CURRENT TRANSFORMER (CT)

  • Definition– A current transformer is a device that is used to produce low ac current in the secondary winding which is proportional to the high ac current in the primary winding.

It is used for both metering & protection system. So, with the help of CT, high value of current can be measured easily by using low value meters.

 

Construction – It consist of a primary winding & secondary winding. The primary winding may be a conductor, strip etc whose current is to be reduced. The secondary winding is wounded on a core. The number of turns of secondary winding depends on the primary current & output of secondary current. Standard secondary output current is either 1A or 5A.

Working principle –It works on the principle of transformer; It is just like a small step-up transformer which reduces the current in secondary winding & increases the secondary voltage. In CT, primary winding has few turns only such as either one turn or two turn etc.

CTR (CURRENT TRANSFORMER RATIO)

CTR stands for Current Transformer Ratio. It is the ratio between primary & secondary current of a CT. For example, a CT is available of 100/5A where 100A represents maximum value of primary current & 5A represents maximum value of secondary current. Value of secondary current varies in proportion to the primary current. So, here CTR is 20 (100/5 = 20). It means primary current is 20 times greater than the secondary current.

For CT 400/5A, CTR = 80 (which is 400/5 = 80), it means primary current is 80 times greater than secondary current.As CT is just like any other transformer, it must satisfy following equation –

T.R = Np/Ns = Is/Ip

Where Np = No of turns of primary winding,

Ns = No of turns of secondary winding,

Ip = Current in  primary winding,

Is = Current in secondary winding,

EFFECT OF NUMBER OF TURNS OF PRIMARY WINDING

TYPES OF CURRENT TRANSFORMER

There are three types of Current Transformer available – a) Wound CT, b) Toroidal CT & c) Bar-type CT  –

  • Wound Current Transformer– The transformers primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. The magnitude of the secondary current is dependent on the turn ratio of the transformer.
  • Toroidal Current Transformer– These do not contain a primary winding. Instead, the line that carries the current flowing in the network is threaded through a window or hole in the toroidal transformer. Some current transformers have a “split core” which allows it to be opened, installed, and closed, without disconnecting the circuit to which they are attached.
  • Bar-type Current Transformer– This type of current transformer uses the actual cable or bus-bar of the main circuit as the primary winding, which is equivalent to a single turn. They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device.

IMPORTANT TERMS RELATED WITH CT

CT BurdenThe burden of a CT is defined by the load of the device which is connected with CT and impedance offered by the secondary winding of CT. It is represented by the VA.The rated VA indicates the load that transformer can take.

The CT should never be 100% loaded as the burden of CT can increase with age due to increase of resistance of connecting wires, change in temperature, loosening of connections etc.

Accuracy of CTs – Accuracy defines the highest permissible percentage error at the rated current. CTs are classified into two categories –a) metering CTs & protection CTs.

Metering CT – High accuracy CTs are used for metering as they have low saturation point. CTs are available in following accuracy class – 0.1, 0.2, 0.5,1,3,5. These values indicate percentage error at the rated primary current. It means a CT of 50/5A with 0.1 accuracy will have a max error of 0.1 when 50A current passes through the primary.

Metering CTs are designed in such a way that CT is not damaged by high current during fault. During fault, CT gets saturated & output stays in the range of measuring instruments.

Protection CT – Protection CTs have different characteristics than metering CT. Protection CTs have high saturation point as it has to continuously sense fault current even during fault also. These CTs have low accuracy & are classified as 5P10, 10P10 etc.

Example – CT with class 10P10 – Where first letter 10P indicates the maximum(10%) percentage error & last number 10 indicates the number of times the rated current.

Application of CTs

  • Class 0.1 or 0.3 – for Process metering,
  • Class 0.5 or 1.0 – for Commercial metering,
  • Class 1 or 3 – for ammeters,
  • Class 5P10 or 5P20 – for protection,

KNEE POINT SATURATION VOLTAGE

Definition– It is defined as the voltage at which 10% increase in voltage of CT secondary results in 50% in secondary current.

