Electrical installation handbook Protection, control and electrical devices

Rajat Mehta

Electrical installation handbook Protection, control and electrical devices

Rajat Mehta

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Abstract

In order to ensure the proper function in such environments, the shipping registers require that the apparatus has to be tested according to specific type approval tests, the most significant of which are vibration, dynamic inclination, humidity and dry-heat tests.

Figures (502)

This technical collection takes into consideration only the bodies dealing with electrical and electronic technologies.

Flow diagram for the conformity assessment procedures established by the Directive 2006/95/CE on electrical equipment designed for use within particular voltage range: When the CE marking is affixed on a product, it represents a declaration of the manufacturer or of his authorized representative that the product in question conforms to all the applicable provisions including the conformity assessment procedures. This prevents the Member States from limiting the marketing and putting into service of products bearing the CE marking, unless this measure is justified by the proved non-conformity of the product. gisters require that the apparatus has to be tested according to specific type

It is always advisable to ask ABB SACE as regards the typologies and the per- formances of the certified circuit-breakers or to consult the section certificates in the website http://bol.it.abb.com. ABB SACE circuit-breakers (Tmax-Emax) are approved by the following ship- ping registers:

Moulded-case circuit-breaker: SACE Tmax XT

‘ Not available for Ekip N and Ekip |; only for XT2 In=10 A not available with Ekip G The following table shows the available rated currents with the SACE Tmax XT Tmax T and Emax circuit- breakers.

The following table shows the available rated currents for motor protection with the SACE Tmax XT and Tmax T circuit- breakers.

® Also for lu = 1000 A (not available for E8V and E2L). 1 Protection and control devices

The following table summarizes the types of electronic release and the functions they implement:

Generic distribution system (IT, TT, TN) The operating principle of the residual current release makes it suitable for the distribution systems TT, IT (even if paying particular attention to the latter) and TN-S, but not in the systems TN-C. In fact, in these systems, the neutral is used also as protective conductor and therefore the detection of the residual current would not be possible if the neutral passes through the toroid, since the vectorial sum of the currents would always be equal to zero.

installation conditions are particulary restrictive, such as with circuit breakers already installed, limited space in the circuit breaker compartment etc. Thanks to the settings characteristics of the residual current and of the trip times, the residual current relays with external transformer can be easily installed also in the final stages of the plant; in particolar, by selecting the rated residual current I[An=0.03A with instantaneous tripping, the circuit-breaker guarantees protection against indirect contact and represents an additional measure against direct contact also in the presence of particulary high earth resistance values. Such residual current relays are of the type with indirect action: the opening command given by the relay must cause the tripping of the circuit-breaker through a shunt opening release (to be provided by the user).

The following table shows an overview of the MCBs, for further details please refer to the technical catalogue.

‘ 90kA @ 690V only for XT4 160. Available shortly, please ask ABB SACE ® XT1 plug in version only with In max = 125A

SACE Emax air circuit-breakers with full-size neutral conductor

Besides, other program features are the verifications of cable protection, of human beings’ protection and of discrimination. The algorithms for the verifica- tion of the cable protection are described in the international standards. The algorithms for the verification of discrimination are implemented in accordance with the guidelines provided in ABB SACE Technical Application Papers, specifically “QT1: Low voltage selectivity with ABB circuit-breakers” (QT1 from now on). The software “Curves” displays tripping and limiting characteristics according to the catalogues. This program allows the visualization of :

The figure below shows the time-current tripping curve of a circuit-breaker equipped with an electronic release having the protection functions LSIG which are described in the following pages: To simplify the reading of the examples, the tolerance of the protection func- tions has not been considered. For a correct setting it is necessary to take into consideration the tolerances relevant to the different protection functions referred to the electronic trip unit used; for this information please consult the technical catalogue.

Protection against undervoltage (function UV)

Protection against overvoltage (function OV)

Protection against residual voltage (function RV)

Protection against reversal of power (function RP)

Curve typology The curve L complying with Std. IEC 60255-3 is available both for the electronic trip units type PR3832-PR8333 for T7 and X1 series circuit-breakers, as well as for the electronic trip units tyoe PR122-PR123 for Emax series circuit-breakers.

