3 Phase Short Circuit

Short Circuit Analysis Program ANSI/IEC/IEEE & Protective Device Evaluation User’s Guide

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Version 6.60.00

May 2011

Short Circuit Analysis Program ANSI/IEC/IEEE

Table of Contents 1

Unique Features of Paladin DesignBase Short circuit Program ........................................................ 1 1.1 WHAT’S NEW IN THIS RELEASE ......................................................................................................... 1

2

Introduction.............................................................................................................................................. 2 2.1 2.2 2.3 2.4 2.4. 2.4.2 2.5 2.5.1 2.5.2

3

ANSI/IEEE Standard Based Device Evaluation (PDE IEEE) .............................................................. 30 3.1 3.2 3.3

4

Standard Ratings for HV and MV Circuit Breakers (CB)............................................................ 30 Standard Ratings for Low Voltage Circuit Breakers (LV-CBs) ................................................... 34 Standard Ratings for Low/High Voltage Fuses, and Switches................................................... 36

IEC Standard Based Device Evaluation (PDE IEC) ............................................................................ 40 4.1 4.1.1 4.1.2 4.2 4.2.1 4.2.2

4.2.3 5

Type of Faults ............................................................................................................................... 2 Terminology .................................................................................................................................. 3 Sources in Fault Analysis ............................................................................................................. 6 ANSI/IEEE Standard .................................................................................................................... 7 1Multiplying Factors (MF) ............................................................................................................. 7 Local and Remote Contributions .................................................................................................. 8 IEC 60909 .................................................................................................................................. 10 System Parameters .................................................................................................................... 10 Short Circuit Current Calculus .................................................................................................... 24

CIRCUIT-BREAKERS ................................................................................................................ 40 Rated characteristics to be given for all circuit-breakers ........................................................... 40 Circuit Breaker Name Plate Data ............................................................................................... 47 FUSES........................................................................................................................................ 48 General considerations .............................................................................................................. 48 Fuse IEC Characteristic Quantities [IEC 60269-1] ..................................................................... 49 Breaking range ........................................................................................................................... 49 Cut-off current ............................................................................................................................ 49 Cut-off current characteristic; let-through current characteristic ................................................ 49 Peak withstand current ............................................................................................................... 49 Pre-arcing time; melting time ..................................................................................................... 50 Arcing time of a fuse................................................................................................................... 50 Operating time; total clearing time ............................................................................................. 50 I2t (Joule integral) ....................................................................................................................... 50 Fuse nameplate data.................................................................................................................. 51

Protective Device Evaluation Based on IEC Standard ...................................................................... 52 5.1 5.2 5.3

Fuses Evaluation ........................................................................................................................ 55 LVCB Evaluation ........................................................................................................................ 55 HVCB Evaluation........................................................................................................................ 56

6. DesignBase Short Circuit Calculation Method ........................................................................................ 57 A. B. C. D. E. F.

Calculation Methods and the Corresponding Tools............................................................. 57 AC ANSI/IEEE Standard – Paladin DesignBase Short Circuit Tools: ................................. 58 AC Classical Short Circuit Method....................................................................................... 72 AC IEC 60909 Short Circuit Method .................................................................................... 73 AC IEC 61363 Short Circuit Method .................................................................................... 82 AC Single Phase Short Circuit Method ................................................................................ 94

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Short Circuit Analysis Program ANSI/IEC/IEEE 7.

Managing the Paladin DesignBase Short Circuit Program ............................................................... 94

7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 8

Network Reduction/Equivalent .......................................................................................................... 176 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9

