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The Hong Kong Polytechnic University

Department of Electrical Engineering

Project ID : FYP_21

Anti-islanding Protection for Distributed Generation

by

CHEUNG Kok Wai Raymond

14116265d

Final Report

Bachelor of Engineering (Honours) in

Electrical Engineering

Of

The Hong Kong Polytechnic University

Supervisor: Dr. Kevin Ka-wing CHAN Date: March 31, 2018

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Abstract

While a power grid was suddenly unstable and unintentional disturbance, the commonly used

protection relays (voltage and frequency) would detect the islanding of the distributed

generator (DG) which is connected to the power grid. Meanwhile, the ROCOF and frequency

relays are common adopted to detect the islanding and tripped the circuit breaker of loads

automatically. Unfortunately, these relays are not fully reliable because of their associated

non-detection zone (NDZ). The main contribution of the project is to demonstrate the simple

and practical analytical methods to implement the islanding detection. Passive method

(frequency relays & ROCOF) are separately selected to built-in to the power supply system, and

tried to suspend the power supply to the load. Then, find out the range of NDZ in frequency

relay. These simulation results are carried out on MATLAB / SIMULINK software.

Keywords: Islanding detection, NDZ, RoCoF & frequency replays.

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Table of Content

ABSTRACT……………………………………………………………………………………2

TABLE OF CONTENT...……………………………………………………………………..3

LIST OF ACRONYMS…………………………………………………………………….…4

CHAPTER 1 INTRODUCTION……………………………………………………………..5

1.1 OVERVIEW……………………………………………………………………………5

1.2 PROPOSAL OUTLINE…...………………………………………………………..…10

CHAPTER 2 OBJECTIVES………………………………………………………………...11

CHAPTER 3 BACKGROUND…………………………………………………….………..12

CHAPTER 4 METHODOLOGY…………………………………………………………...15

4.1 ROCOF PROTECTION RELAY…...…………………………………………………18

4.2 FREQUENCY PROTECTION RELAY………………………………………………29

4.3 NON DETECTION ZONE…………………………………………………………....34

4.4 THRESHOLD SETTING & OPTIMIZATION………………………………………..37

4.5 OTHER DETECTION METHOD…………………………………………………….39

CHPATER 5 RESULTS……………………………………………………………………..41

CHPATER 6 CONCLUSION AND FUTURE DEVELOPMENT.……………………….47

REFERENCES…………………………………………………………………………….…49

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List of Acronyms

DG Distributed Generation (DG)

ROCOF Rate of Change of Frequency

FR Frequency-based Relays

NDZ Non Detection Zone

AI Anti-islanding

DWT Discrete wavelet transform

PCC Point of Common Coupling

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Chapter 1 Introduction

1.1 Overview

Over the past few decades, fossil fuels (oil, coal and natural gas) have been traditionally used

to generate the sources of electrical energy. In fact, burning fossil fuels will release the

environmental pollutions, these pollutions cause smog, acid rain, greenhouse effect and

climate change. In addition, the electrical energy from the generating plants will be

transmitted to the customer load through long distances transmission lines and the certain

kind of step down transformers. Although the higher voltage of transmitting electricity can

reduce the current and resistive losses, others factors will also affect the resistance of the

conductor of transmission line. For example, the electrons would be collided together in the

transmission line and the energy lost (heat and crackling sound) would also be produced

under the sunlight. To replace the thermal energy sources, many countries are willing to put in

more resources to develop renewable energy to meet the energy consumption. Renewable

energy resources-such as geothermal, hydro, solar and wind energy-are naturally replenished

and will never terminate in the earth. Thus, the amount of the renewable energy generations

are increased to the distributed network to share of electrical supply in many countries.

Moreover, the advantages of the distributed generation (DG) system are energy conservation,

reducing electrical losses due to no need of large transmission line and as backup power

supply for important loads. The DG systems in parallel with the electric utility network will

typically generate the small-scale energy in size from 3 kilowatts (kW) to 10 megawatts

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(MW). Normally, DG systems are connected to the power grid through power electronic

converters [1]. Although DG systems are going to play an important role in mitigation of

climate change, rising the sustainable electric power for demanders, it causes the islanding

phenomenon while the electric utility is suddenly down.

If the electrical energy are only supplied from the power station, the reactive power and others

energy loss will be produced, as shown in Fig 1.1.

