Protection Current Transformer

Ring Type Current Transformer Protection Class Sensor

Item No.: PR
Protective Current Transformers are designed to measure the actual currents in power systems and to produce proportional currents in their secondary windings which are isolated from the main power circuit.
Description
Description
Introduce
 
Replica currents are used as inputs to protective relays which will automatically isolate part of a power circuit in the event of an abnormal or fault condition therein, yet emit other parts of the plant to continue in operation. 
Satisfactory operation of protective relays can depend on accurate representation of currents ranging from small leakage currents to very high overcurrent’s, requiring the protective current transformer to be linear, and therefore below magnetic saturation, at values up to perhaps 30 times full load current. 
This wide operating range means that protective current transformer requires to be constructed with larger cross-sections and heavier cores than equivalent current transformers used for measuring duties only. 
 
Features
Tough resilient flame retardant. 
Tropicalized design with Insulation Class A and thermal 105°C 
Totally enclosed in tough with self-extinguishing. 
 
Specification
 
Dimensions
 
Type Ratio ID(mm.) OD(mm.) Burden of class 5P10 Burden of class 5P20
5 7.5 10 15 20 5 7.5 10 15
HT Thickness(mm.) HT Thickness(mm.)
PR-30 60 30 80 35 35 - - - 35 35 - -
80 30 80 35 35 - - - 35 35 - -
100 30 80 35 35 - - - 35 35 - -
150 30 80 35 35 35 - - 35 35 35 -
200 30 80 35 35 35 - - 35 35 35 -
 
Type Ratio ID(mm.) OD(mm.) Burden of class 5P10 Burden of class 5P20
5 7.5 10 15 20 5 7.5 10 15
HT Thickness(mm.) HT Thickness(mm.)
PR-40 150 40 100 35 35 35 - - 35 35 35 -
200 40 100 35 35 35 - - 35 35 35 -
250 40 100 35 35 35 - - 35 35 35 -
300 40 100 35 35 35 - - 35 35 35 -
400 40 100 35 35 35 - - 35 35 35 -
 
Type Ratio ID(mm.) OD(mm.) Burden of class 5P10 Burden of class 5P20
5 7.5 10 15 20 5 7.5 10 15
HT Thickness(mm.) HT Thickness(mm.)
PR-45 200 45 100 35 35 35 - - 35 35 - -
250 45 100 35 35 35 35 - 35 35 - -
300 45 100 35 35 35 35 - 35 35 - -
400 45 100 35 35 35 35 - 35 35 35 -
 
Type Ratio ID(mm.) OD(mm.) Burden of class 5P10 Burden of class 5P20
5 7.5 10 15 20 5 7.5 10 15
HT Thickness(mm.) HT Thickness(mm.)
PR-60 400 60 100 35 35 35 35 - 35 35 35 -
500 60 100 35 35 35 35 35 35 35 35 -
600 60 100 35 35 35 35 35 35 35 35 35
800 60 100 35 35 35 35 35 35 35 35 35
 
Type Ratio ID(mm.) OD(mm.) Burden of class 5P10 Burden of class 5P20
5 7.5 10 15 20 5 7.5 10 15
HT Thickness(mm.) HT Thickness(mm.)
PR-70 600 70 115 35 35 35 35 35 35 35 35 35
800 70 115 35 35 35 35 35 35 35 35 35
1000 70 115 35 35 35 35 35 35 35 35 35
1200 70 115 35 35 35 35 35 35 35 35 35
 
Type Ratio ID(mm.) OD(mm.) Burden of class 5P10 Burden of class 5P20
5 7.5 10 15 20 5 7.5 10 15
HT Thickness(mm.) HT Thickness(mm.)
PR-85 800 85 125 35 35 35 35 35 35 35 35 35
1000 85 125 35 35 35 35 35 35 35 35 35
1200 85 125 35 35 35 35 35 35 35 35 35
1600 85 125 35 35 35 35 35 35 35 35 35
 
Type Ratio ID(mm.) OD(mm.) Burden of class 5P10 Burden of class 5P20
5 7.5 10 15 20 5 7.5 10 15
HT Thickness(mm.) HT Thickness(mm.)
PR-110 1200 110 155 35 35 35 35 35 35 35 35 35
1600 110 155 35 35 35 35 35 35 35 35 35
2000 110 155 35 35 35 35 35 35 35 35 35
2500 110 155 35 35 35 35 35 35 35 35 35

 

Type Ratio ID(mm.) OD(mm.) Burden of class 5P10 Burden of class 5P20
5 7.5 10 15 20 5 7.5 10 15
HT Thickness(mm.) HT Thickness(mm.)
PR-120 1600 120 165 35 35 35 35 35 35 35 35 35
2000 120 165 35 35 35 35 35 35 35 35 35
2500 120 165 35 35 35 35 35 35 35 35 35
3000 120 165 35 35 35 35 35 35 35 35 35

 

