VT guard Frequntly Asked Questions

Transcription

1 VT guard Frequntly Asked Questions I.Technical Informations a) Another possibility of connecting ( screws?) or suppling together with steel connector (would it be possible?) VT guard is connected via DIN TS 35 rail. Is a standart rail used for connecting of other equipment. We think that every other type of connection will be more complicated. b) No information in the catalog list and in instruction list about red lamp ( function) The Red LED diode indicate earth fault c) Do they have enough space in the cubicle (task for the customer) Please check on Your side d) Phase test - how long time it is tested e) 16 2/3Hz is possible No problem to use it by frequenz 16 2/3Hz f) Application for temperatures from - 25 C to - 50 C The test are at this moment not done. g) how does it change its characteristic under ferroresonance conditions? The ferroresonance phenomenon may take place in the case when the core of an inductive device becomes saturated, and its current-flux characteristic becomes non-linear. In practice the ferroresonant oscillations may be initiated by momentary saturation the core of the inductive element resulting from e.g. switching operation or other type of event resulting in a transient overvoltage in the system. The undamped ferroresonant osillations in power system are dangerous to the equipment installed due to large overcurrents and/or overvolatages which may ultimately lead to permanent equipment damage. For elimination of ferroresonance phenomenon is used normaly dumping resistor. Nominal value of dumping resistor is important for effective and quick dumping - see pictures. There is current versus time dependance. Dumping resistor 17ohm in first case - not effective long time. Dumping resistor 13ohm in the second case - effective dumping. 20KV 0V -20KV 0s 0.5s 1.0s 1.5s V(V2:-) Time - 1 -

2 20KV 0V -20KV 0s 0.5s 1.0s 1.5s V(V2:-) Time h) is there dissipated power during damping? VT guard combine low ohm dumping resistor for effective dumping and high ohm thermal protection (for earth fault). The thermal protection is activated in 1,5s for 100V(depends on voltage). There are used PTC thermistor for encreasing resistive value - generating heat. The temperature of VT guard cover is about 50C during earth fault. VT guard is insensitive to natural network asymetry - threshold voltage is between 20-24V. i) Typical earth fault clearing time for earth faults in XXX are about YY seconds, considering that saturation could occur during the overvoltage produced by the eath fault (sustained 3 seconds until protection operates), can effectiveness of the VT guard be assured?? VT guard funcionality: - During the first part (about 1,5s) the equipment has low resistance then after warming PTC thermistors, the equipment rise its resistive value to protect voltage transformer against overloading. After the earth fault is eliminated (if takes more than 1,5s), the VT guard needs about 2 minutes to cool down and lower its resistive value. - The voltage transformer is designed to withstand earth fault (normaly 1,9Un/8h) - voltage transformer is still in linear part of its charakteristick. - The ferroresonance phenomenon can occure only in ungrounded or not directly grounded networks. During earthfault the system is directly grounded and ferroresonance phenomenon can not occure. j) You have sent to me time - impedance characteristics for 100V, our customers asked if you have developed curves withing the device operating range (from 24 to 110 V) We have complete charateristic for 100V. Because PTC thermistor are used, the input's impedance is dependand on feed voltage (the input impedance is 450Ohm for 190V ) The input's impedance of VT Guard versus time for 100V - 2 -

3 k) The customer has recently normaliza a value of 150 ohm for the fixed damping resistor, is it possible to show using test results, simulations and comparisons that the VT Guard solution is more advantageous than the 150 ohm resistor? Simulations show that in many cases the resistance value needed for efficient damping of the ferroresonant oscillations is very small (<20Ohm). Using a damping resistor of such a low value results in a risk of thermal damage of the VTs during abnormal network asymmetry resulting from prolonged earthfaults. Simulation model for testing the effectiveness of ferroresonance damping with a resistor was done. See the case of phase disconnection at 0,2s time for different value of damping resistors. The effective damping was done by 15 Ohm resistor. But 15 Ohm resistor would cause overloading VT during earth fault, that is why VT Guard incraeases its Ohm value up to 200Ohm for 100V feed voltage

4 40A Secondary current at R=15Ohm 0A - 40A 0s 0. 2s 0. 4s 0. 6s 0. 8s 1. 0s -I(L1a) Ti me l) Is VT Guard apllicable to any VT regardless of its rated burden (VA), because if we take into account that during the first seconds the resistance has a value of about 15 ohms (this means a 600 VA burden for 100V), can this load cause thermal stress to the VT? Shoult it have a minimum rated VA? Time when VT guard has low input's impedance is short to could cause thermal stress to the VT. If the residual winding is 100/3V, there is 100V on VT guard during earth fault. The input impedance is 13 Ohm during first second - current is about 7,7A, after 3s the input's impedance of VT Guard is about 130Ohm - current is about 0,8A. Example: ABB VT use 1,6mm2 wire for secondary (residual winding). The secondary winding whitstand 206A for 3s. m) Can this very low initial resistance cause a voltage drop in the measured residual voltage? can it cause protective relays measuring residual voltage from the open delta winding to register smaller values during earth faults? The earth fault protection should be in paralel connection with VT guard - then there is the same voltage on VT guard and earth fault protection n) Are there minimum or special requierements for the VTs to be used with VT Guard? The VT guard is applicable to any type of voltage transformer equiped by auxiliary winding. VT guard should be used with cooperation with voltage transformers connected to open delta. o) Are available test procedures to check that VT guards are OK when doing periodic maintenance and testing of MV switchgear? The test can be done according to routine teststing (test of insensibility zone, threshold voltage, effctive dumping time - this three tests verify equipment funcionality) p) Are FerroTool and FerroSim software (depicted in the article) available? Ferro tool is under development and it isn't available in official application. q) Life time for VTGuard assuming frequent or not frequent ferroresonance corrections? No problem - no influence for operation and life time. r) Withstand time during earth fault? VT guard was tested for earth connection 110V ( voltage on residual winding when one phase is grounded - earth fault) for 8 hours and the temperatures according to temperature rise tests standard - 4 -

5 were below the required temperature given by this standard. It means that operation during earthfault is not a problem. II.Waranty a) Life time warranty 24 months warranty since the day of putting the apparatus into operation, but not longer than 36 months since the day of delivery. III. Delivery conditions a) Delivery time ( which delivery time we will set up as a standard) 14 days DDU Germany up to 100 pcs. Higher amounts has to be agreed