RU2508587C1 - Protection method of synchronous generators against fault to ground at one point of excitation circuit - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: poles of an excitation winding are shunted by resistors, some part of which is shunted from time to time with controlled switches for the purpose of changing the state of a measuring circuit; voltages are measured on certain elements of the circuit and insulation resistance is calculated as per the formula given in the invention description. A flow chart of the method includes excitation winding (1); controlled insulation resistance (3); two shunting resistors (6) and (8); two controlled switches (5) and (9) of shunting resistors (6) and (8) respectively; ballast high-ohmic resistors (2) and (4) connected between poles of the excitation winding and switches (5) and (9); switch control unit (10); measuring unit 11; output relay unit (12); control unit (13) of good state of switches; earthing resistor (70). The technical results is achieved by decreasing the level of voltages on the shunting switches by their separation with outputs of the excitation winding by means of high-ohmic resistors, and the switches themselves for control of their good state are connected in parallel to the resistors on which voltages are measured.

EFFECT: improving operating reliability of a control system of the good state of insulation and relay protection of excitation circuits.

2 dwg

Description

The present invention relates to the field of electrical engineering, namely to relay protection of synchronous generators, and can be used at power plants to protect synchronous generators from shorting the field winding to earth at one point, as well as to control the insulation resistance.

A known method [1] of protecting excitation circuits from earth faults is made using a measuring bridge, one of the arms of which includes an excitation winding when an auxiliary voltage source is connected to one of the diagonals of the bridge and the measuring organ is connected to the other.

The main disadvantage of this method is the complexity of its technical implementation, including due to the use of an auxiliary voltage source. In addition, the accuracy of the measuring body is negatively affected by a possible change in the capacity of the excitation circuit in operation.

The closest to the claimed technical solution, selected as a prototype, is a method for determining the equivalent insulation resistance of a direct current circuit [2], in which periodically and alternately, using a high-voltage switch based on electronic keys, they are shunted with the additional resistance of a certain pole value of the controlled circuit and voltage is measured at at each pole of the network in steady state, average the measured voltages and calculate the equivalent insulation resistance ty DC.

A significant disadvantage of the prototype is the need for high-voltage semiconductor switches, which are subject to increased requirements for permissible voltages, which complicates and increases the cost of the circuit, reduces its reliability.

The purpose of the invention is to simplify the system for monitoring the state of insulation and relay protection of excitation circuits, increasing the reliability of its operation. This goal is achieved in that they reduce the voltage level on the shunt switches by separating them with the terminals of the field winding using high-resistance resistances, and the keys themselves are connected in parallel to the resistances on which the voltages are measured to monitor their health.

Figure 1 shows the block diagram of the proposed method.

The circuit includes a field winding 1; controlled insulation resistance 3; two shunted by keys of resistance 6 and 8; two controlled keys 5 and 9, shunt resistances 6 and 8, respectively; additional high-resistance resistances 2 and 4 dividing the poles of the field winding and switches 5 and 9; grounding resistance 7; key management unit 10; measurement unit 11; output relay block 12; unit 13 monitoring the health of the keys.

The method is implemented as follows.

, где t - текущее время, посредством которого осуществляется периодическая коммутация ключей 5 и 9. Периодический сигнал управления $$s\left(t\right)$$

с периодом τ представлен на фиг.2. Когда $$s\left(t\right)=1$$

, ключ 5 замкнут, а ключ 9 разомкнут. Когда $$s\left(t\right)=0$$

, ключ 5 разомкнут, а ключ 9 замкнут. Период τ выбирается исходя из условия полного затухания переходных процессов, обусловленных коммутацией ключей, за время τ/2. Блок измерения 11 измеряет напряжения на сопротивлениях 6, 7 и 8 в установившемся режиме, усредняет измеряемые напряжения и производит вычисление сопротивления изоляции по формуле:Block 10 issues a control signal $$s\left(t\right)$$

, where t is the current time by which the periodic switching of keys 5 and 9 is carried out. Periodic control signal $$s\left(t\right)$$

with a period of τ is presented in figure 2. When $$s\left(t\right)=\mathrm{one}$$

, key 5 is closed, and key 9 is open. When $$s\left(t\right)=0$$

, key 5 is open, and key 9 is closed. The period τ is selected on the basis of the conditions for the complete attenuation of transients caused by the switching of keys over time τ / 2. The measurement unit 11 measures the voltage at the resistances 6, 7 and 8 in the steady state, averages the measured voltages and calculates the insulation resistance according to the formula:

