Improving Vector Shift Relaying for Islanding Detection

S. Martinez de Lafuente

1, F. Uriondo

1, J. Tavallo

2 and J.M. Garca

2

1Department of Electrical Engineering

ESI Bilbao, University of the Basque Country

Alameda Urquijo s/n 48013 Bilbao (Spain) Phone/Fax number:+0034 946 012 000, e-mail: sony.89.mtz@gmail.com, felipe.uriondo@ehu.es

2INGETEAM POWER TECHNOLOGY

Parque Tecnolgico de Vizcaya

Edificio 108 48170 Zamudio (Spain) Phone/Fax number:+0034 946 018 900, e-mail: josu.tavallo@ingeteam.com , juanmari.garcia@ingeteam.com

Abstract The two main dedicated passive methods for detecting distributed generation islanding are the rate-of-

change-of-frequency (ROCOF) and vector-surge (VS)

relays. The paper presents an enhanced new algorithm for

VS relaying. The new algorithm takes into account that the

real voltage shift can happen in a wave zero-crossing or that

it can be split into two different half cycles. It also measures

the true vector shift magnitude in the three phases, allowing

for future changes of settings in order to adequate them to

the real relay working situation.

Keywords: Distributed generation, islanding detection,

vector-surge (VS) relays, vector-shift (VS) relays,

synchronous generators.

1. Introduction The problem of islanding happens when a part of the

distribution network is disconnected from the rest of the

system but the distributed generators keep feeding the

islanded part of the network. This situation involves

several problems, for the equipment and for the personnel

safety also, so, it is common practice to disconnect all

distributed generators to minimize the risks associated to

the islanding situation.

The methods to detect the islanding situation can be

classified into active and passive [1]. The paper deals

with one of the most popular passive methods used to

determinate that the islanding has happened, the so called

vector shift, vector jump or vector surge relay.

The principle of operation of the vector shift relay is

quite simple [2]. During normal operation, the terminal

voltage of an embedded synchronous generator lags the

synchronous electromotive force by an angle . If the grid supply is suddenly disconnected from the section of

the network partially supplied by the embedded

generator, the load on the generator increases or

decreases and this causes a shift in the rotor displacement

angle. The terminal voltage jumps to a new value and the

phase position changes as shown in figure 1.

Vector

Shift

T T T

U(t)

t

Fig 1- Vector Shift and variation in the wave period

The vector shift in angle and in time are related by the

following expression:

T

TT

'2 (1)

Where T is the wave period (typically 20ms) and T' is the

period during the vector shift. Usually relays measure the

period calculating the time difference between two

consecutive rising/falling zero edges of the voltage wave.

The calculation is done by the relays [3], [4] every half

cycle and the relay setting threshold must be exceeded

twice consecutively to issue a trip command.

2. Problems in the measure of the vector shift.

The vector shift presented in figure 1 is purely

theoretical, if we simulate with certain degree of detail

the vector shift using the models (to be explained and

referenced in detail in the full paper) presented by Freitas

and other authors we see that the result is the one

presented in figure 2.

1

U(t)

t

VS1 VS2

R S T

T

T Fig 2- Simulated three-phase vector shift

The problems when trying to measure the vector shift are

the following:

The vector shift in two phases (R & T) is divided in two instants of time (VS1 and VS2) in

different half cycles.

The vector shift in phase S (VS2) happens in a zero crossing, so it is also divided in two half

cycles.

A manufacturer [4] solves the problem using a 5 out of 6

tripping method, this is, a measurement of each phase

voltage angle is performed every half-cycle and the

decision as to whether a trip condition exists is made

every full cycle. The angle difference between the

present and previous measurement is calculated. This

process yields six results at the end of each power system

cycle (2 half-cycles x 3 phase voltages). If 5 of those 6

results are above the setting threshold, a trip signal is

initiated.

3. Summary of the new algorithm.

3.1. Basis

The key of the new algorithm is that its decision criteria

is based on three measurements in each phase, this is, on

a total of nine measurements to decide if there has been a

breaker opening or not.

The algorithm measures the period every half cycle but

takes into account that only in one of the measures the

vector shift is exactly measured. This is due to the fact

that the vector shift can be divided in two different half

cycles so only the measure that includes both half cycles

measures the vector shift in its real value.

In order to see it more clearly, in figure 3, we have

isolated the vector shift in one phase for an opening of

the breaker at full load. As illustrated in the figure the

total (and real) vector shift is divided in two parts, the so

called Vector Shift 1 and Vector Shift 2, that happen in

two different half cycles. If we calculate the period for

each zero crossing, we see that T1 does not include the

total vector shift and neither does T3, only T2 measures it.

