MIpower frequently asked questions

General

Can number of contingencies be more than one?

Yes. In each contingency user defined no. of elements can be outaged.

Whether Transformer primary should always be H.V.?

Primary voltage can be either H.V. or L.V. depending on the taps provided.

Why Cost Coefficients are given in Generator data?

These are the cost coefficients used in real power optimization study.

Why breaker rating is given in all menus?

The breaker rating in all menus is just to verify whether the fault MVA is above or below the breaker rating.

Where participation factor field is used?

Participation factor is considered in Flat Frequency Control option to simulate the secondary control effect.

Difference between Schedule power and real power.

Schedule power is the power generated by the generator at that instance and it should be less than or equal to maximum real power generation.

What is the significance of Contingency Weightage?

Contingency Weightage is used to increase the severity of given outage for a particular bus or line. Normally the weightage value is unity. However, if a particular bus voltage deviations should not occur for any outage, and even for small deviations, that outage need to be given higher ranking, then the bus weightage no. is given greater than unity say, 5. Then the voltage deviation is multiplied by 5 times, and hence the contingency ranking will go high. For 400 kV buses and buses connected to neighbouring grid, the weightage can generally be increased, then line weightage is given more than unity. For tie lines and important lines in the grid, which should never be overloaded, line weightage can be increased.

Why primary grounding resistance or reactance is used?

Primary grounding resistance or reactance is used to add neutral ground resistance or reactance to reduce the fault current or to introduce any earthing transformer. It is in elements data of transformer.

How to convert H value from machine base to system base?

What is the significance of Phase shifting transformer / Phase shift angle?

Phase shifting transformer is used in certain systems to push more power in a specific line by introducing angle difference . The phase shift angle in the element data corresponds to the shift given to boost the power flow. Phase angle in the library is the phase shift introduced due to vector connection i.e. 1'O clock position or 11' O clock position etc.

Load Flow Study

Concepts of tie line with reference to MiPower.

The Tie line refers to those lines connecting one utility to other or those lines, which island the system, when kept open. In inter connected operation of power system, it is the responsibility of individual utility to operate the system such that tie line exchange remains constant, Consider two systems A& B connected by a tie line as shown in figure.

Following an increase in load in area B, say by 25 Mw, after the primary control (governor action), a case may arise wherein A will be exporting 10MW extra to B. Hence the representation becomes.

If the flat tie line control were to be there, then A to B should be brought back to 50 MW I.e. the tie line scheduled power. In order to do that, rise signal should be given to generator in area B to increase the generation. The control is called secondary control or AGC (Automatic generation control). In MiPower, it is possible to schedule the tie line flow, by changing the generator set points of all generators trying to participate in the secondary control.

A simple system with two generators and 4 loads all connected at same voltage through bus couplers. The total load is 4x7.5MW. One generator is scheduled for 20 MW and other is slack with generation capacity of 10 MW, the program is not saying converged but it shows generation output of slack generator as 29.54 MW with 0 MVAR and other generator with 0 MW and 12 MVAR while MVAR load total is 13 MVAR. What is the reason ?

·In Elements - Generator Data there is one Heading called as "Real power optimization Data under which Give Real Power Min = 0.0, And Real Power Maximum = ( your specified power or scheduled power )

· If in the system, generation is more and load is less, slack bus takes / absorbs additional power.

· Observe the following in the LFA report.

Total Real Power Generation:

Total Reactive Power Generation:

Total Real Power Load

Total Reactive power Load

The above values should almost match to get converged result.

Which are the primary parameters considered in a load flow study?

P Active power into the network.

Q Reactive power into the network.

|V| Magnitude of bus voltage

& Angle of bus voltage referred to a common reference.

Short Circuit Study

Which are the parameters considered for short circuit study?

Generator

·Transient(Xd') or sub-transient (Xd'') reactance is considered for +ve sequence.

-ve sequence reactance X2 approximately equal to Xd''

·Zero sequence reactance is 0.1 to 0.7 times Xd''

Transmission Line

·+ve seq impedance Z1 equal to -ve seq impedance Z2

· Zero seq impedance Z0 depends upon return path, ground wires and earth resistivity.

X0 is 2 to 2.5 times +ve seq reactance X1

R0 is 5 to 10 times +ve seq reactance R1

B0 is 0.6 to 0.8 times +ve seq susceptance B1

Why Generator MVA rating, transformer MVA rating are not appear in short circuit report?

In the load flow study the transformer, Generator MVA rating are relevant as the loading on the equipment are compared with the rating.In short circuit study the fault level should be compared with breaker rating . Hence only breaker ratings are given in short circuit study.

In short circuit study R / X ratio of short circuit path is mentioned. What does it signify and how is it calculated?

R/X ratio of short circuit path is computed in case of 3 phase fault. The value is used in the selection of asymmetrical braking capacity from the symmetrical braking capacity. The asymmetrical braking capacity should be selected based on the R/X ratio ( i,e resistance to reactance ratio of driving point impedance and the breaker operating cycles. Please refer Chapter 10, Section 10.1 & 10.6 of "Elements of Power System Analysis", Fourth edition, McGraw Hill publication by W.D Stevenson, for further details.

Transient Stability Study

A 3phase to ground fault at a particular bus , then the program is giving "Divide by zero" Error.

Divide by zero error which encountered is due to fault impedance value which might be zero. If you want to neglect the resistance give fault impedance value as 0.000001.

When a frequency of bus is plotted where generator is removed, a steady frequency of 50 Hz is shown for total duration of study. Why is it so ?

Once the generator is removed from the system the program assume that the user is not interested in its bahaviour, rather he is interested in rest of the system. Hence generator machine equations are not solved.

For Transient Studies MiPower should be capable of simulating fault at any length of line.

Simulation of fault at any length of line in transient stability study is not possible with the POWERTRS. However, a dummy bus can be created at the required location and fault can be created at that bus.

Give frequency calculation procedure when different generators are swinging at different frequencies.

Frequency at any bus is computed by the expression

In transient stability we can not see the plots at the load buses. Only at generator buses plots can be seen.

To view load bus plots select those buses in VIZS buses. These plots are stored in the database area under TRS directory. In TRS directory bin files with M or L with .bin extension will be there . Open graph utility and press import button and browse the bin file 1AecoL.bin file in the database area under TRS directory . In this bin file you can see the line flows and r-x plots and load buses plots.

On clicking Graph button from Database Manager, program opens the machine plots(eg. 1Aeco0M.bin) bin file. So to view load buses plot click New button and press import button and browse the bin file 1Aeco0L.bin file in your database area under TRS directory. In this bin file you can see the line flows and r-x plots and load buses plots (if you open Graph through Database Manager).