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The Principle Of Selection And Application Of Current Transformer
Jun 26, 2018

1, rated primary voltage and current

The rated primary voltage of the current transformer should be equal to or greater than the rated primary voltage of the circuit, and the insulation level should meet the relevant standards.

The rated primary current (Ipn) of the current transformer should be selected according to the rated current or maximum working current of the primary equipment, and should be able to withstand the rated continuous thermal current (Icth) of the circuit, the rated short-time thermal current (Ith) and the dynamic stable current (Idyn).

At the same time, the selection of the rated primary current should make the two current under the rated flow ratio condition under normal operation and short circuit, and meet the requirement of the selectivity and accuracy of the setting value of the circuit protection device or the requirement of measurement and measurement accuracy.

The standard values of rated primary current (Ipn) are: 10, 12.5, 15, 20, 25, 30, 40, 50, 60, 75, and their decimal multiplier or decimal.


2.1 rated two current

The current transformer rated two times current (Isn) has two types, 1A and 5A. For new power plants and substations, the rated current transformer rated two times at all levels of voltage should be 1A to reduce the two load of the current transformer, the section of the two cable can be reduced, and the investment is saved. For example, when the original current transformer used in the expansion project is 5A, the rated two current can be 5A.

The current transformer in a factory station is rated two times current, and 1A and 5A are allowed simultaneously. However, the same current value is generally adopted for the rated two current of the current transformer at the same voltage level.

2.2 two times load

The two load available impedance of current transformer is Zb (omega) or capacity Sb (VA). The relationship between the two is


When the current transformer is rated at a two current Isn of 5A, the value Sb = 25Zb, when the current transformer is rated at the two current Isn 1A, Sb = Zb.

The accuracy of the current transformer and the allowable limit current are all related to the two load. It is necessary to choose the rated value of the two loads reasonably and carry out the corresponding checking.

Due to the widespread application of electronic instruments and microcomputer relay protection, the rated two current of the transformer is widely used 1A, as well as the protection and control of the local and other factors. The load of the two circuit is greatly reduced, and the corresponding two load of the current transformer should also choose a lower rating to reduce the cost and improve its structure and performance. For example, the inverted structure is used.

The two load rating of current transformer (Sbn, expressed in VA) can be selected 2.5, 5, 7.5, 10, 15, 20, 30 and 40VA according to requirements. In some special cases, larger ratings can be used.

Brief introduction of technical performance of 3 current transformer

As a measuring instrument, a measuring device and a current source for relay protection, the current transformer is divided into measuring level and protection level according to its basic functions.


4 protection current transformer

Classification of 4.1 protective current transformers

The protective current transformer is divided into two main categories:

(1) the P class (P is protected) current transformer. Including PR and PX classes. The accuracy limit of this kind of current transformer is determined by the composite error or the inflexion point of the excitation characteristic when the primary current is a steady symmetrical current.

Class P protected current transformer protection current transformer class P

The current limit transformer, which is defined as a composite error (c) under steady-state symmetrical primary current, is not limited to residual magnetism.

Class PR protected current transformer protection current transformer class PR

The remanence coefficient has a current transformer with a specified limit. In some cases, the constant value of the two loop time and / or the limit of the two winding resistance can also be specified.

Class PX protected current transformer protection current transformer class PX

A low leakage current transformer is sufficient to determine the performance of the current transformer, when the two excitation characteristics, the two winding resistance, the two load resistance and the turn number ratio are known.

1) the accuracy level of the P and PR current transformers is named as the percentage of the maximum permissible composite errors under the rated accurate limit of primary current, and the standard accuracy level is 5P, 10P, 5PR and 10PR.

2) current and phase errors and composite errors of class P and class PR current transformers under rated frequency and rated load should not exceed the limits specified in table 4-1.

Table 4-1 error limits for class P and PR current transformers



3) the residual magnetic coefficient of class PR current transformer should be less than 10%. In some cases, two time constant Ts value should be specified to limit compound error.

4) generator or transformer main circuit, 220kV and above voltage line should adopt 5P or 5PR class current transformer with small compound error (small wave distortion). 10P or 10PR class current transformers can be used in other circuits.

