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 FAULTSTypes 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 BACKSIn 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 sideThe bypass

valve has higher current

rating than ordinary valves

ARC THROUGHIt 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 asRI=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 pressurePr= reference pressure level

The positive polarity conductor is the primary source of AN.