Let us understand this by this method – An AC voltage applied to the secondary of the CT with primary open, when voltage increased by 10% which causes 50% increase in magnetizing current. This happens because E2άØ (magnetic flux) as E2= 4.44 ØfT2 where Ø is produced by exciting current (Ie). There is a nonlinear relationship between Ø &Ie, after a certain period of exciting current, flux will not increase so rapidly further as the core of CT is made up of CRGO steel material which has its own saturation level.

It is an important factor for protection CTs & protection CTs are also termed as PS (Protection Class). This is related with the saturation of core. Knee point is very important for differential & restricted earth fault protection schemes because there should not be tripping of transformer when fault occurs outside the protection zone. Even If normal CTs are provided with high accuracy (not PS rated) & fault occurs outside the protection zone, the fault current will travel towards protected zone (from secondary side)  then due to different Knee point saturation voltage of both normal CTs, Power transformer may trip. That is whyit is very important that for differential & REF protection only same PS rated should be used.

INSTRUMENT SECURITY FACTOR (ISF) OF CURRENT TRANSFORMER

ISF is the ratio of applied maximum primary current to the rated primary current of a CT. The applied maximum primary current is the value beyond which core of CT gets saturated. ISF is an important factor of CT while selecting an instrument which is to be connected with secondary of CT. A low value of ISF is always better.

Ex – ISF-1.5 & ISF-2. Suppose a CT of ratio 100/5A has ISF 1.5, it means CT gets saturated at 150A of primary current (100X1.5). Similarly, CT gets saturated at 200A with ISF -2. In other words we can say that in ISF – 1.5 case, instrument connected with secondary will get maximum 7.5 A & 10A in ISF – 2 CT because after 7.5A & 10A CT will get saturated & secondary current will not increase with primary current after 150% (for ISF 1.5)  & 200% (ISF – 2). ISF factor is helpful in overload & short circuit condition.

ACCURACY LIMIT FACTOR (ALF) OF CT

It is related with the saturation of core for protection CTs. It is the value of primary current beyond which core starts saturating, once core of CT is saturated, current in secondary winding will not increase with increase in primary winding. So saturation limit of core should be high many times (not infinite) in protection CTs as CT has to follow fault current to operate the relay. Low value ALF is used for metering CTs.

CORE BALANCE CURRENT TRANSFORMER (CBCT)– It is a kind of current transformer which is used for earth fault protection. It uses all phases of a circuit as primary winding when there is no fault the vector sum of all 3 phases is zero so there is no current in secondary winding. But under earth fault condition, the vector sum of all three phases would not remain zero & an unbalanced resultant current will set up a flux in the core &a current would flow in the secondary winding. A CBCT uses 3phsae 3core or 3 nos single core cables only which pass through the hole of the CBCT.

CBCTs are manufactured with one core & one secondary winding. The number of turns of secondary winding does not need to be related to rated current of feeder/cable as no secondary current flows under normal balanced conditions.

SELECTION OF CBCTFollowing points to remember while purchasing a CBCT –

  • Cable size or dia of cable as cable has to pass through the hole of CBCT,
  • Minimum earth leakage to be detected & relay combination,
  • Specification of relay including setting range & burden,
  • Distance between relay & CBCT (if greater than 10 meters).
  • Check Relay ratio ex 12.5/5A or 25/5A…….

DUAL RATIO/CORE CTs

These are two types of CTs – 1) Dual Ratio CT & 2) Dual Core CT.

  • Dual Ratio CT – This type of CT is having center tapped secondary winding. It is applicable where two secondary outputsis required from one CT. This is achieved by adding a tape in the secondary winding & the ratio obtained by the tape is one-half of the full secondary winding.

In this CT, both full & half secondary winding can’t be used simultaneously. Only one winding either full or half will be used at a time.

  • Dual Core CT– Such type of CT has two cores, it means CT has two secondary windings & one primary winding. Dual core CT can be used for two simultaneous applications like metering & relay protection. In case if one winding is used & other is un-used then other winding must be short circuited to avoid high induced voltage in unused winding.