Restricted Earth fault Protection in Transformers & Generators

Restricted Earth fault Protection in

Transformers & Generators

1.0            General :

Transformers and generators are voltage sources. They are traditionally protected by an Over current + Earth fault relay , normally mounted in the breaker panel. This is shown in fig. 1. It should be noted that this protection alone is not adequate. 

When an earth fault occurs within the zone defined as A in the fig-1, or within the machine, the fault current will circulate within the zone or within the machine. The fault current will not flow through the CTs connected to the O/C +E/F relays near the breaker. This will cause a no trip situation when there is a fault in the zone A (Internal fault). Consequently, a separate scheme is required to detect internal earth faults in zone A. This scheme is called Restricted Earth Fault (REF) scheme.

                                                                        

It should be noted that the fault currents in Zone A is limited by the  impedance of the equipments in the zone – for transformers and generators it is very low – the fault currents can rise very fast and damage the equipment. Consequently REF protection is of utmost importance for generators and transformers.

2.0            Why is this called Restricted E/F ?

The name Restricted is derived – since the objective of the protection is to detect the earth fault in the specific zone , restricted to the zone, starting from the breaker to the machine terminals. In case of a generator, the machine terminal is the neutral point. In case of a transformer , the machine terminal becomes the star point of either the primary or secondary winding or both. In case of delta winding, it is the winding itself.                                                                                                                  

3.0            How can we detect internal earth faults ?

It is well established that the sum of currents at the beginning of the zone A should be equal to the sum of currents exiting the zone.    Two sets of CTs are

used to derive these sum of currents at the inlet and exit. A fault in the zone will result in a difference in current .

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An over current relay is used to measure the  difference in the sum of these currents.   

Please refer to fig.2 where a typical REF scheme shown . The REF relay is connected between P & S . It will pick up if there is enough voltage across P-S to drive the pick up current through the relay.

       

There are two current sources in the CT secondary circuit PQRSTU :

a)       the current produces by CT  X ( loop RSPQ) 

b)      the current produced by CT  Y ( loop UPST) 

Under normal conditions, vectorial sum of the currents of these two CTs will be equal and opposite in the branch P-S (ie) the resultant current in the branch PS will be zero. In this situation , the currents of CT X and CT Y will circulate in the loop PQRSTU. The voltage across P-S will be zero.

When there is an earth fault within the zone A, the currents of  X & Y CTs will not be equal – a small difference of current will flow in the branch P-S. This current will result in a voltage across P-S – since the current flows through the relay impedance. If this voltage is adequate to operate the relay , the relay will pick up – thus detecting a fault within the zone.

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4.0            What if a fault occurs outside the zone ?

For a fault outside the zone, the currents through CTs X & Y will be the same – the resultant current through P-S will still be zero and the relay will not not pick up. In this case the fault has to be cleared by another O/C + E/F relay connected near the breaker.

5.0            Why is a Stabilising resistor required in REF scheme ?

We have said in the sections 3 & 4 that the REF relay will detect a fault within a zone restricted between two CTs and it will not detect a fault out side the zone defined by the CTs. This is true, only for ideal conditions- where the two CTs are perfectly matched. Following mis matches will occur under practical situations in the field :

a)                 the CT secondary impedances may not be equal

b)                the lead wires connecting the CT secondaries to the relay may not have equal resistances

c)                 the CTs may have different ratio error and phase angle error – due to this , the secondary cuurents will not be equal even if the primary currents are same..

d)                the CTs may have different saturation characteristics – this will cause small difference in the secondary currents for a same primary current

The cumulative effect all the above, can make the relay trip even when there is a full load current flowing in the primaries – though the primary side currents are same, the secondary side currents need not be the same – a voltage sufficient to trip the REF relay may develop across P-S and hence the relay will trip.

To make the relay insensitive to this voltage produced by CT mismatch, a resistor is added in series with the relay.  Once this resistor is added, the relay will need a voltage which is higher than  the voltage produced by the CT mismatch. This resistor is called stabilizing resistor – this is an important component in REF scheme –since this ensures stability in the scheme by avoiding spurious tripping.

6.0            What is knee point voltage ?

It should be noted that actual input crcuit to trip mechanism (consisting of the

relay + stabilizing resistor) has become a high impedance circuit.   If the relay

has  to  trip,  the CT  secondaries   should  produce  sufficiently  high  enough

voltage to activate the relay,  after allowing for the drop across the stabilizing

resistor.