L: motor protection function against overload according to the indications and classes defined by the Std. IEC 60947-4-1 L: motor protection function against overload according to the indications and classes defined by the Std. IEC 60947-4-1

The following diagram shows the limit curve for Tmax XT2L160, In160 circuit- breaker. The x-axis shows the effective values of the symmetrical prospective short-circuit current, while the y-axis shows the relative peak value. The limiting effect can be evaluated by comparing, at equal values of symmetrical fault current, the peak value corresponding to the prospective short-circuit current (curve A) with the limited peak value (curve B).

Temperature rise limits Description of part*

Circuit-breakers with electronic release Caption

Caption F= Front flat terminals FC= Front terminals for cables HR= Rear flat horizontal terminals VR= Rear flat vertical terminals R= Rear terminals

For switchboards with the following dimensions (mm): 2000x400x400

Note: the reference temperature is the ambient temperature

Note: the breaking capacity |,,, at the maximum rated use voltage, by means of external protection relay, with 500 ms maximum timing, is equal to the value of I, (1s). Table 4: Emax switch disconnectors

The example above shows a plant wired so as to guarantee zone selectivity with Emax CB equipped with PR832/P-PR333/P-PR122/P-PR123/P releases. Each circuit-breaker detecting a fault sends a signal to the circuit-breaker im- mediately on the supply side through a communication wire; the circuit-breaker that does not receive any communication from the circuit-breakers on the load side shall launch the opening command. In this example, with a fault located in the indicated point, the circuit-breakers D and E do not detect the fault and therefore they do not communicate with the circuit-breaker on the supply side (circuit-breaker B), which shall launch the opening command within the selectivity time set from 40 to 200 ms. 3 Protection coordination

Zone selectivity for circuit-breakers type Tmax (T4L-T5L-T6L) with PR223 EF releases

From the curves it is evident that between breakers E2N1250 and T5H400 time discrimination exists, while between breakers T5H400 and XT1N160 there is energy discrimination.

Supply side circuit-breaker 4P (load side circuit branched between one phase and the neutral Load side circuit-breaker 1P+N (230/240) ’ Value valid only for magnetic only supply side circuit-breaker 2 Value valid also with neutral 50%

| Value valid only for magnetic only supply side circuit-breaker > Value valid only for PR232/P, PR331/P and PR332/P trip units > Available only with lu < 1250A

Table valid for Emax circuit-breaker only with PR121/P, PR122/P and PR123/P releases ' Emax L circuit-breaker only with PR122/P and PR123/P releases 2 Available only with lu < 1250A

‘Value valid only with 11 (MCCB) <= Ith (MCS) 2 Only for T7 1000 and T7 1250

Notes for the correct reading of the coordination tables:

- MCCBs type Tmax XT * with these typologies of pole connection the possibility of a double fault to earth is considered unlikely.

The positive pole (+) can be inverted with the negative pole (-).

Network with one polarity connected to earth

4 Special applications Earthing shall be carried out on the supply side of the circuit-breaker.

* for the use of three-phase circuit-breakers please ask ABB “for the use of three-phase circuit-breakers (T4-T5-T6) please ask ABB - MCCBs type Tmax T

= Multiplier factor of 13 due to the induced magnetic fields

K,_, = Multiplier factor of I8 due to the induced magnetic fields

ABB | Protection and control devices 187

K,, = Multiplier factor of I3 due to the induced magnetic fields

K,, = Multiplier factor of 13 due to the induced magnetic fields

= Multiplier factor of I3 due to the induced magnetic fields

Table 8: Tmax performance T4N 80-250 A TMA 11 (400Hz) 13 setting (MI 13 setting (MIN=5xIn) K,, = Multiplier factor of 13 due to the induced magnetic fields 4 Special applications

I3 setting (MIN=5xiIn) = Multiplier factor of I3 due to the induced magnetic fields 4 Special applications

The following diagrams can be used (the polarity may be inverted). Networks insulated from earth Possible connection diagrams with reference to the type of distribution system in which they can be used follow. Connection diagrams

In the presence of an earth fault of positive or negative polarity, the poles involved in the fault work at U/2 (500 V); the following diagram must be used: Correction factors for tripping thresholds

