Instrument Transformers
Instrument transformers are essential parts of many electrical metering and relaying systems. The quality of instrument transformers will affect directly the overall accuracy and performance of these systems. Instrument transformer performance is critical in protective relaying, since the relays can only be as good as the instrument transformers. They serve two basic functions:
• To change the magnitude (but not the nature) of primary voltage and current to secondary values to 120 V and 5 or 1 A where relays, meters, or other devices can be applied
• To provide isolation between primary and secondary circuit for equipment and safety of personnel
When relays compare the sum or difference of two or more currents or the inter- action of voltages and currents, the relative direction of the current must be known. The direction of current flow can be determined by knowing the instru- ment transformer polarity. Polarity markings are normally shown on instrument transformers; however, they can be determined in the field if necessary. Several aspects of current and voltage instrument transformers are discussed next.
Current Transformers
Current transformers (CTs) are designed for connection in the primary cir- cuit (either in series or around the primary circuit). The secondary current of the transformer bears a known relationship with the primary current. Any change in the primary current is reflected in the secondary circuit. Relays, meters, and other devices are connected to the secondary terminals of the CTs. CTs are made in many different ratios, different voltage insulation sys- tems, and for different environmental conditions such as indoor or outdoor use. Generally, the following types of construction are used for CTs.
• Wound type:
In this type more than one primary turn is frequently used to obtain low exciting current and high accuracy. The usual current ratings for this type of transformer are 800 A and below.
• Bar-primary type:
In this type, the primary consists of a single bar extending through the core, which is connected in series with the circuit conductor. This type of construction is suited to withstand the stresses of heavy overcurrent. The usual current rating for this type of transformer is 1200 A or above in order to provide sufficient ampere-turns for good accuracy.
• Window type:
The window-type CT contains no primary winding.
The CT has an insulated hole through the core and secondary windings. The circuit conductor is inserted through the window of the CT, and thus this conductor then becomes the primary of the CT.
• Bushing type:
The bushing-type CT is similar to the window-type.
It has a circular core that is designed to fit on the bushing of a power transformer, circuit breaker, or other apparatus. The secondary windings are wound on the circular core and can be tapped to give multiple ratios. This transformer is mostly used for relaying purposes where
high accuracy at normal current value is not extremely important.
• Double–secondary type:
A double-secondary transformer is actually two transformers, each transformer having its own core. This type of transformer occupies less space than two single-secondary winding transformers. The double-secondary winding transformer permits instruments, relays, or other devices to be separated if required.
• Split–core type:
This is a window-type CT with hinged cores, which
permit them to be installed on buses or other circuits.
• Air–core type:
The air-core CT is used where saturation of the iron
core due to high fault currents is a problem. The air-core transformer
has relative constant error over a wide range of overcurrent and transient conditions.
• Tripping transformers:
Several types of small and inexpensive trans-
formers are available for protective control functions. These transformers are not made with the same accuracy as instrument transformers.
• Auxiliary transformers:
Auxiliary transformers are used to adjust the difference in ratio between different CTs. These transformers are connected in the secondary circuits of main CTs.
CT Accuracy Standards
CTs can be divided into two categories for purposes of establishing accu- racy standards: (1) accuracy standard for metering CTs, and (2) accuracy standard for relaying CTs. Since accuracy is a function of the burden on the CT, standard burdens have been established. Accuracy has been established at various burden values. The standard burdens established by American National Standard Institute (ANSI) C57.13-1993(R2003) are shown in Table 9.1. The performance rating is based on 5 A secondary current unless otherwise specified.
Accuracy Classes for Metering
The ANSI accuracy classes for metering state that the transformer correction factor (TCF) should be within specified limits when the power factor of the metered load is from 0.6 to 1.0 lagging for a specified standard burden, at 100% of rated primary current. CTs for metering service have accuracy classes of 0.3%, 0.6%, and 1.2%.