Type Ratio ID(mm.) OD(mm.) Burden of class 5P10 Burden of class 5P20
5 7.5 10 15 20 5 7.5 10 15
HT Thickness(mm.) HT Thickness(mm.)
PR-130 2500 130 175 35 35 35 35 35 35 35 35 35
3000 130 175 35 35 35 35 35 35 35 35 35
3500 130 175 35 35 35 35 35 35 35 35 35
4000 130 175 35 35 35 35 35 35 35 35 35

 

Type Ratio ID(mm.) OD(mm.) Burden of class 5P10 Burden of class 5P20
5 7.5 10 15 20 5 7.5 10 15
HT Thickness(mm.) HT Thickness(mm.)
PR-140 2500 140 185 - - - 35 35 - - - 35
3000 140 185 - - - 35 35 - - - 35
3500 140 185 - - - 35 35 - - - 35
4000 140 185 - - - 35 35 - - - 35

 

Type Ratio ID(mm.) OD(mm.) Burden of class 5P10 Burden of class 5P20
5 7.5 10 15 20 5 7.5 10 15
HT Thickness(mm.) HT Thickness(mm.)
PR-170 3000 170 205 - - - 35 35 - - - 35
3500 170 205 - - - 35 35 - - - 35
4000 170 205 - - - 35 35 - - - 35
5000 170 205 - - - 35 35 - - - 35
6000 170 205 - - - 35 35 - - - 35

 

Type Ratio ID(mm.) OD(mm.) Burden of class 5P10 Burden of class 5P20
5 7.5 10 15 20 5 7.5 10 15
HT Thickness(mm.) HT Thickness(mm.)
PR-190 3500 190 230 - - - 35 35 - - - 35
4000 190 230 - - - 35 35 - - - 35
5000 190 230 - - - 35 35 - - - 35
6000 190 230 - - - 35 35 - - - 35
8000 190 230 - - - 35 35 - - - 35

 

Type Ratio ID(mm.) OD(mm.) Burden of class 5P10 Burden of class 5P20
5 7.5 10 15 20 5 7.5 10 15
HT Thickness(mm.) HT Thickness(mm.)
PR-220 4000 220 260 - - - 35 35 - - - 35
5000 220 260 - - - 35 35 - - - 35
6000 220 260 - - - 35 35 - - - 35
8000 220 260 - - - 35 35 - - - 35

 

Type Ratio ID(mm.) OD(mm.) Burden of class 5P10 Burden of class 5P20
5 7.5 10 15 20 5 7.5 10 15
HT Thickness(mm.) HT Thickness(mm.)
PR-250 4000 250 290 - - - 35 35 - - - 35
5000 250 290 - - - 35 35 - - - 35
6000 250 290 - - - 35 35 - - - 35
8000 250 290 - - - 35 35 - - - 35

 

Type Ratio ID(mm.) OD(mm.) Burden of class 5P10 Burden of class 5P20
5 7.5 10 15 20 5 7.5 10 15
HT Thickness(mm.) HT Thickness(mm.)
PR-270 4000 270 310 - - - 35 35 - - - 35
5000 270 310 - - - 35 35 - - - 35
6000 270 310 - - - 35 35 - - - 35
8000 270 310 - - - 35 35 - - - 35
10000 270 310 - - - 35 35 - - - 35

 

Type Ratio ID(mm.) OD(mm.) Burden of class 5P10 Burden of class 5P20
5 7.5 10 15 20 5 7.5 10 15
HT Thickness(mm.) HT Thickness(mm.)
PR-300 4000 300 340 - - - 35 35 - - - 35
5000 300 340 - - - 35 35 - - - 35
6000 300 340 - - - 35 35 - - - 35
8000 300 340 - - - 35 35 - - - 35
10000 300 340 - - - 35 35 - - - 35

 

CAUTION: RELAY MANUFACTURER’S RECOMMENDATIONS SHOULD ALWAYS BE FOLLOWED.
 
For space and economy reasons, equipment designers should , avoid over-specifying protective current transformers. To specifying protective CT's and require some or all of the following information:
(a) Protected equipment and type of protection. 
(b) Maximum fault level for stability. 
(c) Sensitivity required. 
(d) Type of relay and likely setting. 
(e) Pilot wire resistance, or length of run and pilot wire used. 
(f) Primary conductor diameter or bulbar dimensions. 
 
Accuracy
 
Accuracy Limit Factor is defined as the multiple of rated primary current up to the transformer will comply with the requirements of 'Composite Error'. 
Composite Error is the deviation from an ideal CT (as in Current Error), but takes account of harmonics in the secondary current caused by non-linear magnetic conditions through the cycle at higher flux densities Standard Accuracy Limit Factors are 5, 10, 15, 20 and 30. 
The electrical requirements of protection current transformer can therefore be defined as: Ratio / VA Burden / Accuracy Class / Accuracy Limit Factor. 
Class 5P and 10P protective current transformers are generally used in overcurrent and unrestricted earth leakage protection. With the exception of simple trip relays, the protective device usually has an intentional time delay, thereby ensuring that the severe effect of transients has passed before the relay is called to operate.
 