5) the P and PR class protective current transformers can meet the requirement of compound error. The accuracy limit coefficient Kalf is generally desirable for 5, 10, 15, 20, 30 and 40.

(2) class TP (TP means transient protection) current transformer. The accurate limit of this type of current transformer is determined by taking into account the periodic and non periodic components of the primary current, and the peak error of a specified transient working cycle. This type of current transformer is suitable for considering the transient effect of the non periodic components in short-circuit current.

Class TP protected current transformer protection current transformer class TP

A protective current transformer that satisfies the transient performance requirements of short-circuit current with non periodic components. Class TP current transformers are classified into the following levels and are defined as follows:

Class TPS: low leakage magnetic current transformer, whose performance is specified by the two excitation characteristic and the limit value of turn ratio error. There is no limit to the remanence.

Class TPX: the accuracy limit is specified as the peak instantaneous error in the specified transient working cycle. There is no limit to the remanence.

Class TPY: the accuracy limit is specified as the peak instantaneous error in the specified transient working cycle. The remanence does not exceed 10% of the saturation magnetic flux.

TPZ level: the accuracy limit is defined as the peak instantaneous AC component error with the maximum DC offset under a specified two loop time constant. There is no DC component error limit. Remanence can actually be ignored.

Type selection of 4.2 protection current transformer

(1) 330kV ~ 1000kV system protection, high voltage side 330kV to 1000kV transformer and 300MW and above generator transformer unit differential protection of current transformer, because the system once time constant is larger, the current transformer transient saturation is serious, resulting in the serious consequences of the protection misoperation or rejection. Therefore, the selected current transformer should ensure that there is no transient saturation in the actual short-circuit operation cycle, that is, the transient error does not exceed the specified value. TP type transformer is generally used.

(2) 220kV system protection, high voltage side 220kV transformer differential protection, 100MW to 200MW generator transformer unit and current transformer used for large capacity motor differential protection, transient saturation problem and its influence result relatively light, can be calculated according to steady state short-circuit condition, and reduce possible transient saturation. There is an adequate allowance for margin. P, PR and PX current transformers are generally chosen. Class PR can limit the influence of remanence, and it can be widely used when conditions are available. In order to consider the transient effect, the accurate limit coefficient Kalf of the selected current transformer should be improved, and the given transient coefficient Ks = Kalf /Kpcf should be determined according to the application and operation experience.

1) the transient coefficient of external faults of 100MW to 200MW generator transformer set should not be lower than 10.

2) the transient coefficient of the 220kV system should not be lower than 2. See IEEE Std C37.110-1996 regulations.

(3) the current transformer used for system protection under 110kV and below is generally chosen according to the steady state condition, and P type mutual inductor is adopted.

(4) the selection of current transformer used in high voltage bus differential protection, the short circuit current is very large in the case of bus fault, and the current difference of each transformer may be very big when the external short circuit is short circuited. Even if the current transformers with the same characteristics are chosen on each side, the degree of transient saturation may be very inconsistent. For this reason, busbar differential protection has the capability of saturating transient transformer. In engineering applications, suitable transformers can be selected according to the requirements of steady state short-circuit current or protective devices.

(5) the transformer of grounding protection for non direct grounding system can adopt zero sequence filter, special cable type or bus type zero sequence current transformer, which is composed of three phase current transformer.

(6) the application of PR current transformer

The PR current transformer is a protective current transformer with the remanence coefficient less than 10% under the steady state symmetrical primary current. The other characteristic parameters are the same as that of the P type current transformer. The type of current transformer has a small air gap on the iron core to ensure that the remanence coefficient is less than 10%.

In the formulation of the DL/T866-2004 "current transformer and voltage transformer selection and calculation guidelines", since there is no experience in manufacturing PR current transformers at that time in China, the "PR type current transformer can be popularized when conditions are available" in the guideline 6.2.2.3.

In the twenty-first issue of "power automation" magazine in 2007, Jing Minhui published "analysis and Countermeasures of the reason of saturation of type P Current Transformers" in the 2007 issue of "power automation" magazine. In view of the misoperation of 3 220kV line fault longitudinal differential protection in the actual power grid since 2006, the remanence of the current transformer at one end of the line is produced by the analysis of the current transformer. The current transformer is saturated, and the current transformer is not saturated at the other end of the line, so that the differential current causes the protection action when the fault is out of area. Therefore, it is suggested to change the DL/T866-2004 6.2.2.3 rule to "recommend the use of PR type current transformer".

