In the past 10 years, the neutral point of generators has been widely used in China through the grounding method of distribution transformers. How to properly configure the distribution transformer and its grounding protection setting calculation and verification method is an important issue. The configuration of the distribution transformer and the grounding protection setting calculation mainly include selecting the appropriate secondary resistance, the grounding protection action current, Voltage value and calibration protection sensitivity and total current value of single-phase grounding short-circuit point, etc. This paper provides a more complete calculation method for this device, and combines with the example of Hubei Paradise Pumped Storage Power Station to calculate the calculation. 1 Generator Neutral Point grounding method selection For large generator sets, the most widely used domestic and international is the grounding method of the distribution transformer and the arcing coil grounding method. The arcing coil grounding method must be considered more factors, choose the appropriate compensation degree and arc suppression coil parameters. More difficult if one-sided pursuit of reduction Yin Jiangang (1971-, male, Hubei Wuhan, engineer, master, mainly engaged in power system relay protection setting calculation and operation work; Cai Min (1964-), male, Hubei Wuhan, senior engineer, Master, mainly engaged in power system relay protection operation and management work. Less ground current The use of smaller detuning degree and larger arc-suppression coil quality factors will inevitably result in a large offset of the neutral point voltage due to the imbalance of the relative capacitance of the generator three. In addition, when the compensation degree is not suitable, Dangerous transmission of overvoltage, threatening the safety of the generator, grounding or secondary re-ignition will occur during the start-up and shutdown of the unit, and there will be a large transient over-voltage relative to the arc-suppression coil grounding method. The distribution transformer grounding will not cause a large offset of the neutral point voltage during most of the normal operation of the unit, which will limit the overvoltage. It can be seen that the high resistance grounding is beneficial to the insulation and normal operation of the unit. It has the effect of preventing the insulation damage of the unit or the weakening of the insulation and causing the grounding of the stator. In addition, the high-resistance energy-absorbing element, large zero-sequence circuit damping, no transmission over-voltage and transient over-voltage danger. Therefore, the distribution transformer is grounded. The method is widely used 2 distribution transformer configuration configuration distribution transformer grounding device configuration, if its key is to choose the appropriate secondary resistance and transformer and in place According to the need of stator grounding protection, the corresponding rated voltage of the CT and PT transformers is the rated voltage of the generator terminal, and the rated voltage of the secondary side is generally 230V (110V is also taken in some places). The tap is set at the position of 173V, and the access is made. Protect the 3U* stator grounding protection so that when the machine is single-phase grounded, the tap voltage is 100V or the neutral point CT can be set on the primary side to form the 3/0 stator grounding protection. The neutral point of the generator is indicated by the grounding of the distribution transformer. The principle of the arrangement of the secondary resistor is that the resistor connected to the secondary side of the distribution transformer is an energy dissipating component, which can increase the zero-sequence loop damping, suppress the transient over-voltage and increase the grounding current, and require the stator winding of the generator. When the single-phase ground fault occurs, the unit is quickly removed. According to the requirements of ANSI.IEEEC37.1011985 <Guidelines for Generator Grounding Protection, the load resistance value should be such that when the single-phase ground fault occurs at the generator end, the power loss in the resistor is equal to or greater than the generator voltage loop to ground. Reactive power loss of the capacitor to limit the generator overvoltage to 26 times the phase voltage; the fault current at the grounding point does not exceed 25A to prevent damage to the core. The load resistor should have an adjustable tap. The position of the tap should be based on the generator voltage. The capacitance value of the loop is determined. The neutral grounding device parameters are designed and manufactured based on the calculated values ​​of the generator set design, which is different from the measured parameters of the generator. This should be fully considered. 2.2 The capacitance value of the generator voltage loop determines the capacitance value of the generator voltage loop mainly includes the generator stator winding, the generator bus, the anti-impact capacitor connected to the busbar, the transformer low-voltage winding connected with the generator, and the outlet pair of the generator end Ground capacitance. The capacitance value generally has two methods to determine the 22.1 calculation method. The generator stator winding capacitance value is provided by the motor manufacturer. The rest of the calculation method is the usual calculation method (this article is omitted). This method is generally applicable to the generator neutral point grounding device manufacturing. plant. 22.2 Test method When the generator set is installed as the stator winding AC power frequency withstand voltage test, the test voltage and current value can be measured. After calculation, the generator capacitance value is obtained. The Hubei Heavenly Generating Motor Unit is used as the stator single-phase winding external processing frequency voltage. For 24kV, the measured current is 3. 