Protection Current Transformers used for such applications are normally working under steady state conditions.
Three examples of such protection is shown. 
 
Three Phase Overcurrent Protection.

Unrestricted ground fault protection

(Detects ground fault on CT load side)

Combined 3Phase & Unrestricted EarthFault Protection

In some systems it may sufficient to simply detect a fault and isolate that circuit. 

However, in more discriminating schemes, it is necessary to ensure that a phase-to-phase fault does not operate the earth fault relay.

 

Phase Fault Stability 

 

Current transformers which are well matched and operating below saturation, will deliver no current to the earth fault relay, since 3-phase currents sum to zero.

If however, the transformers are badly matched, a spill current will arise which will trip the relay. Similarly, current transformers must operate below the saturation region, since, in a 3-phase system, third harmonics in the secondary are additive through the relay, thereby creating instability and erroneously tripping the earth fault relay .

 

Balanced Protection

 

In balanced systems of protection, electrical power is monitored by the protective CTs at two points in the system as shown.

 The protected zone is between the two CTs.

 If the power out differs from the power in, then a fault has   developed within the protected zone and the protection relay   will operate. A 'Through Fault' is one outside the protected   zone.

 Should such a fault occur, the relay protecting the protected   zone will not trip, since the power out will still equal the power   in. 

Numerous different types of balanced systems exist and advice may often have to be obtained from the relay manufacturer. However, in all cases Sensitivity and Stability must be considered.

 

Transient Effects 

 

Balanced protective systems may use time lag or high speed armature relays. Where high speed relays are used, operation of the relay occurs in the transient region of fault current, which includes the d.c. asymmetrical component. The buildup of magnetic flux may therefore be high enough to preclude the possibility of avoiding the saturation region. 

The resulting transient instability can fortunately be overcome using some of the following techniques:

(a) Relays incorporating capacitors to block the dc asymmetrical component. 

(b) Biased relays, where dc asymmetrical currents are compensated by anti-phase coils

(c) Stabilizing resistors in series with current operated relays, or in parallel with voltage-operated relays. Threes limit the spill current (or voltage) to a maximum value below the setting value. 

For series resistors in current operated armature relays. 

Rs = Vkp - VA 

 2Ir/Ir 

Where      Rs = value of stabilizing resistor in ohms  Vkp = CT knee point voltage  VA = relay burden (typically 3VA)  Ir = relay setting current

The value of Rs varies with each fault setting. An adjustable resistor is therefore required for optimum results. Often a fixed resistor suitable for mid-setting will suffice.

 

Sensitivity

 

Sensitivity is defined as the lowest value of primary fault current, within the protected zone, which will cause the relay to operate. To provide fast operation on an in-zone fault, the current transformer should have a 'Knee-Point Voltage' at least twice the setting voltage of the relay.

The 'Knee-Point Voltage' (Vkp) is defined as the secondary voltage at which an increase of 10% produces an increase in magnetizing current of 50%.It is the secondary voltage above which the CT is in magnetic saturation. Differential relays may be set to a required sensitivity but will operate at some higher value depending on the magnetizing currents of the CTs, for example: 

 

The diagram shows a restricted earth fault system with the relay fed from 400/5 CTs.

 The relay may be set at 10%, but, it requires more than 40A to operate the relay,   since the CT in the faulty phase, has to deliver its own magnetizing current and that   of the other CTs in addition to the relay operating current.

 Where 

Primary Operating Current (P.O.C.) = K(Ir + nle) 

K = CT Ratio 

Ir = Relay set current 

Ie = CT magnetizing current 

n = Number of CTs in parallel.

 E/Fault Relay, set to trip at 10% Nutral CT Supplies ie. 

 40A Pri. Fault, will not trip until Relay+ Mag. 

 Current fault is 56A. Of four CT’s

That quality whereby a protective system remains in-operative under all conditions other than those for which it is designed to operate, i.e., an in-zone fault. Stability is definded as ratio of the maximum though foult cuurent at which the system is stable to nominal full load cuurent. 
Good quality current transformers will produce linear output up to the defined kneepoint voltage (Vkp).
Typically, 
Vkp = 2lf (Rs + Rp) for stability, 
Where    If = max. through fault secondary current at stability limit     Rs = CT secondary winding resistance     RP = loop lead resistance from CT to Relay
Technical Parameter

 

Temperature range  –20°C to 70°C 
Protection class  5P10, 5P20 etc. 
Test voltage  4Kv 50Hz. 1min. 
Operating voltage  0.66 KV 
Secondary  1A or 5A
Standard EC44-1, IEC185, BS3938, DIN42600 

 

Accuracy test

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Tel:13968747975
Add:(Zhejiang Lierde Relay Co., Ltd.) Xinguang Ind. Zone, Liushi Town, Wenzhou, Zhejiang, China
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