27.3 · hd 637 s1 (cenelec) / en 50522/ sfs 6001 fundamentalfrequency overvoltages- risk of...

27.3.2012

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Electrotechnical aspects and novel solutions for earth fault management

Ari Nikander27.3.2012

SET-1520 New Applications in Electrical Energy Engineering Content

1. Introduction to earth faults

2. Aspects of neutral earthing• Neutral isolated MV system

• Neutral compensated MV system

3. Modern phase earthing system

4. Aspects of large rural cable networks

5. Novel method for indication of high-resistance earth faults

6. Summary

Earth fault management

Safety aspects –protection of

humansProtection of network Minimising of the costs

due to interruptions

HD 637 S1 (CENELEC)/ EN 50522/ SFS 6001

Fundamental frequencyovervoltages - risk of secondary faults

Costs for customersand electricitycompanies – regulationmodel

WHY?

Introduction to earth faults

• Earth faults or secondary faults developing from earth faults form the most common fault group in MV networks.

• An earth fault can be either temporary or permanent in nature.In Finnish overhead line networks, about 90 % of faults have a temporary nature.

• Temporary phase-to-earth faults are caused by e.g.lightninganimalsflashovers

• Permanent phase-to-earth faults are caused by e.g.faulted MO-arresterfaulted cable terminalfaulted cable insulationtree contactsconductor breaks or out-fallen conductor

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Earth fault types

Phase-to-earth fault Phase-to-phase-to-earth fault

Double line-to-earth fault (cross country fault)

Phase-to-earth fault and a broken conductor (earth contact is on the load side)

Introduction to earth faults- background

• Phase-to-earth faults may cause a hazard to the customers and thepublic.

• Most of the outage costs are accumulated during the time taken tolocate and isolate the fault.

• According to long long-term statistics about 90 % of all faults inFinnish MV systems are cleared by ARs on feeders where they areapplied and about 10 % of faults remain permanent leading to finaltripping.

• Even short interruptions deteriorate the power quality which is notonly significant to the customer but also to the distribution systemoperator (DSO).

• These short interruptions have been included in the regulationmodel.

Thus they have also direct economic value from the DSO’s point of view.

Aspects of neutral earthing

• The way the neutral is connected to the earth is the most importantsingle parameter that determines the behaviour of a power systemduring a single phase-to-earth fault.

• The earthing of the neutral of the MV network has a major influenceon the system not only steady state but also dynamic behaviourduring an earth fault.

• The neutral point treatment affects e.g.

requirements for overvoltage protection

need to restrict touch potentials

properties of relay protection

requirements for supply continuity

Neutral earthing practices

• Neutral isolated networks• Neutral compensated networks

Centrally compensated systemsComplete automatic tuning

Partial fixed tuning

Distributed compensation (nowadays especially large rural cablenetworks)

• Networks with resistance earthingHigh-resistance earthing

Low-resistance earthing

• Solidly earthed networks• This presentation is restricted to neutral isolated and compensated

systems.

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Neutral earthing practices- high impedance earthing

Three-phase equivalent circuit for a system during an earth fault

E1

ZN0 C0

C0

C0

E2

I1

I2

FI3

Ie

RF

E3

U1

U0

U2

U3

= ?

Neutral isolated system

Neutral isolated system

• The currents of single phase-to-earth faults depend mostly on thephase-to-earth capacitances of the conductors.

• When the fault happens the capacitance of the faulty phase isbypassed and the system becomes unsymmetrical.

• The fault current is composed of the currents flowing through theearth capacitances of the two healthy phases.

• The connected high voltage and low voltage networks do not affectthe fault currents since in ungrounded systems there is no path forthe zero sequence components through the substation ordistribution transformers (YNd11, Dyn11).

Neutral isolated systemPhase-to-earth voltages and neutral voltage when

an earth fault is in Phase A

A

B

C

0º

30º 150º

270º

UAF UA

UCF

UBF

U0

UC

UB

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Neutral isolated system- benefits and drawbacks

• AdvantagesFault currents are typically small and single phase-to-earth fault does notrequire immediate de-energization of power supply

Longer tripping delay can be applied

Especially industrial power systems where continuity of the service is important

Low fault currents are benefit in countries where earthing conditions are difficultand low impedance earthing would be costly.