Accuracy Classes for Relaying
The ANSI standard 57.13-1993 has standardized on the accuracy classes and the conditions under which instrument transformers are to be applied. Prior to 1968, the accuracy classes and the conditions under which the instrument trans- formers were applied were based on ANSI standard C57.13-1954. The ANSI standard 57.13-1954 was revised in 1968 and this standard set one accuracy class, instead of the two that was in the older standard. The revised standard also changed the designations for the older class of CTs. Hence, the ANSI 57.13-1968 standard is different than the older standard of ANSI 57.13-1954 in many respects. Since many older CTs and voltage transformers (VTs) are still in use today, it is appropriate to discuss the older and revised ANSI C57.13 standards.
Older standard (ANSI C57.13-1954): In this standard, the accuracy ratings were on the basis of the standard secondary terminal voltage, a transformer would deliver without exceeding a standard percent ratio error. The classifica- tion of CT performance was based on the ratio error at 5 to 20 times secondary current. Therefore, the CTs were divided into classes, H and L. Class H had a nearly constant percentage ratio error when delivering a fixed secondary volt- age over a wide range of secondary current. Class L had a nearly constant magnitude error (variable percentage error) when delivering a fixed secondary voltage over a wide range of secondary current. Standard percent ratio error classes for class H CTs were 2.5% and 10%, whereas for class L CTs, the stan- dard percent ratio error was 10%. Secondary voltages were 10, 20, 50, 100, 200, 400, and 800 V. For example, CTs were classified as 2.5 H 200 or 10 L 200. The first term described the maximum percent ratio error, the second term (H or L) described the transformer performance characteristics, and the third term described the secondary voltage. The class H transformer could deliver a secondary voltage equal to its voltage class at 5 to 20 times secondary rated current. A class L CT, on the other hand, could only deliver a secondary volt- age within its voltage class at 5 to 20 times secondary rated current at fixed burden. In other words, class L cannot be used with proportionately higher burdens at lower secondary current without exceeding its classified ratio error.
Newer standard (ANSI 57.13-1968): A relaying accuracy under the new stan- dard is designated by symbols C and T.
• C-type CTs have single primary turn and distributed secondary windings. For C-type CTs the ratio error can be calculated. Majority of
these CTs are bushing type and are rated at 600 V since they do not have any physical connection with the primary circuit.
• T-type CTs are constructed with more than one primary turn and undistributed windings. The primary windings are insulated and braced for the primary voltage. Because of the physical space required and the fringing effects of nonuniformly distributed winding, because there is flux that does not link both the primary and secondary wind- ings. The leakage flux has significant effect on the CT performance and it is not possible to calculate the ratio correction error using the burden and excitation characteristics. Therefore, the ratio correction must be determined by test for the T-type CTs.
• The secondary terminal voltage rating for the C-type and T-type CTs is the voltage which the transformer will deliver to a standard burden (as listed in Table 9.1) at 20 times normal secondary current.
• The transformer ratio error must be limited to 10% for all currents from 1 to 20 times normal current for burdens not to exceed those listed in Table 9.1.
• K-type:
The K classification was established in the 1993 revision of the C57.13 standard. The K-type CTs are designed to have knee-point voltage at least 70% of the secondary terminal voltage rating.
The K-type is similar to C-type CTs, i.e., their design is based on the principle that the leakage flux in the core does not have an appreciable effect on the ratio of the CT within the limits of current and burden.
To specify a CT under the new standards, one needs only to select either a class C or T transformer and then specify the burden. The first term of the burden classification of a CT identifies the construction type of CTs as dis- cussed in Section 9.2.1, and the second term describes the voltage rating that can be delivered by the full winding at 20 times rated secondary current with- out exceeding 10% ratio correction (error). The ANSI voltage rating applies to the full winding, and if less than full winding is used, the voltage rating is then reduced in proportion to turns used. As an example, a CT C200 or T200 means that a ratio correction error will not exceed 10% for values from 1 to 20 times rated secondary current (5 A) with a standard 2.0 Ω burden.
• The C classification covers bushing transformers, and the T classifi- cation covers any other transformer
• The secondary voltage values are 10, 20, 50, 100, 200, 400, and 800V based on 1 to 20 times the normal current standard burdens listed in Table 9.1