is the ability to maintain steady voltages at all the buses in the power system after being exposed to a disturbance. The disturbances may be:

  1. Line or Generator outages,
  2. Increase in loading,
  3. Generators, synchronous condensers and other reactive power sources inching close to their reactive power limits.
A power system is voltage stable if the magnitude of  voltage at a bus increases as the reactive power injection (at the same bus) is increased. At a given operating condition, this is true for every bus in the system.

When the reactive power demand of the load is not fulfilled, voltage collapse occurs. Voltage stability of a power system is on the verge of collapse when a disturbance increases the reactive power demand beyond the available capacity of the system components. Voltage collapse is a usual phenomenon in a heavily loaded power system or a system having shortage of reactive power. The voltage drop in the line impedance during power flow is the main cause of voltage instability. This reduces the power transfer ability of the transmission system and also reduces voltage support ability.

A system is "voltage unstable" if the magnitude of voltage at one or more bus decreases when the reactive power injection at the very bus or buses is increased.

Thus, for a power system, if the V-Q sensitivity is positive for every bus, the system is voltage stable, otherwise for a negative V-Q sensitivity, the system is voltage unstable.

Voltage Stability Index:

Voltage stability analysis of a power system involves determination of an index called the “voltage stability index” which is used as a measure of inclination of Power system towards voltage collapse. These indices are helpful in determining the weak bus so that adequate reactive power allocation can be done.

Methods of determining the Voltage Stability Index:

There are few methods of determining the voltage stability index and “L-index method” is one such method. 

In L-index method, one has to partition the Y bus matrix as YGG, YGL, YLG, and YLL, where ‘G’ stands for generator and ‘L’ stands for Load. Matrix YLG, and YLL are required to calculate the matrix FLG needed for calculation of L-index. Detailed theory can be seen in many research papers.

MATLAB coding for Y bus partition:

The MATLAB coding for Y bus partition is as given below:

 clear; clc;
% File gives the partition of Y bus.
num=6;   % specify the bus system if you to work with many examples.
%  a function file “volt_ang” gives the admittance, magnitude and angle of bus voltage.
%  This file is a part of the NR load flow code  and not given here.
[Y, Vm, Va]= volt_ang(num)
linedt= line_data(num);                     % calling the line data for the system
busdt= bus_data(num);                     % calling the bus data for the system
nb= max(busdt(:,1)) ;                        % gives the total number of buses in the system
type =busdt(:,2) ;                    % identify the type of bus i.e. ref., generator, and load      
pv = find(type==2 l type==1);           % identify the PV bus           
npv = length(pv);                               % gives the number of PV buses
pq = find(type==3);                          % identify the PQ bus 
npq = length(pq);                               % gives the number of PQ buses

for m=1:npq,

for n= 1:npq,
YLL (m,n) = Y (pq(m), pq(n));

for m=1:npq,

for n = 1:npv,
YLG(m,n)= Y(pq(m), pv(n));

FLG = (YLL)^-1*YLG

Monday, 3 August 2015

Special arrangements for transportation of Large Power Transformer

Large Power Transformers (LPT) are large in dimension, and heavy in weight. They can cost millions of dollars and weigh between 100 to 400 tons. For example a 765 kV, 750 MVA, three phase transformer with size 56 ft (W) x 40 ft (L) x 45 ft (H) can weigh 410 tons  They pose unique requirements to ensure safe and efficient transportation. Hence the weight and dimension of large power transformers need careful planning and the critical transportation aspect should be kept in mind.  

Power transformers can be transported by rail, road, air and sea route. Depending on the size of the transformer unit and on the route and transport conditions, a transformer may be transported completely or partially assembled. LPTs have to be transported with bushings, conservator, cooling arrangements and all other minor accessories removed. If the transformer tank has been drained for transportation, it is necessary that the oil should be replaced by dry air or nitrogen maintained at a slightly positive pressure above the atmosphere. This ensures the dryness of the winding during the entire transportation.

Large power transformers cannot be transported on normal rail cars. The heaviest load a rail-road normally carries is 100 tons whereas the LPT can be 4 times of that weight. A specialized rail-road car called Schnabel car, is used to transport extremely heavy loads. Some LPT are designed and made as an integral part of the Schnabel car. The transformer is designed so that it can be attached to rail car frames with the help of a pinning system. These cars may have 20 or more axles depending on the weight of the transformer to be transported.     

When LPTs are to be transported via road, special permits are also required from various government agencies. Before issuing these special permits, careful inspection of the entire route through which the transformer has to pass, is carried out. Inspection of bridges and their load bearing capacity are of prime importance while issuing such permissions. Hasty permits may lead to serious accidents as one happened in Madhya Pradesh in year 2011, in which a huge trailer carrying a 380 ton power plant equipment on Sagar-Bhopal road was washed away when the bridge through which the consignment was passing collapsed due to the heavy weight killing at least 3 persons and damaging the power plant equipment. 


Sunday, 2 August 2015

Use of Vegetable oil as dielectric medium in Power Transformers

Electric transformers while in operation produce heat due to iron and copper losses (although stray losses are also there). The heat thus produced must be carried away swiftly to avoid excessive temperature rise in various parts of the transformer such as winding and insulation. The cooling medium used must prevent excessive rise in temperature in any portion of the transformer and should avoid formation of “hot spots” within the transformer.

Mineral oil is normally used as an insulating and cooling medium in power transformers. The oil covers the core and coil assembly completely and fills small voids in the insulation to enhance the transformer performance. 

Advantages of Vegetable Oil:

Over the years mineral or silicone oil has been used as insulating and cooling medium in the transformers. Vegetable oil such as rapeseed, sunflower oils etc. are bio-degradable and have a much higher flashover point, are environment friendly and less inflammable. Vegetable oil has higher flash and fire point when compared to mineral oil. Similarly the dielectric strength is also higher. 

Properties of  Vegetable Oil-based Envirotemp Insulating fluid:

The flash point and dielectric strength of “Envirotemp FR3” vegetable oil-based insulating fluid is 330oC and 56 kV at 25 oC whereas for mineral oil the values are 147 oC and 45 kV respectively.  Transformers using vegetable oil will require lesser fire safety systems. Since transformers with vegetable oil are better in terms of fire hazard protection, hence can be used in environment sensitive and densely populated areas.

First EHV class Power Transformer with Vegetable oil:

Siemens has successfully produced and commissioned, in 2014, the world’s first EHV class power transformer that uses vegetable oil as the dielectric medium. The transformer, which is a 380/110 kV power transformer, uses nearly 100 tons of vegetable oil and is commissioned in Bruchsal-Kandelweg substation in Germany. 

Although using vegetable oil in power transformers is not new. Siemens have produced and commissioned more than 30 transformers that use vegetable oil as a dielectric medium up to 69 kV class transformers with individual capacity of 30 MVA.