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To ensure the CTs produce enough voltage,  an additional specification – the

Knee poit voltage - is included for the CTs used for REF protection .

Knee point voltage (KPV)is defined as the point on the magnetizing curve (of

the material used for the CT core) where the core will need 50% increase in

the magnetizing force (ampere turns) to cause a 10% increse in the flux

density.(voltage build up across secondary). In effect, KPV defines the end of

the linear portion of the BH curve .  Higher the KPV, larger is the linear zone

and better will be secondary output for higher fault currents. Higher the KPV,

better are  the chances of a high impedance relay trip.

7.0            How can we calculate the value of the stabilizing resistor and KPV ?

The value of stabilizing resistor and KPV will depend on the following parameters which are unique to a given feeder:

a)                 Impedance of the CT secondaries

b)                Lead wire resistance between the CT secondaries and the REF relay

c)                 The impedance of the REF relay -  this can vary with respect to pick up setting .  We have to consider the relay impedance at the pick up setting being contemplated for the feeder.

d)                The maximum fault current which can occur on the CT secondary side – CT should not saturate under this maximum fault conditions. If the CT saturates, it will offer an alternate path for the resultant current – and the REF relay may not trip.

A detailed method is provided in annexure 1, for calculation of stabilizing resistor and KPV

8.0            What are L&T solutions for REF protections ?

L&T manufactures a high impedance Over curret relay – which is ideally suited for the REF protection of generators and transformers. Typical schemes are shown in fig. 3 &4.

L&T offers a unique feature in the REF relay SC14S – in additon to instantaneous trip, user can select a definite time delay of either 100 millisec or 200 millisec. In case of small transformers & generators ( upto 5 MVA), the feeder trips during breaker closing. This mainly is due to the large inrush current causing momentary difference in CT secondary currents due to mismatch in saturation characteristics. A 100 millisec time delay will help in this case.

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Fig.3 shows a typical scheme for REF protection for generators. The scheme envisages the following ; a) 3 nos. phase CTs b) 1 no. neutral CT c) 1 no.  Relay SC14S d) 1 no. Stabilising resistor

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Fig.4 shows a typical scheme for REF protection for transformers. It should be noted that transformers will need two REF schemes – one on the primary side and the other on the secondary side.

For the transformer primary side , which is usually delta connected, following are envisaged in the REF scheme :

a) 3 nos. phase CTs

b) 1 no.  Relay SC14S

d) 1 no. Stabilising resistor

For the transformer secondary side, which is usually star connected, following are envisaged in the REF scheme :

a) 3 nos. phase CTs

b) 1 no. neutral CT

b) 1 no.  Relay SC14S

d) 1 no. Stabilising resistor

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NEGATIVE SEQUENCE PROTECTION FOR AC MOTORS

NEGATIVE SEQUENCE PROTECTION FOR AC MOTORS

1.0              Need for Negative Sequence Protection

Primary cause of motor failure is excessive heating, which if sustained over long time periods will result in motor burn out.  Over heating also reduces the life of motor. If a motor is continuously over heated by just 10 degrees, its life can get reduced by almost 50%.

Over heating normally occurs due to over current, which in turn may be due to over loads or locked rotor condition or low voltage or phase failure or repeat starts or phase unbalance.

Bimetallic relays are most economical solution for heating due to over loads. However they suffer from inherent deficiencies like poor accuracy, rigid inverse time characteristics, poor repeatability etc. They are totally insensitive to current unbalance, which is one of the major contributors to over heating in motors.

Though the three-phase motor is supposed to be a balanced load, current unbalance occurs frequently in motor feeders due to following:

a)         voltage unbalance in the feeder supply

b)                  phase reversal

c)                  single phasing

Current unbalance in a motor is best represented by the presence of excessive negative sequence component in the motor current. Consequently it is necessary to protect motors against negative sequence .