Isolated networks have been common among the public distribution systems inthe Nordic countries

Low investment costs

• DisadvantagesArching faults need often autoreclosing which cause interruption to customers.

Restriking faults leading high overvoltages are possible.

Fundamental frequency overvoltages which may lead to secondary failure.

Because of overvoltages the use of an isolated system is restricted to the MVsystems.

Neutral isolated system- earth fault arc extinction

• Fault current is typically low but the rising speed of the recoveryvoltage is high.

Self extinction conditions of the arc are not good.

Re-ignition is probable.

Recovery voltage (Ur) and neutral voltage (U0) in an isolated system

time [ms]

volta

ge [k

V]

50 100 150 200 250 300 10 -20

-10

0

10

20

30 Ur

U0

Neutral compensated system

• Compensated networks have been on the increase among thepublic distribution systems in Finland.

Neutral compensated system

Equivalent circuit for an earth fault in a compensated system

Resistance of arc in the case of temporary fault

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Neutral compensated systemPhase-to-earth voltages and neutral voltage when

an earth fault is in Phase A

A

B

C

0º

30º 150º

270º

UAF UA

UCF

UBF

U0

UC

UB

Neutral compensated system- benefits and drawbacks

• AdvantagesFault currents are typically very small and single phase-to-earth fault does notrequire immediate de-energization of power supply

Longer tripping delay can be applied.Low fault currents are benefit in countries where earthing conditions are difficultand low impedance earthing would be costly.Temporary earth faults can be cleared mostly without autoreclosings.

• DisadvantagesHigh investment costs

In addition to Petersen coil needs normally additional loading resistanceMore complicated earth fault protectionRestriking faults which are difficult to detect are possible especially in cablenetworks.Fundamental frequency overvoltages which may lead to secondary failure.Indication of the high resistance earth faults is more challenging than withisolated neutral.

Neutral compensated system- earth fault arc extinction

• Fault current is typically low and the rising speed of the recoveryvoltage is low.

Self extinction conditions of the arc are good

Re-ignition is not probable

Recovery voltage (Ur) and neutral voltage (U0) in compensated system

0

U r

time [ms]

volta

ge [k

V]

50 100 150 200 250 300

0

U0

-5

-10

-20

5

10

15

20

Neutral compensated system- effect of the compensation degree on earth fault

arc extinction

Neutral voltage (U0) and recovery voltage in compensated system

Neutral voltage with 50 % compensation degree Recovery voltage of faulty phase with 50 % compensation degree

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Cross-country fault

• Normally secondary failure when the primary fault is one phase-to-earth fault

• Fault current at most the same order of magnitude as with two phase short-circuit

• Can lead high touch voltages and damages in telecommunication systems.

• Fault locations can be along the same feeder or along separate feeders.

Initial transients of earth fault- phenomenon

Initial phase-to-earth voltages with 49.8 fault resistance

Neutral voltage and sum current of faulty feeder with 49.8 fault resistance

Devices - earth fault protection and active compensation of residual current

• RCC ground fault neutralizerCentralized E/F protectionResidual current compensationMeasuring of phase-to-earth admittancesPD based diagnostic

• First version introduced in 1993 (Swedish Neutral AB)

• Total response time less than 3 cycles

• Extensive cablingSafetyRestriking faults

Improving the quality of supply in MV distribution network applying modern shunt circuit-breaker

• Development of the modern cost-effective phaseearthing system (PES)

• The target was reducing the harmful short interruptionsto the customers and electricity producers with phase-to-earth faults of an neutral isolated MV system.

• The novel shunt circuit-breaker (SCB) was testedapplying RTDS (Real Time Digital Simulator)environment before field tests in the real network.

• Now the novel SCB has been installed, tested withartificial earth faults and ready for service in one primarysubstation (110/20 kV).

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Description of phase earthing

Simplified diagram of phase earthing system


• The earth fault arc can be extinguished by connectingthe faulty phase temporarily to earth at a feedingprimary substation.

The major part of the fault current is transferred away from thefault location.

The recovery voltage of the fault location after the arc currenthas tripped out is very advantageous for the extinction of the arcwhen the fault disappears during phase earthing (PE).

