Psp Lab2 Print

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Power systems protection 302

Name:Ubayeda Shaqer

Student number:15975669

Title of the experiment:Restricted Earth Fault Protection For A____________________Three Phase transformer

Laboratory group:Monday (1400-1600)(Even)

Laboratory supervisor:Hadi

Laboratory partners: Scott

Date performed the experiment:-09-2014

Due date:06-09-2014

Date submitted:06-09-2014

I hereby declare that the report presents entirely my own work and have not copied from any

other student or past student.

Student signature: ________________________________________


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Table of Contents

Introduction: ........................................................................................................................................... 3

Aim: ......................................................................................................................................................... 3

Summary of report: ................................................................................................................................. 3

Theory ..................................................................................................................................................... 3

Procedure ................................................................................................................................................ 5

Equipment ........................................................................................................................................... 6

Results ..................................................................................................................................................... 7

Test 1:No Load Tests ........................................................................................................................... 7

Test 2:Balanced Load Test .................................................................................................................. 7

Test 3:Unbalanced Load Tests ............................................................................................................ 8

Discussion................................................................................................................................................ 9

Answers to the questions ..................................................................................................................... 11

No Load Tests .................................................................................................................................... 11

Balanced Load Test ........................................................................................................................... 13

Unbalanced Load Tests ..................................................................................................................... 16

Conclusion ............................................................................................................................................. 20


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Restricted Earth Fault Protection of a Three-Phase

Power Transformer (Using Current Sensitivity Relay)

Introduction:

This lab tests the restricted earth fault protection for a three-phase power transformer. The

sensitivity is adjusted and the current is measured across various points.

Aim:

This lab has been designed to test the restricted in the fault which is used to protect earthed

and wye-connected three phase machines but cant be used deltaconnected setting due to the

fact this type of protection works on the neutral phase of the three phase setting. The

protection scheme was tested on three different circuits one with no load in the system,

balanced load and unbalanced load.

Summary of report:

The experiment was successfully completed. Faults that occurred in the protection zone was

identified and cleared- but any external faults (i.e. outside the reach of the relays) did not trip

the circuit.

Theory

An earth fault relay generally provides protection against faults by measuring the neutral or

the residual current or both. Ideally the neutral and residual currents of a system are zero.

Under normal conditions and by application of Kirchhoffs laws the sum of currents in both

current transformers (CTs) equals zero. The REF under external faults, the current in the line

CTs are balanced with the neutral CTs; however this is not the case under internal faults

the current is not balanced and current flows in the neutral circuit therefore this is an

indication of an fault and the REF operates.

The REF is a combination of residual current relay which detects the current differential of

the threephases (in a three phase system) and a second relay thats operates on the neutral

line of the wye connected scheme. The currents in both the lines are observed and this is how

faults are detected and then cleared; this is very accurate approach for protection.


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Figure 1: Protection Scheme

Three current transformers for each phase and current transformer for the neutral line is usedto measure current in this protection scheme. As seen in the figure above the secondary

windings of the line current transformers are connected in parallel to obtain the vector sum of

the three line currents. These secondary windings are also connected in parallel with a

protective relay and the secondary winding of the neutral current transformer. This is

similar to a differential protection however a REF protection scheme is comparatively

cheaper than differential protection scheme.

Any unbalance between the two current initiates the protection system causes the protective

scheme to trip and isolated the transformer. However this is true only for the secondary sideof the transformer. According to requirements and budget a REF can be placed in the primary

side of the transformer too. This would provide a complete protection for the entire system

against both external and internal faults in the system. REF protection is fast and can isolate

winding faults extremely quickly, thereby limiting damage and consequent repair costs.

Figure 2: Protection Scheme and Single Line Diagram for REF


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Procedure

The EMS Workstation and the Protective Relaying Control Station was set up according to

the schematic provided.

Figure 3: Schematic For REF protection

The circuit was connected according to each connection i.e. for the red wire phase A of the 3

phase circuit was selected and connected from the input to its end at RL (1). Following colour

one by one starting and ending its connection was procedure used to begin and complete the

circuit.

Faults were then manually initiated for different 3 different setting of the circuitunder no

load, a balanced load and unbalanced. The time and current in the ammeter were observed

and recorded. The sensitivity settings were set according to the instruction (Sensitive Relay to

20 % and 50%). In this protection scheme the time setting were already placed internally within the relay.


