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.
Bill, 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, Daniel
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).