Neutral earthing in an industrial HV network

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                                       Neutral earthing in
                                       an industrial HV

François Sautriau

Having graduated from the Ecole
Supérieure d'Electricité in 1968, he
joined Merlin Gerin in 1970. After
working on the design of networks
and protective devices, he was head
of the design office for industrial
projects and then for naval
equipment projects.
He is now, consultant in the
Marketing Division of the Protection
and Control Department

E/CT 62 up dated April 1996
Merlin Gerin Technical Specification n° 62 / p.2
neutral earthing in an industrial HV network                                             HV electrical networks can be earthed
                                                                                         in different ways. This document
                                                                                         analyzes the constraints imposed by
                                                                                         the different parameters of the
                                                                                         installation (overvoltages, network,
                                                                                         receivers) and calculates the fault
                                                                                         Different protection modes are
                                                                                         described along with the settings and
                                                                                         adjustments suggested according to
                                                                                         the requirements.

1. Introduction                                                              p. 4
2. Earthing                            Direct earthing                       p. 4
                                       Earthing through a reactor            p. 4
                                       Earthing through a resistor           p. 4
3. Requirements imposed by             Earthing through a current            p. 5
overvoltages                           limiting reactor
                                       Earthing through a resistor           p. 4
4. Requirements imposed by networks                                          p. 5
5. Requirements imposed by receivers                                         p. 6
6. Calculating fault currents                                                p. 6
7. Earth protection mode               Earth protection adjustment           p. 7
                                       Earthing with accessible neutral      p. 8
                                       Earthing with an artificial neutral   p. 8
Appendix 1: Comments on determining                                          p. 10
network capacitance values
Appendix: Bibliography                                                       p. 11

                                                                             Merlin Gerin Technical Specification n° 62 / p.3
1. introduction

When designing an industrial HV                    implies the following:                      neutral generally implies mandatory
network, a suitable neutral earth                  s the risk of high overvoltages likely to   tripping on the first fault, however:
arrangement must be selected: the                  favourize multiple faults,                  s it reduces overvoltages,
neutral can either be insulated, or it can         s the use of superinsulated equipment,      s it provides a simple, reliable,
be connected to earth. The use of an               s compulsory monitoring of the              selective means of protection,
insulated neutral in an HV network has             insulation,                                 s it allows the use of equipment, and in
the advantage of ensuring operational              s protection against overvoltages,          particular cables, with lower insulation
continuity since it does not trip on the           which will become compulsory in the         levels than for an insulated neutral.
first fault, however the network                   near future,
capacitance must be such that an earth             s the need for complex, selective
fault current is not likely to endanger            protection against earth faults which
personnel or damage equipment.                     cannot usually be ensured by simple
On the other hand, an insulated neutral            current-measuring relays. An earthed

2. earthing

The purpose of this study is not to                However, in the event of an earth fault,    earthing through a resistor
compare the different neutral earth                the current is not limited, damage and
arrangements, but rather, once the                 interference occur and there is             This is often the most satisfactory
neutral earth solution has been                    considerable danger for the personnel       solution.
adopted, to determine the earthing                 during the time the fault persists.         A study is necessary to choose
mode by finding a compromise                       This solution is not used for HV            between these two earthing, (through a
between three often contradictory                  distribution.                               reactor or through a resistor) :
requirements:                                                                                  accurate determining of these earthing
s to sufficiently damp overvoltages,                                                           modes depends on the voltage level,
s to limit damage and disturbances                 earthing through a reactor                  the size of the network and the type of
caused by an earth fault,                          Tuned reactor (Petersen coil)               receivers.
s to provide simple, selective protective          This solution is sometimes used for         Depending on the earthing mode, a
devices.                                           public HV networks. It is rarely used for   criterion then determines a maximum
Earthing can be of different types:                industrial distribution.                    impedance value corresponding to the
s direct (without impedance-dependent              Protective relays sensitive to the active   overvoltage problem.
current limiting),                                 component of the residual current must      Next, it is necessary to check its
s through a reactor,                               be used to obtain selectivity.              compatibility with the requirements of
s through a resistor.                                                                          the network and the receivers.
                                                   Current limiting reactor
                                                   This solution can result in serious
direct earthing                                    overvoltages, as demonstrated by Le
This type of earthing is the most                  Verre (the Research and Development
efficient in limiting overvoltages;                division of the E.D.F.) [1]. It can be
protection selectivity presents no                 used only where there are low limiting
difficulties.                                      impedances.

