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Practical Power Distribution

Welcome !

Jerry Walker (IDC Technologies)

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IDC Profile

h International training & consulting company

hOperations in United States, Canada, United Kingdom, Southern Africa, Australia, New Zealand, Singapore, Malaysia.

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Workshop Contents

h Practical Short Circuit Calculations

h Power Cables

h Earthing

hDiscussions

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Medium Voltage Switchgear

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Distribution Equipment Ratingsh Rated Current

h Rated voltage

h Rated insulation level

h Rated short time withstand current

h Rated peak withstand current

h Symmetrical and asymmetrical rating

h Rated supply voltage of closing or opening devices

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Ratings (cont)

h Fault current: Types of electrical faults

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Fault current

hAssumptions for simple fault current calculations:

hIgnore cable between switchgear and fault

hCable between transformer and fault?

hIgnore arc resistance

hIgnore complex algebra

hIgnore source impedance

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Example 1 (no cable impedance)

Transformer Ratings: Vp = 132 kV

Vs = 11 kV

S = 20 MVA

Z = 10 %

Source

3-ph fault

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Example 1 (cont)

Quick formula:

I fault = S/ (√3 x Z% x Vs)

= 20 x 106 / (1.732 x 0.1 x 11 x 103)

= 10 497 A

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Example 2 (with cable impedance)

Cable impedance: 0.1548 Ω/km

Four cables in parallel over 1 km

Ztrf = 10% (0.1p.u)

Zc = 0.0387 Ω

= 0.0387x20x106 / (11x103)2

= 0.0064p.u.

Ztot = 0.1064p.u.

I fault = 20x106 / (√3x0.1064x11x103)

= 9 866 A

Source

3-ph fault

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Effects of fault current

h Thermal energy

hI2t

h Electromechanical stress

hPhysical construction

h Ratings: Withstand, breaking, making

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The effect of “DC Offset”

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DC Offset

h Peak determined by Power Factor

h Rate of decay determined by L/R

hcomplex calculation

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Computer Software

h Load Flow Studies

h Fault Calculations

h Protection Settings

h Stability Studies (Transient)

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Power Cables

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Cables mainly used for power distribution purposes

h PILC

h XLPE

hAlso:

hPVC

hElastomeric

hOverhead lines

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Typical cable construction

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Cable selection criteria

hApplication

h Load

hVoltage Drop

h Fault current

h Protection

h Installation

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Low Voltage Distribution

h PVC vs XLPE

h PVC normally less expensive

h XLPE higher current carrying capacity

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Application - Medium Voltage

hOverhead vs below ground

hOverhead lines:

hLess expensive initially

hEnvironmental & safety hazard

hMaintenance intensive

hLightning

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Application - MV (cont)

h Copper vs Aluminium

h Copper cables smaller/lighter

h Copper more expensive

h Corrosion

hAvailability

h Standardisation

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Application - MV (cont)

h PILC vs XLPE

h PILC more expensive

h XLPE higher current density

h XLPE larger due to insulation

h PILC higher chemical resistance

h PILC longer life span

h XLPE can be moved frequently

h Standardisation

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Application - MV (cont)

h Single core vs Three-core

hHigh current applications

h Installation for 3-phase

hTrefoil formation

hAvoid metal gland plates or break path

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Load to be supplied

hObtain or calculate FLC

h Size according to manufacturer’s tables

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Voltage Drop

hAllow for starting current

h Calculate Voltage Drop using manufacturer’s data (two methods)

h Continue Exercise to calculate volt drop over cable

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Fault Current

h Calculate fault current

h Rate according to manufacturer’s tables

hAdjust according to fault clearing time, if necessary

hUse formula for SC-rating if required

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Distribution Example

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Withstanding short circuit currents

h Short circuit current usually flows for a very brief period

h Stopped by the operation of protection system

h Less than a quarter cycle for an HRC cartridge fuse/current limiting breaker

hA few cycles or a relay-breaker combination

h Initial temperature assumed as maximum permissible conductor temperature

h Final temperature should be lower than the value at which cable insulation will suffer permanent damage

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Energy dissipation in cable

h E is the energy in watt seconds

h I is the fault current in amperes (known by short circuit calculation)

h R is the resistance of a unit length of the cable in Ohms

h T is the time for fault being cleared in seconds

TRIE ⋅⋅= 2

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Calculation for withstand

h I is the fault current in amperes (known by short circuit calculation)

h T is the time for fault being cleared in seconds

h S is the cross section of the cable conductor in sq. mm

h Value of K is as per the next slide (* for sizes of 300 sq.mm and greater)

222 SKTI ⋅=⋅

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K factor as per BS 7671Conductor Material Insulation Initial temperature

Deg CLimiting Final Temp. Deg

CK

Copper 70 Deg C Thermoplastic Gen. Purpose PVC

70 160/140* 115/103*

Copper 90 Deg C thermoplastic (PVC) 90 160/140* 100/86*

Copper 60 Deg C thermosetting (Rubber) 60 200 141

Copper 85 Deg C thermosetting (Rubber) 85 220 134

Copper 90 Deg C thermosetting (XLPE) 90 250 143

Copper Impregnated paper 80 160 108

Copper Mineral-Plastic covered or exposed to touch

70 (sheath) 160 115

Copper Bare and not exposed to touch and not in contact with combustible materials

105 (sheath) 250 135

Aluminium 70 Deg C Thermoplastic Gen. Purpose PVC

70 160/140* 76/68*

Aluminium 90 Deg C thermoplastic (PVC) 90 160/140* 66/57*

Aluminium 60 Deg C thermosetting (Rubber) 60 200 93

Aluminium 85 Deg C thermosetting (Rubber) 85 220 89

Aluminium 90 Deg C thermosetting (XLPE) 90 250 94

Aluminium Impregnated paper 80 160 71

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Practical Method

h I12 x t1 = I2

2 x t2h I1 = Manufacturers published short circuit current.

h t1 = Manufacturers published allowable time.

h I2 = Calculated Fault Current (3-ph / 1-ph)

h t2 = Protection trip time (primary or back-up?)

