Protection of Power Systems 3. Instrument Transformers

There are two basic types of instrument transformers: voltage transformers (VTs), formerly called potential transformers (PTs), and current transformers (CTs). Figure 10.2 shows a schematic representation for the VT and CT.

The transformer primary is connected to or into the power system and is insulated for the power system voltage. The VT reduces the primary voltage and the CT reduces the primary current to much lower, standardized levels suitable for operation of relays.

600V and 1000V Current Transformers Donut or Window-Type Bar-Type 600V and 1000V Current Transformers 

Dead-Tank Circuit Breaker with Bushing-Type CTs

A 110 kV CT

SF6 110 kV Current Transformer

Current Transformers and Live-Tank Circuit Breakers

SF6 insulated Inductive Voltage Transformers

500 kV Capacitor Voltage Transformer

Capacitor Voltage Transformer

VTs For system-protection purposes, VTs are generally considered to be sufficiently accurate. Therefore, the VT is usually modeled as an ideal transformer, where V is a scaled-down representation of V and is in phase with V.

A standard VT secondary voltage rating is 115 V (line-to-line). Standard VT ratios are given in Table 10.1

Ideally, the VT secondary is connected to a voltage-sensing device with infinite impedance, such that the entire VT secondary voltage is across the sensing device. In practice, the secondary voltage divides across the high impedance sensing device and the VT series leakage impedances. VT leakage impedances are kept low in order to minimize voltage drops and phase-angle differences from primary to secondary.

CTs The primary winding of a current transformer usually consists of a single turn, obtained by running the power system’s primary conductor through the CT core. The normal current rating of CT secondaies is standardized at 5 A in the United States, whereas 1 A is standard in Europe and some other regions. Currents of 10 to 20 times (or greater) normal rating often occur in CT windings for a few cycles during short circuits.

Standard CT ratios are given in Table 10.2

Ideally, the CT secondary is connected to a current-sensing device with zero impedance, such that the entire CT secondary current flows through the sensing device. In practice, the secondary current divides, with most flowing through the low-impedance sensing device and some flowing through the CT shunt excitation impedance. CT excitation impedance is kept high in order to minimize excitation current.

Current transformers are normally equipped with two cores—one for measurements and one for protection purposes. The measurement core will give accurate measurements during normal power system operation, but will saturate for the much higher fault currents. The protection core, on the other hand, is not capable of providing accurate measurements for low currents, but will not saturate for fault currents.

An approximate equivalent circuit of a CT is shown in Figure 10 An approximate equivalent circuit of a CT is shown in Figure 10.7, where The total impedance ZB of the terminating device is called the burden and is typically expressed in values of less than an ohm. The burden on a CT may also be expressed as volt-amperes at a specified current.

Associated with the CT equivalent circuit is an excitation curve that determines the relationship between the CT secondary voltage E and excitation current Ie. Excitation curves for a multiratio bushing CT with ANSI classification C100 are shown in Figure 10.8.

FIGURE 10.8 Excitation curves for a multiratio bushing CT with a C100 ANSI accuracy classification

Current transformer performance is based on the ability to deliver a secondary output current I that accurately reproduces the primary current I. Performance is determined by the highest current that can be reproduced without saturation to cause large errors. Using the CT equivalent circuit and excitation curves, the following procedure can be used to determine CT performance.

For simplicity, approximate computations are made with magnitudes rather than with phasors. Also, the CT error is the percentage difference between (I + Ie) and I, given by: The following examples illustrate the procedure.

Note that for the 15-A secondary current in (c), high CT saturation causes a large CT error of 57.1%. Standard practice is to select a CT ratio to give a little less than 5-A secondary output current at maximum normal load. From (a), the 100 : 5 CT ratio and 0.5  burden are suitable for a maximum primary load current of about 100 A.

Homework 1

Optical CTs Other means of providing power system information for protective relays are developed and finding applications. One is the magneto-optic current transducer. This uses the Faraday effect to cause a change in light polarization passing through an optically active material in the presence of a magnetic field.

The passive sensor at line voltage is connected to a stationary equipment through a fiber-optic cable. This eliminates the supports for heavy iron cores and insulating fluids. The output is low energy, and can be used with microprocessor relays and other low-energy equipment.

These are most useful at the higher voltages using live tank circuit breakers that require separate CTs. In the meantime, iron-cored devices are ubiquitous in power systems and do not appear to be easily replaced.

Optical CTs offer increased accuracy, reduced size, and wider bandwidth. Optical CTs are also safer to work with than conventional CTs. When the secondary circuit of a conventional CT is opened while the primary current is flowing, very high voltages can be developed at the location of the opening.

These voltages can be damaging to the equipment and a safety hazard to the personnel working in the area. Panel boards have been completely destroyed when CT secondary circuits had been inadvertently opened. Test personnel must take special precaution when working on CT secondary circuits which can be time consuming or dangerous if an error occurs.

It is obvious that optical sensors provide significant operational advantages over conventional CTs and will probably receive greater utilization as protective systems are converted to digital devices, fault current levels continue to increase on power systems, costs are reduced, and engineers become more familiar and confident with the technology.

Besides, the advantages associated with accuracy and freedom from saturation, the fundamental characteristics of such measuring systems can be changed by simple program changes in the associated software.

CCVTs VTs would have primaries that are either connected directly to the power system (VTs) or across a section of a capacitor string connected between phase and ground (Coupling-Capacitor VT (CCVTs )). VTs are used at all power system voltages and are usually connected to the bus. At about 115 kV, the CCVT type becomes applicable and generally more economical than VTs at the higher voltages.

Usually, the CCVTs are connected to the line, rather than to the bus, because the coupling capacitor device may also be used as a means of coupling radio frequencies to the line for use in pilot relaying.

The metering CCVT, shown in the figure, consists of a modular capacitive divider which reduces the line voltage V1 to a voltage V2 (10–20 kV), with a series-resonant inductor to tune out the high impedance and make available energy transfer across the divider to operate the voltage transformer which further reduces the voltage to VM, the metering level.