8A, and the calculated generator capacitance value is 0.504<10). F) The test of each device of the generator voltage circuit is troublesome. Generally, the test is simple before the generator set is put into operation. During the test, the other side of the transformer connected to the generator is opened, and the neutral point grounding device of the generator is not connected. When the voltage rises to 20%~30% of the rated voltage, it will be short-circuited relatively, and the capacitance current is measured. Calculate the total capacitance value of each phase of the generator voltage loop from the generator voltage and the short-circuit current to the ground. 2.3 Set the resistance value. According to the "Generator Grounding Protection Guidelines", the grounding resistance and the capacitance of the genset voltage circuit should meet the following requirements. Requirements: to the primary side resistance value of the grounding transformer P; C relative capacitance of the genset voltage circuit, the capacitance of the R2 genset voltage circuit to the ground is calculated The secondary side current value of the transformer P; R one to two secondary load resistance value only; R one to one grounding transformer resistance value P; U. one grounding transformer primary side rated voltage, kV; U2 one by one grounding transformer two Secondary side rated voltage, kV; APd one-to-one grounding transformer load loss (generally take the secondary side rated current of /2n one-to-one grounding transformer, A according to the above calculation formula, calculate the secondary load resistance value, the actual setting should be less than the above Calculated value. The setting of the resistance of the paradise generator motor group is as follows: 500V neutral point grounding device parameters are grounding transformer capacity 10kVA, high voltage side is 10500V, low voltage side is 230V, relay protection tap voltage is 173V, secondary load resistance is connected to 230V transformer impedance The voltage percentage is 3.6, the load loss is 154W, and the primary side is 5/1A current transformer grounded to limit the dynamic overvoltage below 2.6 times the phase voltage, the grounding resistance R<1/3jC; the measured C=0.504MF/K Consider the other capacitors, take the relative capacitance of the genset voltage loop C=0.6MF/fe, because the generator neutral point grounding device design has a margin, the factory setting value R is 0. 6n, meet the requirements of the power station 3 Grounding protection setting calculation 3.1 Relay protection current or voltage action value setting 95% stator grounding protection, the calculation of the action value depends on the current or voltage value of the stator winding from the neutral point 5% short circuit to calculate the grounding transformer The voltage and current value of the single-phase grounding short-circuit at 5% from the neutral point are calculated to the transformer equivalent circuit on the secondary side, which is equivalent to the external phase 5% generator phase voltage. Applied to the series circuit of the internal resistance and internal reactance of the load resistance transformer, the equations are listed: I, h—the current of the grounding transformer, the secondary side, A; Iset 95% stator grounding protection action current setting CTR one current Transformer ratio; Uset 95% stator grounding action voltage setting value, U-one grounding transformer secondary side relay protection tap voltage, V heaven generator neutral point current or voltage action value setting: 3.2 single-phase grounding short circuit Current value test When the generator end is short-circuited, the short-circuit current of the grounding point needs to be less than 25A to avoid burning the generator stator core, and the generator capacitor current is capacitive current, while the grounding transformer current is inductive current, two The sum of the currents is the total current calculation of the single-phase grounding short-circuit point. The following formula: * generator rated phase voltage, V; the capacitor current of the genset voltage loop to ground when the generator terminal is short-circuited relative to ground, A; /r The resistive current of the primary side of the grounding transformer when the generator terminal is short-circuited relative to ground, A; / l - one inductive current of the primary side of the grounding transformer when the generator terminal is short-circuited relative to ground, A; When the motor end is short-circuited relative to the ground, the total current of the short-circuit point, the total current of the short-circuit point when the generator end is short-circuited is less than 5A, which satisfies the requirements of the "Generator Grounding Protection Guidelines". 4 Conclusion The neutral point of the generator is adopted. For the grounding method of the distribution transformer, it is necessary to properly configure the secondary resistance of the distribution transformer. It must be adjusted according to the specific conditions of the generator voltage loop and the parameters of the grounding device, and the fault current of the grounding point should not be verified to exceed 25A to prevent If the fault current of the core is too large or exceeds 25A, consider the inductor on the secondary side of the distribution transformer, so that the current to ground is less than 25A, and the action trip time should be shorter. Accurately calculate the operating current and voltage value and test. The total current value of the single-phase grounding short-circuit point must be considered in the internal resistance and reactance of the distribution transformer. The exact calculation in this paper is to illustrate the influence of internal resistance and reactance. The actual setting calculation can be simplified according to the specific situation. Yin Jiangang, Peng Feng, Wang Weiwei. Reasonably configure the neutral grounding mode of the generator. Power Equipment, 2001, (4). Wang Weiwei. Principle and application of relay protection for electrical main equipment. Beijing: China Electric Power Press, 1996. 10-1985, generator grounding protection guide IJ
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