The Common connection in both neutral and phase side of CTs is done in high imepedance REF as shown below.
There are many different between low and high impedance REF inherently. The most important difference between classical high impedance REF protection and new low-impedance REF protection is the input impedance. As with all numerical relays, the input impedance of the low-impedance REF is very low compared to high-impedance relays. Low-impedance REF protection does not have the same inherent stability against CT saturation for external faults as does high-impedance REF protection.
A second significant difference is that the operating current of the low-impedance REF protection is not realized by CT connection. With low-impedance REF, the relay measures all four CTs necessary to realize the element.
A very important advantage of low-impedance REF protection is the fact that the CT ratios for the phase CTs and neutral CTs do not have to be the same.
Most low-impedance REF relays use an operating and a restraint current. The difference between different relays from different manufacturers lies in the way these relays determine the restraint quantities and in the CT saturation detection algorithm of each relay. Note that, in the case of low-impedance
REF protection, there is no inherent immunity to CT saturation, as is the case with high-impedance REF protection. The following different methods are used to determine the restraint and operating current:
1. Use of the residual current Ir = Ia + Ib + Ic as the restraint current and the differential current Id = Ia + Ib + Ic – In as the operating current.
2. Use of the residual current Ir = Ia + Ib + Ic as the operating current and the neutral current In as the restraint current.
Because of the nature of the protection, classical low impedance REF protection cannot be used as a balanced earth fault protection on an unearthed transformer or on a transformer with only three phase CTs and no neutral CT. In short, it is because the low-impedance REF protection requires a restraint and an operating current, at least one of which is also derived from the neutral CT. When a neutral CT is not provided, a low-impedance REF protection cannot be used to protect an unearthed transformer.
Most relays have a minimum pickup level of 50 mA. Although most low-impedance REF relays have additional supervision for improved security, many manufacturers recommend a pickup setting greater than the steady-state neutral current resulting from load unbalance (quiescent zero-sequence current). This ensures that the relay picks up for actual faults, not for load unbalance. This practice reduces scheme sensitivity, because a greater operating current setting increases the minimum primary operating current.
In cases where a biased earth differential protection is provided, the bias setting serves mainly to prevent the relay from operating for external faults resulting from CT saturation and other lesser important factors. These characteristics are fairly fixed, and security against operation for external faults is almost guaranteed.
For example, assume that the CTs in a high-impedance REF scheme draw 15 mA magnetizing current at the operating voltage, and the relay operating current is 20 mA. It follows then that the total secondary current should be 4 • 15 + 20 = 80 mA. The corresponding primary current must drive sufficient operating current through the relay to produce the magnetizing current necessary for the CTs to operate the relay.
With a 200/1 CT ratio (impedance-earthed transformer), there is an implied minimum primary operating current of 16 A. For a typical 355 A NER, the only part of the winding that is not covered, assuming zero fault resistance, is the bottom 16 / 355 • 100 = 4.5 percent.
Taking the same example, assume that the CTs in a low impedance scheme draw only 2 mA magnetizing current because of the lower voltage across the CTs and the relay draws 50 mA. It follows then that the total secondary current should be 4 • 2 + 50 = 58 mA. With the same CT ratio and NER as in the previous example, the minimum primary operating current is 11.6 A. Clearly, the low-impedance REF function is more sensitive in this case. However, if the CTs used with the high impedance REF were of better quality and the magnetizing current were also 2 mA, the high-impedance REF relay would be more sensitive. In this case, (assuming zero fault resistance) the bottom 11.6 / 355 • 100 = 3.3 percent of the winding is not covered.