Alternating-current filter circuit arrangement

In this three-phase filter circuit system for static converter systems and particularly high-voltage direct-current short ties, at least two high-pass filters are provided which are connected in parallel. Each high-pass filter consists of a choke connected in parallel with a resistance and, in series with both, a capacitor and can be individually connected to the three-phase busbar via a switch. The filter circuits are used not only for reducing harmonics but also as compensating devices, which can be switched in steps, for the reactive power requirement of the static converters so that in the lower partial load range of the system one of the filter circuits is disconnected for reasons of reactive power balance. In order to improve the filtering effect in this lower partial load range, a bus-coupler switch connecting in each case the common junctions of capacitor, resistance and choke of the high-pass filters is closed. When the switch is closed, this widens the band width of the filter circuit which is in operation and increases the resonance frequency, and individual distortions, the total distortion and telephone interference factors on the busbar are reduced.

BACKGROUND OF INVENTION 
The invention relates to an alternating-current filter circuit arrangement 
for static converter systems and a method for filtering harmonics in 
power-supply networks. 
A previously known alternating-current filter circuit arrangement is 
disclosed in Austrian Pat. No. 329,677. There, one high-pulse filter tuned 
to lower-order harmonics and one high-pulse filter tuned to higher-order 
harmonics each are connected between each phase conductor of a three-phase 
power system in order to attenuate harmonic currents originating from a 
static converter circuit having a three-phase static-converter bridge. No 
provision is made for switching high-pulse filters on or off in dependence 
on reactive-power demand or on changes in the harmonics content. 
Static converter systems generate a plurality of current harmonics which 
can give rise to undesirable distortions of the alternating or three-phase 
voltage and to telephone interference. In order to prevent this, tuned 
and/or wide-band filters are usually connected to the three-phase power 
rail. 
A second peculiarity of a static converter is its requirement for 
fundamental-frequency reactive power. This is dependent on the effective 
power transmitted and on the terminal voltage ratio of the 
static-converter reactifiers. As a rule, the fundamental-frequency 
reactive power is made available wholly or partially by suitable 
compensating devices. It is possible to use several three-phase filter 
circuits tuned to different frequencies for this purpose and, if 
necessary, additional capacitor banks which are connected in parallel with 
the static converter. These compensating devices are switched in steps in 
order to match the total compensating power to the reactive-power 
requirement of the static converter. 
As a result of this dual function of the three-phase filter circuits, the 
problem arises that, for example, when operating a tight high-voltage 
direct-current (HVDC)-transmission coupling with partial load, on the one 
hand the amplitudes of some harmonics increase and, on the other hand, it 
becomes necessary to switch off tuned filter circuits for reasons of the 
reactive-power balance. Especially in the lower partial-load range of an 
HVDC-transmission coupling (below 30% of nominal load), the problem of 
conducting the operation with partially disconnected filters and increased 
firing angle arises. As a rule, the increase in firing angle in this load 
range causes an increase in the current harmonics of the order n=23, 25, 
35 and 37 fed into the supply network. These order numbers apply to a 
twelve-pulse HVDC-transmission static converter circuit. 
Acting in conjunction with the above-mentioned loss in filter action if the 
partial-load range, system perturbations and telephone interferences can 
occur in this operating mode which decisively exceed the usual limits of 
permissible individual distortions, the total distortion and telephone 
interference factors. 
OBJECT AND SUMMARY OF THE INVENTION 
An object of the invention is to improve the filtering of harmonics in a 
power-supply network by simple means and to specify an alternating-current 
filter circuit system, especially for static-converter systems, which is 
also used for providing fundamental-frequency reactive power and which 
shows good filtering action also in the lower partial-load range of the 
power to be transmitted when filters are partially switched off for 
reasons of the reactive-power balance. 
The advantages which can be achieved by means of the invention consist 
especially in the fact that an improvement in the filtering action for the 
higher harmonics is achieved due to the choke and resistance of a 
disconnected filter being connected, by the action of a coupling switch 
which is closed during partial-load operation, in parallel with the 
operational alternating-current filter circuit. In particular, the 
resonant frequency and the band width of the filter circuit are increased. 
Thus the distortions caused by the higher harmonics can be adequately 
reduced during partial-load operation. In addition, the reactive effect on 
communications equipment is considerably reduced. 
A further essential advantage of the invention is based on the fact that 
when the coupling switch is retrofitted into already existing filter 
circuit arrangements, the original filter-circuit characteristic is 
retained when the coupling switch is open.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, the basic single-line circuit diagram of a three-phase filter 
circuit arrangement for a twelve-pulse static converter of a tight 
HVDC-transmission coupling is shown which is composed of a first high-pass 
filter for the eleventh harmonic, a second high-pass filter for the 
thirteenth harmonic and an additional capacitor bank. Each of these 
reactive-power units is designed as a three-phase unit for one-third of 
the reactive power required by the HVDC-transmission static converter at 
rated power. 
In detail, the second high-pass filter for the thirteenth harmonic consists 
of a filter-circuit capacitor C1, a series-connected filter-circuit choke 
L1 and a damping resistor R1 which is located in parallel with the choke 
L1. The high-pass filter C1/L1/R1 is connected to the bus bar via a switch 
S1 which is connected to the capacitor C1. 
The first high-pass filter, arranged in parallel with the second high-pass 
filter C1/L1/R1, for the eleventh harmonic consists of a filter-circuit 
capacitor C2, a series-connected filter-circuit choke L2 and a damping 
resistor R2 which is connected in parallel with the choke L2. The 
high-pass filter C2/L2/R2 is connected to the bus bar via a switch S2 
which is connected to the capacitor C2. 
