ELEC3575: Electric Power Systems Laboratory

(Software Lab)

Part B: Distribution Network

Reinforcement, Expansion and (N-1)

Security Analysis in

MATLAB/Simulink

University of Leeds

Spring 2020 Session

School of Electronic & Electrical Engineering The University of Leeds

Contents

1 Review 3

2 Introduction 3

2.1 Problem Statement and Tasks . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 Learning Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Getting Started 5

4 Task 1: Analysis and Reinforcement of Existing Network 5

5 Task 2: Network Expansion and (N-1) Security Analysis 6

5.1 Task 2.1: Expansion Planning . . . . . . . . . . . . . . . . . . . . . . . . 6

5.2 Task 2.2: Implementation of the new Power Station and Loads in MAT-

LAB/Simulink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

5.3 Task 2.3: Network Expansion and (N-1) Security Analysis in MATLAB/Simulink 8

6 Lab Report Content 9

2 ELEC3575: Electric Power Systems Laboratory

School of Electronic & Electrical Engineering The University of Leeds

1 Review

Please review the following items before the lab:

• (N-1) criterion;

• Load flow calculation;

• Reactive power compensation.

2 Introduction

2.1 Problem Statement and Tasks

In this lab, we will analyse and expand the 110 kV electric distribution network shown

in Fig. 1 using the software MATLAB/Simulink. The network is composed of 4 sub-

stations A, B, C and D (called buses hereafter). Buses B and C are each connected

to bus A via an overhead line (OL), while bus D is connected to bus A via a cable.

The line characteristics are indicated in Table 1. At each bus a load (A1,B1,C1,D1)

is placed, representing the aggregated power demand of the customers at that node.

The load demand is given in Table 2. The distribution network is supplied from a high

voltage (220 kV) transmission system via a 220/110 kV transformer (power rating of

250 MVA) that is connected to bus A.

A new area composed of an industrial (load C2) and a residential (load C3) load

are supposed to be built at bus C. To accommodate the increase in load demand,

a new power station is planned to be constructed at bus E. Bus E is a well-suited

location for the new power station, as it is in close proximity of a river (for cooling

water) and the railway line (for fuel transport). To connect the new power station to

the existing infrastructure, the distribution network has to be expanded. Your main

task is to plan, implement and successfully test this expansion. To do this, you may

use 110 kV overhead lines of the same type as those connecting buses A, B and C.

In addition, your solution needs to satisfy the (N-1) criterion for buses B and C.

To accomplish the above objectives, you need to solve the following tasks.

Task 1) Analysis and possible reinforcement of existing distribution network;

Task 2) Distribution network expansion: line planning, dimensioning, implementation

and analysis guaranteeing (N-1) security at buses B and C.

Important. You need to submit a lab report. The specific tasks to be included in the

report are summarised in Section 6.

3 ELEC3575: Electric Power Systems Laboratory

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External

GridGrid

110/220kV

A

C

B

D

LoadC1

LoadB1

LoadD1

LoadA1

20km

40km

40km

E

Railway Line

River

80km

80km

40km

Figure 1: Considered 110 kV distribution network

Line Type (110 kV) Resistance Inductance Capacitance Length

[Ω/km] [mH/km] [µF/km] [km]

L AB OL ELEC3575, 185-Al/30-St 0.0155 1.37 0.009 40

L AC OL ELEC3575, 185-Al/30-St 0.0155 1.37 0.009 40

L AD Cable ELEC3575, 120-Cu 0.017 0.414 0.25 20

Table 1: Line characteristics

Load Bus Active power demand [MW] Power factor

A1 A 50 0.90 (ind.)

B1 B 50 0.90 (ind.)

C1 C 50 1.00

C2 C 100 1.00

C3 C 50 0.8 (ind.)

D1 D 50 0.82 (ind.)

Table 2: Load demand

4 ELEC3575: Electric Power Systems Laboratory

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2.2 Learning Objectives

After successful completion of this lab, you should . . .

• . . . be able to perform load flow calculations and load flow analysis in MAT-

LAB/Simulink;

• . . . conduct a network expansion design ensuring (N-1) security of specific net-

work buses;

• . . . implement new equipment in MATLAB/Simulink and verify its functioning ac-

cording to your design specifications.

3 Getting Started

To get started, do the following.

1. Download the ”ELEC3575 PowerSystemsLab.slx” file from the VLE and save it

on your local PC.

2. Find ”MATLAB” in the program manager of Windows and start it.

3. Open the downloaded ”.slx” file with MATLAB or you may easily open by double-

click on it.

4. Now you should see a network in the Simulink work space. The network rep-

resents the 110 kV distribution system shown in Fig. 1.

5. Check that the lines ”L AB” and ”L AC” are set with the parameters of the type

”Overhead line ELEC3575” and that the line ”L AD” is set with the parameters

of the type ”Cable ELEC3575”.

6. Check the load demand and power factor at buses A, B, C and D. What is par-

ticular about the load demand C1 at bus C? Can you explain how such situation

could arise in a real-world situation?

4 Task 1: Analysis and Reinforcement of Existing

Network

In this task you will evaluate the performance of the existing distribution network.

Task 1.1 Run a load flow calculation for the system. To do this, double-click on the box

”powergui” and then go into tab ”Tools” and select ”Load Flow”. A window for

power flow results will pop-up, click on the button ”Compute”. The power flow

results are shown in the last five colums of this window as shown in Fig. 2.