With regard to overload protection, no correction factors need to be applied. However, for the magnetic threshold values in use with 1000 Vdc with the previously described applicable diagrams, refer to the corresponding values for alternating current, multiplied by the correction factors given in the following table:

Note: The breaking capacity Icu, by means of external protection relay, with 500 ms maximum timing, is equal to the value of Icw (1 (1) The performances at 750 V are: for E1B/E MS Icw = 25 kA, for E2N/E MS Icw = 40 kA and for E3H/E MS Icw = 50 KA. The following table shows the available versions and their relative electrica performance:

The following diagrams may be used (the polarity may be inverted).

Circuit-breakers with thermomagnetic release for alternating currents " Thermal threshold adjustable from 0.7 and 1 x In; fixed magnetic threshold ® Thermal threshold adjustable from 0.7 and 1 x In; magnetic threshold adjustable between 5 and 10 x In.

Circuit-breakers with electronic release for alternating currents

Air switch disconnectors (up to 1150 Vac)

The following scheme shows a plant having a safety auxiliary power supply:

The breaking capacities of the apparatus inside the switchboard shall be verified to be compatible with the short-circuit values of the plant.

e If the protective device on the supply side of the distribution system is not known The following condition must be fulfilled: From the Icw value of the distribution system it results:

Annex A: Protection against short-circuit effects inside low-voltage switchboards it is necessary to verify that, in the case of a short-circuit on any outgoing conductor, the limitations created by the circuit-breaker are compatible with the busbar system; to comply with this requirement, at the maximum allowable prospective short-circuit current, the circuit-breaker with higher cut-off current and let-through energy must have an adequate current limiting capability for the busbar system. In this case the circuit-breaker is tyoe SACE Tmax XT3S250 In250. The verification shall be carried out as in the previous paragraph:

% Short-circuit protective devices of the same manufacture but of a different series may be considered equivalen’ where the device manufacturer declares the performance characteristics to be the same or better in all relevan’ respects to the series used for verification, e.g. breaking capacity and limitation characteristics (l2t, Ipk), and critical distances. In compliance with the new Std. IEC 61439-1, the compliance of the assembly under short-circuit conditions can be proved in addition to laboratory tests (Icw) also by applying appropriate design-rules, which are pointed out in the following Table (Table 13 of the Std. IEC 61439-1).

Annex B: Temperature rise evaluation according to IEC 60890

The following diagram shows the procedure to evaluate the temperature rise.

Table 7: Enclosure constant k for enclosures without ventilation openings, with an effective cooling surface A, > 1.25 m?

Table 5: Factor d for enclosures without ventilation openings and with an effective cooling surface A, > 1.25 m?

where h is the height of the enclosure, and A, is the area of the base. For “Type of installation”:

Table 9: Enclosure constant k for enclosures with ventilation openings and an effective cooling surface A, > 1.25 m?

Table 10: Temperature distribution factor c for enclosures with ventilation openings and an effective cooling surface A, > 1.25 m?

where g is the ratio of the height and the width of the enclosure. Table 12: Temperature distribution factor c for enclosures without ventilation openings and with an effective cooling surface A, < 1.25 m’

Table 11: Enclosure constant k for enclosures without ventilation openings and with an effective cooling surface A, < 1.25 m?

The values indicated in the table refer to balanced loads, with a current flow equal to the In, and are valid for both circuit-breakers and switch-disconnectors, three-pole and four-pole versions. For the latter, the current of the neutral is nil by definition. Table 13: MCCB power losses

given in the Table 2. Here below the values for each section:

From the geometrical dimensions of the switchboard, the effective coolinc surface Ae is determined below: Making reference to the procedure described in the diagram at page 247, it is possible to evaluate the temperature rise inside the switchboard.

Thus, the total power loss inside the enclosure is: P = 784 [W] Here below the power losses for each connection:

ABB | Protection and control devices 257

Annex C: Application examples Advanced protection functions with PR123/P and PR333/P releases

Figure 1 shows a fault on the load side of an Emax circuit-breaker: the fault current flows through one phase only and, if the vectorial sum of the currents detected by the four current transformers (CTs) results to be higher than the set threshold, the electronic release activates function G (and the circuit- breaker trips). Example:

Solution with a release without “double S”

NOTE 1. The illustrations are not intended to depict actual product or installation practices but are indicative o the method described.