The residual fault current is low.

Improved probability that the temporary earth fault will becleared without the operation of the feeder circuit-breaker.

Extinction probability of earth fault arc

• The insulation level of the spark gap depends on theionization velocity of the exhaust and is the higher the longertime has elapsed after the interruption of the current.

• The rising speed of the recovery voltage immediately afterarc extinction is of absolutely crucial significance consideringthe re-ignition probability.

• The magnitude of the residual fault current and arcing timealso affects the extinction because they affect the degree ofionization.

• Wide deviation of arcing times is very typical.It is due to the nature of the arcing phenomenon burning in thefree air.

Recovery voltage (UA)

Neutral isolated (UA, U0) Neutral compensated (UA, U0)

0.90 1.00 1.10 1.20 1.30 1.40 1.50 1.60 ... ... ...

-40.00k

-30.00k

-20.00k

-10.00k

0.00

10.00k

20.00k

30.00k

40.00k

[V]

UA U0_as

0.90 1.00 1.10 1.20 1.30 1.40 1.50 1.60 ... ... ...

-40.00k

-30.00k

-20.00k

-10.00k

0.00

10.00k

20.00k

30.00k

40.00k

[V]

UA U0_as

2.40 2.50 2.60 2.70 2.80 2.90 3.00 ... ... ...

-30.00k

-20.00k

-10.00k

0.00

10.00k

20.00k

30.00k

[V]

UA

Phase earthing (UA)

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• No any interruption or voltage dip for the customers orgenerators of the electricity producers

• PE can be done temporarily before the functioning of thenormal feeder protection in the corresponding way as HSARof the feeder circuit-breaker.

It requires that the tripping delay of the earth fault protection islong enough.After the functioning of the SCB normal AR sequences can bedone and thereby the PE does not affect the functioning of thenormal feeder protection or the settings of the earth faultprotection.

• The evaluation of the touch voltage at the fault locationduring the PE is needed.

Description of phase earthing- applying a phase earthing system

• Adopting the PE method requires installing one SCB,the programmable logic controller (PLC) and its controlrelaying in the 110/20 kV substation.

TYPICAL OPERATION SEQUENCE OF THE PE AND E/F PROTECTION

t

IE

0.1s 0.3s 0.1s 0.3s t + 0.1s t + 0. 1s 120s

Fault appears

PE HSAR DAR

Defin itive opening

Permissible touch voltages UTP - EN 50522: Earthing of power installations exceeding 1 kV a.c.

(corresponding Finnish standard SFS 6001)

• The curve represents the value of voltage (UTP) that may appearacross the human body, bare hands to bare feet.

No additional resistances have been considered in the calculations

• The highest permissible earthing voltage is double compared tovoltage UTP or quadruple if the touch voltage UTP is proved bymeasurements to remain below the values of the curve.

Development of modern phase earthing system

• The SCB for the field tests was made by modifying Noja PowerOSM27-203 pole recloser (vacuum circuit-breaker) so that all threephases can be operated separately.

• Short open and close pulses were generated by a small SiemensPLC, which was equipped with PNP transistor output stage.

• PLC was programmed so that the earth fault is detected by theprotection relay, which causes automatically the main contact toclose control and then open control with fixed time delay.

• Because the protection relay can detect in earth fault or undervoltage cases simultaneous or sequential two phase earth faultsignals, the PLC program was equipped with 5 s blocking time thatprevents after the first signal all other coming signals.

• Two phase simultaneous or successive controls were naturally alsoblocked.

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Shunt circuit-breaker- prototype

RTDS (Real Time Digital Simulator)- tests

• The operation of the SCB was tested in practice utilizing RTDSequipment and a RSCAD network model corresponding thenetwork fed by one primary substation of Fortum Oyj, namedKalkulla.

• Feeder terminal VAMP 255 was applied for controlling the PLC ofthe SCB.

• The method for the indication of the faulty phase developed wasimplemented in the feeder terminal.

• The IED, PLC and SCB operated predictably in RTDS environmentas a part of network model of Kalkulla.

Connections of RTDS hardware,RSCAD model, SCB and IED

Fault current and current of SCB (upper),neutral voltage and sum current of faultyMV feeder (lower).

