Converter Faults & Protection

CONVERTER

FAULTS &

PROTECTION

INTRODUCTION

Faults in DC

systems are

caused by

the malfunction of the equipment and

controllers

The failure of insulation caused by external

sources such as lightning ,pollution etc…

In a

converter

station

Valves are the most critical

equipment needed to be

C

ONVERTER

F

AULTS Types of Converter Faults Faults due to malfunctions of valves and controllers Arc backs Arc through Misfire Quenching or Current Extinction Short Circuits in converter station Commutation Failure

A

RC BACKS

In this phenomena the valve losses its capability

to block in the reverse direction

Hence conduction takes place in reverse

direction also

This is non-self clearing fault

When this fault is detected we need to block the converter valves and open

the backup AC breaker

This can be eliminated by using a bypass valve placed across converter bridge on the valve side

The bypass valve has higher current rating than ordinary valves

A

RC

T

HROUGH

It is the failure to block a valve during a scheduled non conduction period

A malfunction in the gate pulse generator can fire a valve which is actually not supposed to conduct, but is forward biased

This malfunction is mainly because of failure of

a) Negative grid pulse b) early occurrence of positive grid pulse

M

ISFIRE

This takes place when the required gate pulse is missing and the incoming valve fails to ignite

This can occur in both rectifier

and inverter stations, but effects are more

in inverter

Effects are commutation failure and arc through. This is

a self clearing fault

C

URRENT EXTINCTION

This takes place when the current

through a valve reaches a value

less than the holding current

This fault may cause

overvoltage's to take place in the

C

OMMUTATION

F

AILURE

 It is nothing but the failure of the completion of

commutation before the reversal of commutating voltage takes place.

 The minimum value of extinction angle is defined by

Ƴ=180-α-µ

 The overlap angle is a function of the commutation voltage

and the DC current.

 The reduction in voltage or increase in current or both can

result in an increase in the overlap angle and reduction of Ƴ below Ƴmin.

 Consider the circuit shown above.

 Assuming initially valves 1 and 2 are conducting.

 Now because of increased DC current or decreased AC

voltage or any case valve 1 fails to extinguish.

 Therefore valve1 carries full link current and the current in

valve 3 becomes zero.

 Hence valve 3 extinguishes and valve 1 continues its

 Next when valve 4 fires the short circuit of the bridge takes

place as valves in the same arm conducts.

 This causes the voltage across valve 5 to be negative

hence it does not conducts.

 Valve 4 gets extinguished and valve 6 is fired next.  Hence the normal operation is retained back.

 Therefore it can be said that single commutation failure is

 The effects of single commutation failure are,

 There is no AC current for the period in which the two valves in an arm are left conducting.

 The bridge voltage remains zero for a period exceeding 1/3 of a cycle, during which the DC current tends to increase.

 Double commutation failure can also takes place in a

converter station.

 A commutation failure in a bridge can cause several

sequence commutation failures in the series connected bridges.

 Hence the initial rate of rise of current has to be sufficiently

S

HORT

C

IRCUIT IN A BRIDGE

 This fault has very low probability of occurrence.

 As the valves are kept in a valve hall with air conditioning.  They may sometime occur because of flashover in

bushings.

P

ROTECTION

A

GAINST

O

VER

C

URRENTS It provides basic protection against faults in a converter It compares the rectified current on the valve side of converter transformer to DC current on line side smoothing reactor This is used as

backup. The level of overcurrent required to trip must be set higher than VGP to avoid tripping This is mainly used to detect the ground faults, such as neutral faults.

 The faults producing overcurrents are classified into

3 categories:

 The first one being line faults. They occur frequently and can be controlled by controlling the current.

 The second being the internal faults. They cause high overcurrents. These are infrequent.

 The third fault may be commutation failure at inverters. They occur quite frequently.

P

ROTECTION AGAINST OVER

V

OLTAGES

 The sources of over voltages in converter station are:  Switching operations

 Lightning strokes

 Sudden load rejection

 Resonance between filter and system when suppressing lower order harmonics.

 Symmetrical faults in AC yard

 Errors in voltage control

S

WITCHING

O

PERATIONS

 These over voltages are of short duration.

 Switching surges are on account of circuit breaker

operation while switching inductive and capacitive loads.

 Protection schemes:

 Using surge absorbers with circuit breakers.

L

IGHTNING

S

TROKES

 The primary cause of this over voltage is lightning strikes.  These occur for a very short duration but causes more

damage to the system.

 Protection schemes:

 Using surge arresters and spark gaps.

 Using overhead ground wire.

O

THER FAULTS

 Sudden load rejection,resonance,symmetrical faults in AC

yard and other causes temporary over voltages in the system.

 This occurs at power frequency and lasts for a few

seconds.

 Protection schemes:

 Using surge over voltage relays and circuit breakers.

 Using fast acting static VAR sources.

S

URGE

A

RRESTERS

 It is a device connected between a conductor and ground,

to protect the equipments against high voltage surges.

 It is also known as lightning arrestors.

 It diverts the lightning or switching surges from the

equipment towards the ground.

 Under normal operating voltage, the impedance offered by

a surge arrester is very high.

 As the current always chooses the low resistance path

S

URGE

A

RRESTERS CONTD

…

 When an over voltage occurs it causes the drop in the

impedance of surge arrester.

 Thus the flow now will be through the surge arrester rather

than the main path.

 Two types of arresters are there:  Gapless arresters

 Zinc oxide arresters

 Zinc oxide arrester is widely used as they have high

energy absorbing capability.

S

MOOTHING

R

EACTORS

 It is a high inductance coil connected in series with the

converter to reduce the ripple current on the DC side of the system.

 Basically the DC current from the rectifier has harmonic

components called ripple.

 As SR is in series with rectifier whole load current flows

through it.

 Then their magnitude is reduced and current becomes

C

ORONA ON DC LINES

 The phenomena of hissing sound, violet glow

accompanied with the production of ozone gas due to ionization of air surrounding the conductor, when voltage gradient exceed a particular value is called corona.

 In DC transmission system, due to the discharge a current

pulse is generated resulting in increase in power loss.

 The effects of corona are:  Radio Interference

 Audible Noise

R

ADIO

I

NTERFERENCE

 It is also known as radio influence.

 It occurs in the band region of 0.5 to 1.6Mhz.

 In HVDC lines, RI effect is more in positive conductor rather

than in negative conductor.

 It is expressed in millivolts per meter.

 Mathematically it is expressed as

RI=25+10logn+10logr+1.5(g-go)

 In negative conductors the value of radio interference is lower by

20dB.

A

UDIBLE

N

OISE

 The corona discharges from the conductor produce

compressions and rarefactions that are propagated through the medium as acoustical energy.

 The portion of the acoustical energy spectrum that lies

within the sonic range is perceived as audible noise.The sound level is expressed in decibels'.

 It is defined as

dB=20log(P/Pr)

where P= measured sound pressure Pr= reference pressure level