Fig 1.1 – Power flows in traditional network

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When embedded generators are newly increased to the distribution network, it can reduce the

capacity of power output from power stations and the extent of complementarity between

power stations and embedded generators will be greatly enhanced in the grid network system,

as shown in Fig 1.2.

Fig 1.2 – Power flows with distributed generator

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Although the advantage of the distribution generation can decrease the energy loss from the

transmission line, there is the disadvantages associated with using embedded generators

during the power stations provides the unstable electrical energy.

In normal condition, both the power plants and the embedded generators will generate the

electrical energy to the loads through power grid network, as showed in Fig 1.3

In unintentional condition, the embedded generators continue to keep working even through

the utility grid has suddenly fault. Unwanted islanding will be appeared, as shown in Fig

1.4.

Embedded Generator

Distribution Feeder to Loads

Overhead Line

Fig. 1.3 - Normal Condition Network

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There are common causes of islanding in the distribution networks:

- Some distributions are isolated from the reset of the power system but still energized by

the connected DG systems in the distribution networks.

- The utility power supply is accidently shut down.

- For maintenance operations, utility switching of the loads and distribution system.

- Vandalism

- Natural disasters.

Generally, the influence of the islanding phenomenon will adversely cause the problems:

- Safety hazard for line-workers who may assume the DG systems are not energized during

the inspection of power grid failure.

Embedded Generator

Fig. 1.4 - Unintentional Condition Network

Distribution Feeder to Loads

Overhead Line

Fault occurred

before step down

transformer.

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- Without the voltage/frequency rating of main grid network for reference, the DGs might

generate unstable voltage and frequency (under or over), it is a probability of damage the

customer’s loads or power grid network.

- After islanded is solved, a large current may flow to the DG due to out of voltage phase of

circuit breaker. It may trip the utility power supply again.

- Islanding may distribute the restoration of power to the customers

To effectively isolate the operation of DG during islanding, each DG must have the anti-

islanding protection systems to detect islanded and to prevent the unstable rating of voltage

and frequency to the loads immediately.

1.2 Proposal Outline

Chapter 2 will discuss the objective about the selection of the better islanding protection

method, simulation model design and observation of the NDZ size at different load.

Chapter 3 will briefly introduce the advantage and disadvantage of the detection methods to

monitor between the grid network and DG.

Chapter 4 will study the procedure of selected methods (Passive method : RoCoF & FR) to

the islanded.

Chapter 5 will plan the preliminary result to the project.

Chapter 6 is a brief conclusion of the proposal regarding the benefits and drawback of the

methods and future development.

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Chapter 2 Objectives

For islanding detection methods, it is developed in the distributed power system so far. The

objectives of this proposal are :

1. Selection of the better islanding protection method

To consider the advantage and disadvantage of both active and passive methods, it will try

to combine both methods in this proposal. In normal electrical supply, passive method

(either Rate of Change of Frequency (ROCOF) or Frequency-based Relay(FR)) will the

detection devices. In ideal concept, it is anticipated to solve the islanding phenomone in

different conditions.

2. Simulation model design

Using the Matlab software, it will simulate the power grid network with DG. The circuit

breaker of the loads will receive the rating of frequency change or phase shift at the

instant from the Point of Common Coupling (PCC) to control the the electrical supply.

Moreover, the rating of reactive power will be detected by the active method during

islanded.

3. Observation of the NDZ size

According to the IEEE 929[2] and UL1741 standard[3], it states that the loads will be set

to the power output. The reason of different loads are observed which of passive methods

that is more suitable for the detection of the NDZ size.

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Chapter 3 Background

According to the international standard of the IEEE 1547, islanding should be detected and

disconnected within 200ms[2]. Moreover, there are 2 nos. of main methods have been

propsed to monitor the islanding: Remote and Local Methods[4]. Generally, the remote

islanding detection method depends on the communication between the power grid network

and the DG. The local islanding detection mehtod depends on the measurement of some

parameters on the DG side, such as voltage, frequency and current. Local methods are also

classified as active and passive methods. Active methods : it will intentionally peturb a

disturbances signal to the output of the inverter and observe the influence of parameters

outlined; Passive methods : it uses the local measurments of the parameters to montior the

operatrion of both DG and

grid networks. The Fig. 3.1 is

shown the classification of the

islanding detection methods.

Remote Methods

(Communication methods) is a most effective, high reliability to approach protection

detection, but it is very expensive for small system compared to local methods and need

communication infrastructure.