® For higher ambient temperatures, consult manufacturer. Table 4: Correction factor for ambient air temperature other than 30 °C

Cables in layers: a) spaced; b) not spaced; c) double layer

Bunched cables: a) in trunking; b) in conduit; c) on perforated tray

Table 6: Reduction factor for single-core cables with method ot installation F

NOTE 1 Values given are averages for the cable types and range of conductor sizes considered in Tables 8 to 9 (installation methods E, F and G). The spread of values is generally less than 5 %.

Table 8: Current carrying capacity of cables with PVC or EPR/XLP insulation (method A-B-C)

Table 8: Current carrying capacity of cables with PVC or EPR/XLP insulation (method E-F-G)

Insulation Table 10: Correction factors for ambient ground temperatures other than 20 °C

Table 11: Reduction factors for cables laid directly in the ground (installation method D2) NOTE 1 Values given apply to an installation depth of 0,7 m and a soil thermal resistivity of 2,6 K-m/W. They are average values for the range of cable sizes and types quoted for Tables 8-15-16. The process of averaging, together with rounding off, can result in some cases in errors up to +10 %. (Where more precise values are required they may be calculated by methods given in IEC 60287-2-1.)

NOTE 1 Values given apply to an installation depth of 0,7 m and a soil thermal resistivity of 2,6 Kim/W They are average values for the range of cable sizes and types quoted for Tables 8-15-16. The process of averaging, together with rounding off, can result in some cases in errors up to +10 %. (Where more precise values are required they may be calculated by methods given in IEC 60287-2-1.)

Table 13: Reduction factors for single-core cables laid in single way ducts in the ground (installation method D1)

Table 15: Current carrying capacity of cables buried in the ground (installation method D1)

Table 16: Current carrying capacity of cables buried in the groun (installation method D2)

Table 1: Resistance and reactance per unit of length of copper cables

Table 2: Resistance and reactance per unit of length of aluminium cables

Table 4: Specific voltage drop at cosy = 0.9 for copper cables

Table 3: Specific voltage drop at cosy = 1 for copper cables

Table 6: Specific voltage drop at cosg = 0.8 for copper cables

Table 8: Specific voltage drop at cosy = 1 for aluminium cables

Table 10: Specific voltage drop at cosy = 0.85 for aluminium cables

1 Protection of feeders Table 12: Specific voltage drop at cosy = 0.75 for aluminium cables

Table 1: Resistance values [Q/km] of single-core and multi-core cables in copper and aluminium at 80 °C

and this means that the cable is not fully exploited.

The formula (1) must be verified along the whole length of the cable. Due to the shape of the specific let-through energy curve of a circuit breaker, it is generally sufficient to verify formula (1) only for the maximum and minimum short-circuit current that may affect the cable. The maximum value is normally the value of the three-phase short-circuit current at the beginning of the line, while the minimum value is the value of the phase to neutral short-circuit current (phase to phase if the neutral conductor is not distributed) or phase to earth at the end of the cable. Table 2: Maximum withstood energy for cables k? S? [(kA)? s]

1 Protection of feeders After calculating the minimum short-circuit current, verify that

Multiply the length value obtained from the table by the correction factor k,:

Figure 1: Disconnection of the neutral conductor

Figure 2: Three-phase alternative power supply with a 4-pole switch

‘for TN-C systems, the Standards specify a minimum cross section of 10 mm? for copper and 16 mm‘? for aluminium conductors

Table 4: Values of k for protective conductors as a core incorporated in a cable or bunched with other cables or insulated conductors

Table 3: Values of k for insulated protective conductors not incorpo- rated in cables and not bunched with other cables

Table 2: Current carrying capacity |,, of copper BTS

*phase resistance at I,, Table 3: Current carrying capacity |,, of aluminium BTS

Table 5: Values of the withstood energy and peak current of aluminium BTS

Table 6: Current distribution factor 1 Protection of feeders

Table 1: Incandescent and halogen lamps ® For calculation see Annex B Calculation of load current |,