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In Restricted Earth Faultscheme the common terminals of phase CTs are connected to the

secondary of Neutral CT in such a manner that secondary unbalanceelectric current of phase

CTs and the secondaryelectric current of Neutral CT will oppose each other. If these both

electric currents are equal in amplitude there will not be any resultantelectric

current circulate through the said close path. Ammeters were used to measure this current.

Equipment

Current sensitive relay

Relay test set

Figure 4: Completed Connected Schematic

This figure displays the complete connected circuit.
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Results

Test 1:No Load Tests

7.1The primary line current (indicated by ammeter I1), the secondary line current (indicated

by ammeter I2), the neutral current (indicated by ammeter I3), and the current flowing in the

AC/DC Current Sensitive Relay (indicated by ammeter 14)

Table 1: Ammeter Currents (No Load)

7.2An earth fault near the neutral end of the secondary winding of transformer T1.

Table 2: Ammeter Currents (Internal Fault at T1)

I1= 40mA I2=4,0mA

I3= 600mA I4 = 600mA

7.3

A phase-to-earth fault beyond the line current transformers.

Table 3: Ammeter Currents (External Fault)

I1= 650mA I2= 650mA

I3= 800mA I4 = 0mA

Test 2:Balanced Load Test

8.1An earth fault near the middle of the secondary winding of transformer T1.


11= 200mA 12= 200mA

13= 600mA I4 =420 mA why zero?? balanced

I1= 0 A I2= 0 A

I3= 0 A I4=0 A


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I1= 350mA I2= 350mA

I3= 600mA I4 = 550mA

8.3A phase-to-earth fault beyond the line current transformers.


I1= 1.05A I2= 1.05A

I3= 600mA I4 = 0mA

Test 3:Unbalanced Load Tests

9.1This unbalances the three-phase load currents.

Table 7: Ammeter Currents (Unbalanced Load)

I1= 200mA I2= 200mA

I3= 200mA I4 = 0mA



I1= 200mA I2= 200mA

I3= 150mA I4 = 180mA



I1= 0A .8 I2= 0A.8

I3= 0A .8 I4 = 600mA 4ma

9.4A phase-to-earth fault beyond the line current transformers.


I1= 750mA I2=750mA

I3= 800mA I4 = 0mA


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Discussion

This laboratory investigates the function and limits of the Restricted Earth Fault. The REF

requires 4 CTs to be connected in parallel. The CTs compare the difference in the neutral and

residual current in order to detect a fault. As seen in the schematics (figure 1) the protection

system has been designed where under normal conditions the neutral current and the CT

currents should cancel out (Ammeter 4 should always be zero).

This lab primarily deals with two different faults internal and external faults. The faults test is

conducted under three different configurationsNo load, Balanced load and Unbalanced

load. In all three cases the internal faults are detected and cleared by the REF protection

system. However in all three cases the external faults could not be detected nor cleared by the

REF. This was simply because the faults were outside the scope and reach of the CTs

therefore they completely avoid detection. This suggest the REF system is highly stable andreliable as it operates when required and within its zone.

The internal faults that were investigated were located in two different locations in T1 or T3

and in the middle of the winding or the neutral. Faults in the secondary winding cause an

imbalance between the neutral and residual current; this is detected by Ammeter 4 which then

triggers the relay according to the current setting of the relay (sensitivity). 0% means the

relay is most sensitive and will trip the circuit when any amount of current is detected under

any conditions (not necessary a fault present in the circuit). The current setting is setting up a

standard or reference fault current which will allow the relay to distinguish between fault

current and normal current; when to or not to trip the circuit. Therefore the relay must beadjusted according to the rated power and current setting taking into consideration the

maximum fault current that can be expected.

Most of the ammeter reading are in Millie-Ampere which is very difficult to decipher

between whether the detections actually error or saturation or mismatch but however since

the relay operated during faults and cleared the internal faults; it can safely be assumed the

currents during the fault are the actual fault current, where a percentage of the current has

been contributed by the various error issues of the circuit. CT mismatch, CT saturation,

tolerance of the equipment and spill currents are sources of error that affect the recorded

current values.

Ammeter 1 and Ammeter 2 display the phase currents on the primary and secondary winding

of the transformer. Since there is no protection on the primary side faults on the primary cant

be detected or cleared. Ammeter 3 displays neutral current which is essential in detection of

the faults or unbalances in the system. As the winding fault position moves towards the

neutral, the magnitude of the current seen on the primary rapidly decreases and could

potentially not be detected (limiting the amount of winding which can be protected).


