Technical information General Regulations WTL current transformers are special transformers which transfer the primary circuits with prescribed accuracy in smaller secondary circuits. Primary and secondary circuits are galvanic separated. Generally current transformers are distinguished for measuring purpose (instrument transformer) and protection purpose (protection transformer). Instrument transformers are assigned for connection direct reading, writing and counting measuring and protecting equipment; see the additional description for generator protection transformer. WTL current transformers correspond to the international standards such as VDE 0414/01.94; IEC 61869-2; NFC 42-502/2.74; BS 7262; ANSI C57.13; DIN 42600 and to the basic requirements of insulation coordination for equipment of use under low and medium voltages. Explanation of the general meanings for current transformer acc. to VDE / IEC rated current: is the nominal value of the primary and secondary current for which the CT is designed; standard specificity primary currents are 5, 10, 15, 20,25, 30, 40, 50, 60, 75 A and the decade multiple of these value. Standard specificity secondary currents are: 5 A, 2 A and 1 A. transformation ratio: is the ratio of the primary rated current in regulation to the secondary rated current, marked ratio is described as a not shortened faction. burden: the impedance of the secondary circuit in ohms ( O ) and power-factor. The burden is usually expressed as the apparent power in voltamperes absorbed at a specified powerfactor and at the rated secondary current. rated burden: the value of the burden on which the accuracy requirements of this specifi cation are based. rated output: the value of the apparent power ( in volt-amperes at a specified power-factor ) which the transformer is intended to supply to the secondary circuit at the rated secondary current and with rated burden connected to it. accuracy class: current transformer will be divided accordingly to the accuracy in different classes. In each class certain limits of error shall be respected. There is a difference in current transformer for testing and protection purpose. 1
Some application-samples: current transformer class application for measuring 0,2 counting 0,5 accuracy measuring + counting 1 counter and writer 3 rough measurement, relay, regulatorr for protection 5 P protection device with large 10 P protection device with normal Instrument security factor: indicates the multiple of the rated primary current of a metering CT when loaded by rated burden and exceeding the rated value of the composite error of 10 %. The instrument security factor shall be indicated following the indication of corresponding rated output. Example: 10VA class 0.5FS5 min. 10 % composite error at 10 times rated current. Accuracy limit factor: indicates the multiple of the rated primary current of the CT for protection purpose when loaded by rated burden and not exceeding 5 % or 10 % of composite error. The accuracy limit factor shall be indicated following the corresponding output an accuracy class. Example: 5VA 10P10 max. 10 % composite error at 10 times rated current. 10VA 5P20 max. 5 % composite error at 20 times rated current. rated continuous-thermal current: is the value of the current which can be permitted to flow continuously in the current transformer when loaded with his rated burden without exceeding the limit of the allowed temperature rise. rated short time thermal current: is the r.m.s. value of the primary current which the CT can withstand for one second without suffering harmful effects, secondary winding being shortcircuited. The mentioned current transformers in the list are designed for min. Ith = 60 times primary rated current. rated dynamic current: is the peak value of the primary current which the CT can withstand without being damaged electrically or mechanically by the resulting electromagnetic forces. 2
Designation of current transformers connecting terminal The primary terminals of the current transformers are marked by the letters P1 und P2. The secondary terminals are marked by the letters S1 and S2. Executive forms We separate bushing-type current transformer and winding-type current transformer. The advantage of the bushing-type current transformer is, that no separate primary current circuit is needed. This transformer will be slide and fixed on conductor-rail or cables. The bushing hole is adapted to the usual rail dimensions, resp. cable-diameter. The necessary fi - xing material as terminal piece or base-fixing has to be mentioned with the order if required. Winding-type current transformer will be inserted by lower primary current diameter and larger power demand. The winding current transformer has got a fi xed installed primary winding which must be integrated in the power course. The tunnel current transformer is a normal bushing-type current transformer through which the primary conductor is threaded several times. The number of threadings determines the ratio of transformation. observe this: Primary current of the current transformer: measuring current = the number of threadings. Output and class accuracy remain unchanged. Example 1: Current transformer for primary rated current 20 A, secondary 5 A, output 10 VA in class 1FS5 Select: WTL current transformer S 70/30, 100/5 A, 10 VA in class 1 primary rated current of the CT measuring current 100 A 20 A 5 threadings must be taken to get a correct measurement. calculation: = 5 Example 2: Current transformer 50/5A, output 5 VA in class 1FS5 Select: WTL current transformer S 60/30, 150/5 A, 5 VA in class 1 primary rated current of the CT measuring current 150 A 50 A 3 threadings must be taken to get a correct measurement. calculation: = 3 3
Special Design frequency coverage: If an other frequency than 50-60 Hz is required the frequency range goes up to max. 400 Hz, without any clarification. At 16 2/3 Hz the transformer output is reduced to 1/3 of its value at 50 Hz. The pricelist should indicate a 3-times higher output at 50 Hz. change over: WTL current transformers can be delivered with primary or secondary switch-over. CT s without primary winding ( bushing-type CT ) can be manufactured with secondary taps. Relevant burden and accuracy class shall be required for each transformation ratio if not otherwise specified. Power-consumption of the conventional measuring instruments and relay by 5 A Instruments: indicating moving-iron current transformer indicating bimetal current transformer Self-consumption: 3-4 VA indicating power meter recording power meter indicating phase meter each phase active power meter reactive volt-ampere meter measuring transducer for current measuring transducer for power, each current circuit 1,5 VA 6-12 VA 5 VA 1,2 VA 2,5 VA 1-3 VA Measurement-line self-consumption: by 5 A by 1 A 10m two-wire line 1,5 qm copper 6 VA 0,24 VA 10m two-wire line 2,5 qm copper 3,6 VA 0,14 VA 10m two-wire line 4 qm copper 2,2 VA 0,09 VA 10m two-wire line 6 qm copper 1,5 VA 0,06 VA 10m two-wire line 10 qm copper 0,9 VA 0,04 VA 4
Relays: dependent current relay electronic independent current relay electronic dependent current relay electro-mechanical independent current relay electro-mechanical power watchman electronically reverse power relay electronically Self-consumption: 0,5-3 VA generator-differential protection thermical overload relay magn. quick-response release active power adjuster electronically quick distance relay generator-earth leakage relay power relay with transformer current release 0,5-3 VA 2 VA 2-30 VA 0,1-2 VA 10-30 VA 5