Connections current transformers

In some instances, it is required to measure the sum of the energy supplied along several feeders. This is the case, for example, when a factory site is fed by more than one cable and the tariff is based on the overall energy and maximum demand used. The usual method of doing this is to summate the currents in the individual feeders by installing separate current transformers in each and connecting their secondaries in parallel (Figure 17.3). 

The current transformer is arranged with its primary winding in series with the supply (Figure 17.8). It thus carries the load current. The measuring instrument is connected across the secondary as shown. The ideal theoretical relationship between the currents and the number of turns on the primary and secondary is ... 

The star-point connection to earth is provided with a current transformer and a sensitive relay. The relay (51 G) is of a definite time delay or inverse time delay type so that it can be graded as back-up protection to earth fault relays at downstream feeders and consumers. 

In order to restrict the detection of earth faults to those within the stator winding, and those from the stator terminals to the switchgear current transformers, a sensitive relay (64) is used. Three current transformers are used in the stator live lines and one in the star to NER connection. All four current transformers are connected in parallel such that any unbalance in the currents due to an internal fault is detected by the restricted earth fault relay (64). A sensitive high impedance relay is used to achieve an instantaneous response. However, if a high impedance is connected across a current transformer it is possible that very high voltages will appear across the impedance. This is due to the action of... 

Where a bus-section circuit breaker is used to divide the busbars (during abnormal operating conditions) each set of busbars is protected as a separate zone. Each zone consists of the incomers, the outgoing circuits and the bus-section circuit breaker(s). An accurate current transformer is connected in each line of each circuit. All the current leaving the zone must be balanced by current from the incomer circuits. A fault in the zone will be detected by the (87) relay. Rapid operation is required... 

It is therefore necessary to provide a sensitive method for detecting earth fault currents. The most common method is to provide a core balance current transformer at the circuit breaker or contactor. This current transformer has a current or turns ratio, which is independent of the ratios used by the transformers connected in the three-phase conductors. This is because a particular level of current is to be detected rather than a fraction or multiple of the stator load current. The switchgear manufactnrer will normally recommend the ratio of the core balance transformer and the matching relay. The relay will be either instantaneous 50 N or an inverse time 51 N type depending upon whether the motor is controlled by a circuit breaker or a contactor. 

A core balance current transformer functions more reliably and is more sensitive than a set of three current transformers connected in parallel. A three-transformer system is prone to responding to the initial inrnsh cnrrent of the motor. To avoid this the current setting needs to be higher than would be preferred. 

It should be noted that this function is assigned only to a relay which detects the flow of current from the frame of a machine or enclosing case or structure of a piece of apparatus to ground, or detects a ground on a normally ungrounded winding or circuit. It is not applied to a device connected in the secondary circuit or secondary neutral of a current transformer, or in the secondary neutral of current transformer, connected in the power circuit of a normally grounded system. 

Due to the open delta connection of PT s, it follows that the switchgear fire was started by the failure phase (between phases 2 and 3) of PT. According to the records of the fault currents, it is estimated that this event had duration of about 2 cycles (see Fig. 25). As shown in the figure, because the fault is a fault-phase, unbalanced residual current is generated through the common point of the current transformers. From the oscillograms it is concluded that the evolution of biphasic failure to three-phase fault was about 30 to 40 ms. 

A current transformer (CT) has the large load currents connected to the primary winding of the transformer and the ammeter connected to the secondary winding. The ammeter is calibrated to take account of the turns ratio of the transformer, so that the ammeter displays the actual current being taken by the load when the ammeter is actually only taking a small proportion of the load current. 

If the secondary coil is connected to a load, a current I2 flows in the load and a corresponding current I flows in the primary coil. Since energy is conserved, the current transformation ratio is the inverse of the voltage transformation ratio ... 

The operation of a transformer ratio arm bridge is shown schematically in Figure 3.1.2. Briefly, voltage 180° out of phase is fed from the secondary winding of the input voltage transformer to the cell or unknown impedance and to resistance and capacitance standards. The arms of the bridge consist of a series of ratio taps of the primary windings of an output current transformer. The standard and unknown impedances are connected to the output transformer in such a way that a detector null is achieved wheu the sum of the flux induced by the unknown and standard currents in the output transformer is zero. In this condition... 