** also Emax CB type E1 can be used for this application

Starting with autotransformers is the most functional of the methods used for reduced voltage starting, but is also the most expensive. The reduction of the supply voltage is achieved by using a fixed tap autotransformer or a more expensive multi tap autotransformer. Applications can be found with squirrel-cage motors which generally have a power from 50 kW to several hundred kilowatts, and higher power double- cage motors. Starting with autotransformers

Tripping time in seconds (Tp) Trip Class

MA: magnetic only adjustable release MF: fixed magnetic only release

MA: magnetic only adjustable release MF: fixed magnetic only release Table 4: 400 V 50 kA DOL Normal Type 2 (Tmax XT/T - Contactor - TOR/EOL)

MA: magnetic only adjustable release MF: fixed magnetic only release Table 5: 400 V 70 kA DOL Normal Type 2 (Tmax XT/T - Contactor - TOR/EOL)

MA: magnetic only adjustable release MF: fixed magnetic only release Table 6: 400 V 80 kA DOL Normal Type 2 (Tmax XT/T - Contactor - TOR/EOL)

Table 11: 440 V 50 kA DOL Normal Type 2 (Tmax XT/T - Contactor - TOR/EOL) MA: magnetic only adjustable release MF: fixed magnetic only release

Table 12: 440 V 65 kA DOL Normal Type 2 (Tmax XT/T - Contactor - TOR/EOL) MA: magnetic only adjustable releass MF: fixed magnetic only release

(*) Connection kit not available. To use connection kit provide E800DU800 Table 15: 500 V 50 kA DOL Normal Type 2 (Tmax XT/T - Contactor - TOR/EOL)

Table 17: 690 V 25 kA DOL Normal Type 2 (Tmax XT - Contactor - TOR/EOL) MA: magnetic only adjustable release MF: fixed magnetic only release

(*) Type 1 coordination (**) Cable cross section equal to 4 mm2 (***) No mounting kit to contactor is availabl. Table 18: 690 V 50 kA DOL Normal Type 2 (Tmax T - Contactor - TOR/EOL)

Comments: (A) Provide by-pass contactor during motor start-up (‘) Set EOL tripping characteristic to class 30 usable also for 415V (*) Connection kit not available. To use connection kit provide E800DU806 Table 19: 400 V 35 kA DOL Heavy duty Type 2 (Tmax XT/T - Contactor - TOR/EOL)

Table 20: 400 V 50 kA DOL Heavy duty Type 2 (Tmax XT/T - Contactor - TOR/EOL) MA: magnetic only adjustable release MF: fixed magnetic only release

Table 27: 400 V 50 kA Y/A Normal Type 2 (Tmax XT/T - Contactor - TOR/EOL)

Table 26: 400 V 35 kA Y/A Normal Type 2 (Tmax XT/T - Contactor - TOR/EOL)

Table 29: 440 V 65 kA Y/A Normal Type 2 Tmax XT/T — Contactor - TOR/EOL)

Table 28: 440 V 50 kA Y/A Normal Type 2 (Tmax XT/T - Contactor - TOR/EOL)

Table 31: 690 V 25 kA Y/A Normal Type 2 (Tmax XT - Contactor - TOR/EOL) Table 30: 500 V 50 kA Y/A Normal Type 2 (Tmax XT/T - Contactor - TOR/EOL)

Comments: *size wire 4mm *“connect TOL at line-delta supply side

Table 32: 690 V 50 kA Y/A Normal Type 2 (Tmax T - Contactor - TOR/EOL) Comments: *size wire 4mm “connect TOL at line-delta supply side

Table 33: 400 V 35 kA DOL Normal and Heavy duty Type 2 (Tmax XT/T with Ekip M/PR222MP - Contactor)

Table 34: 400 V 50 kA DOL Normal and Heavy duty Type 2 (Tmax XT/T with Ekip M/PR222MP - Contactor)

Table 36: 690 V 25 kA DOL Normal and Heavy duty Type 2 (Tmax T with Ekip M - Contactor)

Table 37: 500 V 50 kA DOL Normal and Heavy duty Type 2 (Tmax XT/T with Ekip M/PR222MP - Contactor) vomments. * For Heavy start, select the class 30 on the EKIP M or MP release ** In event of normal start, choose AF300 *”* EKIP M-LIU also available in M-LRIU version

MVVITTTIET Ilo. * For Heavy start, select the class 30 on the MP release Table 38: 690 V 50 kA DOL Normal and Heavy duty Type 2 (Tmax T with PR222MP - Contactor)

Further to the above consideration, the follwing diagram shows the inrush current curve for a 20/0.4kV of 400kVA transformer. This transformer has an inrush current during the very first moments equal to about 8 times the rated current; this transient phenomenon stops after a few tenths of a second.

