Reles de Proteccion

Secondary distribution protection relaysc do you know about them?c do you know how to specify them?

date

07/99

- C31 -

revised

07/99

WHAT YOU NEED TO REMEMBERProtection devices are divided into 2 main categories: c with auxiliary power supply;c without auxiliary power supply*.

Our choice in specifying protection devices is guided by customerrequirements and technical and economic constraints.

Glossary of ANSI codes:49: thermal overload50: instantaneous overcurrent50G: instantaneous earth fault on core balance CT50N: instantaneous earth fault on CT51: time delayed overcurrent51G: time delayed earth fault on core balance CT51N: time delayed earth fault on CT*Sometimes called: self-powering protection.

SECONDARY DISTRIBUTIONSUBSTATION TYPES

11 criteria used to choose a protection system:c secondary distribution substation types; c customer practices;c current standards; c auxiliary power supplies; c protection device characteristics; c settings and selectivity conditions; c resistance to the environment; c price;c incorporation and use; c maintenance ; c dependability.Each of these criteria are explained in detail later in the document.

THE CHOICE OF PROTECTION DEVICE DEPENDS ON THE SUBSTATION TYPE

Secondary distribution substations can be defined as follows:All of the equipment going to make up the substation is groupedwithin the same walls, either inside an existing building, or in afactory-built substation installed outdoors.

Secondary distribution protection relays are used on 6 to 36 kVnetworks. They may also be combined in a protection systemincluding protection devices, metering, and possibly automaticcontrol and communication functions.

The requirements of secondary distribution customersare to enhance continuity of service and guaranteethe safety of people and property.

Secondary distribution protection relays

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revised

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SECONDARY DISTRIBUTIONSUBSTATION TYPES (contd)

In MV/LV substations, relays protect the transformer.Example of a simplified MV/LV substation, equipped with: c cubicle with switch on the 2 MV incomers;c 1 circuit breaker combined with a protection system on the MV/LVtransfo feeder.

Faults specific to the MV/LV transformer are picked up by detectorsplaced on the transformer and by current sensors placed on the MVside of the transformer. These detect:

c gas release, pressure and temperature increases (DGPT);c winding temperature increases (probes);c transformer short-circuits;c transformer earthing faults.


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Protection

Transformer

Sensor

Breakingdevice

MV

LV

SECONDARY DISTRIBUTIONSUBSTATION TYPES (contd)

In MV/LV substations, relays protect the network from overloads,short-circuits and earthing faults.

Example of a simplified MV substation, equipped with: c cubicle with switch on the 2 supplies;c 1 circuit breaker combined with a protection system on each feeder.

MV substation overcurrent faults are detected by current sensorsplaced on the power conductors. They detect:

c overloads;c short-circuits;c earthing faults.


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ProtectionProtectionProtection

FeederFeederFeeder Back-upsupply

Normalsupply

*NC *NC*NO

* : 2/3 interlocking

CUSTOMER PRACTICES

AUXILIARY SUPPLY

PROTECTION DEVICES ARE GREATLY INFLUENCED BY UTILITIES COMPANIES

In public distribution, protection systems generally have to beapproved. Utilities companies have their own protection plans; i.e. in France pole-mounted low power MV/LV transformers are notprotected against MV-side overcurrents.

In industrial distribution, customer specifications are sometimesinfluenced by Utilities companies.Influencing actions on utilities companies should therefore be anongoing concern.

PROTECTION DEVICES MUST MEET SPECIFIC STANDARDS

Standards define the conditions which have to be established and maintained to ensure the safety of people, the conservation of property and to limit interruptions in network operation wheninstallations are connected to a public distribution network.Each country has standards adapted to the characteristics and operating rules of the electrical network (e.g.: IEC - ANSI).

SOME PROTECTION DEVICES FUNCTION WITH AUXILIARY POWERSUPPLY AND OTHERS WITHOUT AUXILIARY POWER SUPPLY

In secondary distribution, protection relays can function in variousways: with or without auxiliary power supply.

Relays without auxiliary power supply provide a solution which:c is economic: no battery or battery charger;c functions independently: using fault energy to control the openingof the breaking device;c is integrated in the breaking device: the whole of the protectionchain (relays, sensors, release-trip units) are integrated in the factory;In this case, the fault energy is used by special current sensors, to control the tripping of the breaking device (please note these relayshave minimum current requirements to operate (actuating current).

Relays with auxiliary power supply provide: c wide-ranging and accurate settings;c settable indicators;c logical selectivity management;c information exchange with a monitoring and control system;c metering.The power supply characteristics must be known with this type of relay(voltage, power, etc.).


