Discussion on voltage problems in backup protectio

Discussion on voltage problems in backup protection of main transformer

Abstract: in order to reflect the overcurrent caused by external short circuit of transformer and PolyOne TPE material as internal short circuit of transformer & nbsp; For the backup of innovative wearable intelligent devices, transformers should be equipped with current protection as backup. According to the transformer capacity and short-circuit current level, considering the requirements of protection sensitivity, the backup protection of transformer phase to phase short circuit is generally set as overcurrent protection, overcurrent protection of composite voltage start, negative sequence overcurrent and unit voltage start overcurrent protection and directional overcurrent protection. In these configurations, in addition to simple current protection, Others involve voltage elements as locking or starting elements. Next, we will analyze and explain several problems related to voltage

key words: transformer backup protection in order to reflect the overcurrent caused by the external short circuit of the transformer and as the backup of the internal short circuit of the transformer, the transformer should be equipped with current protection as the backup. According to the transformer capacity and short-circuit current level, considering the requirements of protection sensitivity, the backup protection of transformer phase to phase short circuit is generally set as overcurrent protection, overcurrent protection started by composite voltage Negative sequence overcurrent and unit voltage start overcurrent protection and directional overcurrent protection. In these configurations, except for simple current protection, other voltage elements are involved as locking or starting elements. Next, we will analyze and explain several problems related to voltage. 1. All low-voltage components should adopt line voltage instead of phase voltage. Whether it is low-voltage components in composite voltage or low-voltage components in negative sequence overcurrent and overcurrent protection of low-voltage start-up of unit components, their voltage should adopt line voltage. Voltage components should be installed on the low-voltage side. Why can't three-phase voltage be used? We know that if the three-phase voltage is used as the low-voltage starting element, the sensitivity is very high when the low-voltage side is short circuited between phases, but the sensitivity will be reduced when the high-voltage side is short circuited between phases. Take the short circuit of transformer a and B as an example (transformer y/d11). When a and B phases are short circuited, ù ab=0, that is, ù a= ù B transformer ù B corresponds to low-voltage ù AB, and ù a corresponds to low-voltage ù Ca, then there is ù ab= ù Ca, that is, ù a- ù b= ù c- ù A. changing this formula can deduce ù a+ ù B + ù c=0=3 ù a, so there are three low-voltage sides.Phase voltage, ù c=- ù B, ù a=0. In this case, using three phase voltage elements as low-voltage starting elements, the protection will act, but the sensitivity will be reduced. More importantly, because the neutral point on the secondary side of the TV of the phase voltage we connect is grounded, for the small current system, when the a phase on the low voltage side is grounded and the transformer operates under overload, the a phase voltage a=0, and the protection may malfunction, which is not expected by us according to the performance and requirements of the flexible packaging film test; Using line voltage as low voltage starting element can completely avoid this problem. The vector diagram is shown in Figure 1. Figure 1 phase voltage and line voltage vector when phase a on the low voltage side is grounded Figure 2 why negative sequence current + fly ash, calcium carbonate/formaldehyde adsorption performance of filling masterbatch is used to compare the low-voltage starting overcurrent of unit parts, instead of three-phase line voltage starting overcurrent, we have analyzed the reasons why we do not use three-phase line voltage starting overcurrent protection. When three-phase AB, BC and Ca on the low voltage side are used as low-voltage component signals, In case of phase to phase short circuit on the high-voltage side (taking phase A and phase B as examples), we can infer from the above that the voltage of the three lines on the low-voltage side ùab= ù B, ù bc=- ù c= ù a, ù ca=-2 ù C are all high, and the sensitivity of the low-voltage components is very low, so the protection cannot be started. If the low-voltage starting elements connected to the three-phase line voltage are installed on both sides of the transformer, the protection sensitivity is not a problem, but the wiring of the protection device is too complex and too many elements are used. It is better to use negative sequence overcurrent plus single element low-voltage starting overcurrent, which can fully meet the requirements of protection sensitivity. 3 selection of voltage components in directional protection for step-up transformers with power on three sides or step-down transformers with power on high and medium voltage sides, considering the needs of operation mode and protection coordination, directional overcurrent may be used as backup protection. The voltage of the component in the power direction of the protection should be taken from the voltage transformer on the other side. For example, the voltage of the component in the medium voltage side should be correctly selected according to the regulations, and the voltage of the component should be taken from the high voltage or low voltage side. However, special attention should be paid to the transformer connected with yyd12-11. When the voltage at the low voltage side is taken as the component at the middle voltage side, if the power direction adopts the 90 ° wiring mode, the usual mode of í a, ù BC, í B, ù Ca, ù C, ù AB should not be used, but í a, ù BD should be used; íB、ùcd； í C, ù ad mode (D in the lower corner is △ junction side voltage), so as to ensure that the connection mode between current and voltage is still 90 °. The vector diagram is shown in Figure 2. Figure 2 90 ° connection vector analysis