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 protected

CONVERTER FAULTS

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

ARC 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

ARC THROUGH

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

This fault mainly takes place at inverter station

MISFIRE

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

CURRENT 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

valve

COMMUTATION FAILURE

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.

This gives rise to commutation failure.

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

conduction .

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

self clearing.

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

limited by connecting the smoothing reactor in the circuit.

SHORT CIRCUIT 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.

This fault mostly occurs in rectifiers.

PROTECTION AGAINST OVER CURRENTS

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.

PROTECTION AGAINST OVER VOLTAGES

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

Converter faults

SWITCHING OPERATIONS

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.

Using SF6 breakers.

LIGHTNING STROKES

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.

With the help of neutral grounding.

OTHER 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.

Using On Load Tap Changers.

SURGE ARRESTERS

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

equipment can perform in normal operation.

SURGE ARRESTERS 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.

SMOOTHING REACTORS

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

smoother.

CORONA 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

Space charge field

RADIO INTERFERENCE

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.

AUDIBLE NOISE

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

The positive polarity conductor is the primary source of AN.