I have created and simulated the following energy system using PowerWorld Simulator 17 (please download the picture if it's not clear). energy

Now, I was given the requirement that every bus should have per-unit voltage larger than 0.94. So bus number 8 does not satisfy this condition (its per-unit voltage is 0.88) and I want to do something to correct it. I tried the following methods all of which did it but I am not sure if I am in the right direction.

Method 1: I decreased the MW value of the load connected to bus 8.
Method 2: I decreased the Mvar value of the load connected to bus 8.
Method 3: I attached a generator of 1 MW to bus 8.
I am wondering if any of these methods is correct or if they violate the design of the system. Any other ideas?


The complex voltage at bus 8, \$V_{R}\$, is the voltage at the receiving end of a single transmission line where \$V_{S}\$ is the sending-end voltage (at bus 7) and the relationship between the two voltages is given by $$ V_{R}=V_{S}-IZ_{T}=V_{S}-IR_{T}-IjX_{T} $$ where \$I\$ is the current flowing in the transmission line towards the receiving bus with the impedance \$Z_{T}\$ (or its components resistance \$R_{T}\$ and reactance \$X_{T}\$.

The current flowing \$I\$ is a set by \$\frac{V_{R}}{R_{L}+X_{L}}\$, the voltage on the bus and the load impedance.

From these equations we can see that the per-unit voltage of bus 8, the receiving bus, can be affected by the reactance of the transmission line and the current flow in the line - this current is caused by the net active and reactive power consumed or produced on the bus. Real power use and reactive power consumption decreasing the bus voltage. Real power production and reactive power supply, however, increase the bus voltage as they cause the current to flow in the opposite direction. This gives us three main variables we can change to change the bus voltage, net real power, net reactive power and line impedance.

A nice way to remember this is that the current always flows down the voltage gradient from the higher voltage bus to the lower voltage bus.

You've identified half of these, but the main changes you can make to the simulation to increase \$V_{R}\$ are:

  • Decrease the net real power use at the bus
    • Decrease the real load (MW value)
    • Increase the real power generated at the bus (attach a generator)
  • Decrease the net reactive power use at the bus
    • Decrease the reactive load (MVar value)
    • Increase the reactive power produced at the bus (attach a capacitor bank or a generator operating with a leading power factor)
  • Decrease the transmission line impedance.

Of these, the adding a capacitor bank to the bus is the most practical in the real world as capacitors are cheap and changing the reactive load has a much greater effect than the real load on the per-unit voltage.

It should also be noted that as \$V_{R}\$ is also dependant on \$V_{S}\$ taking similar measures at bus 7 would also raise the voltage at bus 8.

As to which is the "correct" answer, that really requires more information in your assignment's question!

  • \$\begingroup\$ OK, but I have some questions: First, why real power production and reactive power supply increases the bus voltage? I mean, it seems to me that they increase current but from the first equation you wrote this means that the bus 8 voltage is reduced. Same question for the opposite case. Second, I don't know if are familiar with PowerWorld Simulator's interface but by "capacitor bank" you mean a "Switched Shunt", right? \$\endgroup\$ – mgus May 24 '14 at 20:39
  • \$\begingroup\$ Secondly first: yes, "switched shunt" is what PowerWorld calls a bank of capacitors. I used PowerWorld in university, a couple of years ago now. \$\endgroup\$ – Xcodo May 25 '14 at 10:06
  • \$\begingroup\$ I edited to clarify the first point a bit. I have a nice diagram in front of me showing the vectors for all this, but my Power Systems Engineering 5 notes aren't available publicly. I could redraw it for you - do you have your own course notes on this? \$\endgroup\$ – Xcodo May 25 '14 at 10:22
  • \$\begingroup\$ Basically, you answered my question. My notes don't have any vector diagram. So only if it's not too much trouble, could you redraw it? \$\endgroup\$ – mgus May 26 '14 at 13:23

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