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THE HONG KONG POLYTECHNIC UNIVERSITY

DEPARTMENT OF ELECTRICAL ENGINEERING

Subject Code : EE570

Subject Title : Design and Analysis of Smart Grids

Session : Semester 2, 2021/22 Venue : Online Examination

Date : 3 May 2022 Time : 19:00 – 22:00

Time Allowed : 3 Hours Subject Examiner(s) : Dr I. Kocar, Dr U. Karaagac

This question paper has a total of 6 pages (attachments included).

Instructions to Candidates:

This paper contains 7 questions. Students need to attempt all questions.

Questions carry different marks as follows:

Q1: 30 Marks Q4: 20 Marks Q7: 10 Marks

Q2: 10 Marks Q5: 10 Marks

Q3: 10 Marks Q6: 10 Marks

Physical Constants: Nil

Other Attachments: Nil

Available from Invigilator: Nil

Semester 2, 202 1/22 Subject Code: EE570

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Q1. A dispatchable source is connected to a 50 Hz, 11 kV distribution grid through a three-phase grid

converter and a step up transformer as shown in Fig.Q1.a. Real and reactive power outputs of the

converter are controlled with vector control technique. The converter and step-up transformer

parameters are given in Table.Q1.a. The grid equivalent impedance at nominal frequency: Zgrid

= (0.15 + j0.5) Ω

Suppose that the converter control converts the measured voltage and current signals (Vc, Vo,

Iconv and Io) into per unit values. The base quantities are the rated peak voltage and current values

for dq reference frame signals. The phase angle θ of the rotating reference frame is derived by an

inverse park transformation based PLL using Vc. q axis is aligned to the phase axis.

Fig.Q1.a Three-phase grid converter connected 11 kV grid

Table.Q1.a Converter and Step-up Transformer Parameters

Parameter Value

Grid Voltage (line-to-line) 11 kV

Grid Frequency 50 Hz

Converter AC Voltage (line-to-line) 575 V

Converter Rated Power 1000 kW

DC Link Voltage 1150 V

Switching Frequency 10 kHz

Step up Transformer Rated Power 1000 kVA

Step up Transformer Total Winding Resistance 0.002 pu (on 1000 kVA base)

Step up Transformer Total Winding Reactance 0.04 pu (on 1000 kVA base)

(a) Design an LCL filter (consider step-up transformer as grid side inverter inductance) with

the following procedure: (7.5 marks)

- Select inverter side inductance (Li ) to limit the output current ripple by up to 10% of

nominal current,

- Select filter capacitance 5% of the base capacitance,

Calculate f

res

(filter resonant frequency). Is f

res

within acceptable range?

(b) The simplified schematic diagram of the converter control is shown in Fig.Q1.b. Adjust the

PI parameters of the inner and outer controls using internal model control (IMC) method

for 5 ms and 150 ms rise times, respectively. (15 marks)

Semester 2, 202 1/22 Subject Code: EE570

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Fig.Q1.b Simplified schematic diagram of the converter control

converter control for the frequency and voltage support illustrated in Fig.Q1.c.(7.5 marks)

Fig.Q1.c Desired frequency and voltage support

Q2. Consider the power synchronization loop implementations shown in Fig.Q2. For which controller

parameters condition, they become equivalent? (10 marks)

(a) Synchronverter (b) Synchronous Power Control (SPC)

Fig.Q2 Power synchronization loops in grid forming converters

Q3. Explain the differences in the response of grid forming (GFM) and grid following (GFL)

converters to the grid voltage perturbations. (10 marks)

Q4. This problem relates to a small power system with 3 generators at three different buses.

System Data

Semester 2, 202 1/22 Subject Code: EE570

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Consider the three-bus system shown in Fig.Q4. Table.Q4.a provides the key data for the loads

(Pd) and generators (Pg). All values are in MW unless otherwise specified. The data in Table.Q4.b

describe the transmission lines for this system. Reactance values are in per unit on a 100MVA

base.