Table 1: Protection and switching of 230 V transformers

Circuit-breaker “B” (Feeder circuit-breaker)

* also Tmax series CBs equipped with electronic releases can be used for this applicatior “ Emax type E1 can be used for this application Table 2: Protection and switching of 400 V transformers

* also Tmax series CBs equipped with electronic releases can be used for this applicatiot ** Emax type E1 can be used for this application Table 3: Protection and switching of 440 V transformers

* also Tmax series CBs equipped with electronic releases can be used for this application “ Emax type E1 can be used for this application *“* For XTAV with Icu = 90kA at 690V, please ask ABB SACE Table 4: Protection and switching of 690 V transformers

Active power increase with equal dimensioning factors

for different values of the power factor before and after the correction.

In the case of direct connection (diagrams 1 and 2) there is a risk that after disconnection of the supply, the motor will continue to rotate (residual kinetic energy) and self-excite with the reactive energy supplied by the capacitor bank, acting as an asynchronous generator. In this case, the voltage is maintained on the load side of the switching and control device, with the risk of dangerous overvoltages of up to twice the rated voltage value. The usual connection diagrams are shown in the following figure:

Cast Resin Distribution Transformer MV-LV Example For a 630 kVA oil-distribution transformer with a load factor of 0.5, the necessary PFC power is 17 kvar.

* For plug-in version reduce the power of the capacitor bank by 10% Table 5: Coordination type 2 circuit breaker-contactor for the switching of capacitor banks at 400 V, 50 kA 3 Power factor correction

Table 6: Coordination type 2 circuit breaker-contactor for the switching of capacitor banks at 440 V, 50 kA Table 7: Coordination type 2 circuit breaker-contactor for the switching of capacitor banks at 500 V, 50 kA 3 Power factor correction Contactor * For plug-in version reduce the power of the capacitor bank by 10%

* For plug-in version reduce the power of the capacitor bank by 10% 3 Power factor correction

Three types of distribution system are considered:

These distribution systems are used for particular plants, where the continuity of supply is a fundamental requirement, where the absence of the supply can cause hazards to people or considerable economical losses, or where a low value of a first earth fault is required. In these cases, an insulation monitoring device shall be provided for optical or acoustic signalling of possible earth faults, or failure of the supplied equipment.

In TN systems, the fault current flows towards the power supply neutral point through a solid metallic connection, practically without involving the earth electrode (Figure 2).

Note 1: Overcurrent protective devices are not shown in this figure.

An earth fault in a TT system involves the circuit represented in Figure 1: 4 Protection of human beings

An earth fault in a TN system involves the circuit represented in Figure 2:

According to IEC 60364-4, the automatic disconnection of the circuit in case o the first earth fault is not necessary only if the following condition is fulfilled: As represented in Figure 3, the earth fault current in an IT system flows through the line conductor capacitance to the power supply neutral point. For this reason, the first earth fault is characterized by such an extremely low current value to prevent the overcurrent protections from disconnecting; the deriving touch voltage is very low. 4.6 IT System 4 Protection of human beings

Figure 1: Operating principle of the residual current device The operating principle of the residual current release is basically the detection of an earth fault current, by means of a toroid transformer which embraces all the live conductors, included the neutral if distributed.