Field experiments

• The novel PES was tested arranging the field experiments in thereal network when the prototype SCB was installed and ready forservice in the primary substation (110/20kV).

• Artificial earth faults were made along the 20 kV feeder in every phase.

• The PES operated well and predictably with the field experiments.

• In the future it will be brought into play during continuous operation of the network.

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Field experiments

Following figure presents how the fault current and thus also earthingvoltage at the fault location decreases during the PE (400 ms).

Fault current at the fault location with the field testsof the PES

Conclusions of PES

• The pilot implementation of the modern shunt circuit-breaker hasbeen developed and the prototype device has been installed in thereal 110/20 kV primary substation.

• The SCB and its control system were tested applying RTDSenvironment.

• Field experiments arranging artificial earth faults in the realnetwork were carried out.

• The detection of the faulty phase could be carried out reliably withlow ohmic phase-to-earth faults.

• In the future, the field installation will be brought into play duringcontinuous operation of the network.

Electrotechnical aspects of large rural cable networks- Effects of long cable feeders on earth fault phenomena

• Features of MV underground cables differ considerably compared to MV overhead lines.

Phase-to-earth capacitance of underground cable is considerably higher than overhead line.

Considerable increase in capacitive earth fault current.

Aspects of large rural cable networks- Effects of long cable feeders on earth fault phenomena

• Zero sequence series impedance of cable is considerably higher than overhead lines and non-negligible on contrary to overhead lines.

Effect of zero sequence impedance emphasizes with long cable feeders which produce notable resistive fault current component in addition to capacitive fault current component with phase-to-earth fault.Resistive fault current component can not be compensated with usage of Petersen coil which is used to compensate purely capacitive earth fault current.Increase of earth fault current may cause hazard for human safety generating higher touch voltages.

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• Features of MV underground cables differ considerably compared to MV overhead lines.

When earth fault current flows through the series zero sequence impedance of the line it causes the voltage drop in zero sequence system.

This phenomenon cause differences in neutral voltages measured from different locations of the network.

This is against the traditional earth fault analysis which supposes that the earth fault current is purely capacitive and neutral voltage is of the same size in all over the network.

Voltage drop in the zero sequence system can lead lower neutral voltage at the feeding bus bar in the case of long cables lower inductive current produced by centrally located Petersen coil.

In the case of high-resistance earth fault low level of neutral voltage at feeding bus bar makes fault indication more difficult.


• Compensation practicesCentralized

Distributed (e.g. one 15 A unit per every 7 km cable )

Centralized + distributed (10 –15 km cable sections from the substation is compensated centrally, the rest of the feeder distributed way)

Aspects of large rural cable networks- Growth of reactive power

• Cable can be considered as a capacitor which generates capacitive power.

• Because the capacitance in the cable is much larger than OHL the generation of the reactive power is much larger in the cabled network.

Cabling of the rural MV network can increase the reactive power generation even some MVArs which should be compensated locally.

• This reactive power influences the voltages in the distribution system.

Voltage rise due to the small loading in the partly cabled feeder.

Indication of high-resistance earth faults

• Motives for indication of high-resistance earth faultsElectrical safety

Reliability

• Method developed in TUT about ten years agoExperiences

Sensitivity

RTDS test environment and centralized functionality

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Indication method for high-resistance earth faults

• Main motive for indication is electrical safetyNo live parts that can cause dangerous hazard voltages for humans

• Secondary target is anticipation of developing phase-to-earth faults

Avoiding the interruption for customers

Limiting the affecting zone of the developing fault before an interruption


• In overhead line networks high-resistance earth faults may appear due to e.g.

trees leaning against a conductorwhen conductor falls to the ground with very high resistivityconductor breaks when the load side end of the conductor has an earth contactfaults with covered conductors or other network component faults

metal oxide surge arrestersoverhead line pin insulatorscable terminals

• This type of faults tend to evolve gradually into a full-scale earth fault.

• Thus early identification and location of such faults is of increasing importance in improving the reliability of electricity distribution.