Fig. 3.1 - Classification of islanding detection methods

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The common remote methods are included :

- Transfer Trip Scheme

- Power Line Carrier Communication (PLCC)

- Supervisory Control and Data Acquisition (SCADA)

Active Methods (Local Methods) are characterized by their small NDZ size, useful for

multiple DG and apply to inverter based DG. Due to the intentional injection of perturbation

in the grid network, the methods will affect the power quality, long time for detection and

implement difficulty. The common active methods are included :

- Impedance Measurement

- Active Frequency Drift (with positive feedback)

- Sandia Frequency/Voltage Shift

- Sliding Mode Frequency Shift

- Negative Sequence Current Injection

- Variation of Active and Reactive Power

- Detection of Impedance as Specific Frequency

Passive Methods (Local Methods) has fast response during detection, non-influence of power

quality, without perturbation in system and cost-effective. However, it is useless for multiple

DG and nuisance tripping when maintaining work. The significant problem of passive

method has a large NDZ size without false trips. The common passive methods are included :

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- Rate of Change of Frequency (RoCoF)

- Phase Jump Detection (Vector Surge)

- Voltage Unbalance

- Over/Under Voltage and Frequency

- Harmonic Distortion

- Rate of Change of Output Power

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Chapter 4 Methodology

Before adopting the anti-islanding protection, it will focus on the characteristics of the

distributed generator unit. The power generated by distributed generator should be keep in

constant equilibrium of power supply and demand to prevent the power deviation. However, the

operation of the generator must produce the electrical speed from the same mechanical speeds.

Both of the speeds are related to the number of machine poles “p”,

The equation is We = [p / 2 * Wm] … (1) (We – electrical speed, Wm – mechanical speed)

To ensure electrical speed from one to another generator, the synchronous generators must keep

constant of the mechanical speed. The equation of the synchronous speed is n(sync) = [f * 60 / p]

(f – frequency of 3 phase power supply, p – no. of pole pairs)

When the amount of pole pair is increased, the speed of synchronous generator will be

decreased, the result is shown in the table 4.1.

Pole Pair number 1 2 3 4 5 6

Synchronous speed [rpm]

at 50 Hz

3000 1500 1000 750 600 500

Synchronous speed [rpm]

at 60 Hz

3600 1800 1200 900 720 600

Table 4.1 - Relationship of pole pair no. and Synchronous speed (rpm)

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In addition, the mechanical speed of the rotation in rad/s would also be changed to the

synchronous speed, the equation is Nm = [We * 2 / p (rad/s) * 60 / (2 * π) (sec/min)

/(rad/rev)] ... (2). The result is shown in Table 4.2.

No. of

poles(P)

Synchronous

speed(Ns)

No. of

poles(P)

Synchronous

speed(Ns)

No. of

poles(P)

Synchronous

speed(Ns)

2 ( 30/π )*We 8 ( 7.5/π )*We 14 ( 4.285/π )*We

4 ( 15/π )*We 10 ( 6/π )*We 16 ( 3.75/π )*We

6 ( 10/π )*We 12 ( 5/π )*We 18 ( 3.333/π )*We

In fact, it is difficult to maintain the constant speed during the operation. For example, the

amount of the power demand is more than the system generation, the rate of the speed will be

fallen, and the power demand is less than other, the rate of the speed will be risen.

According to the block diagram of speed governing (Fig 4.1), it illustrates the power variations

between ∆Pm and ∆Pe :

Table 4.2 Mechanical speed and synchronous speed at different poles

Fig. 4.1 Block diagram of speed governing

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The speed deviation (∆w) formulation becomes:

∆w(s) = (1/2*Hs)( ∆Pm - ∆PL - D∆w) or ∆w(s) = (1/(2*Hs + D))( ∆Pm - ∆PL) … (3)

∆Pe = ∆PL + D*∆w … (4)

Pm – Mechanical power input to generator,

Pe – Electrical power output of generator,

PL – Non-frequency sensitive load change,

H – Inertia constant of the generator,

D – Load damping constant

If the Pm is step down, it may cause the unbalance of the power: Pe > Pm. The speed w(s) will

be decreased below nominal until the Pe is reduced to the Pm. Using the Laplace transformed,

the steady-state speed deviation (∆wss) can be calculated by change in time domain ∆w(t) from

frequency domain ∆w(s). The feedback of speed governing is determined by H & D.