                                                                                               [1] See bibliography

Merlin Gerin Technical Specification n° 62 / p.4
3. requirements imposed by overvoltages

earthing through a current                         earthing through a resistor                            current IC in the event of an earth fault.
                                                                                                          Hence the relation IL ≥ 2 IC.
limiting reactor                                   As recommended by EDF for
                                                   hydroelectric power networks. The                      Determination of the cable capacitance
(see fig. 1)                                                                                              values depends on their design (see
                                                   resistance value r is détermined in
The study of overvoltages that occur               order to obtain a total active power                   appendix for this calculation).
when short-circuits are eliminated from
networks with the neutral earthed                              U
through a reactor gives the following              loss :           equal to or greater than the
s let, I0ω be the earth fault current              capacitive power 2 C ω U in the event
limiting reactance,                                of a phase-earth fault, i.e.:
s and Lω the network three-phase                       2
short-circuit reactance.                           U         2

The neutral-to-earth overvoltage                      ≥ 2 CωU .
occurring when short-circuits are
eliminated is:                                                               U
                                                   When dividing by                 , this become
∆V      1          I0
   =                    for a radial field cable    U                       U
V          2        L                                          ≥ 2.3Cω
network,                                            3r                      3

∆V   1         I0                                  where:
   =                    for all other cases.
V    2         L                                           U
                                                   s       is the value of the earth fault
In practise, the earth fault current is               3r
limited to at most 10 % of the three-              current IL in the earthing connection,
phase short-circuit current, as applied                             U                                     fig. 1 : a zigzag or neutral point coil provides
by the EDF to its HV power distribution            s   3Cω              is the network capacitive         an earth fault current limiting reactor.
network.                                                            3

4. requirements imposed by networks

The above criterion is used to define              its path and in particular to the cable
the lower limit of the phase to earth              shields. The maximum current
fault current.                                     withstood by the cable shields may be
To determine the upper limit, it is                specified by the constructors. As a
necessary to check that the fault                  general rule, the value used is between
current does not cause damage along                500 and 3 000 A for 1 second.

                                                                                                    Merlin Gerin Technical Specification n° 62 / p.5
5. requirements imposed by receivers

In HV networks, receivers are                      between 3 kV and 15 kV, most                          machine when the metal plating is
transformers which have no particular              frequently 5.5 kV in France; the earth                damaged is much more time
requirements as concerns the neutral               fault current should not exceed 20 A in               consuming and more costly.
earthing in a power supply network.                order to avoid damage to the steel
However, industrial HV networks can                plating of the machines, for if reworking
supply rotating machines with voltages             a winding is a regular repair, repairing a

6. calculating fault currents

The currents in the different circuits are
easily calculated using a simple
approximative method.                                                                     3
This consists in ignoring the short-                                    aE                1
circuit impedance of the source and the                             VN       E
coupled impedances with respect to the                                aE                  2
neutral earth impedance and the
                                                                                                                   I rD                      IrS
network capacitances. In other words,
we consider that earth fault currents are                                                          1
much lower than three-phase short-                                                            G=
                                                                    1                              Z          CD                       CS
circuit currents (see fig. 2).                                   g=
To calculate the neutral-to-earth
potential, the sum of the currents
flowing to earth is considered to be                               IN
zero (see diagram).
IN + IrD + Σ IrS = 0
0 = g VN + [G + jω C] (VN + E)
    + jωC (VN + a2E) + jωC (VN + aE)                  1
                                                   z=              Neutral earth impedance
0 = VN [g + G + 3jωC] + GE
    + jω CE (1 + a2 + a)                              1
                                                   Z=              Phase to earth fault impedance
Since 1 + a + a = 0
                                                   CD              Phase to earth capacitanc e of the faulty outgoing
This gives:                                                        feeder
                                                   CS              Phase to earth capacitance of a sound outgoing
           − GE
VN =                                                               feeder
       g + G + 3jω C                               C = Σ CS        Total phase to earth capacitance of the network
                  − zE                             E               Network phase voltage
where VN =                                         VN              Neutral point to earth potential
           z + Z + 3 jω CzZ                        IN              Neutral to earth current
                                                   ID              Fault current
                                                   IrD             Residual current of the faulty outgoing feeder
                                                   IrS             Residual current of a sound outgoing feeder

                                                   fig. 2: Earth fault current calculation parameters.