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Protection plays an important role

h Cable sizing is influenced by the type protection and tripping device used

hHRC fuses allow a very small let through energy

h Cables can be more economically sized in certain cases where fuse protection is employed

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I2.T ‘let through’

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Effect of prolonged short circuit currents

h Failure of insulation due to overheating

h Relative movement between core and sheath due to electrodynamic force (paper insulation)

h Formation of voids and subsequent failure of insulation (solid dielectric cables)

hMechanical integrity loss and disintegration of insulation

h Ignition

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Final cable size selection

hHighest of the sizes required for

hThermal withstand of normal load current (derated ampacity)

hLimiting voltage drop (normal/peak current) to permissible values

hShort circuit withstand for the time required for back up protection

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Environmental Conditions

hUnfriendly conditions may determine cable sheath, insulation, armouring, conductor material

h Bedding material

hDerating Factors

h Intermittent operation

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Handling and Installation

h Transportation

hOff-loading

h Storage

hMechanical forces during installation

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Grounding

Explanation and comparison of different systems of system grounding

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Ungrounded system

hAn electrical system which is not intentionally connected to the ground at any point

hA virtual connection to ground does exist through capacitances between the live conductors and earth

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Ungrounded system-Eqv. circuit

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Virtual ground in an ungrounded system

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Ungrounded system-Advantages hWhen there is a ground fault in the system the fault current is very low

hNo immediate problem to the system

h The system can continue operating without interruption, for some time

hNo elaborate protective equipment and earthing systems

h Low overall cost of the system

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Ungrounded system - Disadvantages

h Capacitances between the system conductors and the ground results in fault current (small magnitude)

h Can cause repeated arcing and build up of excessive voltage with reference to ground (Arcing ground)

h Locating a faulty circuit is time consuming

hA simultaneous second earth fault in a different phase results in a short circuit

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Earth fault detection-Ungrounded systems

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Earth fault detection-Ungrounded systems

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Solidly grounded systems

hA system whose neutral is directly connected to ground without any intentional resistance

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Solidly grounded system Eqv. circuit

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Solidly grounded systems - Advantages

hA fault is readily detected and therefore isolated quickly by circuit protective devices

h It is easy to identify and selectively trip the faulted circuit

hNo possibility of transient over voltages

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Solidly grounded systems -Disadvantages

h High ground fault currents due to low ground circuit impedance (higher than even 3-phase faults)

h Needs equipment of high rupturing capacity

h Faults inside a device (a motor or generator) results in major damage to active magnetic parts

h High repair cost and long outages

h Not used in medium voltage circuits feeding to such equipment

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Reactance grounded systems

hAn inductor is used to connect the system neutral to ground

h Limits fault current to 25% to 60% of the three-phase fault current

hWill result in extensive damage to active magnetic parts in case of internal equipment faults

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Reactance grounded system Eqv.

circuit

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Tuned (Resonant) grounding

h Resonant grounding is a special type of reactor grounding

h Reactor impedance matched to system capacitance (tuned)

h Results in very low ground fault current

hAvoids arcing grounds

h System configuration changes will need re-tuning

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Tuned grounded system Eqv. circuit

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Tuned grounding-Normal condition

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Tuned grounding-Fault condition

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Resistance grounding

h The most common type of grounding method adopted in medium voltage networks

h The system is grounded by a resistor connected between the neutral point and ground

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Resistance grounded system Eqv.

circuit

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Resistance grounding-Advantages

h Reduces damage to active magnetic parts by reducing the fault current

h Minimizes the fault energy

h Flash or arc blast effects are minimal thus ensuring safety of personnel near the fault point

h Avoids transient over voltages and resulting secondary failures

h Reduces momentary voltage dips on faults unlike a solidly grounded system

h Fault current permits easy detection and isolation of faulted circuits

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High resistance grounding

h Limits the ground fault current to about 10 amps

h To avoid arcing ground the fault current value should be more than the system capacitance current

hTypical application: Utility generators in MV range

hNot advisable with systems involving extensive cable networks

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Grounding of power utility generators

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Neutral in systems without a star point

h In Delta-connected transformer substations a neutral may not always be available

h In this case it will be necessary to obtain a virtual neutral using a device called grounding transformer

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Zig-Zag grounding transformer

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Zig-Zag grounding transformer

Ground fault current path in a zig-zag transformer

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Star-delta grounding transformer

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Star-delta grounding transformer

Ground fault current path in a star-delta transformer

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Other special cases of grounding

h Power distribution systems with multiple sources

hTransformers and generators

hTwo or more generators

hTwo or more transformers

hDistribution systems with sources of different voltages

hMobile equipment with a power transformer mounted

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Thank You !