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date

07/99

- C31 -

revised

07/99

CURRENT STANDARDS

PROTECTION DEVICECHARACTERISTICS

PROTECTION SYSTEMS HAVE TO COVER A WIDE RANGE OF SOLUTIONSTO MEET THE REQUIREMENTS OF VARIOUS PROTECTION PLANS

The trip curves have various gradients to meet the operatingconstraints of various users and to provide selectivity between the LV and upstream protection trip curves. (see the various trip curves presented in appendix 1).

c SI Standard Inverse time;c VI Very Inverse time;c EI Extremely Inverse time; c UTI Ultra Inverse Time;c LTI Long Inverse Time;c RI which represents the characteristics of definite time trip curvesfor electromechanical relays (trip curve essentially used byScandinavian countries and Belgium).

Overcurrent protection devices can be time delayed, ANSI code 50.

Overcurrent protection devices for earthing faults can function on the sum of 3 CT measurements, ANSI code 50N or 51N, or a core balance CT measurement (specific core balance sensor),ANSI code 50G or 51G.

Thermal overload protection devices, ANSI code 49, protect againsttemperature rise, memorising the previous temperature rise values.Value calculations take account of temperature rise and fall timeconstants of the monitored equipment (see the manufacturerscharacteristics for this equipment).


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revised

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Without auxiliary power supply,protection devices have characteristics depending on their operating principle:using fault energy.

Overload

Short-circuitphase/phase

Short-circuitphase/earth

Others

Key

With auxiliary power supply,certain digital protection devices can integrateall of the characteristics shown below.

Management of internal transformer protection devices (DGPT, Buchholz, PTC temperature probes, etc.). Protection devices against temperature rise (PT100 temperature probes, etc.)

non-functioning zone for the protection device (in this zone, the energy is insufficientto operate the protection chain).DT: Definite Time.IDMT: Inverse Definite Mean Time.

Specific IDMT (SI, VI, EI, etc.)

DT

51

50-51

50-51 50-51

51

Thermal overload IDMT (SI, VI, EI, etc.)

DT

51

5149

DT Specific

DT

51

50N

DT

51N

50-51

51

IDMT (SI, VI, EI, etc.) and DT

51

50N


51N

50-51

51


DT

50N-51N or 50G-51G

51N or 51G

50N-51N or 50G-51G

51N or 51G


DT


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The settings and selectivity conditions are dealt with in various ways according to whetherthe protection devices are required with or without auxiliary power supply.

SETTINGS AND SELECTIVITYCONDITIONS

The protection device must be set to ensure selectivity relative to upstream MV network protection devices and downstream LVnetwork protection devices. This ensures that only the protectionmechanism closest to the faulty section is opened.

Application examples: Selectivity between protection devices upstream anddownstream of the transformer(IDMT* trip curve).

Selectivity is:ampermetric between the MV2 and LV trip curveschronometric between the MV1 and LV trip curveschronometric between the MV2 and upstream trip curves

*IDMT = Inverse Definite Mean Time.


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MV 1MV 2

UpstreamUpstream

Key:LV protection device trip curveMV protection device trip curveupstream protection device trip curvesettings margin to ensure the selectivity of protection devices

MV 1

MV 2

LV

Time

Current

I transfo Isc LV Isc MVLV

LV

SETTINGS AND SELECTIVITYCONDITIONS (contd)

Another application example:Selectivity between upstream transformer trip curve and MVfeeders (specific trip curve).Selectivity is:ampermetric between the MV and LV trip curves

(I LV feeders

RESISTANCE TO HARSHENVIRONMENTS

PRICE

PROTECTION DEVICES ARE EXPOSED TO MANY DIFFERENT TYPES OF DISTURBANCE

Protection relays are exposed to electromagnetic disturbances,basically due to electrical arcing and the proximity of high currents.These devices are likely to be installed in kiosks which may besubjected to harsh climatic conditions (resistance to dust andhumid atmospheres).


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date

07/99

- C31 -

revised

07/99

SEARCHING FOR AN ADAPTED SOLUTION AT THE LOWEST COST

Protection devices are installed on networks which can be low power.They are mounted in cubicles that are mass-produced and low cost.

Consequently, when choosing a protection solution, you must takethe cost of the whole system into account (including relays, sensors,cubicles) relative to the cost of the equipment requiring protection:c relays without auxiliary power supply save on the cost of a battery,a charger and their installation;c two current sensors are sufficient if protection against earthingfaults is not required;c one type of relay covers a large number of configurations andtherefore reduces operating and maintenance costs (large choiceand wide range of settings of protection trip curve settings on the same relay).When continuity of service is of prime importance, relays withauxiliary power supply will be recommended to provide:c an accurate and wide range of protection settings;c the possibility of choosing protection against thermal overloads;c more accurate and more sensitive protection against earthingfaults (core balance CT measurements).

In conclusion, according to the product type, the cost of protectionsystems will be optimised by the choice of options (see following page).