If a delta/wye connected power transformer is installed between the power source and the load, the power factor at the transformer input generally will reflect the average PF of the loads on the secondary. This conclusion works on the assumption that the low PF is caused by inductive and capacitive reactances in the loads. However, if the load current is rich in harmonics from rectifiers and switching regulators, some of the harmonic currents will flow no farther toward the power source than the transformer delta winding. The third harmonic and multiples of three will flow in the delta winding and will be significantly reduced in amplitude. By this means, the transformer will provide some improvement in the PF of the total load. 

Two types of phase/current sample pickup elements are commonly used the sample loop and torroidal current transformer (TCT). The sample loop consists of a single turn unshielded loop of rigid construction, with a fixed gap at the open end for connection of the sample line. The device must be mounted on the tower near the point of maximum current. The loop can be used on towers of both uniform and nonuniform cross-section. It must operate at tower potential, except for towers of less than 130 electrical degrees height, where the loop can be operated at ground potential. 

Electrical Connections. Electric current is brought from the transformers by air-cooled copper busbars and close to the electrode by water-cooled bus tubes and flexible cables, connecting to water-cooled copper contact plates at the electrode. The plates are held against the electrode by hydraulic pressure. The connectors are as short and as balanced as possible to allow cancelling of magnetic fields associated with individual conductors. 

Reduced-voltage starting. A reactor, resistor, or transformer is temporarily connected ahead of the motor during start to reduce the current inrush and limit voltage dip. This is accompanied by reduced starting torque. For reactor or resistor start, the torque decreases as the square of current for transformer start, the torque decreases directly with line current. The reactor, resistor, or transformer can be adjusted to give a proper balance between torque and current. 

When there is no dedicated transformer and these circuits are connected on the system bus directly a large inductor will be essential at the incoming of the static circuits, sufficient to absorb the trapped charge within the transformer and the interconnecting cables up to the converter unit. The size of the inductor can be calculated depending on the size (kVA) of the distribution transformer, its fault level and the characteristics of its current limiting protective device. An inductor sufficient to absorb //, L of the transformer and the cables may be provided at the incoming of the sialic circuits. 

The philosophy to assume the impedance of the source of supply (generator or a transformer) as the impedance of the faulty circuit may be far from reality and may give a very high fault current. In actual operation, the fault intensity may be far less, as every device and component connected in the circuit will tend to add to the effective impedance of the faulty circuit and limit the magnitude of the fault current. Figure 13.15 also subscribes to this theory. But it is customary to design the systems for the worst fault conditions which, in all likelihood, may not arise, and decide the protective scheme and the current settings of the protective relays for the minimum possible fault current. 

Through the tap-offs of the bus, the unit auxiliary transformers (UATs) are connected to feed the station auxiliary services. For more clarity we have taken out the portion of the tap-offs from Figure 13.21 and redrawn it in Figure 13.18 to illustrate the above system and its interconnections. The tap-offs are now subject to the cumulative inOuence of the two supply sources. In the event of a fault on this section, both the sources would feed the same and the fault current through the tap-offs would add up. The tap-offs should thus be designed for the cumulative effect of both fault levels. For the sake of an easy reference, Table 13.8 suggests a few typical values of fault currents, worked out on the basis of data considered for the G and GT. One such example is also worked out in Example 13.3. 

This is the ratio of instrument limit primary current to the rated primary current. Consequently a high SF will mean a high transformation of the primary current and can damage instruments connected to its secondary. For measuring instruments therefore it is kept low, as it is required to measure only the normal current and not the fault current. 

These are employed to detect a fault, rather than measuring the current of a power sysletn or the connected equipment. There is a fundamental difference in the requirement of a measuring and a protective transformer in terms of accuraev. saturation level and VA burden. Unlike a... 

The preferred current lalings may follow series R-ll) of lEC 600,39 and tis disctissed in Section 13.4.1(4). They mtiy inereti.se to 6000 A or so. depending upon the application as when required to connect a large LT alternator or the LT side of a large transformer to its switchgear. The preferred shoil-lime ratings may be one of those indicated in Table 13.7. 

For the tap-offs, connecting a UAT through the main bus section between the generator and the generator transformer, however, as discussed above, the momentary peak current will depend upon the short-time rating of such tap-offs. The likely ratings are noted in Table 13.8. 

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