UA, UB, UC are balanced phase-to-earth voltages UA', UB', UC’ are unbalanced phase-to-earth voltages of healthy stateU0’ is neutral voltage of healthy stateUAF, UBF, UCF are unbalanced phase-to-earth faults during an earth faultU0M is neutral voltage during an earth fault

U0 is change of neutral voltage

A

B

C

0º

30º 150º

270º

UAF

UA

UCF

UBF

U0F

UC

UB

U0 = U0F – U0’

U0

U0’


• The phase whose voltage lags the reference phasor U0 by 90º is interpreted as faulty.

• Thus if Phase 1 is supposed to be faulty its phase angle is 90º referenced to U0.

• According to the field measurements, the error margin is typically about 2º. The phase angle typically exceeds the theoretical value 90º by 1 - 2 degrees in isolated systems.

• The neutral voltage U0 can be calculated in the following way:

'000 UUU M

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where U0 is the change of neutral voltageU0' is the measured neutral voltage before a changeU0M is the measured neutral voltage after a changeUth is the specified threshold value of neutral voltageI0r is the reactive component of phasor I0i

I0i is the change of sum current of Feeder iis the phase angle of phasor I0i referenced to phasor U0

U F is the phase-to-earth voltage of the faulty phaseis the phase angle of phasor U F referenced to phasor U0is the phase angle of phasor U0is the phase angle of phasor I0i referenced to phasor U0

B0i is the phase-to-earth susceptance of Feeder i (sum of three phases).

UBI

UR

iIi

UFFi

i

F

000 0sin

sin

000000 arg , 'arg UiiIMU IUU

thM UUUU '000 iIir II 000 sin

iI0

0U

FU

iI0

Indication method for high-resistance earth faults – results from the field tests

Calculated shunt resistances when the length of the faulty overhear line feeder was 18 km.

FU 0UFU 0U

0UFU 0UFU 0U

0U

Staged resistance

Faulty feederB0 = 1.0·10-4 S

Healthy feederB0 = 4.0·10-4 S

Phase angle of U F

Neutral voltage

Phase angle of

RF [k ] RF [k ] RF [k ] [deg] [V] [deg.]

10.2 10.84 1558 90.56 582.7 -63.3

21.1 22.77 2874 90.70 289.1 -62.0

56.6 54.16 5778 91.84 123.6 -62.4

97.0 89.96 110620 91.18 75.4 -61.5

118.0 119.68 11595 91.33 57.0 -61.5

186.0 203.22 153770 91.37 33.7 -61.5

FU 0UFU 0U

0UFU 0UFU 0U

0U

Indication method for high-resistance earth faults – results from field tests

Calculated shunt resistances when the length of the faulty overhead line feeder was 27 km.

FU 0UFU 0U

0UFU 0UFU 0U

0U

Staged resistance

Faulty feederB0 = 1.4·10-4 S

Healthy feederB0 = 4.0·10-4 S

Phase angle of U F

Neutral voltage

Phase angle of

RF [k ] RF [k ] RF [k ] [deg] [V] [deg.]

10.2 10.73 1010 90.53 592.2 -63.3

21.1 22.17 2650 90.50 292.7 -61.9

56.6 53.0 5317 91.99 125.7 -62.5

97.0 85.60 4886 91.94 78.2 -62.2

118.0 120.68 5855 90.59 53.8 -60.8

186.0 197.36 11960 91.55 33.9 -61.6

FU 0UFU 0U

0U

Indication method for high-resistance earth faults - implementation as a part of feeder protection

• The prototype installation containing two REF 54_ Feeder Terminals and MicroSCADA was carried out at one 110/20 kV substation.

• The network was mainly of overhead construction.• The neutral of the network was isolated from the earth in this

110/20 kV substation. • These two REFs were available for configuration changes

and software updates constantly without interruption to the protection of the feeder lines as would the case be if they were also taking care of the protection itself.

• It was possible to re-configure and monitor REFs remotely using the modem connection to MicroSCADA. The REFs communicated with MicroSCADA using LON bus.