The equation of the inertia constant H = Stored kinetic energy in megajoules at synchronous

speed / machine MVA rating = (1/2) * (J* {w(rad/s)}² / Sbase) … (5)

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4.1 ROCOF protection relay

ROCOF is another popular protection relay to detect the islanded in the power network.

However, if the load and generation on the part of the network that becomes islanded are closely

matched; the limitation is recognised in Engineering Recommendation (ETR) 113 [5] section

3.4.5. The trip signal will transfer to the circuit breaker to cut off the power supply of distributed

generated immediately whether the result of ROCOF is out of the pre-set range (threshold). The

settings of ROCOF are between 0.1 Hz/s and 1.2 Hz/s in 50 Hz [6].

The ROCOF is proportional to the value of active power imbalance between the generator

output and local load. The equation of ROCOF

is ∆f/∆t = [- (PL - Pdg) * f(rated) / (2H *

Sn) ] … (6), drawing is shown in Fig 4.2

H - inertia constant of the generator[s]

f (rated) - rated frequency of the generator[Hz]

PL – Local load[MW]

Pdg – generator output[MW]

Sn – nominal generator rating[MVA]

Using the Matlab / Simulink to design power network with main power supply, customer load

and the synchronous generator, it is the common method to simulate the situation of power

distribution. The model of main power supply (swing type) is shown in Fig 4.3. The power

Fig 4.2 - ROCOF

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source provides 1000V rated voltage to the 15kW load through a 11000V/380V transformer.

On the other hand, the model of distributed generator network is shown in Fig 4.4. It also

provides the electrical power to the customer load through another 3-phase transformer.

Fig 4.3 Main power supply network

Fig 4.4 - distributed generator network

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The circuit breakers (CB) in the network acts as a part of ROCOF relay. When the main power

supply is suddenly broken down in the main grid. The CB on distributed generation side will

receive the order to turn off the power supply to customer load. Then, each parameter has

connected to the 3 phase measurements in the circuit, the function of the measurement is to

indicate the value of the voltage and current in the network instantaneously. The scope will

collect the measured data and display signals produced during simulation in terms of time. The

signal is displayed in the scope, as shown in Fig 4.5.

In the power network, 3 phase delta / star (Dy) transformers are used to step up the voltage

rating, the information of two transformers are shown in Fig 4.6:

The turn ratio of (Dy) transformer (a) = V1/V2 * (3)^(½)

= (11000/380) * (3)^(½) = 50.1

The current of apparent power of main power supply = 15000 / [(3)^(½) * 11000] = 0.787A.

The current of apparent power of customer load = 15000 / [(3)^(½) * 380] = 22.8A.

Fig. 4.5 - Scope displays the signals

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In normal condition, both main power supply and distributed generator supply the electricity to

the customer load. However, the main power grid is unable to supply the power to the load due

to unpredictable disasters or power fault. The 3 phase measurement of main power grid will

obtain the no power signal on 0.3 second and display on the screen, as shown on Fig 4.7.

Fig 4.6 - 11kV/380V transformer

Fig 4.7 No current supplied from the main grid

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Meanwhile, the distributed generator has increased the current rating to the customer load after

the main grid has suspended the power supply on the 0.3 second, as shown on Fig 4.8.

In the distributed generator, the typical synchronous generator will also provide the electricity to

customer load. The information of synchronous generator is shown in Fig 4.9.

The inertia constant H = (1/2) * (J(w(rad/s))² / Sbase) = (1/2) * ( 4.86 * (50 * π)² / 15000) = 4s

The synchronous speed of the generator = (50 * π)rad/s / (2 * π / 60) = 1500 rpm.

The synchronous generator has 2 important parts: Hydraulic Turbine & Governor (HTG) block

and Excitation System block (refer to Fig 4.2.). In HTG block, the machine actual speed (we),

machine actual electrical power (Pe0) and speed deviation (dw) are connected to receive the

data of feedback loop from the synchronous generator.

Fig 4.8 - Larger amount of current supplied from Distributed Generator

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Reference speed (wref) and reference mechanical power (Pref) are indicated to the constant

valve. The output of HTG acts as the mechanical power (pm) for the synchronous generator

block. For Excitation System, the vd and vq inputs are connected to receive the data of feedback

loop from the synchronous generator. The desired valve of the stator terminal voltage (vref) is

indicated to the constant valve and the power system stabilizer (vstab) is connected to ground

block. The output of excitation system acts as the field voltage (Vf) for the synchronous

generator. The machine initialization of the powergui tool should be set up before running the

synchronous generator.

