lab3 manual

EE433 LAB#3: Voltage Stability Assessment W2102

EE433 LAB# 3

VOLTAGE STABILITY ASSESSMENT

Important Note:

The nature of this lab requires you to fully complete the pre-lab questions. It is impossible to carry out the experiment without preparations. Any student who fails to

complete the pre-lab work will not be allowed to proceed with the lab. A 50% mark reduction will be imposed even if the student completes the lab on its own time.

Student Name:

ID No.:

Lab Group: H

1. Objective Voltage stability is concerned with the ability of a power system to maintain acceptable voltages at all buses in the system under normal conditions and after being subjected to a disturbance. It is an important consideration in the design and operation of power systems. In this lab, you will learn the basic concepts of voltage stability and associated assessment methods. Case studies will be performed to understand the P-V and Q-V characteristics of a power system under different circumstances. Through this lab, you will gain an understanding on the various limiting factors that affect the capability of a power transmission system to transport power to loads.

2. Voltage Stability Theory

2.1 The Voltage Stability Problem Voltage stability is also called the load stability. It is a characteristic of a power system that is required to transmit sufficient power to meet load demand. The voltage stability is the ability of the system to maintain acceptable voltage at all buses in the system under normal and abnormal operating conditions. A power transmission network has an inherent limit as to how much power it can deliver to loads. When this limit is exceeded, the voltages experienced by loads become too low to be practically useful. In many cases, the voltages will go straight from normal to zero in a matter of a few seconds or minutes. This process is called voltage collapse. Massive voltage collapses across several interconnected power systems are not unusual. For example, at the summer of 1996, two voltage collapses occurred in the west coast Canada-US-Mexico interconnected system, causing blackouts for millions of customers. The capability of a transmission network can be loosely visualized as the size of a pipe or the number of parallel pipes. The power transfer capability of this system would be affected if one or more pipes (transmission lines) are lost. Loss of transmission lines or other supporting components such as shunt capacitors will cause the reduction of system capability. When system capability is reduced, loads have to be shed. Otherwise the demand and supply cannot match and voltage collapse will occur. The simplest form of voltage stability assessment involves the determination and characterization of a network's power transfer capability. This capability must be greater than the anticipated load. The


network capability will change if some of its components are not available due to failure or maintenance. The capability assessment therefore needs to be conducted for a number of contingency scenarios for planning and managing the system. PV and QV curves are two most commonly used techniques to assess the capability of a network. These curves show the voltages of selected buses as functions of increased system load. The “nose points” of these curves are the system limit.

load

voltage

limit

existing load

voltage

limit

Q load

direction

of

load increase

PV curve QV curve

2.2 Procedure for Voltage stability Analysis

In this lab, voltage stability analysis is just limited to the assessment of system capability using PV and QV methods. The capability will be determined for various operating scenarios. The results will help you to understand what are the impact factors that affect network transfer capability and hence the voltage stability of a system. The PV curve method is a process to determine the PV curves of a network. While there are many advanced and automated methods to determine the curves, the following procedure is sufficient to determine simple PV curves of a network:

1. Develop a load flow base case. The case, reflecting existing system loading and operating conditions, must be solved successfully by your load flow program (The PowerWorld Simulator).

2. All loads (both real and reactive power parts) in the system are scaled to a new level. The generator output may also need to be scaled. In this lab, generation scaling is not performed to simplify the process. Thus, the power mismatch is supplied by the slack generators. Record the new total load level, as P1 (the real power part).

3. The case is solved by using the load flow function again. If the case solves, record the voltages

of key buses (called monitored buses) as VA1, VB1 etc. If the case cannot be solved or diverges, go to step 6.

4. Save the solved case. Scale up the loads again on the basis of this solved case and solve it. If it

solves, record the results as P2, and VA2, VB2 etc.

5. Steps 2 to 4 are repeated until the case cannot be solved, i.e. the solution of load flow diverges. 6. If the load flow diverges, it typically means that the limit (nose-point) has been reached. The PV

curve process is now completed. The results, recorded P and V data points, can be plotted as PV curves.