Vector

Shift 1

Vector

Shift 2

T1

T2

T3

t

U(t)

Fig 3- Simulated Vector Shift

The algorithm proceeds as follows: every half cycle it is

checked whether the measured vector shift exceeds the

set threshold or not. If the setting is exceeded this vector

shift is compared to the ones in the previous and the

following half cycles and it searches for the biggest one.

The biggest value and the half cycle when it has taken

place are memorized. Then, the values of the vector shifts

in the previous and the following half cycles to the

recorded one are added (this must be a similar value to

the recorded one). If both values are higher than the set

threshold, it is accepted that a vector shift has happened

in that phase.

The same process is repeated in the other two phases and

if the setting is exceeded in the three phases a three pole

trip is provoked.

Another important aspect to point out, that the new

algorithm also takes advantage of, is that the addition of

the vector shifts of the periods T1 and T3 is equal to the

measured vector shift in the period T2.

3.2. Ancillary elements

The vector shift is calculated with the following formula

in each phase:

average

average

T

TT

'360

Where the average period is calculated based on the

periods measured in the previous cycles.

Not to distort the measure, the following measures are

taken:

In the calculus of the average period, the periods where the vector shift has a higher value than

the established one are not included. For

example, if we take a value of 2 degrees/cycle,

that fits with a ROCOF of 0.278 Hz/s, if the

ROCOF has a higher value than this, the queue

is not refreshed and the unit ends up operating

after a certain time. This avoids the distortion of

the average period due to faults and serves as a

backup of the ROCOFs unit.

Neither are included in the average period the values of the period when voltages are below

75% of the rated value because it is assumed

that these values are polluted by the effect of a

fault or the connection of the source.

The value of the voltage is calculated using a half cycle Fourier transform, in that way when

the decision of including or not a cycle in the

calculus of the average is taken, you already

know the effective value of the voltage with

precision, something that would not happen with

a full cycle Fourier transform.

In order to provoke the trip order in the three phases, the voltages have to exceed the 75% of

the rated value, to avoid the three pole trip in

three phase faults.

4. Simulated model.

The system selected to make all the simulations is

the one used in most of the articles dedicated to the

topic of the vector shift[5][6].

GS2MVA0.69kV

VSRelay

Yg/0.69/13,8kV

Line

Yg/13,8/230kV

230kV100MVA

Fig 4- Model used in the simulations

The implemented MATLAB model is shown in the

next picture.

Fig5- MATLAB model used in the simulations

Due to the size of the picture, the details of the simulated

model cant be appreciated in the best way. It has been included a voltage regulator and it also has been

simulated the turbine with its control. This last point

could seem unnecessary for the study of the vector shift

but is interesting to study the algorithms stability in large simulations.

The model also gives the chance of connecting and

disconnecting different loads, provoking faults in order to

analyse the stability and provoking islands with different

unbalances between the load and the generation.

5. The algorithms sensitivity.

As it has been mentioned before, one of the advantages

of the algorithm is the capacity of measuring the vector

shift precisely. This advantage has been used to analyse

the algorithms sensitivity. Starting with a situation of full load in the source, we form islandings with different

levels of instability between generation and consumption

in the islanding and we save the data of the measured

vector shift getting the curve shown in the picture below.

In the picture, for example, it can be seen that for a

setting of 4 of the vector shift, it is needed a instability

of 20% between the generation and the consumption to

make the unit act. For the typical setting of 8 it is needed

an instability of 40% between generation and

consumption to make the unit act. In the same way, the

rest of the points of the curve can be read.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 20

2

4

6

8

10

12

14

16

18

20

22V

ecto

r S

hif

t (d

egre

es)

Consumption (p.u)i

Island with lack of

generation

Island with excess of

generation

Perfect Island

Fig 6- Sensitivity of the algorithm of the vector shift

6. Security of the algorithm.

As important as the sensitivity is the security of the

algorithm. It has been proved that for a setting of 4, the

simulated system does not act against:

Single phase-faults.

Double phase faults.

Three phase faults: It depends on the duration of the faults, because the longer it lasts, the

measured vector shift is higher. For example, for

a setting of 4, doesnt act for faults with a duration shorter than 100ms.