Fig.Q4 One line diagram of a three-bus transmission system

Table.Q4.a Generator and Load Data

Bus No Pd Pg [0] Pg_min

Pg_max Marginal Cost $/MWh

1 100 200 0 350 10

2 300 200 0 300 50

3 200 200 0 400 20

Table.Q4.b Generator and Load Data

From Bus To Bus R X Limit Rating MVA

1 2 0 0.4 100

1 3 0 0.2 100

2 3 0 0.3 100

(a) DC Power Flow: compute the DC power flow solution for the data described above. Bus 1

is the reference bus. Report the following: (8 marks)

- Nodal admittance matrix without considering the voltage sources and loads. Reduced

nodal admittance matrix considering that Bus 1 is the reference. Reduced B matrix and

P vector in pu.

- The voltage phase angles (converted to degrees).

- The power flowing on each transmission line and the actual generation at Bus 1

considering power balance of the system, in pu and MW.

- Are any of the transmission line flow limits violated?

(b) Economic dispatch: compute the economic dispatch solution for the power system

described above. Report the following: (6 marks)

- The amount of power generated by each generator at the optimal solution.

- The total cost of running the system in this state for a period of 4 hours.

- Does economic dispatch violate line flow limits?

BUS2

0.4j

0.2j

BUS1

0.3j

BUS3

Semester 2, 202 1/22 Subject Code: EE570

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Table.Q4b Optimal Power Flow Results

From Bus To Bus R X Limit Rating MVA P transfer Pg

1 2 0 0.4 100 100 250

1 3 0 0.2 100 50 100

2 3 0 0.3 100 -100 250

- Why are the solutions different among the three cases (Power Flow, Economic Dispatch

and OPF)?

- Compute the LMP at each bus considering incremental generation costs and the OPF

solution. Is the cost of operating the system higher compared to economic dispatch

solution?

- In what ways might smart grid technologies help us to get to better solutions than those

above? Which particular technologies might help?

Q5. Consider the three-phase distribution feeder in Fig.Q5 with line-to-line system voltage of 22 kV.

The cable sections and their ampacities (max current they can carry due to thermal limits) are

given in Table.Q5.a. The load data on each node is also provided below in Table.Q5.b. Assume

nominal voltage on all buses and unity power factor distributed generation. Ignore system losses.

The system is a three-phase balanced system: use (√3) ́V(line-to-line) ́(Current rating) to

compute the power transfer capacity of line sections.

Among the nodes B, C, and D which node can host maximum distributed generation without

violating thermal limits of the system? Demonstrate your answer with calculations. Is it possible

to supply power to the substation? (10 marks)

Table.Q5.a Cable sections and ampacities

Line section : AB BC CD

Conductor size 185 mm² 95 mm² 35 mm²

Current rating 388 A 268 A 151 A

Table.Q5.b Cable sections and ampacities

Node: B C D

Maximum active power 2.0 MW 3.5 MW 2.5 MW

Minimum active power 0.5 MW 0.9 MW 0.7 MW

Maximum reactive power 1.3 MVAR 2.0 MVAR 1.3 MVAR

Minimum reactive power 0.3 MVAR 0.5 MVAR 0.5 MVAR

Fig.Q5 One line diagram of a three-phase distribution feeder

Semester 2, 202 1/22 Subject Code: EE570

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Q6. Consider various distribution grid layouts discussed in class and provide short answers or select

the right answer for the following questions.

(a)Consider an underground MV network layout with loop design and radial operation. Why

SCADA is a must for such a system for system recovery after faults? Explain in your own

words in three lines. (2 marks)

(b) (4 marks)

(c)

(4 marks)

Q7. Write a sufficiently detailed response to demonstrate a depth of knowledge about the topics below.

(a) List three benefits of advanced metering infrastructure? What new opportunities for rate

design does AMI present? Why do we need new rates? (5 marks)

(b)Voltage regulation in distribution circuits with large scale integration of distributed

generation is challenging. Why? Explain how to handle voltage regulation with

explanations. (5 marks)

--- End of Paper ---