power world

Experiment 4

INTRODUCTION TO POWERWORLD AND POWER FACTOR CORRECTION

I. Purpose

In this experiment, a brief introduction of POWERWORLD will be presented and effect of power factor correction and transformer taps on voltage regulation and losses will be studied.

II. Background for Power Factor Correction and Tap Changer Transformer

A. Power factor correction

Consider a single phase load fed from a source as in Figure 4.1.

Figure 4.1. Phasor diagram of a simple circuit

Let and where, is positive, the current lags behind voltage

Complex power flow in the direction of current indicated is given by:

where, P is real power, Q is reactive power and S is apparent power.

Complex power can be represented by phasor diagram as shown in Figure 4.2.

Figure 4.2. Phasor diagram of reactive power compensation

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Figure 4.2. clearly shows that power factor can be corrected by decreasing the Q. After decreasing the reactive power from Q to Q’, while real power is same as P or P’, new power factor will be

B. Voltage regulating or tap changing transformer

Voltage regulating or tap changing transformers are those, which have variable secondary side voltage.

With tap setting 1.0, secondary side voltage VS will be where Vp is primary side voltage and n

is the turn ratio. With tap setting ‘a’ secondary side voltage will be

depending on whether that tap is in primary side or in secondary side. In this experiment, it is assumed that the tap is in the primary winding, so if ‘a’ < 1, the secondary voltage will be raised.

III. POWERWORLD Introduction

PowerWorld can run power flow studies and perform symmetrical fault analysis on large-scale power systems. These basic studies will be illustrated here using the 3-bus system in Fig. 4-3. In addition, these instructions are based on PowerWorld Simulator 13, and they are different from older versions.

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Figure 4.3 Power system configurations

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Click the Application Button and then go to New Case (Ctrl+N) to create a new diagram. HINT: System buses must be added first. You can either go to Draw\Network\Bus (Fig. 4.4). Then, the dialog shown in Fig. 4.5 will open. HINT: Bus 1 has the slack generator connected to it, so when adding Bus 1 checks the “System Slack Bus” checkbox. By selecting Bus 1 to be the slack bus, its voltage will be assumed to be 1.00o pu. For all the other buses, leave the checkbox unchecked.

Figure 4.4 Shortcut for adding buses

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Figure 4.5 Bus Options dialog box

After all buses are in place, you may start adding generator, load, shunt, transformer and transmission lines as you wish, since the order does not matter. Similarly, you can either go to Draw\Network menu.

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When adding the slack generator to Bus 1 as shown in Figure 4.6, insert 0.0 on the “MW Output” field since the slack generator will provide real and reactive power to balance all power flow equations. These values will be determined by the power flow solution method. HINT: To change symbol size and orientation, go to Display tab.

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Figure 4.6 Generator Options dialog box

The dialog boxes for adding the load on Bus 3 and the branch from Bus 1 to Bus 2 are shown in Figs. 4.7 and 4.8. Note that the branch ratings are inserted in the “Limit A (MVA)” field. The dialog boxes for adding the transformer and the switched shunt are shown in Figs. 4.9 and 4.10.

The diagram in Fig. 4.3 is what you should have when you finish creating the 3-bus system. Remember to save your case.

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Figure 4.7 Load Options dialog box

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Figure 4.8 Transmission Line Options dialog box

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Figure 4.9 Transformer Options dialog box

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Figure 4.10 Switched Shunt Options dialog box

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IV. Problems To Do In The Lab

A. Make a new power system case as given in Figure 4.3. This system represents a large industrial plant load drawing power from a generator, to which it is connected through a transmission line and transformer. A shunt capacitor is connected in parallel to the load. Impedances are in pu on 100 MVA base. The transformer has a 10:1 ratio, transforming 138 kV to 13.8 kV. Take the transmission line MVA limit as 250 MVA. The slack bus is the bus that supplies real and reactive power to balance the power flow in the system. The slack bus has a fixed voltage, both in magnitude and angle. The slack bus generator outputs (real power and reactive power generation) are unknown until the power flow has been solved. Once the power flow has been solved, the output of the slack bus generator will be computed by the program.

