CIRCUTOR TSA Series - Manual (Page 7)

1 INTRODUCTION
There is a lot of electrical supply installations where most of the loads are single phase, supplying a
rectifier plus a filter . This case is very common in office buildings, big stores, TV studies, where
there is a lot of computers and electronic devices using DC supply.
In such installations there is a particular type of problem with harmonics of 3
rd
order and their
multiples (called some times “the triplens”). The current of the triplens add on the neutral in phase,
instead of adding like vectors at 120º. Because of that , the current at the neutral may be higher than
the current at the phase cables, even in case that the three phases are perfectly balanced. This causes
unexpected tripping of thermal protections and overheating of the neutral cables.
The TSA filters are the optimal solution to solve such problems , since the filtering of third
harmonic by other types of passive filters may cause the saturation of transformers and may spoil
the situation instead of improving. The basis of the TSA is a three phase D-Y isolating transformer,
which eliminates the third harmonic upstream. At the output there is also a wide band passive filter
to damp down the 5
th
and 7
th
harmonics.
2 LOCATION AND SIZE OF THE TSA FILTERS.
2.1 Where to locate the TSA?
It is important to locate the TSA as close as possible to the disturbing loads. Because of that we
don’t recommend the solution of big centralized TSA filters to compensate the whole power
distribution. It’s better to split up the filter in several smaller filters, placed as close as possible of
the single phase lines causing the disturbances. In that way the currents upstream the TSA will be
practically exempt of 3
rd
harmonic.
2.2 How to size the TSA?
The TSA filters are selected by its total power in kVA , but to calculate the current that they can
deliver , a power factor correction must be applied. Such power factor depends on the distortion of
the current that the TSA must supply downstream to the loads. To calculate the kVA of the TSA ,
the fundamental and the most significant harmonics to be supplied to the load side must be known
and then the rated power may be calculated according to the following formula.
S VA U I n
I
C
n
n
( ) , * * * ( *
%
)
=
∑
1 732
100
1
2
1
Where : U
C
is the phase to phase voltage at the secondary side (usually 220V or 230V)
I
1
is the fundamental current at the load side.
n is the harmonic order
%I
n
is the % of harmonic n (Including the 3
rd
harmonic to be filtered)
The coefficient FS n
I
n
n
=
∑
( *
%
)
100
2
1
is named the “overload factor”
REMARK: The current written in the characteristics plate is the rated current given by the
weighted addition of fundamental and all the harmonics as follows: I n
I
n
n
1
2
1
100
*
( *
%
)
∑