Sequence impedances

Note One should employ only current sensing relays as far as possible for such applications as a iiegatise sequence current has a severe effect on motor windings due to i much lower negative sequence impedance of the motor (.Section I2.2(v)i thtin a corresponding negative sequence voltage. 

The magnitudes of symmetrical and non-symmetrical fault currents, under different conditions of fault and configurations of faulty circuits, can be determined from Table 13.5, where Z] = Positive phase sequence impedance, measured under symmetrical load conditions. The following values may be considered ... 

Zj = Negative phase sequence impedance Zq = Zero phase sequence impedance... 

Table 13.6 Approximate negative and 2ero phase sequence impedances compared to positive phase sequence impedances...

Note The impedances marked on Figure 13.1.6 refer to positive phase. sequence impedances only. Faults that are non-symmelrical alone... 

These transformers are three-phase and may be connected for zig-zag or star/delta connections (Section 20.9.1). The delta may also be made open type by inserting a resistor across it to help adjust the zero-sequence impedance, if required. 

The high impedance is usually a resistance chosen to limit the earth current to about 20 mA. A current detector is used in conjunction with the resistance to raise an alarm if a line-to-earth fault occurs. A zig-zag transformer, or reactor, is sometimes used with three-wire supplies such as used in drilling rigs and emergency systems. It is specially designed and internally connected to create a very low zero sequence impedance to earth currents. Therefore, the current is limited only by the resistance of the neutral earthing resistor. Some special purpose earth current alarm systems are available that inject a small DC current into the three-phase system, which is used to identify the actual location of the fault. 

Step 1. Find the total positive, negative and zero sequence impedances in the circuit. The total positive sequence impedance Z is -... 

It may be noted that the simple treatment of the zero sequence impedances in the example would tend to be more appropriate to a remote switching station fed by an underground cable. 

Often, the analysis of a feeder uses the positive and zero sequence impedances for the line sections. The 3x3 sequence impedance matrix can be obtained by the following transformation ... 

Table 3.1 shows that the positive-sequence impedance is smaller for a solidly bonded cable than for a cross-bonded cable, and the positive-sequence current is larger for a solidly bonded cable. Because of this size differential, the return current flows only through the... 

The impedance calculation in lEC 60909-2 assumes solid bonding. As a result, if the positive-sequence impedance of a cross-bonded cable is derived based on lEC 60909-2, it might be smaller than the actual positive-sequence impedance. 

The phase angle of the zero-sequence current mentioned in Table 3.1 demonstrates that grounding resistance at substations in both cross-bonded and solidly bonded cables significandy affects the zero-sequence current. As a result, there is litde difference in the zero-sequence impedance of the cross-bonded cable and the solidly bonded cable. The results indicate the importance of obtaining an accurate grounding resistance at the substations to derive accurate zero-sequence impedances of cable systems. 

The resistivity of the conductor is obtained from the conductor resistance and the cross section of fhe conductor S (pc= dc 2 dc )- Ir general, the resistivity is greater than that of the intrinsic resistivity of the conductor (e.g., copper 1.8 x 10 O-m) because of the gap within the stranded conductor. If fhe resisfivify of fhe metallic sheath is not provided by the manufacturer, it is obtained % the resistance and its cross section in the same manner as the main conductor. The relative permittivities of the main insulator and the corrosion-proof layer are determined by their materials. For example, the relative permittivity of a cross-linked polyethylene (XEPE) is 2.3. The permittivity of fhe corrosion-proof layer is widely ranging. However, the value has no effect on the positive-sequence impedance and admittance of the cable. 

---