9

A. 3P, LL, LG, LLG Fault, ½ Cycle ........................................................................................... 94 B. 3P, LL, LG, LLG Fault, 5 Cycle ............................................................................................ 97 C. 3P, LL, LG, LLG Fault, Steady state .................................................................................... 99 D. 3 Phase Fault, Steady State .............................................................................................. 101 E. Protective Device Evaluation (PDE) Tool Based on ANSI/IEEE Standard ....................... 103 F. Protective Device Evaluation (PDE) Based on IEC Standard ........................................... 112 G. Report Manager – ANSI/IEEE ........................................................................................... 127 H. Short Circuit Back Annotation ............................................................................................ 142 I. Managing Schedule in Short Circuit .................................................................................. 145 J. Managing Utility / PCC Short Circuit contribution .............................................................. 159 K. Managing MOTOR CONTRIBUTION ................................................................................ 160 L. Managing UPS bypass function during a fault downstream UPS source ......................... 161 M. Three-phase Faults IEC 61363 Method ............................................................................ 163 N. Short Circuit Analysis Input Data ....................................................................................... 167 Power Grid Input Data .............................................................................................................. 167 Synchronous Generator Short Circuit Input Data .................................................................... 168 Induction Motor Short Circuit Input Data .................................................................................. 169 Synchronous Motor Short Circuit Input Data............................................................................ 170 High Voltage ANSI/IEEE Circuit Breaker Short Circuit Input Data .......................................... 171 Low Voltage ANSI/IEEE Circuit Breaker Short Circuit Input Data ........................................... 172 Low Voltage IEC Circuit Breaker Short Circuit Input Data ....................................................... 173 Low Voltage ANSI/IEEE Fuse Short Circuit Input Data ........................................................... 174 Medium / Low Voltage IEC Fuse Short Circuit Input Data ....................................................... 175 Introduction ............................................................................................................................... 176 Sample System Data................................................................................................................ 176 How to Perform Equivalent/Reduction Calculations ................................................................ 177 Separating the Equivalent Part from the Rest of the System................................................... 178 Specifying the Buses for the Equivalent................................................................................... 179 Reporting of the Equivalent System ......................................................................................... 180 Computation of Equivalent System and Inspection of the Result ............................................ 183 Reconstructing the Original System by Using the Equivalent .................................................. 185 Validation and Verification of the Equivalent ............................................................................ 192

TUTORIAL: Conducting a Three-phase Short Circuit Study .......................................................... 195 9.1 9.2 9.2.1 9.3 9.3.1 9.3.2 9.3.3 9.3.4 9.4 9.4.1 9.4.2 9.4.3 9.4.4

The Calculation Tools............................................................................................................... 196 Graphical Selection of Faulted Bus (Annotation) ..................................................................... 197 AC-ANSI/IEEE Method............................................................................................................. 197 Short Circuit Annotation Tool ................................................................................................... 199 3-Phase Fault, 30 Cycles at Bus 18 ......................................................................................... 200 3-Phase Fault Current, ½ Cycle Fault at Bus “MAINBUS”:...................................................... 202 3-Phase Fault Current, 5 Cycle Fault at Bus “MAINBUS”:....................................................... 204 Change the “Fault Type” displayed onto the drawing. ............................................................. 207 Professional Report .................................................................................................................. 215 All types of Faults at bus MAINBUS, 0.5 Cycle Symmetrical:.................................................. 215 All types of Faults at All buses, 0.5 Cycle Symmetrical: .......................................................... 219 All types of Faults at All buses, 5 Cycle Symmetrical: ............................................................. 222 All types of Faults at All buses, 30 Cycle Symmetrical: ........................................................... 224

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Short Circuit Analysis Program ANSI/IEC/IEEE List of Figures Figure 1: Device Evaluation, ANSI Standard, Page 1 .................................................................................... 38 Figure 2: Device Evaluation, ANSI Standard, Page 2 .................................................................................... 39 Figure 3: Percentage D.C. current component in relation to the time interval from initiation of short-circuit current, for different time constantτ. ............................................................................ 44 Figure 4: PDE Flow Chart - IEC standard: ..................................................................................................... 52 Figure 5: Unbalanced system ......................................................................................................................... 66

List of Tables Table 1: Table 2: Table 3: Table 4: Table 5: Table 6: Table 7: Table 8: Table 9: Table 10: Table 11: Note:

‰

Recommended ANSI Source Impedance Multipliers for 1st Cycle and Interrupting Times ............. 6 30 cycles calculation impedance....................................................................................................... 7 Resistivity and equivalent earth penetration ................................................................................... 22 IEC voltage factor ............................................................................................................................ 23 CB rated interrupting time in cycles ................................................................................................ 30 K factor ............................................................................................................................................ 33 Default Device X/R Values Using EDSA’s Library .......................................................................... 34 n factor based on PF and short circuit level .................................................................................... 42 Icu and k factor ................................................................................................................................ 46 CB Name plate data ........................................................................................................................ 48 IEC c factor...................................................................................................................................... 81