On the other hand, the output A, B and C of synchronous generator are connected to the parallel

load (GL15kW). It is because the generator acts as a current source in Simulink software and it

Fig 4.9 - Synchronous Generator

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is unable to connect in series with the 3 phase transformer (the inductive component). The high

value of parallel load can prevent bad effect on simulated system.

To understand ROCOF protection relay in the network, there is a flow chart of ROCOF to

describe the operation in each step, as shown in Fig 4.10.

Fig 4.10 – Flow Chart of ROCOF

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- Different frequency will be measured from Point of Common Coupling (PCC).

- Then, keeping measuring the rate of change of frequency. When the power mismatch occurs,

the frequency will be change.

- If the variation of frequency is detected within the acceptable range of setting, ROCOF does

not detect the change in frequency.

- If the change in frequency is out of the range of setting, ROCOF can detect the changed and

generate the signal to circuit breaker for breakdown the circuit.

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The model of ROCOF protection relay is created in the Matlab / Simulink, as shown in Fig 4.11.

The operation of ROCOF protection relay model in the simulation

The Phase Lock Loop ‘PLL’ block determine will continuously track the frequency of 3 phase

sinusoidal signals. An initial frequency must be specified because the function measures the

frequency of input signal based on the difference between the actual frequency and the initial

frequency [7].

When receiving the voltage signal from the 3 phase measurement of customer load, the ‘PLL’

block will change the input signal from Clarke’s transformation [e.g. 3 phase (ABC) to 2 phase

(ab)] to Park transform [e.g. stationary 2-phase windings (ab) to rotating 2-phase windings (dq)].

The (dp) signal will be transferred to the derivative block and the real time frequency signal will

be displayed in the scope block. After leaving the derivative block, the (dq) signal will change to

the value of ROCOF that will enter to the transfer function block. The transfer function makes

the curve of the data versus time more fluent. The Abs block will adjust the value to “absolute”

Fig 4.11 – ROCOF Relay Simulation Model

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value. Then, the absolute value will compare with the pre-set constant block which is the

threshold. The relational operator will also collect the comparison result to decide the operation

of the circuit breaker for the synchronous generator. If the absolute value of ROCOF is smaller

than the threshold, it means no islanding in the circuit and the relational operator will display to

“1”, the circuit breaker will keep in switch on. Otherwise, if the absolute value of ROCOF is

larger than the threshold, it means islanding will occur in the circuit and the relational operator

will display to “0”, the circuit breaker will switch off to terminate the power supply to the

customer load.

The simulation model of ROCOF protection system is shown in Fig 4.12.

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Fig 4.12 – Simulation Model of ROCOF Protection Relay

Simulation Model

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4.2 Frequency Protection Relay

Generally, the detection time of islanding should be set within 200 to 400ms. Many distributed

synchronous generator systems have built in under/over frequency relays (FR). Although the

IEEE distributed resources interconnection guide recommends that a DG must not be

disconnected due to small frequency variation. [2], the FR is much simpler for the protection

system of the distributed generators. The operation of the FR is dependent on the active power

imbalance which is happened on the islanded system, mismatch between the bus-bar generation

and load. When the mismatch of active power decreases, the capability to quickly detect

islanding of these devices diminishes accordingly [8]. However, the generation and load are

very close, the frequency relays may fail to detect an islanding. When Pm is balanced to Pe, the

rotor speed (w) and angle (δ) of the distributed generator are constant respectively. If the

disturbance is occurred at the distributed network, the frequency will be changed because the

active power imbalance (∆P) will causes the transients in the distributed generators. The swing

equation of synchronous generators is listed below:

(2*H/wo) * (dδ/dt) = ∆Pm - ∆PL = ∆P … (7)

dδ/dt = w – wo … (8) (w : generator rotor speed, wo - synchronous speed)

Substitute (8) into (7)

w = (wo* ∆P / 2*H) * t + wo … (9)

The system angular speed in the time w = wo + ∆w,

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wo + ∆w = (wo*∆P / 2*H) * t + wo => ∆w = (wo*∆P / 2*H) * t … (10)

where w = 2πf.

So, ∆w = (wo*∆P / 2*H) * t => ∆f = (fo*∆P) / (2*H) * t … (11)

Equation (11) is provided the relationship between active power imbalance (∆P), frequency

deviation(∆f) and the detection time(t). The frequency relays can also be adjusted with the time-

delay settings (tset).

t = (2*H*∆f) / (fo*∆P) + tset … (12)

In order to estimate the operation of the frequency relay, the swing equation is used to observe

the performance of frequency relays between the detection time and active power imbalance.