It is important to note the following points:

1. The amount of load increase should be gradually decreasing as one approaches the nose point. Namely, the incremental P becomes smaller and smaller. The purpose is to help one to zero in the nose point more accurately.

2. When solution diverges, don't record the results. The output results are not correct.

3. The PV curve plots V against P, although the system Q has also been changed in proportion to P. The exact name for PV curve should therefore be the SV curve. However, the terminology of PV

curve has been accepted in industry. PV curve does not mean to increase P only. The QV curve method is a process to determine the QV curves of a network. A QV curve represents the system voltage behavior when increased reactive power is withdrawn from a bus called test bus. This is done by connecting a synchronous condenser (a generator with zero active power) to the test bus. The voltage setting of the condenser is decreased gradually, resulting increased withdrawal of reactive power from the network. There is a limit for the amount of reactive power that can be withdrawn. This limit is the nose point. The process of obtaining a QV curve is as follows:

1. Develop a solved load flow base case.

2. Select the test bus. Test bus is typically an important bus in the network. The need to select a test bus is one of the main drawbacks of the QV method. Record the test bus voltage.

3. Add a synchronous condenser to the test bus. A condenser has a zero P output. The voltage setting of the condenser is equal to the test bus voltage recorded in step 2.

4. Run the load flow. The case should be easily solved. Examine the output of the condenser. It should be zero for both P and Q. Save the case.

5. Decrease the voltage setting of the condenser by a small amount, say, 0.03pu. Run the load flow case. If the case is solved, record the Q output from the condenser. The output should be negative, indicating Q withdrawal from the system. Save the case.

6. If the case can not be solved, stop. Otherwise, repeat Step 5.

The above process will result in a series of data points (V1, Q1), (Vn, Qn). Plot of the Q data versus V data is the QV curve. In the QV curve method, one only records and charts the voltage and reactive power output of the test bus.

3. System Description and data The system for this lab is the same as the one used for power system load flow in Labs #2. Please check with the instructor to make sure you have the right case. In this lab, you will focus on the voltage stability analysis aspects.


4. Pre-Lab Questions

Student Name:

ID No.:

Lab Group: H 1. What is voltage stability? How does voltage instability manifest in a power system? 2. What are the main objectives of stability analysis? 3. Give the definitions of PV curve and QV curve. Why there is a "nose point" in these curves?

4. List at least four compensating devices or methods that can be used to maintain or to increase

system voltage stability. Explain the advantages and disadvantages of each. 5. If the reactive power output from a generator reaches its limit, will the generator help voltage stability?

Explain why.


6. Plot qualitative PV curves for the following cases: 1) base case, 2) if a parallel line is added to an existing line, 3) if a shunt capacitor is added to the system, 4) if a series compensation is added to a line.

7. If a shunt capacitor is switched off in an industry plant, what could happen to the bus voltages? Use

PV curve to explain your answer. 8. State the advantages and disadvantages of the QV curve method.


5. Lab Procedure and Results Documentation

Task #1: Determination of P-V Curves

In this task, you learn to use PowerWorld Simulator to carry out voltage stability study. You need to get familiar with at least the following tasks:

1. Open, save and solve cases 2. Understand case data organization 3. How to Scale Case 4. Locate input data and output results

The simulator has a special tool to scale cases. This function is illustrated here. Select “Scale Case” from “Other Tools” group in “Tools” ribbon, the following „System Scaling‟ dialog box will appear.

There are several options to scale the cases: scaling the buses, areas, zones or super areas. In this lab,

we scale the case on selected buses. As shown in the above figure, click „Buses‟ button, all available

buses in the system appear in the table. Double click the cells in the „Scale?‟ column, „no‟ can be changed to „Yes‟, indicating the bus has been selected for scaling. Select load buses 4, 6, 8 and 9.

After selecting the buses, the total load attached to these buses will appear in „Bus Load‟. Shown in the above figure, the original load MW of this lab is 9.99MW.