Figure 3: Vertical discrimination between RCDs

4 Protection of human beings Case B: three-phase + neutral circuits in IT system with neutral distributed

Note for the use of the tables 4 Protection of human beings

WVMPEPOULIVEILIGUtUE IU VANS TI VGA! Vel Vildot.s UIG VANS VII TIGA protected length read in Table 2 (TN system) or Table 3 (IT system) shall be multiplied by the following factor: Correction factor for three-phase voltage different from 400 V: the value of

Correction factor for neutral distributed in IT systems (for Table 3 only): TIVE GIS VIVO SOEUEUVITS SIGLOU TT TANI Fe lo VAIS UI UNIS TGA DIO tected length shall be multiplied by the coefficient corresponding to the phase conductor cross section and to the ratio between the protective conductor (PE) and the phase cross sections: the value of the maximum protected length shall be multiplied by 0.58. Correction factor for protective conductor cross section S,,. different from the cross sections stated in Table 1: the value of the maximum pro-

WYPTPOCUUUILIGULUE PUT QIU VaMItS. UI VAIUS UI UI TIAA IU POLO UlLou length read in Table 2 (TN system) or Table 3 (IT system) shall be multiplied by the following factor:

As regards the exposed conductive parts upstream the parallel point A-B, such as for instance the exposed conductive part of the transformer or of the inverter when the transformer is incorporated, a residual current device® shall be interposed as Figure 5 shows; this residual current device detects the leakage currents coming both from the network as well as from the PV generator. When the residual current device trips due to an earth fault current, the inverter goes in stand by due to lack of network voltage.

5.4.2 Plants without transformer and the network, the PV installation itself shall be insulated from earth in its 5 Photovoltaic plants

Short-circuit power S,__, [MVA] knet Net voltage U, [kV] Generator

Three-phase short-circuit Two-phase short-circuit

The circuit-breakers CB1(CB2) to select, with reference to the rated current of the transformers, are Emax E3N 2500.

Procedure In the row corresponding to the cable cross section 120 mm‘, it is possible to find the column for a length equal to 29 m or right below (in this case 24). In the column of upstream short-circuit current it is possible to identify the row with a value of 32 kA or right above (in this case 35). From the intersection of this last row with the previously identified column, the value of the downstream short-circuit current can be read as being equal to 26 kA. Example Note:

Therefore, it is possible to state that a real three-phase network may be replaced by three single-phase networks related to the three positive, negative and zero sequences, by substituting each component with the corresponding equivalent circuit. If generators can be considered symmetrical as it occurs in plant practice, by considering as a positive sequence set the one they generate, the three single-phase networks are defined by the following circuits and equations:

These relationships applied to the three sequence circuits of Figure 3 allow the definition of the sequence network equivalent to the three-phase network under study and representing the initial fault condition. This network may be represented as follows:

By using the given fault conditions and the formula 1), it follows that: Without going into the details of a theoretical treatment, it is possible to show the procedure to semplify and resolve the electrical network under a pre-estabilished fault condition thruogh an example.

In the following example, a network with a MV/LV transformer with delta primary winding and secondary winding with grounded star point is taken into considera- tion and a line-to-line fault is assumed downstream the cable distribution line.

Table 2: Correction factors for load current with cos other than 0.9 Annex A: Calculation of load current I,

Table 4: Correction factors for load current with cosq other than 1 Annex A: Calculation of load current I,

Harmonics are generated by nonlinear loads. When we apply a sinusoidal voltage to a load of this type, we shall obtain a current with non-sinusoidal waveform. The diagram of Figure 2 illustrates an example of nonsinusoidal current waveform due to a nonlinear load:

phases are equally loaded, the current in the neutral is zero. The presence of

Table 1 shows the values of the parameters described above.

2 This value shall be used for bare cables exposed to touch. Table 2: Values of k for phase conductor

Material of conductor Table 3: Values of k for insulated protective conductors not incorpo- rated in cables and not bunched with other cables

Table 5: Values of k for protective conductors as a core incorporated in a cable or bunched with other cables or insulated conductors

* Temperature limits for various types of insulation are given in IEC 60724. Table 4: Values of k for bare protective conductors in contact with cable covering but not bunched with other cables Annex C: Calculation of the coefficient k for the cables (k?S?)

Metric Prefixes for Multiples and Sub-multiples of Units

The International System of Units (SI) SI Base Units

Resistivity values, conductivity and temperature coefficient at 20 °C of the main electrical materials

Annex D: Main physical quantities and electrotechnical formulas

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