Electrical transmission or interconnection systems – Stabilized – anti-hunting or antioscillation systems
Reexamination Certificate
2001-07-27
2003-07-08
Riley, Shawn (Department: 2838)
Electrical transmission or interconnection systems
Stabilized, anti-hunting or antioscillation systems
Reexamination Certificate
active
06590302
ABSTRACT:
The present invention relates to a method for reducing natural system oscillations with respect to a ground potential in an electrical drive having a voltage intermediate-circuit converter with a controlled input converter and with an input-side inductance, namely a mains system input inductor using the step-up controller mode, and having an electric motor connected thereto, for example a motor using field coil technology, and to a corresponding electrical drive and intermediate-circuit converter voltage.
BACKGROUND OF THE INVENTION
In present-day converter systems with a intermediate circuit voltage, e.g., in multi-shaft converter systems, system oscillations can be formed which are virtually undamped. This relates primarily to converters having a voltage intermediate circuit and having a controlled feeder in the form of a regulated mains-system-side converter, which also referred to as an input converter.
Converters are principally used for operating electrical machines at a variable supply frequency. An intermediate circuit frequency converter allows an electric motor, for example in a three-phase machine such as a synchronous machine, no longer to be operated directly from the mains system and hence at a fixed rotation speed, since the fixed mains system can be replaced by an electronically produced, variable-frequency and variable-amplitude mains system for supplying the electrical machine.
The two mains systems, first the supply mains system, where the amplitude and frequency are fixed, and second the mains system supplying the electrical machine where the amplitude and frequency are variable, are decoupled via a DC voltage store or a direct current store in the form of an intermediate circuit. Such intermediate-circuit converters in this case essentially have three central assemblies:
a mains-system-side input converter, which can be designed to be uncontrolled (for example diode bridges), or controlled, in which case energy can be fed back into the mains system only when using a controlled input converter;
an energy store in the intermediate circuit in the form of a capacitor in a voltage intermediate circuit and an inductor in a current intermediate circuit; and
an output-side machine converter or inverter for supplying the machine, which generally uses a three-phase bridge circuit having six active current devices which can be turned off, for example IGBT transistors, to convert the DC voltage in a voltage intermediate circuit into a three-phase voltage system.
Such a converter system with a voltage intermediate circuit which is preferably used, inter alia, for main drives and servo drives in machine tools, robots and production machines owing to its very wide frequency and amplitude control range, is shown in the form of an outline sketch in FIG.
1
.
The converter UR is connected via a filter F and an energy-storage inductor, whose inductance is L
K
, to a three-phase mains system N. The converter UR has a feeder E, a voltage intermediate circuit with the energy-storage capacitance C
ZK
, and an output inverter W.
FIG. 1
shows a regulated feeder E, which is operated in a controlled manner by means of switching components (for example a three-phase bridge circuit composed of IGBT transistors), as a result of which the arrangement as shown in
FIG. 1
experiences a stimulus A
1
. The inverter W is likewise controlled via further switching components, for example by means of a three-phase bridge circuit having six IGBT transistors. The fact that switching operations also take place in the inverter likewise represents a stimulus A
2
to the system. The capacitor C
ZK
in the voltage intermediate circuit is connected between the positive intermediate circuit rail P
600
and the negative intermediate circuit rail M
600
. The inverter is connected on the output side via a line LT and by means of a protective-ground conductor PE and a shield SM to a motor M, in the form of a three-phase machine.
The fixed-frequency three-phase mains system N feeds the intermediate circuit capacitor C
ZK
via the filter F and the energy-storage inductor L
K
by means of the regulated feeder and via the input converter E, with the input converter E (for example a pulse-controlled converter) operating together with the energy-storage inductor L
K
as a step-up controller. Once current has flowed through the energy-storage inductor L
K
, it is connected to the intermediate circuit and forces the current against the greater voltage into the capacitor C
ZK
. This also allows the intermediate circuit voltage to be kept above the peak value of the mains voltage.
This combination thus effectively represents a DC voltage source. The inverter W uses this DC voltage to form a three-phase voltage system in which, in contrast to the sinusoidal voltage from a three-phase generator, the output voltage does not have an ideal sinusoidal oscillation profile, but also has harmonics since it is produced electronically via a bridge circuit.
In addition to the above-described elements in such an arrangement, it is necessary to remember that parasitic capacitances occur which assist the formation of system oscillations in such a converter system. For example, in addition to the filter F with a discharge capacitance C
F
, the input converter E, the inverter W and the motor M all have discharge capacitances C
E
, C
W
and C
M
to ground. Furthermore, the line LT has a capacitance C
PE
to the protective-ground conductor PE, and a capacitance C
SM
to the grounded shield SM.
It has now been found that these system oscillations are stimulated in a particularly pronounced manner in the feeder E. Depending on the control method chosen for the feeder, two or three phases of the mains system N are in this case short-circuited, in order to cause current to flow through the energy-storage inductor L
K
. If all three phases U, V, W are short-circuited, then either the positive P
600
or the negative intermediate circuit rail M
600
is rigidly locked to the star point of the supply mains system (generally close to ground potential depending on the zero system component). If two phases of the mains system N are short-circuited, then the relevant intermediate circuit rails P
600
and M
600
are rigidly locked to an inductive voltage divider from the two mains system phases.
Depending on the mains voltage situation, this voltage is close to ground potential (approximately 50-60 V). Since the intermediate circuit capacitance C
ZK
is generally large (continuous voltage profile), the other intermediate circuit rail 600 V is lower or higher, and may thus also drag down the remaining mains system phase. In both situations, the intermediate circuit is particularly severely deflected from its “natural”, balanced rest position (±300 V with respect to ground), thus representing a particularly powerful stimulus to system oscillation.
With regard to the production of undesirable system oscillations, the frequency band which is relevant for the application area of less than 50 to 100 kHz allows a resonant frequency to be calculated with concentrated elements. In this case, the discharge capacitances C
F
to ground in the filter F are generally so large that they do not govern the frequency. In this case, it can be assumed that there is a dominant stimulus to oscillations before the described capacitances, and the filter discharge capacitance C
F
can be ignored.
The resonant frequency f
res
(sys) of this system, which is referred to by f
sys
in the following text, thus becomes:
f
sys
=
1
2
π
L
∑
·
C
∑
(
1
)
where
L
&Egr;
=L
K
+L
F
(2)
where L
K
represents the dominant component and L
F
the unbalanced inductive elements in the filter (for example current-compensated inductors) which act on the converter side, and
C
&Egr;
=C
E
+C
W
+C
PE
+C
SM
+C
M
(3)
This expression is shown schematically in FIG.
2
. In this case, L
&Egr;
and C
&Egr;
form a passive circuit, which is stimulated by a stimulus A and starts to oscillate at its natura
Goepfrich Kurt
Raith Sebastian
Segger Bernd
Baker & Botts LLP
Riley Shawn
Siemens Aktiengesellschaft
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