active earthing system for mv networks

International Conference on Renewable Energies and Power Quality (ICREPQ’13)Bilbao (Spain), 20th to 22th March, 2013

Renewable Energy and Power Quality Journal (RE&PQJ)

ISSN 2172-038 X, No.11, March 2013

Active Earthing system for MV networks

F.J. Pazos1, A. Amezua2, I. Gutierrez2, J.M.Garcia3, R.Toledo3, G. Buigues4 and V. Valverde4

1 Iberdrola Distribución Eléctrica, S.A.U.Avenida San Adrián, 48, 48003 Bilbao (Spain)

e-mail: [email protected]

3 Ingeteam Power Technology, S.AParque Tecnológico de Bizkaia, Edificio 110

48170 Zamudio (Spain)e-mail: [email protected]

e-mail: [email protected]

2 OrmazabalBarrio Basauntz, 2 – 48140 Igorre (Spain)

e-mail: [email protected]: [email protected]

4 Faculty of Engineering of Bilbao, UPV/EHUAlameda de Urquijo s/n, 48013 Bilbao (Spain)

e-mail: [email protected]: [email protected]

Abstract.This paper describes and a new technology, the active earthingsystem, based on a multi-frequency power converter, whichcombines a new power electronic device with a protection andcontrol system.This system allows network enhanced operation andmaintenance, overcoming some of the limitations of thetraditional earthing systems, giving rise to relevant improvementin supply continuity.

The key questions and results of the validation along with labtesting data and field details are shown in this paper. Theexperiences of the first real application in a MV distributionsubstation being conducted in Gernika– Spain will be presentedin ICREPQ’13 conference.

Key words

Active earthing, predictive maintenance, fault extinction,fault location.

1. Introduction

The type of neutral treatment has an important influence inthe behaviour of Medium Voltage (MV) networks. Itparticularly affects voltage dips, swells and interruptionsand also fault location, since earthing system determinesseveral aspects of earth-faults such as fault current,duration and, to a certain extent, their probability ofoccurrence.So far, traditionally the MV neutral has been earthed bymeans of passive elements, such as feeders' capacitancesand/or discrete elements namely reactors and resistors or,simply, solidly earthed. In any case, when a fault happens,conventional earthing systems are not able to modify thecurrent that flows through the fault or network neutral. Itdoes not provide any other function while there is no faultin the network.

The active earthing system has the traditional advantagesof a resonant network, such as arc suppression or lowstep and touch voltages. However, additional features areachieved by the injection of a controlled current inmagnitude, phase and frequency. More details can befound in [1].

Two main tasks, described below, were considered forthe system validation.

In the laboratory, a scale model of the active earthingsystem has been used connected to a MV networksimulator. Earth faults have been simulated on differentnetwork configurations simulating several combinationsof overhead line and cable sections, and a wide range offault impedances.

In the field, real tests are being held. First of all, and priorto carry on fault extinction tests, the network is checkedin order to make sure that isolation level and networkcomponent rating are able to withstand the new operatingconditions, which are similar to those necessary forresonant networks. The active earthing system is used tocreate controlled phase-to-earth overvoltages in eachphase and facilitate the process. Secondly, fault detectionand location is tested for provoked (intentional) faults,including those of high impedance. Finally, the activeearthing system performance is tracked and recorded forstandalone operation.

2. The “Active Earthing” concept

“Active Earthing” means that any zero-sequence voltagecan be created injecting the appropriate current, as statesin (1), when desired and with the desired angle andfrequency.

https://doi.org/10.24084/repqj11.248 164 RE&PQJ, Vol.1, No.11, March 2013

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000 33 ZIVZIV NNN (1)

Therefore, phase-to-earth voltages can be modified ondemand, as can be seen in equation (2):

Nneutraltophaseearthtophase VVV (2)

• In normal operation conditions, with no fault in thenetwork, neutral voltage can be modified, thus reducing orincreasing one phase-to-earth voltage to any value, asshown in figure 1.• During an earth-fault phase-to-earth voltages can bemodified as well, being able to eliminate the voltage in theaffected phase.• Different frequencies can be injected in order to improvethe protection system behaviour.

1

3

2

1

3

2

1

3

2

Figure 1: Any phase to earth voltage can be created by adding theappropriate zero-sequence voltage

The aim in developing the active earthing system is toincorporate new features, related to system maintenanceand operation.

For maintenance purposes, it will improve weak insulationdetection, while the operation will take advantage of abetter detection of high impedance faults. A particularimprovement in this area will be the identification ofdangerous situations such as a direct contact of liveconductors to earth. The enhanced earth-fault location willbe useful for both purposes.

As a consequence, some faults will be prevented fromappearing in the distribution network and those thathappen will be promptly detected and located, thusimproving continuity of supply and power quality.

This active system will behave as a low earth-fault currentearthing network, but with additional features whichdramatically improve the weaknesses of isolated orcompensated networks.

Regarding material stress due to overvoltages, the systemisolation can be periodically checked so the duration of theovervoltage will be limited, leading to a reduced impact onmaterial.

The active earthing system comprises an innovativeprotection system for the enhancement of earth-faultdetection, faulted phase and feeder identification andlocation.