2.0       Supply Unbalance & Sequence components :

When the power supply to the motor is unbalanced, the unbalanced voltage and the resulting unbalanced currents in the three phases can be resolved into three balanced components as follows :

a)                  Positive Sequence component : This component is in the same phasesequence as that of the motor current. All its three phases are perfectly balanced – they are equal in magnitude and are displaced by 120 degrees. The positive sequence component represents the amount of balance in the power supply  and consequently  is instrumental in delivering useful power.

b)                  Negative Sequence component : This component has a phase sequence opposite to that of the motor current hence the name negative sequence. It represents the amount of unbalance in the feeder . All its three phases are perfectly balanced – they are equal in magnitude and are displaced by 120 degrees. This component does not produce useful power – however by being present it contributes to the losses and causes temperature rise.

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c)                  Zero Sequence component : This , if present, represents extent of earth fault in the feeder. All its three phases are in the same direction.

3.0       Effect of Unbalance in Motors :

3.1              When the supply voltage is unbalanced, the positive sequence component reduces and results in low power delivery.

An unbalance of only 5% ( R phase = 415 V , Y phase = 415 V, B phase = 436 V) can reduce the out put by 25%, even though the motor continues to draw the same current as at the time of balanced condition. This means that the motor current has to increase under unbalanced condition in order to deliver the same power. This increase will result in motor heating.

3.2              During supply unbalance, negative sequence currents flow through the stator windings. This results in induction of  negative sequence voltage in the rotor windings. Since the rotor is short circuited, this will result in abnormal current flow in the rotor and damage the rotor winding. A voltage unbalance of the order of 3% can increase the heating by nearly 20% in the rotor.

3.3              The negative sequence impedance of the motor is approximately same as the locked rotor impedance which in turn is approximately one sixth of normal motor impedance. Due to this even small voltage unbalance can produce large negative sequence current in the motor.

3.4              The frequency of the negative sequence current induced in the rotor will be equal to (supply frequency) x (2-slip) Hz. This is due to the fact that it is revolving in the opposite direction . This frequency will be typically around 99 Hz during normal motor operation . Due to skin effect, high frequency negative sequence currents encounter high rotor resistance. This inturn results in over heating. The increased resistance is typically 5 times the normal positive sequence resistance.

4.0       Reasons for Unbalance :

Voltage unbalance can be due to many reasons. These include unsymmetrical loading, blown fuses in capacitor banks and single phasing.

5.0              Single phasing :

Single phasing causes worst kind of unbalance – it produces equal amount of positive and negative sequence components. The prime causes for single phasing are :

a)                  broken or burn out of connecting leads to motor

b)                  blown fuses

c)                  faulty contact in switching element

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To understand the effect  of single phasing in motors, consider the condition shown below .

It can be seen that the current in winding C will be larger than that in A and B and consequently it will burn out. Typical currents in the windings are shown below >

 

Motor current              Current in windings when single phasing has occured          

(% of full load)           (% of full load)

 

3 phase current           Lines L1 & L3                        Winding A&B             Winding C

 

            50                                80                               50                               108

            55                                 90                               58                               118

            60                                102                              62                               131

            65                                120                              70                               147

            70                                130                              79                               161

            75                                147                              87                               180

            80                                165                              95                               198

            85                                180                              102                              215

            90                                200                              111                              235

            95                                222                              120                              258

            100                              243                              129                              285

 

From the above table , we can see that in a motor running at 50% load, on single phasing in L2,

a)         the healthy lines L1 and L3 will carry 80% of full load

b)         the windings will carry 50% of full load

c)         the winding C carries more than twice full load (108%)

A thermal over load relay in this case ,  will  not trip since it is seeing only 80% current in healthy lines L1 & L3 while the winding C is already over stressed. This situation gets worse with increasing loads at the time of single phasing.

A negative sequence calculation would have helped in this case.

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6.0              Phase reversal :

Phase reversal in a motor can be very dangerous – the motor will run in the opposite direction resulting in severe damage to gear boxes, material flow problems and hazard to operating personnel.

Phase reversal in a motor feeder produces a negative sequence of nearly 100% and is well detected by a relay with negative sequence protection.

Considering the above details, it can be easily concluded that a motor protection relay will provide an effective protection to the motor only when it is equipped to measure the negative sequence component level in the motor feeder.

