Metering and protection CTs
As you should already know, current transformers are used for metering and relay protection purposes. When we are talking about current transformers used for metering, their performance is of interest during normal loading conditions. Metering transformers may have very significant errors during fault conditions, when the currents may be several times their normal value for a very short time.
Since metering functions are not required during faults, this is not significant.
Current transformers used for relaying are designed to have small errors during faulted conditions, while their performance during normal steady-state operation, when the relay is not required to operate, may not be as accurate. In spite of this difference, all (measuring or relaying) CT performance may be calculated with the same equivalent circuit.
The different values of equivalent circuit parameters are responsible for the difference in performance between the various types of CTs. Note that the performance of CTs when they are carrying the load current is not of concern as far as relaying needs are concerned.
Accuracy can be visualized as how closely the secondary wave shape resembles the primary wave shape. Wave shape and phase difference are both components of the accuracy classification.
The CT accuracy at high overcurrents depends on the cross section of the iron core and the number of turns in the secondary winding. The greater the cross section of the iron core, the more flux can be developed before saturation. Saturation results in a rapid decrease in transformation accuracy.
The greater the number of secondary turns, the less flux that is required to force the secondary current through the relay. This factor influences the burden the CT can carry without loss of accuracy.
Auxiliary current transformers are used in many relaying applications for providing galvanic separation between the main CT secondary and some other circuit. They are also used to provide an adjustment to the overall current transformation ratio.
In particular, the possibility that the auxiliary CT itself may saturate should be taken into consideration. Auxiliary CTs with multiple taps, providing a variable turns ratio, are also available. The burden connected into the secondary winding of the auxiliary CT is reflected in the secondary of the main CT, according to the normal rules of transformation:
If the auxiliary CT ratio is l : n, and its burden is Zl, it is reflected in the main CT secondary as Z1/n2.
Consider the CT connection shown in Figure 1. CT1 has a turns ratio of 1200 : 5, while CT2 has a turns ratio of 1000 : 5. It is desired that when the primary current flows through the two lines as shown, the current in the burden be zero. Assume the primary current to be 600 A.
With the polarity markings as shown, the burden current is zero.
The burden on CT2 is Zb, while that on CT1 is Zb × (1.2)2 = 1.44 Zb. The burden on the auxiliary CT is of course Zb.
CT connections such as these are used in various protection schemes, and utilize the fact that, assuming no auxiliary CT saturation, when the primary current flows uninterrupted through the two primary windings the burden current remains zero, while if some of the primary current is diverted into a fault between the two CTs the burden current is proportional to the fault current.
In three-phase circuits, it is often necessary to connect the CT secondaries in wye or delta connections to obtain certain phase shifts and magnitude changes between the CT secondary currents and those required by the relays connected to the CTs.
Consider the CT connections shown in Figure 2. The wye connection shown in Figure 2(a) produces currents proportional to phase currents in the phase burdens Zf and a current proportional to 3I0 in the neutral burden Zn. No phase shifts are introduced by this connection.
The delta connection shown in Figure 2(b) produces currents proportional to (I’a − I’b), (I’b − I’c) and (I’c − I’a) in the three burdens Zf.
If the primary currents are balanced, (I’a − I’b) = √3|I’a| exp(jπ/6), and a phase shift of 30° is introduced between the primary currents and the currents supplied to the burdens Zf.
By reversing the direction of the delta windings, a phase shift of −30° can be obtained. The factor √3 also introduces a magnitude change which must be taken into consideration. We will discuss the uses of these connections as we study various relaying applications.
Delta connected CTs (VIDEO #1)
Delta connected CTs (VIDEO #2)
Wye connected CTs (VIDEO)
Recall the wye connection of CT secondaries shown in Figure 2(a). Each of the phase burdens Zf carries phase currents, which include the positive, negative and zero-sequence components.
Sometimes it is desired that the zero-sequence current be bypassed from these burdens. This is achieved by connecting auxiliary CTs which provide an alternative path for the zero-sequence current. This is illustrated in Figure 3.
The neutral of the main CT secondaries is not connected to the burden neutral. Instead, a set of auxiliary CTs have their primaries connected in wye and their secondaries in delta.
The neutral of the auxiliary CTs is connected to the neutral of the main secondaries through the neutral burden Zn. The secondary windings of the auxiliary CTs provide a circulating path for the zero-sequence current, and it no longer flows in the phase impedance burdens Zf.
It is possible to obtain the zero-sequence current by using a single CT, rather than by connecting the secondaries of three CTs as in Figure 2(a). If three phase conductors are passed through the window of a toroidal CT, as shown in Figure 4(a), the secondary current is proportional to (Ia + Ib + Ic) = 3I0.
In a connection of three CTs as in Figure 2(a), any mismatches between the three CTs will introduce an error in zero-sequence current measurement.
This is entirely avoided in the present application.
However, it must be recognized that such a CT application is possible only in low-voltage circuits, where the three phase conductors may be passed through the CT core in close proximity to each other.
The ampere-turns produced by the sheath current are now cancelled by the ampere-turns produced by the return conductor, and the net flux linking the core is produced by the sum of the three phase currents. This sum being 3I0, the burden is once again supplied by the zero-sequence current.
- Power System Relaying by Stanley H. Horowitz and Arun G. Phadke