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A restricted earth fault system supplied by a three phase transformers must include a time

delay in order to avoid dubious and nuisance triggering resulting from transients. Generally it

recommended that current setting must not be lower than 15% of the transformer rating at

best, or 20 % of the transformer ratings in the most unfavourable cases. Moreover, if an earth

fault occurs in a star winding near a neutral point, the maximum fault current is only a smallpart of the maximum fault current which is limited by the neutral earth impedance. For this

reason, the current setting is usually 20% of the maximum current limited by the neutral earth

in order to protect 80% of the windings.

Earth fault protection applied to a delta-connected or unearthed star winding is inherently

restricted, since no zero sequence components can be transmitted through the transformer to

the other windings. If a fault occurs on the delta side it cannot be detected or cleared. Build of

fault current can lead to catastrophic failure in the system.

It can be observed that fault currents recorded near the neutral for the primary side have

smaller amplitude comparatively. Hence, although the prospective current level decreases as

fault positions progressively nearer the neutral end of the winding ;a large percentage of the

winding can be covered if the protection system is placed on the secondary side (as seen in

the schematics figure 2). Both windings of a transformer can be protected separately with

restricted earth fault protection, thereby providing high-speed protection against earth faults

Because the fault detection is limited between the lines of CT it is restricted in this zone

which why the protection scheme is called the Restricted earth fault system.

REF are comparatively cheaper than the differential protection scheme therefore it is better

option due to the fact it is reliable, fast and stable protection system

.


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Answers to the questions

No Load Tests

7.1Record the primary line current (indicated by ammeter I1), the secondary line current

(indicated by ammeter I2), the neutral current (indicated by ammeter I3), and the current

flowing in the AC/DC Current Sensitive Relay (indicated by ammeter 14) in the following

blank spaces.

The circuit currents are:

Is the REF protection system almost perfectly balanced? Briefly explain.

Yes, the REF protection system was balanced. Under normal conditions the system was

balanced because there was no fault in the system and by application of Kirchhoffs

laws the sum of currents in both current transformers (CTs) equals zero. This has been

observed and recorded above, all ammeter values equal to zero.

7.2On the Fault able Transformers, set fault switch FS3 of transformer T1 to the Iposition

to insert an earth fault near the neutral end of the secondary winding of transformer T1.

While doing this, observe the circuit currents and the tripping indicator (red LED) on the

AC/DC Current Sensitive Relay. Adjust the current set point of the AC/DC Current Sensitive

Relay to 20 % and 50%, respectively and observe the operation of relay.


I1= 40mA I2=4 0mA

I3= 600mA I4 = 600mA

Describe what happens when an earth fault occurs near the neutral end of one of the power

transformer secondary windings.

Due to an earth faults and imbalance in current is seen through the circuit. The

ammeter placed in the circuits detects currents due to the imbalance and the presence of

a fault. Current through Ammeter 1 and 2 are the same because they are placed across

two different phases of the three phase system and Ammeter 3 and 4 detects the

difference in current therefore subtraction of the phase current and neutral current

results in the same value.

I1= 0 A I2= 0 A

I3= 0 A I4=0 A


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The circuit trips when the difference in current reaches a certain value (level). This

tripping characteristic is set for a certain reference in order to take into account error

of the CT and prevent any false trips.

7.3On the Universal Fault Module, depress the INITIATE FAULT button to produce a

phase-to-earth fault beyond the line current transformers.


I1= 650mA I2= 650mA

I3= 800mA I4 = 0mA

Set the current setting of relay on 20% and observe the operation of relay. After that change

the current setting of relay to 0% (minimum) and apply same fault again. While doing this,

observe the circuit currents and the tripping indicator on the AC/DC Current Sensitive Relay.

Describe what has happened.

0%means the relay is the most sensitiveany differential current in I4 would cause

the protection scheme to operate and the circuit to trip. However the with the manual

initiation of the UFM the circuit did not trip nor was a differential current detected in

I4. This is because from the schematic we can see the fault is outside the protection zone

and scope of the REF therefore even the most sensitive relay settings will not pick up the

fault.

Is the REF protection system stable for faults outside the protection zone defined by the fourcurrent transformers? Briefly explain.

Yes the system is stable. The protection system only operates when it is requiredthat

is the fault is within its protection zone. The UFM is outside the protection zone of the

CTs so the fault is not detected and no difference in current detected by Ammeter 4.

Therefore in case of an external fault the relay is not activated, the CTs dont pick up

any fault and REF is stable.