REF RELAY SETTING CALCULATION

Transformer Data :

  8 MVA , 6.6KV/ 415 ,  Impedance = 0.0835 p.u., Tap + 10% ( 440Kv – 360KV

CTR = 800 /1

Relay  : CAG 14 / P120

 

                                Fault MVA = MVA  / Transformer Impedance

                                        =  8 / 0.0835 = 95.8084

Fault Current  =    Fault MVA  x 1000 / ( 1.732 x  Voltage rating )

                      =   95.8084 x 1000 ( 1.732 x 6.6 )

                      =    8281.06  Amp primary

                          = 8281.06 / 800  = 10.4763  Amp Secondary

Fault Voltage developed across the relay   =  fault current  x loop resistance

 at the time of Maximum fault conditions

                                    V s       =    If ( Rct + 2 RL)     RL  = CT Lead Resistance

                                                 =   10.4763  x (  Rct + 2 RL)

                                                =    10.4763  x ( 4 + 2)  ( assumed RL = 1 ohms)

                                                =     62.8579 volts

The relay has to be set at   Is = 0.1 In for maximum sensitivity

The stabilizing resistor shall be set at value of resistance during fault minus the relay resistance

                                                =   62.85    -    1 VA

0.1                            (0.1) Square       

                                            = 628.5      -  100

                                                    R     =     528.5 ohms

TRANSFORMER VECTOR GROUP

Transformer vector group can gives informations about the phase angle difference between the primary high voltage side winding and the low voltage side secondary winding.

Every time when synchronizing a generator electrical engineer must check transformer vector group because failing to do can cause severe damage to the transformer itself and the whole power system as well.

The most common configuration Yd11 tell us that there are plus 30 degree phase shift while Yd1 indicate that there are minus 30 degree phase shift between HV and LV side so it is not possible to parallel Yd1 and Yd11 if they are feed from the same source.

You can learn more about transformer in detail here including transformer vector group and different type of winding configurations.

Testing Cables with High Voltage

Hipot Testing FAQ What is "hipot" testing? Why high voltage test? What kinds of high voltage tests are there? What is "dielectric breakdown testing?" What is "dielectric withstand testing?" What is "insulation resistance testing?" How do these "hipot" tests affect quality? With all the high voltage being used, what about my safety? Where is the high voltage applied? Where is the current measured?

What is "hipot" testing?

Many people are familiar with a continuity test. A continuity test checks for "good connections." You do a continuity test by seeing if current will flow from one point to another point. If current flows easily enough then the points are connected. Many people aren't familiar with a hipot test. "Hipot" is short for high potential (high voltage). A hipot test checks for "good isolation." You do a hipot test by making sure no current will flow from one point to another point. In some ways a hipot test is the opposite of a continuity test.

Continuity Test: "Make sure current flows easily from one point to another point."

Hipot Test: "Make sure current won't flow from one point to another point (and turn up the voltage really high just to make sure no current will flow)."

In the simple case a hipot test takes two conductors that should be isolated and applies a very high voltage between the conductors. The current that flows is watched carefully. Ideally not much current will flow. If too much current flows the points are not well isolated and they should fail the test.

Why high voltage test?

You use a hipot test to make sure you have good isolation between the parts of a circuit. Having good isolation helps to guarantee the safety and quality of electrical circuits. Hipot tests are helpful in finding nicked or crushed insulation, stray wire strands or braided shielding, conductive or corrosive contaminants around the conductors, terminal spacing problems, and tolerance errors in IDC cables. All of these conditions might cause a device to fail.

What kinds of high voltage tests are there?

There are three common high voltage tests.

• Dielectric Breakdown Test
• Dielectric Withstanding Test
• Insulation Resistance Test

What is dielectric breakdown testing?

With dielectric breakdown testing you are trying to answer the question "How much voltage can I apply between the wires before the insulation fails?" You increase the voltage until the current suddenly increases. You are finding the highest voltage the cable can stand before it fails. Once the cable fails it is usually damaged or destroyed.

What is dielectric withstand testing(DW)?

In dielectric withstand testing you are trying to answer the question "Will this cable withstand a required voltage for a required time?" You apply the voltage for the amount of time and watch the current that flows. Ideally no current flows and the cable is not harmed.

What is insulation resistance testing (IR)?

In insulation resistance testing you are trying to answer the question "Is the resistance of the insulation high enough?" You apply a voltage and very carefully measure the current. You then calculate the insulation resistance using Ohm's Law (R = V/I).

How do these "hipot" tests affect quality?

All of these tests are tools you can use to better understand how a cable will perform and to monitor any changes in the cable's performance.