, $$R_{\mathrm{and}s}=\frac{R_2}{R_{\mathrm{one}}}\frac{R_{\mathrm{one}}{U}_{\mathrm{at}\mathrm{one}}{U}_{\mathrm{at}2}-(R+R_{\mathrm{one}})(U_{12}{U}_{\mathrm{at}\mathrm{one}}+U_{21}{U}_{\mathrm{at}2})}{U_{32}{U}_{\mathrm{at}\mathrm{one}}-U_{31}{U}_{\mathrm{at}2}}-{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="00000013.png"\; state="\{\{state\}\}">R</figure-callout>}}_2$$

,

where R _from - the value of the insulation resistance 3;

R is the value of the additional high-resistance resistances 2 and 4;

R ₁ - the value of shunt resistances 6 and 8;

R ₂ - the value of the grounding resistance 7;

;U ₂₁ is the average voltage across resistance 8 when shunting with key 5 of resistance 6 when $$s(t)=\mathrm{one}$$

;

;U ₃₁ - the average voltage across the resistance 7 when shunting the key 5 of the resistance 6, when $$s(t)=\mathrm{one}$$

;

;U ₁₂ - the average voltage across the resistance 6 when shunting the key 9 of the resistance 8, when $$s(t)=0$$

;

;U ₃₂ is the average voltage across resistance 7 when shunting with key 9 of resistance 8 when $$s\left(t\right)=0$$

;

U _v1 - voltage across the field winding at the time of measuring voltages U ₂₁ and U ₃₁ ;

U _B2 - voltage on the field winding at the time of measuring voltages U ₁₂ and U ₃₂ .

The accuracy of the calculation of the insulation resistance is not affected by a change in the excitation voltage, since it is taken into account that U _in1 ≠ U _in2 . Voltages U _in1 and U _in2 are calculated by block 11 according to the following formulas:

; $$U_{\mathrm{at}\mathrm{one}}=\frac{2R+R_{\mathrm{one}}}{R_{\mathrm{one}}}{U}_{21}+\frac{R}{R_2}{U}_{31}$$

;

. $$U_{\mathrm{at}2}=\frac{2R+R_{\mathrm{one}}}{R_{\mathrm{one}}}{U}_{12}-\frac{R}{R_2}{U}_{32}$$

.

When block 13 detects a failure of keys 5 or 9, a fault signal is issued and the operation of block 12 is blocked. There can be several criteria for detecting a key failure. For example, a malfunction is indicated by the presence of voltage at a resistance of 6 or 8 when it is shunted, or the absence of voltage at a resistance of 6 or 8 when it is shunted.

The output relay block 12 compares the insulation resistance calculated by the block 11 with the value of the tripping parameter and generates an output protection trip signal when the resistance decreases below the permissible limit. The formation of the output signal in block 12 is provided only with a steady decrease in insulation resistance, i.e. observed over a given period of time.

The method can be used to monitor the state of insulation and relay protection of the excitation circuits, ensuring the independence of the results from changes in the excitation voltage, the magnitude of the capacitance of the excitation circuit and the place of deterioration of insulation.

Information sources

1. USSR Copyright Certificate No. 612338, cl. H02H 7/06, 1978, bull. Number 23.

2. USSR author's certificate No. 1569745, cl. G01R 27/18, 1990, bull. No. 21.

Claims (1)

A method of protecting synchronous generators from earth faults at one point in the excitation circuit, which consists in switching on between the poles of the excitation winding of the generator and the ground a measuring circuit consisting of active resistances, controlled keys, a measuring unit and an output relay unit, measuring voltages at certain active resistances during attenuation transients in the measuring circuit due to periodic switching of the keys, the subsequent calculation of the insulation resistance from the measured voltages and in giving a protection trip signal in the case when, as a result of the comparison, the calculated insulation resistance turns out to be less than the value of the response parameter of the output relay block, characterized in that, in order to simplify the monitoring of the insulation state and the relay protection of the excitation circuits and increase the reliability of its operation, between the poles of the field winding and the keys include additional high-resistance active resistances, allowing to reduce the voltage level on the keys, thereby reducing the requirements for them, and the keys themselves for uschestvleniya control their serviceability is connected in parallel to the active resistance, which measure the voltage.

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