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date

07/99

- C31 -

revised

07/99

Without auxiliary power supply

Faultsphase/phase

Faultsphase/phase +phase/earth

With auxiliary power supply

ANSI codes 50-512 CTs suffice

Protection devices50-51

ANSI codes 50-51 and 50N-51N3 CTs are needed to calculatethe earthing fault current

Protectiondevices50-5150N-51N

ANSI codes 49-50-51 and 50N-51N3 CTs are needed to calculate the earthing fault

Protectiondevices49-50-5150N-51N

ANSI codes 49-50-51 and 50G-51Ga specific core balance CT is used to detect low level earthing fault current

Protectiondevices49-50-5150G-51G

ANSI codes 49-50-512 CTs suffice (no I2 measurement)

Protection devices49-50-51

CHOOSING THE TYPE AND THE NUMBER OF SENSORSPRICE (contd)

PRICE (contd) The various types of options Without auxiliary power supply:c only have 2 CTs when there are no earthing fault protection devices;c stress the simplicity of operation (when a fault occurs: reclose the circuit breaker instead of changingthe fuse);c highlight protection efficiency (when a fault occurs: the 3 phases are simultaneously switched) ;c stress maintenance savings (low quantity of equipment in stock, quick to replace, etc.);c underline the established nature of protection systems and ourcompetencies in this field (selection guide and technical protectionguides);c talk about our accumulated experience in LV, MV, HV, EHV, and associated monitoring and control: relays already installed in the nuclear, petrochemical and mining industries, in the navy, etc.(innovation in terms of digital protection devices and protectiondevices without auxiliary power supply).

With auxiliary power supply types:c only have 2 CTs if there is neither earthing fault protection, nor I2measurement; c measurement of residual current using the sum of 3 CTmeasurements is sufficiently accurate if the earthing fault is not or is only slightly limited (for isolated neutral applications in whichspecial selectivity is required, a core balance CT must be used). The core balance CT offers better measurement accuracy than the sum of 3 CTs and detects a fault of approximately 1 ampere;c overloads can be dealt with by the thermal overload protection(ANSI code 49);c stress the possibility of communicating with a monitoring andcontrol system (supervision, telecontrol);c stress performance in terms of safety (automatic controls,automatic calibration, etc.) implementation and use (simplicity,dependability, availability, etc.), the choice of trip curves andsettings, without forgetting test reports available from the qualitytechnician and test certificates for the relays themselves;c talk about our accumulated experience in LV, MV, HV, EHV andassociated monitoring and control systems: relays already installedin the nuclear, petrochemical and mining industries, in the navy, etc.(innovation in terms of digital protection devices and protectiondevices without auxiliary power supply);c stress the simplicity of maintenance (2 screws and a few connectorsto move);c underline the established nature of the protection systems and our competencies in this field (selection guide and technicalprotection guides, etc.).


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date

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revised

07/99

SIMPLIFIED INCORPORATIONAND USE

RAPID AND LOW COSTMAINTENANCE, LIMITED STOCK

In cases where the customer does not have an electrical engineeringdepartment, the protection system must be simple to install,incorporate and use.

All normal use characteristics are given on the relays front paneland the operating data is directly displayed in physical values(Amperes, volts, etc.).

Following breaking, the protection relay can be automatically reset,or reset via a switch or via an external command (depending on thecharacteristics of the relay chosen).


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date

07/99

- C31 -

revised

07/99

Protection relays without auxiliary supply have terminals allowingsimplified test systems to be connected. Some of these tests cantake place when the relay is operational.

With digital auxiliary power supply relays, the testing of only onefunction enables the correct functioning of the whole relay to bevalidated (checking the correct functioning of the current input and calibrating the integrated IT signals). Some indication tests can be performed when the relay is operational.

Whatever the relay type, maintenance by the replacement of the relay, adjusted according to the application, is quick andinexpensive. The spare parts set is limited to a small number ofdevices for the whole installation and even for several installations.

DEPENDABILITY DEPENDABILITY OF THE INSTALLATIONS CRITICAL SECTIONSTo be profitable, industrial installations have to be dependable allthe time whilst guaranteeing the safety of people and property; in other words, they have to have an ongoing main power source:electricity. This is why electrical networks protection systems aredesigned, installed and tested according to precise rules dictatedby reliability experts. These experts study, draw up and validatedependability rules.

Dependability is divided into 4 complementary techniques applied to the section concerned: reliability, availability, safety and maintainability.

DEPENDABILITY RESULTS FROM VARIOUS ASPECTS OF THE INSTALLATION

With relays without auxiliary power supply: the use of faultenergy protects us against possible failure in the auxiliary powersupply (transformer, battery or charger).

With auxiliary power supply relays: permanent automaticmonitoring indicates operational faults (i.e. loss of power supplycauses the disactivating of an auxiliary relay).


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date

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- C31 -

revised

07/99

100

1000

t (s)

I/Is

10

1

10 10010.1

RI

Standard inverse time

Very inverse time

Extremely inverse time

Ultra inverse time

APPENDIX 1 Types of protection trip curve against current faults


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date

07/99

- C31 -

revised

07/99

Documents

Reles de Proteccion