You can view this manual on your CD as an Adobe Acrobat PDF file. The file name is: Short Circuit Analysis Program

3_Phase_Short_Circuit.pdf

You will find the Test/Job files used in this tutorial in the following location: ‰

C:\DesignBase\Samples\3PhaseSC

Test Files: ANSIYY1, IEC-YY; Busfault, EDM5, IEC1-60909, IEC2-60909, IEEE399, IEEEpde, MutualNet, SlidingFault, T123, T123PDE, testma1, Trib, TribNVTAP, UPSexpse, West

©Copyright 2011 Power Analytics Corporation All Rights Reserved

iii

Short Circuit Analysis Program ANSI/IEC/IEEE

1

Unique Features of Paladin DesignBase Short Circuit Program The salient features of the Paladin DesignBase advanced short circuit program:

9 9 9 9 9 9 9 9 9 9 9 9 9 9 1.1

Fault analysis of complex power systems having over 50,000 buses Exact short circuit current and contributions computation using Three-Sequence Modeling Simulate sliding and open conductor faults High speed simulation by utilizing the state-of-the-art techniques in matrix operations (sparse matrix and vector methods) Automated reactor sizing for 3 Phase networks Exporting and importing data from and to Excel Import system data from Siemens/PTI format into Paladin DesignBase Customize reports Professional Reports UPS source bypass Support of ANSI and IEC standards for PDC (protective device coordination) Support of ANSI and IEC standards for PDE (protective device evaluation) Fully integrated with ARC flash program Fully integrated with PDC

What’s new in this release

9 9 9 9

New PDE based on IEC Standards New professional report tool based on Crystal Reports New functions for UPS bypass and motors fed from VFD Minimum and maximum utility fault contribution

1

Short Circuit Analysis Program ANSI/IEC/IEEE

2

INTRODUCTION The short circuit is an accidental electrical contact between two or more conductors. The protective devices such as circuit breakers and fuses are applied to isolate faults and to minimize damage and disruption to the plant’s operation.

2.1

Type of Faults Types of Faults depend on the power system grounding method. The most common faults are: • • • •

Three-Phase Fault, with or without ground (3P, or 3P-G) Single line to ground Fault (L-G) Line to Line Fault (L-L) Line to line to ground Fault (L-L-G)

Estimated frequency of occurrence of different kinds of fault in power system is: 3P or 3P-G: L-L: L-L-G: L-G:

8% 12 % 10 % 70 %

Severity of fault: Normally the three-phase symmetrical short circuit (3P) can be regarded as the most severe condition. There are cases that can lead to single phase fault currents exceeding the three-phase fault currents; however, the total energy is less than a three-phase fault. Such cases include faults that are close to the following types of equipment: • • • •

The Wye side of a solidly grounded delta-wy transformer / auto-transformer The Wye-Wye solidly grounded side of a three winding transformer with a delta tertiary winding A synchronous generator solidly connected to ground The Wye side of several Wye grounded transformers running in parallel

2

Short Circuit Analysis Program ANSI/IEC/IEEE

Type of Short Circuits: a):3P – three-phase; b):L-L, line-to-line; c):L-L-G, line-to-line-to-ground; and d): L-G, line-to-ground

2.2

Terminology Arcing Time - the interval of time between the instant of the first initiation of the arc in the protective device and the instant of final arc extinction in all phases. Available Short Circuit Current - the maximum short circuit current that the power system could deliver at a given circuit point assuming negligible short circuit fault impedance. Breaking Current - the current in a pole of a switching device at the instant of arc initiation (pole separation). It is also known as “Interrupting Current” in ANSI Standards. Close and Latch Duty - the maximum rms value of calculated short circuit current for medium and high-voltage circuit breakers, during the first cycle, with any applicable multipliers with regard to fault current X/R ratio. Often, the close and latching duty calculation is simplified by applying a 1.6 factor to the first cycle symmetrical AC rms short circuit current. Close and latch duty is also called “first cycle duty,” and was formerly called momentary duty. Close and Latch Capability - the maximum asymmetrical current capability of a medium or highvoltage circuit breaker to close, and immediately thereafter latch closed, for normal frequency making current. The close and latch asymmetrical rms current capability is 1.6 times the circuit breaker rated maximum symmetrical AC rms interrupting current. Often called “first cycle capability.” The rms asymmetrical rating was formerly called momentary rating. Contact Parting Time - the interval between the beginning of a specified over current and the instant when the primary arcing contacts have just begun to part in all poles. It is the sum of the relay or release delay and opening time. Crest Current / Peak Current – the highest instantaneous current during a period. Fault – an abnormal connection, including the arc, of relative low impedance, whether made accidentally or intentionally, between two points of different voltage potentials.