Using the equation (12) to find out the performance curve of frequency relays at the detection

time from 0 to 1 second, as shown in Table 4.3 and Fig 4.13.

Table 4.3 – Active power imbalance vs detection time

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In the curve of performance of FR, x-axis is the active power imbalance and y-axis is the

operating time of frequency relays. If the detection time is required within 0.4 second after the

islanding happened, the intersection of 0.4s horizontal line obtains the 0.25 of active power

imbalance level. If the islanded occurs high above 0.25 of the active power imbalance, it will

detect the islanding condition less than 0.4s. If the islanded occurs less than 0.25 level of active

power imbalance, the relay will take more than 0.4s to operate in the moment. The threshold is

called the critical power imbalance.

The variation of frequency is affected by the electrical speed of generator, if the signal is over

frequency or under frequency setting, the relay will send a trip signal to circuit breaker of the

generator. The frequency relays can be operated with a time delay to persist the tripping mode

during the pre-determined time to trigger the frequency relay. In normal condition, the relay will

be set using the multi-stage, the setting of the time delay are operated at the same time.

Non-

Detection

Zone

Detection Zone

Fig 4.13. – Performance Curve of Frequency Relay

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The model of Frequency relay is created in the Matlab / Simulink, as shown in Fig 4.14.

Similarity with the ROCOF simulation model, there are the ‘PLL’ block to collect the voltage

signal from the 3 phase measurement of customer load and change to the (dp) signal. Before

entering to the transfer function block, the frequency value will be displayed immediately. The

transfer function makes the curve of the data versus time more fluent. The output signal from

the transfer function will compare with 2 nos. of constant block which is connected to Over

frequency or Under frequency comparator individually. After comparing with the over / under

frequency comparators, the comparison result will be logically determined by the ‘OR’ gate

logic. Finally, if the “1” number is indicated on the display, the circuit breaker will keep in

switch on. Also, if the “0” number is indicated on the display, the circuit breaker will switch off

to terminate the power supply to the customer load.

The simulation model of Frequency relay system is shown in Fig 4.15.

Fig 4.14 – Frequency Relay Simulation

Model

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Fig 4.15 – Simulation Model of Frequency Relay

Simulation Model

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4.3 Non-Detection Zone

The non-detection-zone (NDZ) method is one of the techniques available to characterize the

capabilities and limitations of the frequency and voltage relays [9]. If the power imbalance

between generation and load is not far away, the protection relays could not detect an islanding

occurrence in a well-timed. Constant current control or constant power control are the 2 used

control system in distribution generation network. In constant current control, the root mean

square valve of current is injected to the network as constant. In constant power control, the

active power and reactive power in each line period are controlled as constant [10]. As shown

Fig 4.16,

Before the disconnection of the network, both active and passive power of the load is :

P(load) = V² / R = P + ∆P … (13)

Q(load) = ∆Q = V² * [[1/(2 * π * f * L)]-[2 * π * f * C]] … (14)

Fig 4.16 – Typical distribution generation connected with power network

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After the disconnection of the network as shown Fig 4.17, the reactive current will remain zero

under the current control. The resonant frequency (f ’) of load is :

f ’ = 1 / (2 * π * (L * C)^½) … (15)

Using equation (14) is divided by equation (13), then

Q(load) / P(load) = [[1/(2 * π * f * L)]-[2 * π * f * C]] * R

= R * (C / L) ^½ * [[1/(2 * π * f * (C * L) ^½ )]-[2 * π * f * (C * L) ^½ ]] … (16)

Then, substituting equation (15) into equation (16)

Q(load) / P(load) = ∆Q / P * (1 + ∆P / P)^(-1) = Q(f) * [(f ’ / f) – (f / f ’)] … (17)

Therefore, Q(f) = R * (C / L) ^½ … (18)

When the new value of frequency remains between the frequency thresholds, the frequency

relay will not operate. The non-detection zone of frequency relay for current controlled is

Q(f) * [(f(min) / f) – (f / f(min))]

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f(max))] … (19) (f(min) and f(max) are the setting of frequency thresholds).

Under the IEEE 1547, the tolerance of the normal voltage frequency is 98.83% to 100.83%.