B. Create a base case system with no shunt capacitors. Solve the power flow. Record the 13.8 kV bus voltage magnitude and angle, and system losses in Table 1.

Hint: you can disconnect the shunt capacitor/load from the system by operating the corresponding circuit breaker connected to it.

C. Run several power flows, adding shunt capacitors to the system in increments of 10 MVAr (10, 20, 30, 40) at the 13.8 kV bus. By adding the capacitor you are changing the effective power factor (PF). Calculate the PF each time you increase the capacitor MVAr. The base case PF (without capacitor) is approximately 0.832 lagging. Find out the change in 13.8 kV Bus voltage and system losses for each case. Record the data in Table 2.

Note that capacitors are passive elements dependent on the voltage magnitude. Therefore, the amount of reactive power supplied by the capacitor rises as V2. So even if you wish to supply 10 MVAr by a capacitor, the actual amount of MVAr supplied should be checked via the capacitor information dialog.

D. Repeat step B and step C with a transformer tap of 134.55 kV (0.975) and record the data in Table 3.

E. Repeat step B, and C with the transformer tap at 0.975 and a new parallel transmission line added from bus 1 to bus 2 with the same parameters as the existing one. Record the data in Table 4.

F. With the configuration in part E., i.e. two transmission lines in parallel, transformer tap at 134.55kV (0.975) and 40 MVAr of capacitor at 13.8kV:

a) Increase the REAL power of the load in increments of 6 MW (6, 12, 18, 24, 30) while keeping the reactive power at 40 MVAr in all cases. Run the power flow for each case and record the data in Table 5.

b) Increase the REACTIVE power of the load by increments of 4 MVAr (4, 8, 12, 16, 20) while keeping the real power at 60 MW. Run the power flow for each case and record the data in Table 6.

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Table 1. Power flow solution at the base case

Vmag, kV

(bus 3)

Vangle, deg

(bus 3)

Transmission line losses

Transformer losses Total transmission losses Bus 3 PF

Ploss Qloss Ploss Qloss Ploss Qloss

Table 2. Power flow solutions with reactive power support

Capacitor

MVAr

Vmag, kV

(bus 3)

Vangle, deg

(bus 3)


Transformer losses Total transmission losses

Bus 3 PF

Nominal Actual Ploss Qloss Ploss Qloss Ploss Qloss

10

20

30

40

Table 3. Power flow solutions with reactive power support and a new tap of the transformer

Capacitor

MVAr

Vmag, kV

(bus 3)

Vangle, deg

(bus 3)



Bus 3 PF


10

20

30

40

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Table 4. Power flow solutions with reactive power support and a new parallel transmission line

Capacitor

MVAr

Vmag, kV

(bus 3)

Vangle, deg

(bus 3)



Bus 3 PF


10

20

30

40

Table 5. Power flow solutions at different real load levels

Pload Increase, MW 6 12 18 24 30

Pgen, MW

Vmag, kV

(bus 3)

Vangle, deg

(bus 3)

Table 6. Power flow solutions at different reactive load levels

Qload Increase 4 8 12 16 20

Qgen, MVAr

Vmag, kV

(bus 3)

Vangle, deg

(bus 3)

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V. Questions

1. Calculate 13.8 kV bus voltage regulation with no shunt capacitor connected to bus 3. Bus voltage regulation can be calculated by applying formula given below, while no load voltage is voltage without any load on bus 3 and full load voltage is voltage of bus 3, when bus 3 is connected to full load.

2. Plot 13.8 kV bus voltage vs. power factor for transformer tap position of 1.0 and 0.975. Different power factors are being obtained by changing the shunt capacitor value.

3. Plot system losses vs. power factor for transformer tap position of 1.0 and 0.975. Explain the relation between losses and power factor.

4. Select tap position and shunt capacitor MVAR for power factor correction such that voltage regulation becomes less than 5%.

5. What effect did parallel lines have on voltage magnitude, angles and losses?

6. Plot Pgen vs. |V3| and Pgen vs. θ3. Measure the average slope, and fit a linear curve to the data points. Which relationship is stronger?

7. Plot Qgen vs. |V3| and Qgen vs. θ3. Measure the average slope, and fit a linear curve to the data points. Which relationship is stronger?

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