3. Active Earthing system description

This new active earthing system for the MV distributionnetwork, as shown in Figure 2, bases its operation onpower electronics and combines them with a protectionand control system, which obtains the necessarymeasurements to detect and locate the fault.

Zero sequence current flow and zero sequence voltagesare determined by the network configuration. For a givenconfiguration, if a current controlled in magnitude, phaseand frequency is injected into the network, it is possibleto determine network parameters by measuring currentsand voltages. Depending on the network parameter to bemeasured, the injected current is adapted, using, forinstance, different frequencies for capacitance orinductance measurements. This can be done both innormal operation and in a faulty network operation.

The active earthing system operates as follows:• It is able to perform network zero sequence impedancecalculations by means of the injection of fundamental andnon-fundamental (other than 50-60Hz) frequencycurrents, as shown in (3). This calculation is used, duringnormal operating conditions of the network, for thecalculations for the detection and location of the fault bythe earthing system.

fN

fN IVf

N

fNf

CI

VX sin

(3)

00

00

f

fXX

f

fX f

Earthingf

Cf

C (4)

• It is also able to provoke a controlled neutral voltage,not only in module and in phase, but also in time, so thevoltage of each phase gets the desired voltage.

During a phase-to-earth fault in a feeder, the activeearthing system performs:• Fault location, both for transient and permanent faults,by means of an innovative and enhanced protectionsystem and fault locator based in measurements ofdifferent frequencies generated by this new system.

0000

0 f

fXRZ f

LLfL

(5)

kmline

fL

Z

ZfaulttocetanDis

00__

(6)

• Fault extinction, injecting zero-sequence powerfrequency current to cancel the voltage of the faultedphase applying the voltage required by solving (2),

Vn

Vn

Vn

Vn


extinguishing transient faults and, as a result, avoiding thetripping of the feeder breaker.

ACTIVE EARTHING SYSTEM

Protection and Control System(Converter control and relaying)

R-S-T

N

MV Feeders

MV Busbar

HV/MV Transformer

MV System Earth

By-pass

Figure 2. Active earthing system scheme

• The reduction of faulty phase-to-earth voltage, in case ofweak insulation points or partially damaged undergroundcables, to keep the line in service until the problem isfixed, applying the voltage required by solving (2) for thedesired line voltage.

In addition, network insulation testing, for condition basedmaintenance purposes:• Controlled voltage changes are used in combination withdetection and location features of insulation failures,allowing the condition based maintenance of MV feeders.•On-line network component monitoring and diagnostic

can be done in combination with partial dischargedetection devices installed in substations or portabledetectors can be used for weak insulation points detection.

4. Scale model system

The objective of the scale model system is guarantying thesuitable response of this system and the correct operationof the network during field tests. The influence of thelevels of load currents and voltages on the validation issmall, since these do not affect the zero-sequencecomponents of actual networks. Therefore, a low powermodel is a useful tool to validate the system, in conditionsmore realistic than simulations.

Thus, the model used comprises the active earthing systemand the distribution network. This model allows testing theoperation principles and the correct operation of thecomplete system. By means of the current injection offrequencies not existing in the network, measurements andcalculations accuracies are checked. On the other hand,injections at network frequency are done in order to checkhow fault currents are cancelled and how controlledovervoltage can be provoked.

Nevertheless, field tests are necessary to determine thebehaviour of the system in a real network, given that inthis model discrete components (resistors, inductors and

capacitors) are used to simulate the distributedparameters of overhead lines and cables of a realnetwork.

Description of the systemNetwork model is constituted by a transformer and threefeeders divided into sections and different types of loads.Several network configurations can be tested modifyingthe type and number of sections of these lines. Besides, itis possible to simulate earth faults for each line sectionwith resistive fault impedances and/or defective MOVsurge arresters. Power electronics and the protectionsystem model is a precise reproduction of the real onewith lower rated power. Finally, feeders and earthingcurrents and busbar voltages are monitored to analyse theresponse of the whole system.

Figure 3, shows the distribution network simulator,which its diagrams in the front, what elements of thenetwork are modelled. In the diagram above thetransformer of the substation, earthed through an inductorand the connection of the active system by a transformeris included. In the one below, three lines composed bydifferent sections, loads and resistances to generate thefault where it is required.

Figure 3: Distribution network simulator

Laboratory testing resultsLaboratory tests, described in [2], with the scale modelinclude:- Measurements accuracy- Controlled phase overvoltage- Detection and location of weak insulation points- Fault location- Fault extinction

In addition, testing of new protection concepts has beencarried out.The system can work in two modes, normal(exploitation) mode and maintenance mode.In the normal mode, faults can be easily detected by theexistence of zero sequence voltage.However, during the maintenance mode a zero sequencevoltage is always present, since it is injected by the activenetwork in order to check the isolation, as can be seen infigure 4.