Scale

Options

Total Load


Determine the scaling factor that you want to use. Enter that number is the „Scale by‟ field. The new load

values will appear in the „New Value‟ field. In this lab, we don‟t scale up the generation in order to simplify the procedure. The following items need

to be unchecked:

Enforce Gen MW Limits

Scale Gen to Keep ACE Constant

Scale Only AGCable Generator and Load To keep changing both P and Q, check following item. × Constant P/Q Ratio

In the “Option” ribbon, select “Simulator Options”„ and set the „Min. pu. volt for constant power load to 0.00.

Also, check the item „Initialize from Flat Start Values‟. (Why?)

Monitoring buses: BUS #4, BUS#8 and BUS #9 In this case, both real and reactive power load demand will be scaled up to determine the PV curve. This

is done one step at a time using the „Scale Case‟ Tool of the program. Consult section 2.2 for more information.

1. Run base case to get the initial voltages at BUS #4, BUS#8 and BUS #9, which is the starting

point when you draw the P-V curves.

2. Proportionally increase the total load demand (both active power and reactive power) by using the Scale Case Tool. The scaling factors to use have been listed in the following table.

3. Run power flow solution for the case. If it converges, save it as the starting point for the next step and record bus voltages.

4. Steps 3 and 4 are repeated until the case diverges, which means it cannot be solved. When the case diverges, the following window will appear:


5. Fill in the following table. Draw P-V curves electronically for Bus#4, Bus#8 and Bus#9. These curves will be used as the base case for comparative studies in subsequent tasks.

Scaling Factor

Total Load

(Real Power) Voltage at Bus#4 Voltage at Bus#8 Voltage at

Bus#9

MW Per unit per unit per unit

1.0 ( original load)


2.0 19.98

3.0

4.0

5.0

6.0

6.6

7

......

......

......

Questions associated with this task:

1. Plot the PV curves obtained in this task and attach to your report.

2. Why software should initialize from flat start for this study?

3. The PV curves can be plotted for different buses. Do all curves have the same nose point? Why?

4. If only active power is scaled up in the process, one can get another set of 'PV' curves. Draw

qualitatively (no simulation is needed, draw based on your theoretical expectation) the following two curves in one chart and explain the difference: the first curve is the standard PV curve, and the second curve is the PV curve with only P scaled up. Please remember that you have to find more scaling factors for active power by trial and error until the case diverges.


Task #2: Impact of Adding a New Transmission Line Monitoring Buses: BUS #4, BUS#8 and BUS #9 Case description: This case is the same as the case in Task#1 except that an identical parallel line

is added between Bus#2 and Bus#4 In this case, a second transmission line identical to the line of Branch 4 will be put into operation between Bus#2 and Bus#4. This means the new impedance of Branch 4 is half of the original impedance of the branch 4. After making modifications to the case and solving it, save the case as a new base case. PV curves will be determined for this new case. Using the PV curve procedure, complete the following table.

Scaling Factor Total Load (Real Power)

Voltage at Bus#4 Voltage at Bus#8 Voltage at Bus#9

MW per unit per unit Per unit

1.0 (original load)

2.0

3.0

4.0

5.0

6.0

7.0

7.35

7.4235

……

……


1. What is impact of adding a parallel line to the system PV curve? 2. Why does adding a parallel line mean the line impedance is halved? If a line has a shunt

admittance (the shunt branch of the common PI circuit for a line), what is the impact of adding a line on the admittance?

3. Are there other methods to reduce the line series impedance? (Name at least one)


Task #3: Impact of Reactive Power Capacity of the Co-generator Monitored buses: BUS #4, BUS#8 and BUS #9 Case description: This case is the same as the case of Task #1 except that there is a limit to

reactive power output of the generator. This limit is 46.8 MVar.

Important notice: Before starting this task, check the „PowerWorld Simulation Option‟ to make sure that the reactive power limit of the generator will be taken into consideration by the Simulator. Uncheck the „Disable Checking Gen MVAR Limits‟ option, shown as follows:

The new case is created by setting the Qmax of the generator at bus #2 to 46.8 MVar. Shown in the following figure, 46.8 MVar is entered into the field „Max Mvars‟ of the generator record. After making modifications to the case and solving it, save the case as a new base case. PV curves will be determined for this new case.