So, f(min) = 50 – 0.6 = 49.4 Hz and f(max) 50 + 0.4 = 50.4 … (20)

Equation (19) will be changed to

Q(f) * -0.024

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4.4 Threshold setting and Optimization

In order to find the reasonable threshold for the Frequency relay and ROCOF relay that achieve

to stable and sensitive in the network, automated islanding and reconnections methods have

been developing in many power plants. If the unpredictable fault has happened in the power grid,

the power plant will terminate the electrical supply to the customer load. Until the fault is

resolved, it will be auto-reconnected to the power supply in the network. The question is how to

detect and breakdown the power supply of the distributed generator during reconnection time, it

will cause the serious damage of the customer loads due to the confliction of current from both

sides. The response of the islanding detecting relay should be within 0.2 second. The ideal

operating time of the relay is recommended to detect the islanding within 0.1 second. Therefore,

both frequency relay and ROCOF relay should be reliable and sensitive to prevent the operation

mistake when disturbance occurs in the main grid.

Several factors may affect the performance of frequency and ROCOF during the islanding. It

includes the different types of load and power factor, temporary fault in main grid, low value

power mismatch.

Different type of load and power factor :

Investigate the different of certain type of constant load (power, impedance and current) under

the same model setting. Then searching the reasonable threshold setting for FR and ROCOF

bases on the acceptable power factor of customer load.

Temporary fault in main grid:

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Many factor will cause the temporary fault in the main grid. Such as, trees growth into

conductors, trees contact can cause the short circuit of overhead distribution line, or animal-

caused faults may occur on overhead distribution lines, underground distribution circuits,

substations, and transmission line [11]. Moreover, the detecting relay has the ability to

distinguish the real islanding and temporary fault.

Low value power mismatch:

The most challenging situation of detecting islanding situation is when the output power of

distributed generator can exactly match the power consumption of customer load [12]. If the

frequency of the supply is vibrated intensely, the value change of the FR or ROCOF relay will

be obscure. Setting the zero output of reactive power can maintain the 100% of the power factor,

the FR and ROCOF method can easily detect the full power supply from the distributed

generator during islanding conditions.

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4.5 Other detection method

Wavelet-transform Islanding detection method

Wavelet transform has been commonly used in power system, like feature power system

protection, data compression of power quality waveforms, extraction, de-noising etc. [13][14].

Actually, transient state of voltages and currents has high frequency waveforms, it is unable to

be detected by general methods on a power frequency. Therefore, wavelet analysis can

distinguish the islanding or non-islanding in normal operation, it will extract a useful feature to

achieve the classification [15].

The structure of wavelet is shown in Fig 4.18[16], there are two paths of filter in each level of

discrete wavelet transform (DWT). In first level, the low pass filter with impulse response (g)

can find out the approximation coefficient (a1(n)) and the high pass filter (h) can find out the

detail coefficients (d1(n)) respectively. The filter outputs are subsampled by 2.

Fig 4.18 – Structure of wavelet filter level.

Fig 4.19 – 3 level discrete wavelet transform

decomposition

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Through iterative methods as shown in Fig 4.19[16] in each level, it can be decomposed to

contain the approximation coefficients and details coefficients in each level.

The equation of approximation coefficients and details coefficients are listed below :

… (22)

… (23)

(am(n) – approximation coefficient at level m and dm(n) – detail coefficient at level m)

Compared with other passive islanding methods, wavelet method can react to the spectral

changes occurring at higher frequency waveform of PCC voltage caused by islanding and detect

the islanding situation in a smaller NDZ.

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Chapter 5 Result

ROCOF protection relay

Before connecting to ROCOF relay, the freqency of synchronous generator has raised and

fallen extremely within 1 second, as shown in Fig 5.1. The crazy stator current is in Fig 5.2

These frequency and stator current from the generator have caused the ugly sinusoidal

waveform of current of customer load when both generator and main grid supplied the power

to the customer load, as shown in (Fig 5.3a, b & c).