0 2000000 4000000 6000000 8000000Time (ms)

inj50_16>4_?-EA09_f_v_an(V) inj50_16>8_?-f_p_neutro(W)

Figure 4: Phase to earth voltage increase vs. active powersupplied by the earthing system in maintenance mode

For this reason, a new detection system has beenimplemented, in which the active power injected by theactive earthing in the zero sequence circuit is examined.Figure 5 compares the active power in a healthy feeder(blue) with the active power of the same feeder having adefective surge arrester. The abnormal active powerincrease is detected and limited by the earthing system,being able to detect the lack of isolation before thedefective element is seriously damaged.

0

5

10

15

20

0 1000 2000 3000 4000 5000 6000 7000 8000

Electrotek Concepts® TOP, The Output Processor®

Mag

nitu

de(M

ag)

Time (ms)

inj50_17-2>1_?-f_p_neutro(W) inj50_16-2>1_?-f_p_neutro(W)

Figure 5: Active power supplied in healthy and low isolationconditions

Another crucial aspect checked in laboratory is thedetection of network topology changes, since the systemmust be aware of the system capacitance. In addition to aperiodical checking, the active earthing detects highfrequency transients, representative of switching events inthe network.

5. Field installation description

One of the most important issues when installing a newdevice, is to assure the correct response of the protectionsystem in the new situation while maintaining highestsafety standards (low level of touch and step voltages) forpersonal and equipment protection.

It has to be taken into account that the network isreconfigured as high impedance earthed while in operation

without faults, and it performs as resonant earthed wheninjecting for fault extinction.

Regarding protection system, when the active earthingsystem is installed in a substation solidly or lowimpedance earthed, as it is the case in Gernika, nochanges are expected to be required in relation with theovercurrent protections system. This protection diagramwill not operate until the active earthing system finishesextinguishing the fault.

For isolated or high impedance earthed networks, minorchanges might be needed in the protection system inorder to maintain the protection operation diagram. Themodification must delay the activation of the overcurrentprotection about 100 milliseconds, the time needed forthe active earthing system to control de current of theneutral. Besides, zero sequence overvoltage protectionwill be inhibited during the process of the extinction ofthe fault.

One of the most important issues when changing theearthing system is to ensure safety of persons and goods.This is accomplished by the correct operation of theprotection system and the level of touch and step voltagesin the new situation. It has to be borne in mind that theactive network is earthed through high impedance whilein normal service and that it behaves like a resonantearthed one when injecting for the extinction of the fault.

On the other hand, with this novel earthing system, theisolation level requirements are similar to those of theresonant networks. Isolation condition has been alwaysan issue when changing an existing MV network fromlow to high impedance earthing. Typically, during thefirst months, or even years, the higher stress for isolationof high impedance earthing makes insufficient isolationappear in elements, with the consequent effect on powerquality.

The traditional solution to this problem is a revisioncampaign of the whole MV system, previous to theearthing change, where any doubtful isolation should bereplaced. However, this process is slow, costly andunreliable, since some failures (i.e. defective cable joints)cannot be easily detected. The active earthing systemfacilitates this process, since it allows checking thenetwork isolation during the commissioning process. Aphase by phase checking process can be applied to eachfeeder, locating defective points either by means of thehigh impedance fault detection included in the systemprotection or using external detection based in partialdischarges.

The use of active earthing system implies using aprotection system able to work with the signals injectedby the converter. This can be done at different levels:• With feeder protection relays with adapted features forearth fault detection (normal characteristics for the rest)• With a protection relay dealing with the earth faultdetection of several feeders, keeping the existing relaysfor the rest of functions.


Figure 6: Protection relay control and monitoring HMI

• Maintaining the existing relays and installing a by-passbreaker, which will close in case of permanent fault,allowing the existing protection system to work normally.In this case, part of the protection functions (i.e. faultlocation) would not be available.

Figure 5: Active earthing converter

6. Conclusion

The novel earthing system presented allows networkoperation and maintenance and continuity of supply ofthe MV distribution network in which it is installed byovercoming the limitations of the traditional earthingsystems used currently in distribution network MVsubstations due to the capability of controlling the neutralcurrent and voltage of the network. It is based on thecombination of a multi-frequency power converter and aprotection and control system.Mentioned improvements, not together achievable bymeans of conventional earthing systems, are summarisedas follows:• Transient earth-fault extinction• Fault detection• Faulted feeder identification and fault location• Network condition oriented maintenance• In case of a weak insulation points or partially damagedunderground cables, the new system will give the chanceto reduce faulted phase-to-earth voltage, to keep the lineenergised while the problem is fixed.

Laboratory tests results allow us to conclude thefollowing:• The network is protected having low step and touchvoltages• The measurement at other frequencies than 50-60Hzimproves accuracy in fault detection and location• Fault current can be cancelled almost completely, tomake its extinction possible• The active earthing system allows operating networkswith permanent faults in a safe way• Response times allow carrying out the location andextinction of transient faults

All the functionalities of this patented system, based onpower electronics technology, lead to an improvednetwork performance for earth-faults, providing a realpossibility to carry out condition oriented maintenance.

References[1] F.J. Pazos, 2009, "Active grounding system for MVnetworks by means of power electronics", CIRED 2009, paper0158[2] A. Amezua, 2011 “Experimental validation results of theactive grounding system for MV networks”, CIRED 2011,paper 560


active earthing system for mv networks

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