The additional capacitor bank, which is located in parallel with the 
high-pass filters, is designated by C3 and can be switched into circuit 
via a switch S3. 
Between the common junction points of C1/L1/R1 and C2/L2/R2, a coupling 
switch S4 (also of three-phase design) is arranged. 
The three-phase filter circuit arrangement described must fulfil a dual 
function. On the one hand, provision must be made for keeping the 
distortion of the network voltage and the telephone interferences as low 
as possible by tuning the filter-circuit resonant frequencies to the 
harmonics generated by the static converter. In particular, the eleventh 
and the thirteenth harmonic occur in this case as characteristic 
harmonics. 
On the other hand, the three-phase filter circuits must wholly or partially 
compensate the inductive reactive-power absorption by the 
HVDC-transmission static converter. For this purpose, the compensating 
reactive power is matched to the reactive-power demand of the static 
converter in steps of 33.3% of the compensating reactive power during 
nominal operation by closing or opening the switches S1, S2 and S3, all 
switches S1, S2 and S3 being closed with full reactive-power demand 
(nominal operation), the switches S1 and S2 being closed and the switch S3 
being opened with a reactive-power demand of 66.6% of the demand during 
nominal operation and the switch S1 being closed and the switches S2, S3 
being open with a reactive-power demand of 33.3% of the demand during 
nominal operation. 
From these two requirements, the problem arises that when the HVDC station 
is operated with small transmission power (for example at 30% of nominal 
power), the higher harmonics occur to a greater extent and can no longer 
be adequately filtered because the three-phase filter circuits are 
partially switched off for reasons of reactive-power balance. This is made 
more difficult, especially by the fact that the HVDC-transmission static 
converter is operated in the lowest partial-load range, but the fact that 
if an increased firing angle is preselected, the harmonics of the order 
n=23, 25, 35 and 37 contribute significantly to telephone interference and 
to system perturbations in the higher-level supply system. 
Therefore, when the tight HVDC-transmission coupling is operated in the 
lowest partial-load range with closed switch S1 and open switches S2 and 
S3, the switch S4 is closed. 
Closing of the switch S4, with simultaneously open switch S2 and 
disconnected capacitor bank C3, causes the choke coil L2 and the damping 
resistor R2 of the high-pass filter C2/L2/R2, which is already 
disconnected with partial load, to be connected in parallel with the 
filter choke coil L1 of the still operational high-pass filter C1/L1/R1. 
As a result of this parallel connection, the effective filter-circuit 
inductance and the effective damping resistance of the operational filter 
C1/L1/R1 is reduced. In consequence, the resonant frequency of the filter 
is increased (detuning of the original resonant frequency) and the quality 
factor is decreased. This simultaneously results in an increase of the 
bandwidth of the high-pass filter. 
In connection with this, FIG. 2 shows the frequency-dependent impedance 
characteristics of the filter-circuit arrangement 
(.vertline.ZF.vertline.=absolute value of the complex filter impedance; 
n=order number of the harmonic). Curve A here applies to a closed switch 
S1 with simultaneously open switches S2, S3, S4 whilst curve B is valid 
for closed switches S1, S4 with simultaneously open switches S2, S3. 
As can be seen from FIG. 2, the resonant frequency shifts from the 
thirteenth harmonic to about the seventeenth harmonic. The filter 
impedance .vertline.ZF.vertline. significantly decreases for the harmonic 
with the order number n&gt;15 while it slightly increases for the harmonic 
with the order number n&lt;15. 
As a result of the measure of connecting the choke coil L2 and the damping 
resistor R2 into circuit, the individual distortions of the higher 
harmonic, the total distortion of the system voltage and the telephone 
interference factors can thus be significantly reduced when operating a 
tight HVDC-transmission coupling from about 30% of the nominal power down 
to minimum power with increased firing angle. The coupling switch S4 
required for this purpose does not represent a significantly increased 
expenditure with respect to the costs for a filter-circuit capacitor, 
choke coils, resistors and circuit breakers of the filter circuits. Since 
this additional switch must be designed in most cases not for the system 
voltage of the supply system but for a series voltage within the range of 
customary medium-high voltage levels (approximately 30 to 72.5 kv), the 
installation of this switch does not have any great space requirements. An 
additional advantage resulting from the low space requirement can be seen 
in the fact that this switch can also be installed subsequently into 
already existing filter-circuit arrangements and thus provides an improved 
filtering action with low load without changing the original filter 
characteristic when the coupling switch S4 is open. 
Since the maximum amplitudes of the harmonics of the order n=11 and n=13 
occur in tight HVDC-transmission couplings approximately within the range 
of 60% . . . 70% of nominal power and the filtering action of the highpass 
filters C1/L1/R1 and C2/L2/R2 continues to be effective when the switch S4 
is open, the harmonics can thus still be adequately filtered in this load 
range. 
A further advantage is based on the fact that capacitors of the same 
constructional size and thus the same capacity are provided in the outputs 
of the tuned filters C1/L1/R1 and C2/L2/R2, respectively. When the switch 
S4 is closed, the arrangement shown in FIG. 1 has optionally the same 
impedance characteristic when the switch S1 is open and the switch S2 is 
closed as when the switch S1 is closed and the switch S2 is open. Due to 
the redundancy of the filter capacitors consisting of many individual 
capacitor cans, this fact increases the operational reliability of the 
filter-circuit arrangement and provides a uniformly divided utilisation 
time for the filter capacitors with partial load. In addition, spare parts 
storekeeping is advantageously simplified.