differential relay transformer protection

DIFFERENTIAL RELAY&

TRANSFORMER PROTECTION

CONTENTS

POWER SYSTEMINTRODUCTION TO DIFFERENTIAL PROTECTIONTRANSFORMERS PROTECTION BUS BAR PROTECTIONGENERATOR PROTECTIONFEEDER PROTECTIONCONCLUSIONREFERENCES

TABLE OF CONTENTSPower system

Differential protection is a very reliable method of protecting generators, transformers, buses, and transmission lines from the effects of internal faults.

In a differential protection scheme in the above figure, currents on both sides of the equipment are compared. The figure shows the connection only for one phase, but a similar connection is usually used in each phase of the protected equipment. Under normal conditions, or for a fault outside of the protected zone, current I1 is equal to current I2 . Therefore the currents in the current transformers secondaries are also equal, i.e. i1 = i2 and no current flows through the current relay. If a fault develops inside of the protected zone, currents I1 and I2 are no longer equal, therefore i1 and i2 are not equal and there is  a current flowing through the current relay.

Introduction

THREE PHSE TRANSFORMER

Differential Relay Set-up for Delta-Star Transformer

CTs in delta

CTs in star

Delta Star

Differential Relay – Transformer Application

• Transformation ratio has to be taken into account.

• Three-phase delta-star or star-delta. The phase shift and 3 have to be taken into account

• Similar types of CTs has to be used.

• Allow for ratio changes due to tap-changing (may be 10-15%)

Transformer Differential Protection Example

• 33/11 kV, star-star transformer. Rating = 25 MVA

• Standard CT ratios available are 200:5, 400:5, 600:5, 800:5, 1000:5, 1600:5, 1800:5

• Allow for 15-20% unbalance

Transformer Differential Protection Example .. 2

• Full–load current = 437 A/ph (33kV side),

=1312 A/ph (11kV side)• Allow 150% overload. Choose 600:5 CT for 33kV. • Transformation ratio = 3. Choose 1800:5 CT for 11kV side• Set HV normal current In = 450 A say• Set bias to 20%.• If delta-star transformer, allow for 3 (in LV CT ratio for

old relays – Idelta = Iline/ 3; taken care of in setting for modern relays)

Buchholz RelaysA Buchholz relay is a gas and oil operated device installed in the pipe work between the top of the transformer main tank and the conservator.  A second relay is sometimes used for the tap changer selector chamber.  The function of the relay is to detect an abnormal condition within the tank and send an alarm or trip signal.  Under normal conditions the relay is completely full of oil.  Operation occurs when floats are displaced by an accumulation of gas, or a flap is moved by a surge of oil.  Almost all large oil-filled transformers are equipped with a Buchholz relay, first developed by Max Buchholz in 1921.

Gas produced within the transformerAn oil surge from the tank to the conservatorA complete loss of oil from the conservator (very low oil level)

Conditions DetectedA Buchholz relay will detect:        Fault conditions within a transformer produce gases such as carbon monoxide, hydrogen and a range of hydrocarbons  A small fault produces a small volume of gas that is deliberately trapped in the gas collection chamber (A) built into the relay. 

Typically, as the oil is displaced a float (B) falls and a switch operates - normally to send an alarm.  A large fault produces a large volume of gas which drives a surge of oil towards the conservator. 

This surge moves a flap (D) in the relay to operate a switch and send a trip signal.  A severe reduction in the oil level will also result in a float falling.

  Where two floats are available these are normally arranged in two stages, alarm (B) followed by trip (C).

Busbar Differential Protection Scheme

Generator Differential Protection

Feeder Differential Protection (Balanced Voltage)

“Translay” protection

Top - external fault, CT current balance, no net voltage across relays

Bottom - internal fault, CT currents add, net voltage across both relays.

CONCLUSION.

BY UNDERSTANDING THE WORKING PRINCIPLE OF DIFFERENTIAL RELAY , IT CAN BE USED IN POWER SYSTEM TO PROTECT MANY DEVICES FROM THEIR INTERNALS FAULTS.

REFERENCES.

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