Using the PV curve procedure to complete the following table:


Voltage at Bus#4

Voltage at Bus#8

Voltage at Bus#9

MW per unit per unit per unit


2.0

3.0

4.0

5.0

5.5

5.61

5.6661

……

……


1. Are there any differences in system voltages between the new base case of this study and the base case used in Task #1? Why?

2. What is the impact of generator Q limit on system capability? Explain why. 3. Why does a generator have a limit for reactive power output (think your EE332 course)?

Task #4: Impact of Shunt Capacitor Monitoring buses: BUS #4, BUS#8 and BUS #9 Case description: This case is the same as the case in Task#3 except that the shunt capacitor at

Bus#4 is switched off.

Make sure that the reactive power Limit of the generator will be taken into consideration. Make corresponding changes as you did in Task #3 before running the case. A new base case for this task is created by switching off of the capacitor at Bus #4 for the base case of Task #3. The capacitor can be switched off by a click on the attached switch (The switch will turn green when it‟s in off status). After making modifications to the case and solving it, save the case as a new base case. PV curves will be determined for this new case. Using the PV curve procedure to complete the following table:



Voltage at Bus#4 Voltage at Bus#8 Voltage at Bus#9

MW per unit per unit per unit


2.0

3.0

4.0

5.0

5.5

……

……


1. Are there any differences in system voltages between the new base case of this study and the base case used in Task #3? Why?

2. What is the impact of disconnecting the capacitor on the reactive power output of the generator?

Why? 3. What is the impact of disconnecting the capacitor on the system capability (i.e. PV curve limit)?

Why? 4. If the generator has no reactive power output limit in this case, what can happen to the PV curve?

Draw qualitative PV curves for the following cases: 1) case of Task 3, 2) case of Task 4 and 3) case of Task 4 but there is no reactive power limit for the co-generator.

5. Plot PV curves for Bus #4 obtained in Tasks #1, #2, #3, and #4 in one chart and attach to the report. Discuss the differences of the curves.

Task #5: Determination of Q-V Curves Test Bus: BUS #9 Case description: This case is the same case as that of Task#1 There are two approaches to get the Q-V curves:

Approach 1: Connect a fictitious synchronous condenser on BUS #9, reduce voltage setting of the condenser gradually. Record the reactive power output and voltage setting of the condenser. Consult section 2.2 for more information.

Approach 2: Scale up the reactive power load of Bus #9 gradually using the PV-curve-like

procedure. Record the voltage and Q load of the test bus


Run power flow solution for the case one by one in both approaches. Complete the following table.

Approach 1 Approach 2

Voltage setting at Bus#9

Reactive output in synchronous condenser

Reactive Scaling Voltage output

(MVar) MVar at Bus#9 per unit

Initial voltage: From tested value

(0.98995)

1.04013 (original reactive load)

0.95 20

0.90 50

0.85 80

0.80 100

0.75 120

0.70 140

0.65 160

0.63 170

0.62 180

0.61 181

0.60

0.58

0.56

……


1. Plot in one figure the curves obtained by both approaches. 2. Is there any difference between the two curves? Why?

3. Why is the starting point of approach #1, the voltage setting of the fictitious condenser, the same

as the test bus voltage of the base case? 4. Discuss the advantages and disadvantages of each approach to obtain the QV curve.

6. Lab Report The lab report should include the following items:

1. Answers to the pre-lab preparation questions. 2. Fully completed tables listed in Section 5. 3. Your responses to the questions listed in Section 5.

Mark allocation of this lab is as follows:

All the students must participate in the lab.

Pre-lab preparation: 20%

Report including data preparation: 80%

lab3 manual

Documents

voltage stability assessment

reactive power output

load flow diverges

power transfer capability

reactive power limit

pv curve procedure

scale case tool

test bus voltage