Merlin Gerin Technical Specification n° 62 / p.6
Since we know VN, the different                s I rD   = I D + 3jω C D VN                      In the event of a short-circuit Z = 0, the
currents (IN neutral to earth current, ID                                                       above formulae become:
                                                            g + 3 jω ( C − C D )
fault current, IrD and IrS residual currents            =                          GE
in outgoing feeders) are calculated as                         g+G+3jω C                        VN = − E
shown below:
                  −gGE                                      1+ 3jω ( C − C D ) z                           −E
s I N = gVN                                             =                          E            s   IN =
             g + G + 3jω C                                   Z + z + 3jω CzZ                                z
      =                                            I rS = 3jω C S VN
                                                                                                    I D =  +3jω C  E
                                               s                                                           1
          Z + z + 3jω CzZ                                                                       s
                                                                                                          z       
                                                              − 3jω C S
                                                        =                    GE
                         g + 3jω C                          g + G + 3jω C
    I D = G ( VN +E) =
                                                                                                    I rD =  + 3jω (C − C D ) E
s                                    GE                                                                    1
                       g + G + 3jω C                          1− 3jω C S z                      s
                                                                                                           z                
                                                        =                      E                                            
         1+ 3jω Cz                                          Z + z + 3jω CzZ
       Z + z + 3jω CzZ                                                                          s   I rS = − 3jω C S E

7. earth protection mode

The neutral earthing impedance affects         used, in particular when these                   10 times less than the saturation value
the required method of protection              3 transformers are already required for          for a steady-state balanced current. An
against phase to earth faults.                 another application. But the                     earth protection device supplied by
As a general rule, the higher the fault        measurement obtained is degraded by              3 transformers must therefore include a
currents, the easier they are to detect;       the inaccuracies of all three                    time delay in order to avoid spurious
the lower they are, the harder they are        transformers, in particular in the event         triggering resulting from transients. The
to detect .                                    of transient overcurrents when the               current setting must not be lower than
Moreover, it is advisable, even                transformers become saturated.                   6 % of the transformer rating at best, or
essential, to ensure protection not at                                                          15 or 20 % of the transformer ratings in
one single point, but in all branches of       earth protection setting                         the most unfavourable cases.
the network, since the relays operate                                                           Moreover, if an earth fault occurs in a
                                               This must be adjusted according to the           star winding near a neutral point, the
                                               measurement accuracy. It must ensure             maximum fault current is only a small
Phase to earth protection is provided by       maximum protection and authorize                 part of the maximum fault current which
overcurrent relays supplied by the earth       selectivity.                                     is limited by the neutral earth
current.                                                                                        impedance. For this reason, the current
                                               If the measurement is carried out using
This current can be measured as                                                                 setting is usually 20% of the maximum
                                               the sum of the secondary currents of
follows:                                                                                        current limited by the neutral earth in
                                               the three transformers, it will be
s either by a core balance transformer         degraded by the dispersion of the                order to protect 80% of the windings.
around the three phase conductors and          transformers. In particular, a residual          But, as the calculation shows, in the
which directly detects the sum of their        current is measured if there is no earth         event of a fault, a residual capacitive
currents (zero if no earth fault) ;            fault when the transformers become               current flows through the sound parts of
s or by three current transformers the         saturated.                                       the network. So, to prevent the
secondary windings of which are                Saturation is caused by excessive                protective device of a sound line from
connected to form a neutral conductor          amplitude of the phase current, but              tripping spuriously, the threshold must
through which the sum of the three             more specifically by the DC component            be set to 30% higher than the
currents flows.                                induced in a short-circuit or unbalanced         capacitive current flowing through this
The core balance transformer solution          inrush current.                                  sound line when a phase to earth short-
is the most accurate, however it can           Note that, in transient conditions, the          circuit affects the network.
only be installed on cables, not on            DC component can induce saturation of            Moreover, we must take into account
busbars or overhead lines. The                 the transformers even though the peak            the possible presence of voltage
3 current transformers solution is often       value of the transient current is around         harmonics likely to produce currents