An external fault in the star side will result in current

flowing in the line current transformer of the affected phase and at the same time a

balancing current flows in the neutral current transformer; hence the resultant electric

current in the relay is therefore zero. So this REF relay will not be operating for

external earth fault.

Has the fault been cleared by the REF protection system? Why?

Yes No

The REF does not clear the fault because it does not see the fault; it is outside the

protection zone and scope of the CTs; the fault must be cleared by another protection

scheme.The system is operative for faults within the region between current

transformers that is, for faults on the star winding in question. The system will remainstable for all faults outside this zone.


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Balanced Load Test


to insert an earth fault near the middle of the secondary winding of transformer T1.


the current setting of relay to 50% and apply same fault again. While doing this, observe the

circuit currents and the tripping indicator on the AC/DC Current Sensitive Relay.


11= 200mA 12= 200mA

13= 600mA I4 =420 mA why zero?? balanced

Describe what happens when an earth fault occurs on one of the power transformer

secondary windings and describe the difference of relay operation for mentioned two

settings.

At 20% a lower current initiates the protection scheme as the relay is more sensitive

(the reference current is lower) than it is at 50%. But the measurement obtained is

degraded by the inaccuracies of all three current transformers, in particular in the

event of transient over-currents when the transformers become saturated. This means

the fault current is detected earlier when the relay is at 20% but protection scheme

takes the same amount of time to clear the fault. When an earth fault occurs at thesecondary winding the neutral current transformer carries the unbalanced fault current

and this initiates Restricted Earth Fault Relay to isolate the transformer against the

internal fault.

For which current setting of relay, the fault has been cleared by the REF protection system?

By both the current setting the fault was cleared by the relay. The setting just adjusted

at what fault current the REF tripped the circuit but it cleared both the faults. The

sensitivity setting must be adjusted in order to in to consideration mismatch, error, and

saturation; make sure there are no nuisance trips.


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8.2On the Fault able Transformers, set fault switch FS3 of transformer T3 to the I position to

insert an earth fault near the middle of the secondary winding of transformer T3.





11= 350mA 12= 350mA

13= 600mA I4 = 550mA


secondary windings.

At 50% current setting the relay (protection and trip) is initiated for smaller current as

it is more sensitive (the reference current is lower) than it is at 70%. But the

measurement obtained is degraded by the inaccuracies of all three current

transformers, in particular in the event of transient over-currents when the

transformers become saturated. Most of the current setting recorded is in milliamps.

This means the fault current is detected earlier when the relay is at 50% but protection

scheme takes the same amount of time to clear the fault. When an earth fault occurs at

the secondary winding the neutral current transformer carries the unbalanced faultcurrent and this initiates Restricted Earth Fault Relay to isolate the transformer against

the internal fault.


The relay functioned for both the current settings. It is usually recommended to keep

the current setting at 20% to 80% is an optimum standard depending on fault levels

this range ignores nuisance trips at the same time functions for high fault levels (high

powered transformers).


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I1= 1.05A I2= 1.05A

I3= 600mA I4 = 0mA





The manual initiation of the UFM the circuit did not trip nor was a differential current

detected in I4. This is because from the schematic we can see the fault is outside the

protection zone and scope of the REF therefore even the most sensitive relay settings

(i.e. 0%) will not pick up the fault.

Is the REF protection system stable for faults outside the protection zone defined by the four

current transformers? Briefly explain.

Yes the system is stable. The protection system only operates when it is requiredthat

is the fault is within its protection zone. The UFM is outside the protection zone of theCTs so the fault is not detected and no difference in current detected by Ammeter 4.

Therefore in case of an external fault the relay is not activated, the CTs dont pick up

any fault and REF is stable. An external fault in the star side will result in current



current in the relay is therefore zero. So this REF relay will not be operate for external

earth fault

On the Universal Fault Module, place the INITIATE FAULT button in the released position.

Has the fault been cleared by the REF protection system?

Yes No


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Unbalanced Load Tests

9.1Set the resistance of resistor R1 to the value given in the following table. (Make all the 3

existing 4800 (ohm) resistors parallel) This unbalances the three-phase load.


I1= 200mA I2= 200mA

I3= 200mA I4 = 0mA

9.2On the Fault able Transformers, set fault switch FS3 of t ransformer T1 to the

I position to insert an earth fault near the middle of the secondary winding of

transformer T1.