Dielectric breakdown testing is used in product design and qualification stages. It helps establish the maximum voltage of the design. It can also be used on a random sample basis to verify that the maximum voltage is not changing. Dielectric breakdown testing may be required during the development of assemblies used in critical applications.

Many test specifications require a Dielectric Withstand Test on every cable produced. The test is usually done at about 75% of the typical breakdown voltage. It is done as a safety net. The test is sensitive to arcs or corona so it often finds terminal spacing problems, over-mold problems, tolerance errors in IDC cables, or any problem that might produce arcs. This test doesn't significantly degrade the cable.

The Insulation Resistance test is typically done on every cable tested. It is usually done at 300 to 500 Vdc with 100 to 500 Megahoms resistance. The test is a very sensitive to contamination in the assembly process. Solder flux, oils, mold release agents, and skin oil all can cause problems. This test excels at identifying insulation that will conduct in the presence of moisture. Doing this test on every cable allows you to detect contamination changes in the manufacturing process.

Additional High Voltage Testing Resources:

AC Hipot Testing
Guidelines for using Voltage to Detect Insulation Defects
High Voltage arc distance. 

With all the high voltage being used, what about my safety?

During a hipot test you may be at some risk. The risk can be reduced by using a tester designed to be safe and by using that tester according the manufacturer's instructions.

Products being designed today usually must comply with product safety regulations. Some of these regulations work to reduce the chance of you receiving a harmful electrical shock. Modern equipment is more likely to follow these regulations. When it comes to hipot charge, energy, and voltage you should select the "safest" machine that will still test your cables.

To minimize your risk of injury from electrical shock make sure your hipot equipment follows these guidelines:

• The total charge you can receive in a shock should not exceed 45 uC.
• The total hipot energy should not exceed 350 mJ.
• The total current should not exceed 5 mA peak (3.5 mA rms)
• The fault current should not stay on longer than 10 mS.
• If the tester doesn't meet these requirements then make sure it has a safety interlock system that guarantees you can not contact the cable while it is being hipot tested.

These guidelines come from the test standard EN61010-1, Safety requirements for electrical equipment for measurement, control and laboratory use, April 1993, CENELEC. Over the last decade many of the safety regulations have been harmonized (standardized) and EN61010-1 is similar to UL 61010A-1 (formerly UL3101-1).

While you are testing cables there are several things you can do to reduce the risk even more:

• Verify the correct operation of the safety circuits in the equipment every time you calibrate it.
• Follow all of the manufacturer's instructions and safety guidelines.
• Don't touch the cable during hipot testing.
• Allow the hipot testing to complete before removing the cable.
• Wear insulating gloves.
• If you have any health condition that can be aggravated by being startled then don't use the equipment.
• Don't allow children to use the equipment.
• If you have any electronic implants then don't use the equipment.

Where is the high voltage applied?

To understand a how hipot testing works you'll need to understand where to connect the high voltage supply. Hipot testers usually connect one side of the supply to safety ground (Earth ground). The other side of the supply is connected to the conductor being hipoted. With the supply connected like this there are two places a given conductor can be connected: high voltage or ground.

When you have more than two contacts to be hipot tested you connect one contact to high voltage and connect all other contacts to ground. Testing a contact in this fashion makes sure it is isolated from all other contacts.

What happens when you test something more complicated than just contacts? A series of contacts that are connected with wires, resistors, capacitors, diodes, and other components is called a "network" of connections (or "net"). To hipot test a net you connect all of the contacts in the net to high voltage and connect all other contacts in the device to ground. For example, if you have a wire that connects two pins, the high voltage will be simultaneously apply to both of those pins and the entire wire will be raised in voltage. All other wires and pins will be held at ground. If you have a resistor that connects two pins, both pins are raised in voltage, the voltage drop across the resistor is always zero. The entire resistor is raised in voltage. In short, all pins of a component see the same voltage at all times. Applying the voltage in this fashion makes sure the body of the component is isolated from the rest of the device.

Where is the current measured?

During the hipot test the current that flows out of the high voltage supply is measured.

What causes current to flow through an insulator?

Insulation "does not conduct." But if you use enough voltage even the best of insulations will allow some current to flow. You may wonder why the current flows? There are several reasons current will flow through insulation during a hipot test. Resistance, capacitance, arcs, electrochemical effects, and corona are all effects that describe current flow. All of these effects add together during a hipot test shape the outcome of the test.

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