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Short Circuit Analysis Program ANSI/IEC/IEEE

Fault Point X/R – the calculated fault point reactance to resistance ratio (X/R). Depending on the Standard, different calculation procedures are used to determine this ratio. First Cycle Duty – the maximum value of calculated peak or rms asymmetrical current or symmetrical short circuit current for the first cycle with any applicable multipliers for fault current X/R ratio. First Cycle Rating – the maximum specified rms asymmetrical or symmetrical peak current capability of a piece of equipment during the first cycle of a fault. Interrupting Current – the current in a pole of a switching device at the instant of arc initiation. Sometimes referred to as “Breaking Current”, I b , IEC60909. Making Current – the current in a pole of a switching device at the instant the device closes and latches into a fault. Momentary Current Rating – the maximum available first cycle rms asymmetrical current which the device or assembly is required to withstand. It was used on medium and high-voltage circuit breakers manufactured before 1965; present terminology: “Close and Latch Capability”. Offset Current - an AC current waveform whose baseline is offset from the AC symmetrical current zero axis. Peak Current – the maximum possible instantaneous value of a short circuit current during a period. Short circuit current is the current that flows at the short circuit location during the short circuit period time. Symmetrical short circuit current is the power frequency component of the short circuit current. Branch short circuit currents are the parts of the short circuit current in the various branches of the power network. Initial short circuit current IK" is the rms value of the symmetrical short circuit current at the instant of occurrence of the short circuit, IEC 60909. Maximum asymmetrical short circuit current Is is the highest instantaneous rms value of the short circuit current following the occurrence of the short circuit. Symmetrical breaking current Ia , on the opening of a mechanical switching device under short circuit conditions, is the rms value of the symmetrical short circuit current flowing through the switching device at the instant of the first contact separation. Rated voltage VR the phase-to-phase voltage, according to which the power system is designated; IEC UR the rated voltage is the maximum phase-to-phase voltage. Nominal Voltage UN – (IEC) the nominal operating voltage of the bus. Initial symmetrical short - circuit power S K " is the product of

4

3 *I K "*U N

Short Circuit Analysis Program ANSI/IEC/IEEE

System breaking power S B is the product of

3 *I a * U N

Minimum time delay t min is the shortest possible time interval between the occurrence of the short circuit and the first contact separation of one pole of the switching device. Dynamic stress is the effect of electromechanical forces during the short circuit conditions. Thermal stress is the effect of electrical heating during the short circuit conditions. Direct earthing / effective earthing is the direct earthing of the neutral points of the power transformers. Short circuit earth current is the short circuit current, or part of it, that flows back to the system through the earth. Equivalent generator is a generator that can be considered as equivalent to a number of generators feeding into a given system. DesignBase Short Circuit Analysis Program is based on ANSI/IEEE and IEC Standards and fully complies with the latest ANSI/IEEE/IEC Standards: • • • • • • • • • • • • • • •