I1= 200mA I2= 200mA

I3= 150mA I4 = 180mA

Describe what happens when an earth fault occurs on one of the power transformer primary

windings.

When a fault occurs on the primary winding it is less likely the fault is going to be

detected by the REF protection scheme. This is because the CTs are all connected on

the secondary side of the transformer not the primary. There no protection placed on

the primary or any neutral current present in the primary winding. If a fault occurs in

the primary then the fault current will circulate in the circuit will build until it leads to

a catastrophic failure in the transformer and circuit.

According to the test 9.2 when a fault occurs in the secondary winding the following

results were observed and recorded. At 10% current setting the relay is initiated

without a fault in the circuit. This is because the relay is very sensitivetherefore it

trips (nuisance trip) due to any difference in current that is present due errors that have

not been take into consideration (i.e. CT mismatch saturation etc.). The current





scheme takes the same amount of time to clear the fault as the time setting are fixed andwe are only changing the current setting.


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When an earth fault occurs at the secondary winding the neutral current transformer

carries the unbalanced fault current and this initiates Restricted Earth Fault Relay to

isolate the transformer against the internal fault.


For both the current setting the faults were cleared by the protection scheme. This is

because the faults are internalbut 10% the relay is too sensitive which lead nuisance

tripssmall difference in currents is considered to be a fault current which is the not

case. Even without manually introducing the fault in the system the protection scheme

operates.

Are the values of I4 same for both faults in steps 8-2 and 9-2? Why?

No they are not the same. This due to the fact the load requirements in the circuit are

different in between 8.2 and 9.2 and due CT mismatch, saturation and presence of spillcurrent directly effects the current values that have been recorded. The current

measurement obtained is degraded by the inaccuracies of all four current transformers.


to insert an earth fault near the middle of the secondary winding of transformer T3.





I1= 0A .8 I2= 0A.8

I3= 0A .8 I4 = 600mA 4ma


secondary windings.

At 30% current setting the relay is activated and protection scheme is initiated for a

smaller fault current as it is more sensitive (the reference current is lower) than it is at70%. 70% current setting is generally considered as the optimum sensitivity setting

where high fault current can detected and cleared by the transformer. Due to the fault

in the phase T3 there is an imbalance in the phase currents which is carried by the

neutral compared with the CT current; the difference of the two is a sign a fault has

occurred and the I4 activates the relay which causes the circuit to trip. But the





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scheme takes the same amount of time to clear the fault. When an earth fault occurs at

the secondary winding the neutral current transformer carries the unbalanced fault

current and this initiates Restricted Earth Fault Relay to isolate the transformer against

the internal fault.


For both the current setting the faults were cleared by the REF protection system as

they are internal faults and in accurate range of the relay sensitivity.

Are the values of I4 same for both faults in steps 8-3 and 9-3? Why?

No they are not the same. This due to the fact the load requirements in the circuit are

different in between 8.3 and 9.3 and due CT mismatch, saturation and presence of spill

current directly effects the current values that have been recorded. The currentmeasurement obtained is degraded by the inaccuracies of all four current transformers.




I1= 750mA I2=750mA

I3= 800mA I4 = 0mA





0%means the relay is the most sensitiveany differential current in I4 would cause

the protection scheme to operate and the circuit to trip. However the with the manualinitiation of the UFM the circuit did not trip nor was a differential current detected in

I4. This is because from the schematic we can see the fault is outside the protection zone

and scope of the REF therefore even the most sensitive relay settings will not pick up the

fault.


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Is the REF protection system stable for faults outside the protection zone defined by the four

current transformers? Briefly explain.

Yes the system is stable. The protection system only operates when it is required (stable)

that is the fault is within its protection zone. The UFM is outside the protection zone

of the CTs so the fault is not detected and no difference in current detected by Ammeter

4. Therefore in case of an external fault the relay is not activated, the CTs dont pick up

any fault and REF is stable.

An external fault in the star side will result in current



current in the relay is therefore zero. So this REF relay will not be operating for

external earth fault.

Has the fault been cleared by the REF protection system?

Yes No


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Conclusion

The REF is a combination of residual current relay which detects the current differential of

the three phases (in a three phase system) and a second relay thats operates on the neutral

line of the wye connected scheme. The currents in both the lines are observed and this is how

faults are detected and then cleared; this is very accurate approach for protection. It is far

cheaper method of protection when compared to the differential protection. Unlike this lab

investigation if REF is used on both sides of the transformerthis will ensure fault protection

throughout the transformer.

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