01 Introduction

In medium voltage systems, besides circuit breakers and other switches, current transformers and voltage transformers (VTs) are commonly used. Although current transformers  seem like a mundane component, making the correct selection in electrical design is not trivial. After reviewing numerous design drawings and handling practical engineering projects, I've noticed some issues with the selection and technical requirements of current transformers  among designers, necessitating a discussion here.

02 Accuracy Class for Protection

The protection accuracy class of a CT is denoted by a percentage of the maximum composite error allowed under the rated accuracy limit primary current, followed by the letter "P." Classes such as 5PR, 10PR, PX, TPS, TPX, TPY, and TPZ are produced to meet the various protection needs in high voltage systems, imposing special requirements on current transformers  under specific circumstances. For instance, TP class is for transient protection, PR class specifies current transformers  with a defined limit for remanence, PX for current transformers  with low leakage, and TPS for low leakage current transformers  with strictly controlled turns ratio. TPX class specifies the peak instantaneous error under designated transient conditions without remanence limit; TPY class, besides meeting TPX error requirements, has remanence limits; TPY class current transformers  maintain accuracy during short-circuit transients without remanence, used in 330kV and above power systems. P class current transformers  are used in 110kV and below power systems. Since this article focuses on medium voltage current transformers , we will only discuss P class current transformers .

In medium voltage systems, only 5P and 10P are considered, where "P" stands for protection, denoting the class for protective applications. The numbers 5 and 10 preceding "P" represent accuracy classes, meaning the composite error does not exceed 5% and 10%, respectively. The numbers following "P" indicate accuracy limit factors, typically 10, 15, 20, with 5, 30, 40 rarely used. The accuracy limit factor signifies that under a short-circuit primary current of the CT's rated current multiplied by the above factors and a secondary load not exceeding the CT's rated load at the rated frequency, the composite error for 5P and 10P does not exceed 5% and 10%, respectively.

03 Accuracy Class for Measurement and Metering

Due to the presence of excitation current, the actual current on the secondary side is less than what would be calculated from the primary side using the transformation ratio, and there's also a phase angle difference and turns ratio error, measured by the CT's accuracy. CT accuracy classes include 0.1, 0.2, 0.5, 1, 3, and 5, with 0.2 and 0.5 typically used for metering, and special purposes served by 0.2S and 0.5S. For measurement, common accuracy classes are 0.5, 1, and 3, with specific error requirements outlined in Table 1. For classes 3 and 5, the CT load is considered at 50% to 100% of the rated load, unlike other accuracy classes which require 25% to 100%.

04 Difference between Accuracy Classes 0.2, 0.5, and Special Classes 0.2S, 0.5S

0.2S and 0.5S are special versions of the 0.2 and 0.5 classes, with "S" standing for Special. These classes have strict accuracy requirements even below 5% of the rated current, unlike their non-special counterparts. For example, at 1% of the rated load current, the 0.2S class has an accuracy of 0.75, meaning the composite error does not exceed 0.75% from 1% to 120% of the rated current, under secondary loads between 25% and 100% of the rated burden and a power factor range from 0.8 to 1.0, meeting the error limit requirements of the "Regulations for the Verification of Current Transformers for Measurement and Metering."

05 Protection Class 10P and 10% Error Curve

For 10P protection class current transformers , the composite error does not exceed 10% when the primary current does not surpass the accuracy limit factor. The 10% error curve essentially matches the curve relating the 10P accuracy limit factor to secondary load and power factor, simplifying use and design. For example, for a calculated short-circuit current of 18 times the rated current and a secondary load of 12VA, a CT with parameters 10P20 and a capacity of 15VA would be suitable. As long as the primary current does not exceed the limit factor range and the capacity does not surpass the rated capacity, the accuracy meets the specified value. Many factors can affect the error not exceeding 10%, such as the size of the primary current, secondary load, and CT capacity. If these values exceed the specifications, further short-circuit and secondary load calculations and accuracy limit factor curve consultations are necessary.