ANSI/IEEE Std. 141 – 1993, IEEE Recommended Practice for Electric Power Distribution of Industrial Plants (IEEE Red Book) ANSI/IEEE Std. 399 – 1997, IEEE Recommended Practice for Power Systems Analysis (IEEE Brown Book) ANSI/IEEE Standard C37.010 – 1979, IEEE Application Guide for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis ANSI/IEEE Standard C37.5-1979, IEEE Application Guide for AC High-Voltage Circuit Breakers Rated on a Total Current Basis ANSI/IEEE Standard C37.13-1990, IEEE Standard for Low-Voltage AC Power Circuit Breakers Used in Enclosures IEC-909 – 1988, International Electro technical Commission, Short Circuit Current Calculation in Three-Phase Ac Systems UL 489_9 – 1996, Standard for Safety for Molded-Case Circuit Breaker, Molded-Case Switches, and Circuit-Breaker Enclosures “A Practical Guide to Short-Circuit Calculations”, by Conrad St. Pierre IEC 60909-0/2001-07, Short-circuit currents in three-phase AC systems, Part 0: Calculation of currents IEC 60909-3/2003, Short-circuit currents in three-phase AC systems, Part 3: Currents during two separate simultaneous line-to-earth short-circuits and partial short-circuit currents flowing through earth IEC 60947-1:2000-10, Low-voltage switchgear and controlgear – Part 1: General rules IEC 60947-2:2003, Low-voltage switchgear and controlgear – Part 2: Circuit breakers EN 60947-3:1999, Low-voltage switchgear and controlgear – Part 3: Switches, disconnectors, switch-disconnectors and fuse-combination units BS EN 62271-100:2001, High-voltage switchgear and controlgear – Part 100: High-voltage alternating-current circuit-breakers IEC 62271-111:2005-11, High-voltage switchgear and controlgear – Part 111: Overhead, padmounted, dry vault and submersible automatic circuit reclosers and fault interrupters for alternating current systems up to 38 kV

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Short Circuit Analysis Program ANSI/IEC/IEEE

2.3

Sources in Fault Analysis Power utilities, all rotating electric machinery and regenerative drives are sources in fault calculation.

½ Cycle Network Duty The decay of short circuit current is due to the decay of stored magnetic energy in the equipment. The main impedances for the first ½ cycle is the sub-transient impedance. It is generally used for the first ½ cycles up to a few cycles; The ½ cycle network is also referred to as the sub transient network, because all rotating machines are represented by their sub transient reactance. ½ cycle short circuit currents are used to evaluate the interrupting duties for low-voltage power breakers, low voltage molded-case breakers, high and low voltage fuses and withstand currents for switches and high-voltage breakers. The following table shows the type of device and its associated duties using the ½ cycle network. Type of Device

Duty

High voltage circuit breaker Low voltage circuit breaker Fuse Switchgear and MCC Relay

Closing and latching capability Interrupting capability Bus bracing Instantaneous settings

Table 1: Recommended ANSI Source Impedance Multipliers for 1st Cycle and Interrupting Times Source Type

1/2-Cycle Calculations

Interrupting Time calculations

Reference

Remote Utility (equivalent)

Z

Local Generator

Z

Synchronous Motor

Z dv"

1.5* Z dv

ANSI C37.010

Large Induction Motors: >1000 HP or 250 HP and 2 poles

Z"

1.5* Z

ANSI C37.010

(1.5 to 4 cycles cpt) " s

Zs

ANSI C37.010

" dv

Z

ANSI C37.010

" dv "

Medium Induction Motors 50 to 249 HP or 250 to 1000 HP 1 kV and SrG ≥ 100 MVA; X" RGf = 0.07 d for generators with UrG > 1 kV and SrG < 100 MVA; X" RGf = 0.15 d for generators with UrG ≤ 1 kV. RGf = 0.05

In addition to the decay of the DC component, the factors 0.05, 0.07, and 0.15 also take into account the decay of the AC component of the short-circuit current during the first half-cycle after the short circuit took place. The influence of various winding-temperatures on RGf is not considered. The values RGf cannot be used when calculating the aperiodic component iDC of the shortcircuit current. When the effective resistance of the stator of synchronous machines lies much below the given values for RGf, the manufacturer’s values for RG should be used. The subtransient impedance Z G of the generator, in the positive-sequence system can be calculated with the formula:

Z G = RG + jX d" ,

22

When calculating initial symmetrical short-circuit currents in systems fed directly from generators without transformers unit, the corrected impedance positive-sequence system:

(

Z GK of the SG has to be used in the

)

Z GK = K G Z G = (K G RG ) + j K G X d" , 23 with the correction factor KG for SG, given by the relationship:

KG =

cmax ⋅ U n , 1 + x ⋅ sin ϕ rG U rG

(

)

" d

24

where: cmax is the voltage factor according to table 2.2. UN - the nominal voltage of the system.

x"d - the relative subtransient reactance of the generator related to the rated impedance, according to the (21) relationship. ϕrG is the phase angle between U rG and I rG . UrG - the rated voltage of the generator.

14

Short Circuit Analysis Program ANSI/IEC/IEEE The correction factor KG (equation 24) for the calculation of the corrected subtransient

(cU

/ 3

)

n impedance Z GK has been introduced because the equivalent voltage source is used instead of the subtransient voltage E″ behind the subtransient reactance of the synchronous generator.

