The mechanical effects of short-circuit currents in - Montefiore

6. D.C. CONFIGURATIONS
6.1. INTRODUCTION
A reliable power supply of substations and power plants
often needs the construction of enlarged d.c.
installations in which batteries act as energy storage
devices. Installations without batteries are fed through
converter and controlled drives. For these systems to be
safe, the knowledge of short-circuit strength is required.
Therefore it is necessary to study the currents and forces
which can occur and to develop simplified methods for
design of d.c. configurations.
As shown in Figure 6.1, such d.c. auxiliary systems may
include the following equipment as possible sources of
short-circuit currents :
– power converters;
– stationary storage batteries;
– smoothing capacitors;
– d.c. motors, e. g. motor-generator sets.
Usually, the connections between the equipment are
done by cables or rigid buses.
D.c. auxiliary systems in substations and power plants
are designed for ±24 V as well as for 220 V. Nominal
currents may be as high as 2,5 kA. The current capacity
of the storage batteries ranges from 200 Ah up to 2000
Ah. The capacity of smoothing device is rather high; it
is about 10 mF to 3 F.
Peak values of short-circuit currents caused by power
converters, storage batteries, smoothing capacitors and
electric motors may be estimated roughly as follows:
– The peak value of the converter current may be 20
times the nominal current, leading to about 50 kA
with nominal currents of about 2,5 kA;
– Estimation of the peak value of the battery current
may be based upon the capacity; the peak value in
amperes is then 20 times the capacity in amperehours,
e.g. 40 kA for 2000 Ah;
– The peak current of the capacitor fed by a converter
may be as high as the peak value of the converter
current;
– The peak value of the motor current is about as high
as the starting current, which is 4 to 10 times the
nominal current.
High peak and steady-state short-circuit currents cause
correspondingly high mechanical and thermal stresses,
which must be taken into consideration at the design
stage. Short-circuit currents and resulting mechanical
and thermal effects in three-phase systems may be
calculated according to IEC Publication 60909 [Ref 11]
and IEC/EN Publication 60865-1 [Ref 2, Ref 3]. These
standards are based upon long-term research, those are
about mechanical effects published in the CIGRE
brochure [Ref 58]; this document has been revised in
1996 [Ref 1]. A standard for the calculation of shortcircuit
currents in d.c. auxiliary systems has been
published in 1997 [Ref 59], it is based upon [Ref 60]. A
97
standard for the mechanical and thermal effects has
been published in 1997 [Ref 61]. In 2000, a Technical
Report with an example of the d.c. installation shown in
Figure 6.1 was also published [Ref 62].
In section 6.2, different short-circuit current
characteristics as measured on a modelling system are
presented and a standardized characteristic is introduced
[Ref 60, Ref 63]. In section 6.3, the curve of the
electromagnetic force, which corresponds to the
standardized characteristic, is transformed into a
rectangular substitute time function which may be used
for determining the mechanical and thermal stress [Ref
64, Ref 65].
Figure 6.1 Circuit diagram of a d.c. auxiliary system in a substation
or a power plant.
6.2. SHORT-CIRCUIT CURRENTS AND
ELECTROMAGNETIC FORCES
6.2.1. Short-circuit current characteristics measured
For reasons of operational safety d.c. auxiliary systems
are not available for short-circuit experiments.
Therefore, a model system for 24 V and 100 A has been
designed, cf. Figure 6.2 [Ref 60, Ref 63]. It consists of a