At present, many domestic current transformer manufacturers have already produced the experience of producing TPY current transformer with air gap, so it is feasible to produce PR type current transformer. The above suggestions are reasonable and feasible. It is beneficial to ensure the safe and stable operation of the power grid and prevent the saturation of the current transformer due to the remanence of the current transformer. The protection misoperation is made to improve the correct action rate of protection. Therefore, it is recommended to adopt class PR current transformers in the new and extended power plants and substation projects. It is suggested that the application in 220kV and 110kV systems with differential current protection and distance protection should be popularized and then extended to other voltage grade systems.

5 type of current transformer for measurement and measurement

The technical specification for the design of DL/T5202-2004 electric energy metering system and the technical specification for the design of DL/T5137-2001 electric measurement and electric energy metering device are given.  The current transformers used for measurement and measurement are 0.2S, 0.5s and 0.2,0.5,1 respectively.

Selection of 6 variable ratio current transformer parameters

Variable ratio current transformer: in a current transformer, a current transformer with a variety of current ratios is obtained by using a series or parallel connection of each section of a primary winding, or / and the method of using two winding taps. When the current transformer has multiple two windings, and the rated current ratio of the two winding is different, it is also called compound transformer ratio current transformer. The current ratio of measurement level and protection stage can be different.

(1) the series and parallel mode of primary windings

Two times of current ratio can be obtained by using primary winding in series or parallel mode. Example: 2x600/5A: the primary winding is 600/5A when connected in series, and the primary winding is 1200/5A when connected in parallel. It is generally used on current transformers with voltage levels of 66kV and above. For 35kV and below voltage level, it is seldom used because of the difficulty in product structure layout.


(2) two winding tap mode

The two winding taps can be used in the theory of any part between the ends of the windings, usually in the middle, and the two winding taps are used only in measuring the current transformer. The protection level is reduced by the current ratio obtained by tap. Therefore, the protection stage generally does not use a two tapping mode to achieve a smaller current ratio.


(3) primary winding series parallel connection and two winding tapping mode are adopted at the same time:

At the same time, a series of parallel and two winding taps can be used to obtain more current ratios.


(4) the current transformer with series parallel connection with primary winding should take into account the steady state performance of the short circuit current.

The dynamic stability current of the primary winding is nearly half of that of the parallel mode. In other words, the dynamic stability current of the primary winding is two times that of the series connection. Therefore, the short circuit stability performance of the current transformer should be determined on the basis of the performance of the series mode of the primary winding in determining the short circuit performance of the current transformer.

The two winding tap mode is independent of the dynamic and thermal stability of the product, but it is related to the current density of the two winding. It is necessary to ensure that the current density of the two winding is not more than 180A/mm2 (copper wire) when the rated short time heat current is rated; 120A/mm2 (aluminum wire).

For the current transformer with two winding taps, the output capacity should be considered.

The output capacity S1 of the two winding tap (tap value is K, K is integer within 10), and the rated output capacity Sn at full turn, the relationship is:

Example: in 1/2 tap, S1= is 1/4 of full turn. The output capacity of the general tap is determined by the upper level. In fact, the product can be made more than the calculated value (the specific output capacity should be negotiated between the user and the manufacturer).

7 Band S level measuring current transformer

0.2S and 0.5S are special purpose current transformers. When connected with a wide load watt hour meter (over 4 times and above the overload meter), the measurement current of the current transformer can be measured between 50mA and 6A (i.e. a certain current between 1% to 120% of the rated current 5A). 0.2S and 0.5S class current transformers should be used at metering points.

From "error limits of current transformers with S and common metering levels"


Add: No.38 South Taoyuan Road, Yaozhuang Town,Jiashan County, Jiaxing City, Zhejiang Province, China
Contact person: Elva Zhuang&Sunny Ni
Tel:+86-573-84775555
Fax:+86-573-84776699
Phone:+86-13732570078
E-mail:sales9@great-relay.com
       sunny.ni@great-relay.com