Another important aspect is the capacity of detecting the

opening of the breaker just when the fault occurs (if we

want to be more precise, when it finishes due to the

opening of the breaker). As different mechanisms are

used to numb the unit against faults, it is important to

detect the opening of the breaker just before the fault, in

this sense we can affirm that in all the cases, when the

breaker opens during a fault, the unit acts properly with

almost every setting between 4 and 12 for one-phase,

double-phase and three-phase faults.

Finally, we have to highlight the influence of the

connection and disconnection of the loads and, in it the

relative strength of the system and the point of

1

Discrete,

Ts = 2.083e-006 s.

1

A B CA B C

A B C

a b c

A

B

C

a

b

c

VabcA

B

C

a

b

c

A

B

C

a

b

c

A

B

C

a

b

c

Vf _

m

A

B

C

Pm

Synchronous Machine

2 MVA 0.69 kV

1

wref

Pref

we

Pe0

dw

Pm

gate

HTG

v ref

v d

v q

v stab

Vf

Excitation

System

A

B

C

100MVA, 230 kV

source

A B C

A B C

Vabc (pu)

Iabc (pu)

connection with the load is fundamental, and the

algorithm does not run away from those influences. The

connections and disconnections of high power loads

make the unit act in a wrong way.

In the simulated system, we can calculate the vector shift

provoked by the connection and disconnection of

different power loads in the exit of the generator or in the

terminals of the generator in low voltage. If we simulate

the connection and disconnection of loads in the exit of

the source, in which changing P2 (power connected en the

AT side of the transformer) we get a table like the

following:

Table 1 Vector shift provoked by the connection and disconnection of

loads

Potencia

(MW/Scc)

Potencia(pu Scc

PCC)

Salto Vector

-20/28 -20% 22.69

-10/28 -10% 14.45

-5/28 -5% 7.77

-2/28 -2% 3.20

+2/28 +2% 3.22

+5/28 +5% 7.81

+10/28 +10% 14.54

+20/28 +20% 23.91

7. Conclusions.

The designed algorithm for vector shift, measures with

precision the pure vector shifts and the zero crossing

ones. Also measures the value of the vector shift to

enable changes in the settings if needed, with criteria of

the vector shift measured by the unit. In the other hand,

the unit is relaying as it does not act against faults and

trips properly in the openings of the breakers caused by

faults. The effect of the connection and disconnection of

the loads and the inaccurate tripping in the abrupt

changes in the loads in weak systems and for loads that

are next to the source is not sorted out, but this is a

problem inherent to the philosophy of operation of the

vector shift.

8. Acknowledgements

This study has been done as a part of the PROINVER

project which is financed by Spanish Ministry of Science

and Innovation (INNPACTO program).

9. References.

[1] "The Impact of Renewable Energy Sources and

Distributed Generation on Substation Protection

and Automation". CIGRE WG B5.34. August

2010.

[2] N. Jenkins, R. Allan, P. Crossley, D. Kirschen, and

G. Strbac, "Embedded Generation", 1st ed.

London, U.K.: Inst. Elect. Eng., 2000.

[3] Crompton Instruments, "Vector Shift and ROCOF

Relay Instruction Manual".

[4] AREVA, "MICOM P341 Relay Instruction Manual

P341"

[5] John Mulhausen, Joe Schaefer, Mangapathirao

Mynam, Armando Guzmn, and Marcos

Donolo, "Anti-Islanding Today,Successful

Islanding in the Future", 2010 Texas A&M

Conference for Protective Relay Engineer.

[6] Walmir Freitas, Wilsun Xu, Carolina M.

Affonso, and Zhenyu Huang,"Comparative

Analysis Between ROCOF and Vector

Surge Relays for Distributed Generation

Applications". IEEE Transactions On

Power Delivery, VOL. 20, NO. 2, APRIL 2005

10. Other references.

[7] Jyh-Cherng Gu, and Cheng-Chun Wang and

Kuo-Chen Hsu, "Islanding Detection and Protection of a Utility connected Wind Turbines

via a novel FPGA-based Relay", APAP2009,

October 18~21, 2009 Jeju, Korea.

[8] F. Katiraei, C. Abbey, I. Da Cunha and Y. Brisette,

"Performance Assessment of Commercial

Relays for Anti-Islanding Protection of Rotating

Machine Based Distributed Generation".

CIGRE CANADA, Conference On Power

Systems, Winnipeg 2008.

[9] Walmir Freitas, Wilsun Xu, and Zhenyu Huang," A Practical Method for Assessing the

Effectiveness of vector Surge Relays for

Distributed Generation Applications". IEEE

TRANSACTIONS ON POWER DELIVERY,

VOL. 20, NO. 1, JANUARY 2005