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Indication method for high-resistance earth faults - implementation as a part of feeder protection

Prototype installation of high-resistance earth fault indication carried out in due course in one 110/20 kV substation

20 kVFeeder 1

Modem connection

MicroSCADA at Substation

LON

R

Feeder 2 Feeder N

REF 54_

R

REF 54_

OI

SP AC3 3 1 C

SPACOM

Network control centre- MicroSCADA monitor

I >I0 >U0 >

I >I0 >U0 >

I >I0 >U0 >

O

I

SP AC3 3 1 C

SPACOM

OI

SP AC3 3 1 C

SPACOM

Staged resistance Faulty feeder Healthy feederRF [k ] RF [k ] RF [k ]

10.2 10.9 1350

21.1 22.8 4500

56.6 55.1 22000

97.0 92.6 5250

118.0 120.5 19000

186.0 195.0 25000

Calculated shunt resistances in relay prototype

Indication of high-resistance earth faults- centralized protection system

• Methods for improving sensitivity of earth fault protection as a function of centralized protection

• RTDS simulations of centralized protection system

• Indication method for high impedance earth faultsMethod is developed in TUT.

Method is implemented as centralized in order to test the new technology and its possibilities.

Indication of high-resistance earth faults- New setup with a central Station Computer

• Regulation rules and drivers in consequence of opened free market of electricity distribution (MV)

legislationnew reporting and monitoring requirements for electricity distribution companiesEven short interruptions in the supply need to be reported.Variations in the power quality need to be monitored.The regulation model highlights the quality of the distributed energy

higher quality will generate higher profits to network operators

• The control of the network moves further away from the actual physical network.

Many company fusions have created bigger players.

• Communication network technology has developed fast over the past years enabling more centralized control

increasing need to gather data from larger networks and pre-process data before it is viewed by the NCC personnel


• Rapid increase of distributed energy generationnew challenges when the MV network is being used in a different way that it was originally designed forRequire fast reactions from electricity distribution companies.

• The maintenance and functional updating measures required by the protection equipment of substations have been time consuming and expensive.

switching off the whole protection system and causing interruptions in supplyunnecessary secondary testing of the switchgear

• Significant amount of Finnish substation installations start to be outdated and must be refurbished.

• Utilizing of the new protection algorithmsneed for easily upgraded system without interruptions to customers

• Introduction and the increasing acceptance of the IEC 61850 standard have made available fast and standardised Ethernet based communication.

Combining the station and process bus enables an approach where part of the protection and condition monitoring is moved from the bay level IEDs to a centralized Station Computer.

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• Measurements from several feeders

• Method calculates fault resistance for all feeders when an earth fault is indicated according to change of neutral voltage.

Better reliability and sensitivity with very high fault resistances.

Method needs up-to-date value for phase-to-earth admittance or phase-to-earth susceptance of appropriate feeder.

Network database

Updated values for phase-to-earth admittances based on measurements during real earth faults

Indication of high-resistance earth faults- centralized protection system

RF1 RF4RF3RF2

If 1 k < RF < 200 k Feeder 1 isconcluded faulty

Typically for earth faultswhere 5 k < RF < 200 k

Indication of faulty phase and calculated values for RF of every feeder

GTNET Cards

DOPTO Card

RTDS®

Voltage & current measurements (3x8) in IEC61850-9-2 format

RTDS is controlled by a PC workstation

Measurements are taken from each feeder in the network model run by the RTDS

Substation computer

HV/MV Transformer

PI

PI

PI

PI

PI

PI

PI

PI

PI

Control signals (i.e. trip signals) to feeder IEDs

Trip and start signals to RTDS

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Summary of high-resistance earth fault indication

• RTDS environment developed for testing centralized functionality suitable to be run in a primary substation.

• Indication method for high impedance earth faultsMethod is implemented as centralized in order to test the new technology.

Preliminary test results are promising.

Novel solutions for earth fault management- Summary

• Aspects of long rural cable networksNeed for novel solutions for earth fault protectionDistributed compensation + centralized compensation with large rural cable networksNeed for distributed reactive power compensation

• Residual current compensation (RCC devices)Active current injection to the neutral pointApplicability in large rural cable networks?

• Development of modern phase earthing systemCost-effective method for reducing short interruptions to customers and DG units

• Indication methods for high-resistance earth faultsFeeder level solution orImplementation as a part of the centralized protection system

27.3 · hd 637 s1 (cenelec) / en 50522/ sfs 6001 fundamentalfrequency overvoltages- risk of...

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