06 The Reasonable Selection of Protection Level

(1) For short-circuit protection, the measuring winding of a current transformer should not be used. When a short circuit occurs, the primary current is extremely large. 

To ensure measurement accuracy and not reduce the reliability of protection, the secondary current needs to increase proportionally with the primary current. This requires that the magnetic flux in the core of the current transformer not saturate, necessitating a large core cross-sectional area. However, the requirements for the core excitation of the measuring winding of a current transformer are exactly the opposite of those for the protection winding. When the primary current increases rapidly, the core is required to saturate quickly, thus significantly increasing the measurement error and causing the secondary current not to increase proportionally with the primary current – in other words, increasing very little. This protects the connected instruments from burning out or damaging their pointers. Therefore, measuring windings have a specific parameter, namely the instrument security factor (FS), which is the ratio of the instrument security current to the rated primary current. The instrument security current refers to the minimum primary current for which the composite error of the secondary winding of the current transformer under rated secondary load is not less than 10% as a multiple of the rated current of the current transformer. The IEC (International Electrotechnical Commission) recommends current transformer instrument security factors of 5 or 10. It is understandable that if a current transformer has an instrument security factor of 5, as long as the primary current exceeds 5 times the rated current, the error will definitely be greater than 10%, thus protecting the instrument from burning out due to excessively high primary current. To meet the requirements of the instrument security factor, the cores of windings for measurement and metering are usually made of amorphous alloys or permalloy, which have high initial permeability and low saturation magnetic flux density. From the analysis above, it is clear that the requirements for the secondary windings of current transformers for protection and measurement are opposite. Thus, even if a measuring winding has a class of 3, it cannot substitute for a protection winding of class 5P.

(2) Determination of Protection Accuracy

Should we choose 5P or 10P for protection accuracy? Generally speaking, 10P level is sufficient for short-circuit protection. Didn’t we have only 10% error curves for current transformers in the past? Moreover, mechanical instruments used at that time were much less sensitive and accurate than today's electronic instruments. Generally, as long as the required sensitivity is met, the reliability of action can be ensured. The required sensitivity already considers the 10% error of the current transformer and other error factors of the protection device.

Common short-circuit protection setpoints are hardly more than 10 times the rated current, and the protection accuracy limit factor is generally above 15. As long as the load does not exceed the rated capacity of the current transformer, the error of the current transformer within the sensitivity range of action does not exceed 10%. Even if the actual current is more than 15 times the rated current and the error exceeds 10%, it does not affect the reliability of protection action, because the sensitivity coefficient is higher at that point. It is worth noting that even if the current exceeds 15 times the rated current, if the load carried at that time is less than the rated load of the protection winding, the short-circuit current can be even higher without necessarily exceeding a 10% comprehensive error, which can be specifically checked against the curve of the accuracy limit factor in relation to secondary load. Some might think of reducing the accuracy limit factor to artificially overload the secondary side load, thereby increasing the load capacity of the current transformer. This approach is not advisable as the secondary winding should not work under overload for a long time.

When high protection action accuracy is required, 5P class can be chosen, but it is not necessary for short-circuit protection. In cases of high sensitivity, a lower accuracy does not affect the accuracy of action; thus, 5P class is rarely used in actual engineering design. However, when the sensitivity coefficient is low, choosing 5P class is reasonable. With other parameters being equal, the price difference between 5P and 10P class current transformers is not significant. Current transformers marked with 10P class, when tested in the field, often significantly exceed their marked accuracy.

(3) The rated capacity should not be chosen too large, nor should the accuracy limit factor be too high.