If the terminal voltage of the generator is different from UrG, it may be necessary to introduce:

U G = U rG (1 + pG ) ,

If the values of

X ( 2 )G

25

" X d" and X q reactances are different, for the negative-sequence reactance

of the SM, their arithmetical mean can be used:

X ( 2)G =

X d" + X q" 2

,

The corrected short-circuit impedance of SG, by the following equation:

26

Z ( 2 )GK

, is given, in the negative-sequence system,

Z ( 2) GK = (K G RG ) + j (K G X ( 2) G ) ,

For the short-circuit impedance with KG from equation (1.20):

Z ( 0 )G

27

of SG in the zero-sequence system, the following applies

Z ( 0 )GK = (K G R( 0 )G ) + jX ( 0 )G ,

28

When an impedance is present between the star-point of the generator and earth, the correction factor KG will not be applied to this impedance.

I"

When calculating the initial symmetrical short-circuit current k , the peak short-circuit current ip, the symmetrical short-circuit breaking current Ib, and the steady-state short-circuit current Ik, synchronous compensators are treated in the same way as SG. If synchronous motors have a voltage regulation, they are treated like synchronous generators. If not, they are subject to additional considerations.

Asynchronous Motors (AM) The rated apparent power of an AM can be calculated from the equation:

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Short Circuit Analysis Program ANSI/IEC/IEEE

S rM =

η rM

PrM , ⋅ cos ϕ rM

29

where PrM, cosϕrM and ηrM are respectively the active rated power, rated power factor and rated efficiency of the motor, in accordance with its nameplate data. The rated current of the AM is given by the relationship:

I rM =

3 ⋅ U rM

PrM , ⋅ η rM ⋅ cos ϕ rM

30

where UrM is the rated line voltage of the AM.

I"

MV and LV motors contribute to the initial symmetrical short-circuit current k , to the peak shortcircuit current ip, to the symmetrical short-circuit breaking current Ib and, for unbalanced short circuits, also to the steady-state short-circuit current Ik. MV motors have to be considered in the calculation of maximum short-circuit current. LV motors are to be taken into account in auxiliaries of power stations and in industrial and similar installations, for example in networks of chemical and steel industries and pump stations.

I k" may be I" neglected if their contribution is not higher than 5 % of the initial short-circuit current k 0 M ,

The contribution of AM in LV power supply systems to the short-circuit current calculated without motors: " ∑ I rM ≤ 0.05 ⋅ I k 0 M ,

31

Where:

∑ I rM

is the sum of the rated currents of motors connected directly (without transformers) to the network where the short-circuit occurs;

I k" 0 M - the initial symmetrical short-circuit current without influence of motors. In the calculation of short-circuit currents, those MV and LV motors may be neglected, providing that, according to the circuit diagram (interlocking) or to the process (reversible drives), they are not switched in at the same time. The impedance module ZM of AM in the positive- and negative-sequence systems can be determined by:

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Short Circuit Analysis Program ANSI/IEC/IEEE

Z rM =

S rM

2 U rM , ⋅ (I LR / I rM )

32

Where: UrM is the rated voltage of the motor; SrM - the rated apparent power of the motor (see relationship (1.25)); (ILR/IrM) - the ratio of the locked-rotor current to the rated current of the motor. The following relations may be used with sufficient accuracy in order to calculate AM parameters: RM/XM=0.10, with XM=0.995⋅ZM for MV motors with rated powers per pair of poles (PrM/p)≥1 MW; RM/XM=0.15, with XM=0.989⋅ZM for MV motors with rated powers per pair of poles (PrM/p)104 (3÷10)⋅103 (1÷3)⋅103 (0.2÷1.2)⋅103 70÷200 50÷100

Clay, loam Marshy soil

Equivalent earth penetration depth δ, m f=50 Hz f=60 Hz >9,300 >8,500 (5.1÷9.3)⋅103 (4.65÷8.2)⋅103 (2.94÷5.1)⋅103 (2.69÷4.65)⋅103 (1.32÷3.22)⋅103 (1.2÷2.94)⋅103 3 (0.78÷1.32)⋅10 (0.71÷1.2)⋅103 660÷930 600÷850 295÷660