                                                                                          Merlin Gerin Technical Specification n° 62 / p.7
which increase as the order of the                 earthing with accessible                       earthing with an artificial
harmonics increases. Note that 3rd
harmonics and multiples of 3 exist even            neutral                                        neutral
in steady-state conditions. Finally, the           The resistor is connected to the neutral       If the neutral of the source is
neutral earth impedance characteristics            output terminal and to the earthing            inaccessible (as for delta-connected
must be coordinated with the protective            system, either directly, or through a          windings), or if there are several
devices so that this impedance itself is           single-phase transformer the secondary         parallel-connected sources, earthing
not degraded by the fault current before           winding of which is loaded by an               can be ensured by an artificial neutral
it is eliminated.                                  equivalent resistance, as is used in           (see figs. 4 and 5) (or earthing
Note: that this concerns circuit                   networks supplied through a                    transformer)
protection and not personnel protection.           transformer which has a star-connected
Conclusion: when the earthed neutral               secondary winding with accesible
solution is selected for a medium                  neutral and for AC generators with
voltage network, it is advisable to use            accessible neutral (see fig. 3).
earthing through a resistor rather than            If the network is supplied through             a)
other systems.                                     several transformers or AC generators,
                                                   it is advisable to have one single
Calculating r and IL                               neutral earth connection to prevent the
This resistance r and the maximum                  maximum earth fault current from
                                                   varying with the number of sources in
current I L =        are determined                service.
taking the following requirements into
s  the current IL must be greater (or                                                                  R
equal) to twice the network capacitive                                                                               I∆
current in the event of an earth fault
IL ≥ 2 LC in order to limit overvoltages,
s the current IL must be less than the
maximum overcurrent that can be                           R                                   r
withstood by the cable shields, normally
between 500 and 3 000 A, depending                                                                b)
on the cable cross section,
s in a network containing HV motors, it              a)                    b)
is preferable to respect the relation:
5 A ≤ IL ≤ 20 A but, if this is
incompatible with the first condition, IL
can be up to 50 A,                                 fig. 3: neutral earthing at the secondary
                                                   winding of a star-coupled transformer,
s in order to ensure correct protection
                                                   through a resistor either connected directly
of receivers, the Ir threshold settings            (a) or connected through a single-phase
should not exceed 0,2 IL, i.e. Ir ≤ 0,2 IL,        transformer (b).
s in order to ensure selectivity with                                                                           I∆
regard to sound connections, the
relation Ir ≥ 1,3 IC, must be respected,                                                                                             r
where IC is the capacitive current of the
line when a phase to earth fault occurs,
s if the earth current is measured by
3 transformers of rating In , then Ir must                                                        fig. 4: earthing a network neutral by means
be ≥ 0,06 In,                                                                                     of a star-delta transformer associated to:
s the thermal withstand of resistor r                                                             a) a resistor connected to the HV side; in
must allow for current IL to flow during                                                          this case the transformer secondary winding
the maximum fault clearing time                                                                   can supply the auxiliaries;
required (1 to 1.5 s) or, conversely, the                                                         b) a resistor series-connected to the
earth fault must be cleared sufficiently                                                          secondary winding.
rapidly to avoid damaging the resistor.

Merlin Gerin Technical Specification n° 62 / p.8
Several solutions are possible:                   The resulting impedance :
s a star/delta transformer with a                 ro + jIoω appears as a resistance
resistor;                                         if ro ≥ 2Ioω,
s a zigzag coil (see figs. 1 and 6): this         with ro and Io referenced to the same
is used in cases where the maximum                voltage.
earth fault current is limited to values
over 100 A;
s or, a special transformer, since to set
up an artificial neutral it may be more
economical to use the transformer
which supplies the substation LV
auxiliaries (see figs. 7 and 8).



                                                  fig. 7: neutral earthing by means of a
                                                  double-star transformer with compensating
                                                  delta windings around a resistor.
fig. 5: special transformer for neutral earting

  R                                                 R

fig. 6: neutral earthing by means of a zigzag     fig. 8: neutral earthing by means of a zigzag
coil.                                             transformer.