During computing the load flow, it indicates the ‘warning

notice’ to prohibit the compute of the load flow in the

powergui, as shown in Fig 5.4. To solve the Load Flow

Fig 5.1 – Frequency curve from the

synchronous generator

Fig 5.2 – Abnormal stator current

from the synchronous generator

Fig 5.4 – warning notice

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data error, it is recommended to change the status of synchronous generator from PV type to

swing type. Then the load flow table could be computed, as shown in Fig 5.5

Fig 5.3a – The output of main supply:

Voltage rms = 1.075e+4 V, Current rms = 1.779 A

The power = 33,124 W

Fig 5.3b – The output of generator:

Voltage rms = 3.67e+2 V, Current rms = 5.754e+1 A

The power = 36,576 W

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After connecting to the ROCOF relay, the frequency of the customer load voltage decreases

because of the system islanding occurrence, as shown in Fig 5.6. The value of df/dt of

ROCOF relay is sufficient to meet the islanding detection. The relay threshold is set to 0.3

second, if the df/dt value is more than 0.3 to 1 second as shown in Fig 5.7, the relay will

transfer the tripping signal to the ciruit breaker of generator.

Fig 5.3c – The customer load :

Voltage rms = 3.67e+2 V, Current rms = 7.08A

The power = 4500 W

Fig 5.5 –Load flow data

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Due to trigger the circuit breaker of the generator, the current supply to the customer load is

decreaed at 0.3 second, as shown in Fig 5.8.

Fig 5.6 – Frequency curve after islanding

Fig 5.7 – ROCOF curve after islanding

Fig 5.8 – Generator current after 0.3 second

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Frequency relay

The situration of results of frequency relay are similar to the ROCOF record before the

connection to ROCOF relay. If the value of frequency touch up the range of preset threshold,

the circuit breaker of generator will be turned off, as shown in Fig 5.7.

Meanwhile, the circuit breaker is received the signal of islanding from frequency relay, it will be

turned off itself to terminate the power supply to the customer load, as shown in Fig. 5.8. And

then, the current supply of generator is zero, as shown in Fig. 5.9.

Fig 5.7 – the frequency touch up the range of preset threshold of the

relay, it will resume quickly to nominal frequency.

Fig 5.8 – touching the boundary of preset range, the signal will be

changed from ‘1’ to ‘0’

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Fig 5.9 –The circuit breaker of the generator will be triggered.

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Chapter 6 Conclusion & Future Development

This project presented the typical methods to detect islanding situation for distribution

generations was proposed. These methods are passive and make decision according to the

measurement of signal of frequency (ROCOF and Frequency relay). These results indicated the

relays how to monitor and disconnect the power output from the synchronous generators during

the islanding occurrence. The results of the protection methods are simulated in Matlab /

Simulink environment, it can be easily observed the operation of protection relay to power

supply network.

In this project, I understood the materiality concept of the anti-islanding protection system to

power grid network, it can reduce the threats to personnel safety, improve the power quality and

enhance the sensitivity of the grid networks in normal operation. In order to prevent to the

islanding effect, the certain kinds of detection method have been developed to solve the any

situations. The detection methods are categorized into 2 groups: Local methods and Remote

methods. Each group of detection methods have their own benefits and drawbacks. In local

methods, passive methods are able to use the inexpensive hardware to activate the protecting

devices through the measurement of the frequency or voltage, but the limitation of it has the

NDZ; active methods can inject the intentional signal or harmonic effects to inspect the

islanding, it can effective minimize the range of NDZ, but more perturbative signals will affect

the quality of electrical grid network. In remote methods, they are more effective to conquer the

NDZ problems but they also tend to be more expensive. In the future, remote methods may be

- 48 -

perhaps the most important devices to monitor the islanding effect.

On the other hand, there are many differences between the fixed parameters and real in change,

such as alternative of supplies and demands for electrical grids, natural change, etc. Under the

mandate of IEEE 1547 standard, the simulation results of detection method should be fulfilled

to the requirement, but the results can only reflect the general condition. In addition, the experts

will extend more effort to improve the islanding method or create the new technology.

Overall, this project has very difficult and challenging for me, I need to understand the meaning

of the islanded situation and the power supply from the utilities power supply and the DG to

customer load. To seek the different kinds of the journals and reference books, it can be useful

for analysing the complication equation of detection methods. Unfortunately, I don’t understand

these complicated journals. It is not much the frequency control of synchronous generator for

the references in the market. And then, I can only find the basic knowledge of power system

network in the teaching notes, it isn’t handle the anti-islanding protection project. On the other

hand, the Matlab / Simulink software is another difficulty and challenging for me to learning.

For example, I have no enough time and knowledge to understand all parameters setting of

synchronous generator. Insufficient knowledge leads to incorrect simulated result, etc. In fact it

is hard to resolves the errors of the simulation result and settle down the mistake.

- 49 -

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