5 ELEC3575: Electric Power Systems Laboratory

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Figure 2: Power flow results

Task 1.2 Evaluate the load flow results.

Task 1.3 Are the voltage magnitudes at all buses within satisfactory limits (i.e., ±5% of

the nominal grid voltage)?

Task 1.4 If you detect any problem, add suitable equipment to improve the network op-

eration. Furthermore, add a shunt capacitor at Bus D (MATLAB/Simulik does

not allow to check loadability; however, the line L AD is loaded to 111.1%). How

does the capacitor(s) need to be dimensioned? Also, make sure to assign an

appropriate nominal voltage to the capacitor element.

Hint:

QLoad = PLoad tan(ϕ), QC,1φ = −ωC|V |2.

Task 1.5 Re-run the load flow calculation. Has the network status improved? Justify your

answer by inspecting the load flow results.

Task 1.6 How could the network operating status be further improved? Only provide

suggestions, an actual implementation is not required.

5 Task 2: Network Expansion and (N-1) Security

Analysis

Now, you will expand the network infrastructure to incorporate the new loads C2 and

C3 at bus C as well as the new power station at bus E. The expansion must also

satisfy the (N-1) criterion for the buses B and C. This task is split into three subtasks:

Task 2.1 Expansion planning;

Task 2.2 Implementation of the new power station and loads in MATLAB/Simulink;

Task 2.3 (N-1) security analysis.

5.1 Task 2.1: Expansion Planning

For the network expansion planning, follow the steps indicated below.

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Task 2.1.1 Explain why a new power station is needed to accommodate the new load de-

mand at bus C.

Task 2.1.2 Calculate the apparent power demand at bus B.

Task 2.1.3 Calculate the apparent power demand of bus C including the new loads.

Task 2.1.4 Calculate the thermal power limit for the ”OL ELEC3575”. Assume the maximal

long-term admissible current density for one conductor is J = 3.5 A/mm2.

Hints:

• Thermal current limit (per conductor, i.e. per phase):

ITh,max = J · A,

where A is the cross section of the conductor strand (see Table 1 and

Lecture Part 5)

• Thermal power limit:

STh,max = 3VLNITh,max

Task 2.1.5 Using your results from the previous task, calculate the required number of OLs

to ensure (N-1) security of the loads at buses C and B. Follow these steps and

use the hints indicated below.

a) Calculate the number of required three-phase OLs of the line type OL

ELEC3575 needed to supply the load at bus B without violating the thermal

power limits of the lines.

b) Based on your result, calculate the number of required three-phase OLs of

the line type OL ELEC3575 needed to supply the load at bus B with (N-1)

security.

c) Perform the same calculations for the loads at bus C.

Hints:

• Number of required three-phase power lines to transmit given apparent

power SL at nominal voltage without violating thermal line limits:

nTh =

|SL|

STh,max

.

• Number of required three-phase power lines to transmit given apparent

power SL at nominal voltage without violating thermal line limits and ensur-

ing (N-1) criterion:

n(N−1) = nTh + 1.

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5.2 Task 2.2: Implementation of the new Power Station and Loads

in MATLAB/Simulink

Task 2.2.1 Download the ”ELEC3575Topology 1.slx” and ”ELEC3575Topology 2.slx” files

from the VLE and save them on your local PC.

Task 2.2.2 These files contain the proposal of two different network topologies for satisfying

the (N-1) security for buses B and C, while also incorporating bus E into the net-

work. Do they coincide with the required number of OLs previously calculated?

Task 2.2.3 Verify the power demand of the new loads C2 and C3 at bus C according to the

values in Table 2.

Task 2.2.4 Verify the new power station at bus E. The new generator operates at 16 kV and

is connected to bus E via a 16/110 kV step-up transformer. Power injection is

set to 150 MW, and the voltage to 1 pu.

5.3 Task 2.3: Network Expansion and (N-1) Security Analysis in

MATLAB/Simulink

In this part of the lab, you will analyse which solution is more beneficial for the (N-1)

secure operation of the system.

Task 2.3.1 Analysing the first provided (N-1) secure network topology.

a) Run a load flow calculation to test the implementation. How would you

judge the resulting network operating state?

b) If necessary, can you think of a way to improve the voltage behaviour at

buses B and C? Test your proposed solution.

c) Now, test the (N-1) security of the proposed network topology. You can sim-

ulate line outages by disconnecting a line. What do you observe (voltages

and power flows)? Do the buses B and C satisfy the (N-1) security cri-

terion?

Note: A line outage is a major disruption in a power system and, therefore,

larger voltage deviations (in the range of ±10%) are still acceptable.

Hint: To calculate the power flow through the line, it is necessary to calcu-

late the current in the line. To do this, use:

|I| = |V1 − V2| × 110kV|ZL|

where: |ZL| = |(r + jXL)× `|

Apparent power is calculated by:

|S| =

√

3|V ||I|

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Task 2.3.2 Perform the same analysis for the second proposed (N-1) secure topology. As-

sess its performance and compare it to the behaviour of the first topology.

Task 2.3.3 According to your analysis, which of the two topologies would you implement in

a real system? Justify your answer.

6 Lab Report Content

Your lab report must contain the following items.

1. Answers to the questions in Tasks 1.3 - 1.6;

2. Answers to the questions in Tasks 2.1.2 - 2.1.5;

3. Answers to the questions in Tasks 2.3.1 - 2.3.3.

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