There have been instances where designers chose current transformers with too large a capacity and too high an accuracy limit factor. For example, in a wind power project, the current transformers in a 40.5kV switchgear were rated at 0.2S/10P30/10P30/10P30 with a capacity of 15/30/30/30(VA) and a ratio of 2500/1(A). Such specifications are difficult for manufacturers to produce, with all three protection windings having an accuracy limit factor of 30 and a capacity of 30VA each. The production challenge lies in the high accuracy limit factor, requiring a large core cross-sectional area to prevent saturation under high short-circuit currents. Moreover, a large capacity requires a large core cross-sectional area to avoid overheating of the core, and a larger wire diameter for the secondary windings. Furthermore, the example requires four secondary windings, and comparing secondary currents of 1A to 5A, the number of turns for 1A is five times that for 5A. Considering all these factors, the current transformer needs to be very large, making it difficult to fit inside the switchgear. Typically, a 35kV level current transformer weighs up to 250kg, and installing three current transformers in a 40.5kV switchgear is challenging. The base plate for installing the current transformers needs to be reinforced with channel steel and insulated partitions added. As can be seen, even if such specifications could be manufactured, fitting them inside the cabinet would be problematic, not to mention the prohibitively high cost. Currently, protection uses microprocessors with a small capacity of 1VA, and short-circuit currents do not reach 30 times the rated current, making the selection of such parameters puzzling. Mechanical relay protection devices are no longer used, replaced by microprocessor-based protection devices requiring less than 1VA of current signal power. Similarly, for measuring or metering electronic instruments, the power of the current signal does not exceed 1VA. Choosing current transformers with excessively large capacities offers no benefits.

07 Determination of the Number and Sets of Current Transformers in the Main Circuit of the Switchgear

(1) Determination of the Number of Current Transformer Sets

In medium voltage systems, it is common to use one set of current transformers (some sets consist of two, others of three current transformers). The number of secondary windings of a current transformer should not exceed five. If more than five are necessary, other parameters such as the transformation ratio, the capacity of each winding, and the accuracy limit factor must be considered. Only after a comprehensive evaluation can the number of secondary windings be determined, as it cannot be arbitrarily decided by the user. If a current transformer manufacturer cannot meet the requirements for the number of secondary windings, and if the switchgear allows, two sets of current transformers can be chosen to be placed on both sides of the circuit breaker, ensuring that the circuit breaker is within the crossover protection range. However, it is challenging to arrange two sets of current transformers within the limited space of a compact switchgear. In cases of necessity, some current transformers are placed in the busbar compartment, or custom-made trolley contact boxes with integrated current transformers are used.

(2) Determination of the Number of Current Transformers per Set

① In ungrounded neutral systems, two current transformers are generally used. Installing current transformers in two phases (usually in phases A and C) is sufficient, as the current of the third phase can be determined by the vector sum of the currents in the other two phases. This arrangement increases the distance between the two current transformers, which is beneficial for safety and cost-saving.

② In directly grounded neutral systems, a current transformer should be installed in each phase because significant single-phase ground fault currents can occur, and the vector sum of the currents in two phases does not equal the current in the third phase.

③ Differential protection generally requires three current transformers, with two sets installed on both sides of the protected equipment, meaning one current transformer per phase.

④ In cases of severe load imbalance, it is advisable to install a current transformer in each phase, i.e., using three current transformers, such as in the circuits of electric arc steel furnaces.

⑤ To increase the load capacity, three current transformers are used, one in each phase, which significantly increases the total capacity compared to using only two current transformers. Of course, increasing the rated capacity of the current transformers is an alternative to increasing their number.

⑥ For overcurrent protection on the high voltage side of transformers and as a backup for ground fault protection on the low voltage side, three current transformers should be installed on the high voltage side, connected in a complete star configuration.

⑦ For detecting zero-sequence currents, dedicated zero-sequence current transformers are widely used, especially in cable feeders. However, three current transformers can also be used by summing the secondary currents' vector of the three transformers, connecting their secondary windings in a star configuration, and deriving the zero-sequence current from the neutral point. In ungrounded systems, the zero-sequence current equals three times the capacitive current of one phase to ground, which is relatively small, making this method impractical for measuring zero-sequence currents due to large errors.

⑧ For the protection and monitoring of critical equipment, such as large generators, regardless of whether their neutral points are grounded, a current transformer should be installed in each phase.

⑨ To enhance the sensitivity of protection against two-phase ground faults at different locations, a current transformer should be installed in each phase, and all fault circuits should be isolated.