CG0021EN1.pdf
(
356 KB
)
Pobierz
protection
and control
<< Back
protection
guide
MERLIN GERIN
mastering electrical power
GROUPE SCHNEIDER
presentation
contents
page
presentation
1
grounding systems
3
short-circuit currents
9
discrimination
15
electrical system protection
21
transformer protection
29
motor protection
35
AC generator protection
41
capacitor protection
47
sensors
53
lexicon
overcurrent protection
undervoltage protection
I >
U <
over and underfrequency
protection
directional overcurrent
protection
I <–
> f >
earth fault protection
overvoltage protection
I
N
>
U >
negative sequence
unbalance protection
real reverse power
protection
P <––
Ii >
thermal overload protection
reactive reverse power
protection
I
Q <––
differential protection
neutral voltage displacement
protection
U
N
>
D
I
I
voltage restrained
overcurrent protection
Buchholz
>
U
generalities
Protection devices continuously monitor the
electrical status of system units and cause
them to be de-energized (e.g. tripped by a
circuit breaker) when they are the site of a
disturbance: short-circuit, insulation fault...
The objectives are:
to contribute to protecting people against
electrical hazards,
to prevent equipment damage (the power
produced by a three-phase short-circuit on a
MV busbar can melt up to 50 kg of copper
within 1 second, the temperature at the
centre of the arc can exceed 10,000°C),
to limit thermal, dielectric and mechanical
stress on equipment,
to maintain stability and service continuity
in the system,
to protect adjacent installations
(for example, by reducing induced voltage in
adjacent circuits).
In order to attain these objectives, a
protection system should have the following
features:
speed,
discrimination,
reliability.
Protection, however, has its limits: faults
have to actually occur in order for it to take
effect. Protection cannot therefore prevent
disturbances; it can only limit their duration.
Furthermore, the choice of a protection
system is often a technical and economic
compromise between the availability and
safety of the electrical power supply.
The choice of a protective device is not the
result of isolated study, but rather one of the
most important steps in the design of the
electrical system.
Based on an analysis of the behaviour of
electrical equipment (motors,
transformers...) during faults and the
phenomena produced, this guide is intended
to facilitate your choice of the most suitable
protective devices.
Merlin Gerin
protection guide 1
2
protection guide
Merlin Gerin
grounding systems
introduction
The choice of MV and HV grounding systems
has long been a topic of heated controversy
due to the impossibility of finding a single
compromise for the various types of electrical
systems. Experience acquired today enables a
pertinent choice to be made according to the
specific constraints of each system.
five grounding sytems
Neutral potential can be grounded using five
methods that differ according to the kind
(capacitive, resistive, inductive) and value
(zero to infinity) of the Z
n
impedance
connection made between the neutral and
earth:
Z
n
=
ungrounded,
no deliberate
connection,
Z
n
is a
resistance
with a fairly high value,
Z
n
is a
reactance
with a generally low value,
Z
n
is a
reactance
designed to compensate
for the system capacity,
Z
n
= 0 - the neutral is directly
grounded.
¥
difficulties and selection
criteria
The selection criteria involve many aspects:
technical characteristics (system function,
overvoltage, fault current, etc...),
operation (service continuity, maintenance),
safety,
cost (investment and operating expenses),
local and national customs.
In particular, there are two major technical
considerations which are, in fact, contradictory:
Reducing the level of overvoltage
Overvoltage is of several origins:
lightning overvoltage, which all overhead
systems are exposed to, up to the user supply
point,
internal system overvoltage caused by
operations and certain critical situations
(resonance),
overvoltage resulting from an earth fault itself
and its clearance.
Reducing earth fault current (
If).
Fault current that is too high produces a whole
series of consequences:
damage caused by the arc at the fault point;
particularly the melting of magnetic circuits in
rotary machines,
thermal withstand of cable shields,
size and cost of earthing resistance,
induction into adjacent telecommunication
systems,
danger for people created by raised frame
potential.
Unfortunately, optimizing one of these
requirements is automatically to the
disadvantage of the other. Two typical
grounding methods accentuate this contrast:
the ungrounded neutral system, which
eliminates the flow of earth fault current
through the neutral but causes the most
overvoltage,
the directly grounded neutral system, which
reduces overvoltage to a minimum, but causes
high fault current.
An intermediate solution is therefore often
chosen: the impedance grounded neutral
system.
Merlin Gerin
protection guide 3
grounding systems
(cont.)
ungrounded
In this type of system, a phase-to-earth fault
only produces a weak current through the
phase-to-earth capacity of the fault-free
phases.
It can be shown that Id = 3 C
ω
Advantage
The basic advantage is
service continuity
since the very weak fault current prevents
automatic tripping.
Drawbacks
The
failure to eliminate overvoltage
through
the earth can be a major handicap if
overvoltage is high. Also, when one phase is
earthed, the others are at delta voltage
(U = V.
e
) in relation to the earth increasing
the probability of a 2
nd
fault. Insulation costs
are therefore higher since the delta voltage
may remain between the phase and earth for
a long period as there is no automatic tripping.
A maintenance department with the equipment
to quickly track the 1st insulation fault is also
required.
Applications
This solution is often used for industrial
systems (
≤
V
c
V being the simple voltage,
c
C the phase-to-earth capacity of a phase,
c ω
the frequency of the system (
ω
= 2
π
f).
The Id current can remain for a long time, in
principle, without causing any damage since
it does not exceed a few amperes
(approximately 2 A per km for a 6 kV single-
pole cable, with a 150 mm
2
cross-section,
PRC insulated, with a capacity of 0.63
µ
F/km).
Action does not need to be taken to clear this
1st fault, making this solution advantageous in
terms of maintaining service continuity.
However, this brings about the following
consquences:
c
if not cleared, the insulation fault must be
signalled by a
permanent insulation
monitor,
c
subsequent fault tracking requires device
made all the more complex by the fact that it is
automatic, for quick identification of the faulty
feeder, and also maintenance personnel
qualified to operate it,
c
if the 1st fault is not cleared, a second fault
occurring on another phase will cause a real
two-phase short circuit through the earth,
which will be cleared by the phase protections.
Id
15 kV) requiring service continuity.
4
protection guide
Merlin Gerin
Plik z chomika:
Qulakin
Inne pliki z tego folderu:
3140746A_E.pdf
(499 KB)
3140747A_F.pdf
(469 KB)
3140748A_E.pdf
(258 KB)
3140750A_F.pdf
(1149 KB)
3140758A_E.pdf
(159 KB)
Inne foldery tego chomika:
Zgłoś jeśli
naruszono regulamin