                                                                                                  Merlin Gerin Technical Specification n° 62 / p.9
appendix 1:how to determine the capacitance values of a network

The capacitance of the cables depends               disturb the network and does not affect       be known is therefore:
on their design:                                    the protection operation.                     Ic = 3 Cω V
s single-pole cable                                 On the other hand, if an earth fault          which only takes into account the
The conductor is surrounded by a                    occurs in the network, in other words         capacitance C regardless of the cable
shield and the capacitance C is that                when a phase is earthed, the cable            type.
measured between the conductor and                  capacitance is equivalent to an
                                                                                                  In practise, cable manufacturers use
the earthed shield.                                 unbalanced load composed of the
                                                                                                  the term star connection capacitance,
s three-pole radial field cable                     capacitance C between the two sound
                                                                                                  for which they indicate the following:
Each conductor is surrounded by a                   phases and the earth under phase to
                                                                                                  s the value of C for radial field cables,
shield and the capacitance C is that                phase voltage U.
                                                                                                  s the value of 3 K + C for belted type
measured between each conductor and                 The two currents CωU which are out of
its earthed shield.                                 phase by 60° flow through the two
s three-pole belted type cable                      sound phases; the sum of these two            They do not normally give this C
A single shield surrounds the three                 currents is known as the capacitive           capacitance value for belted type
conductors. There is a capacitance K                current IC of the network in the event of     cables. On request, they give three
between conductors and a capacitance                an earth fault.                               measurement results for these cables:
                                                                     π                            s the capacitance C1 measured
C between each conductor and the                    I c = 2 Cω V cos     3 Cω U
earthed shield.                                                      6                            between a conducting core and the
                                                    i.e. IC = 3 CωV.                              other cores connected to the metal
                                                    For the three-pole belted type cable,         sheathing; this gives the relation
                                                    if there is no fault, current ic flowing in   C1 = 2 K + C,
                                                    balanced conditions is:                       s the capacitance C2 measured
                     C                              i c = 3 Kω U + Cω V = 3 Kω V + Cω V           between the three conducting cores
                                                                                                  bound together and the metal
                                                    i.e. ic = (3 K + C) ω V per phase, and
                                                                                                  sheathing; this gives the relation:
           K             K                          the sum of the currents of the three
                                                    phases is zero                                C2 = 3 C,
                                                    Cable manufacturers generally give            s the capacitance C3 measured
     C                       C
                                                    this 3 K + C capacitance value for            between two conducting cores, with the
                                                    belted-type cables.                           third connected to the metal sheathing;
                                                    On the other hand, if there is an earth       this gives the relation:
                                                    fault in the network, in other words if a          3K+C
                                                                                                  C3 =
                                                    phase is earthed, the capacitive load                  2
                                                    includes:                                     Thus, capacitance C2 must be known in
                                                    s the three K capacitance values under        order to directly obtain the value of:
                                                    the phase to phase voltage which form                C2
For the single-pole cable and the
                                                    a balanced load;                              C=
three-pole radial field cable, there is                                                                  3
                                                    s the three C capacitance values, two
no ambiguity since there is only one
capacitance, C which is a phase-to-                 of which are under phase to phase
earth capacitance. If there is no fault             voltages and are out of phase by 60°;
and in three-phase steady-state                     the third under a zero voltage.
operating conditions, a capacitive                  The sum of these currents (ic per
current ic flows through each phase                 phase), known as the capacitive current
and is absorbed by the cable                        Ic of the network in the event of an
capacitance C under the phase                       earth fault is:
voltage V, flowing from phase to earth,                                π
                                                    i c = 2 Cω V cos      = 3 Cω U
at the network frequency:                                              6
                                                    i.e. Ic = 3 Cω V.
i c = Cω V = Cω                                     Conclusion: when determining the
                                                    resistance of an earth connection or the
Since this capacitive load is three-                adjustment of an earth protection
phased and balanced, it does not                    device, the capacitive current that must

Merlin Gerin Technical Specification n° 62 / p.10
appendix 2:bibliography

[1] Le Verre: «Overvoltages occurring
when elimining short-circuits in
networks with the neutral earthed
through a resistor». Société Française
des Electriciens (French Electricians'
Guild) Bulletin, 8th series, Volume 1,
No. 4 (April, 1960).
[2] E.D.F.: Memo on protection of
hydraulic power generators. NP 69 03.

                                         Merlin Gerin Technical Specification n° 62 / p.11
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