In many texts the term sequence impedance values are defined as
something which do not really exist in practice but are of theoretical
value with regard to fault current calculations. In my company we
routinely measure the sequence impedances when commisionning lines and
cables. so...surely if something is measurable then it exists in
reality....or am i wrong?
You may be referring to the method of symmetrical components. In this
method, an unbalanced three-phase system is replaced as a sum of
BALANCED in-phase, negative-phase, and zero-phase components. Thus, the
complicated unbalanced problem devolves into three simple problems.
This method is a variation of the application of eigenvalues and
eigenvectors for normal modes.
i am indeed referring to that. sequence components are also referred
to as symmetrical components. My question still remains. i am also
familiar with fortescue's theory...well, as far as using it is
concerned. a qualitative understanding of the actual sequence
impedances still eludes me however. many thx for ur interest,
No. You are not entirely wrong.
Under normal operating (and most real world fault) conditions, the
currents in the system are the phasor sums of positive, negative and
zero sequence components. And the resulting voltage drops are the result
of these currents and their associated sequence impedances.
There are ways to measure each impedance component directly. Positive
sequence is measured by applying a balanced load to your cables,
applying a balanced 3 phase voltage and measuring the phase current(s).
For cables, positive and negative sequence impedances are the same.
Zero sequence impedance can be measured by tying all three phases
together, applying a single phase load between the phases and
neutral/ground and driving the cables with a single phase source.
As Salmon Egg indicates, the use of symmetrical components for analysis of
unbalanced power systems- particularly fault studies is a real labour saver.
Sequence impedances do exist.
The positive sequence impedance is the impedance seen by applying normal or
positive sequence balanced voltages to the equipment. It is the impedance
that you would use for steady state system analysis.
The negative sequence impedance is the seen by applying balanced voltages
with a reversed phase rotation. In most cases such as lines and transformers
it is the same as the positive sequence impedance but in generators and
motors it may differ
Zero sequence impedance can be measured using a single phase voltage on all
phases in parallel, to ground. For transmission lines it includes the
ground return impedance. Note that zero sequence current is 1/3 the ground
current. This will be quite different from the positive and negative
sequence impedance because of the effects of ground returns (in the ground
or in a conductive cable sheath).
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