RU2723911C1 - Method of electrodynamic tests of power transformers - Google Patents

Abstract

FIELD: electric power engineering.SUBSTANCE: invention relates to electric power engineering and can be used in creation of network test benches for testing of power transformers for resistance to short circuit currents. Phase output of the high-voltage winding of the transformer through the in-series connected high-voltage thyristor switch and the capacitor bank is connected to buses of the electric mains, and the neutral and the shorted low-voltage winding are grounded. Capacitor bank is divided into N sections with ratio of capacities of sections 2:2:2:…:2. Current-free switching of separate sections is provided. Capacitive bank capacitance is set equal to the inductance component of the short-circuit impedance of the power transformer. Preliminary charging of capacitor bank from buses of electric mains is carried out along high-voltage circuit, galvanically isolated from short circuit current flow circuit, during M periods of electric circuit voltage, number of which is selected depending on rated power Sof tested power transformer by criterion S/M=const for whole range of tested power transformers. Creasing signal of conductivity interval of high-voltage thyristor switch equal to duration of short-circuit test with equal number of positive and negative half-waves of short-circuit current is generated. Master signal is synchronized with voltage of electric mains and supplied to high-voltage thyristor switch. Short-circuit test is started at the moment of voltage maximum at the buses of the electric circuit, which is opposite in sign to direction of voltage of preliminary charging of the capacitor bank.EFFECT: expanded range of tested power transformers.1 cl, 1 dwg, 2 tbl

Description

The invention relates to the field of electric power and can be used to create network test stands for testing power transformers for resistance to short circuit currents.

There is a method of electrodynamic testing of a power transformer, the phase output of the high voltage winding of which is connected to the busbars of the electric network through a high voltage thyristor switch, and the neutral and shorted low voltage winding are grounded, which consists in generating a driving signal of the conductivity interval of the high voltage thyristor switch, which is equal to the duration of the short experiment short circuits with the same number of half-waves of short-circuit current of positive and negative polarity synchronize the driving signal with the voltage on the buses of the electric network and apply to the high-voltage thyristor switch [1].

The known method allows you to fulfill the requirements of GOST 20243-74 "Power transformers. Test Methods for Short Circuit Resistance ”in terms of normalizing the duration of short circuit experiments (0.2 s) and ensuring a given level of the shock current of the short circuit (impact coefficient of the order of 1.8 ÷ 1.9). However, only if the short-circuit power on the busbars of the electric network is an order of magnitude or more higher than the rated power of the tested power transformer. This is because the total power consumed from the electrical network during the short circuit test is 10 times or more higher than the rated power of the tested power transformer. The ratio of the active and reactive components of the full power is determined by the ratio of the active and inductive components of the short circuit resistance of the tested power transformer. This circumstance limits the range of power transformers in terms of rated power, which can be tested for resistance to short-circuit currents from busbars of the electric network.

There is a method of electrodynamic testing of a power transformer, the phase output of the high voltage winding of which is connected through the high voltage thyristor switch and the capacitor battery to the busbars of the electric network, and the neutral and short-circuited low voltage windings are grounded, namely, the capacitance of the capacitor bank is set equal to the inductive component the short circuit resistance of the power transformer and pre-charge the capacitor bank to a voltage corresponding to the nominal value of the high voltage winding and the given value of the shock current of the short circuit, form the driving signal of the conductivity interval of the high voltage thyristor switch, equal to the duration of the short circuit experience with the same number of positive and negative half-waves of current short circuit, synchronize the driving signal with the voltage of the electrical network and apply to the high voltage thyristor key [2].

The known method allows to reduce the value of the total power consumed from the busbars of the electric network to the level of only the active component, which compensates for the loss in the high-current loop of the test short-circuit current. However, for this it is necessary to set the capacitance of the capacitor bank at the frequency of the voltage of the electric network equal to the inductive component of the short circuit resistance of the tested power transformer. For power transformers with a rated power (25 ÷ 630) MVA and a rated voltage (110 ÷ 500) kV, the inductive component of the short circuit resistance can take values from 6.1 Ohms to 190 Ohms (values given to the high voltage side). Therefore, before each short circuit test, the capacitance of a capacitor bank must be set to one of the discrete values of the specified range, depending on the type of power transformer being tested. The necessary capacitance can be established only by changing the composition and connection scheme of the used sections of the capacitor bank. However, with this installation method, the capacitance of a capacitor bank can take only a number of discrete values, many of which are obviously less than the range of power transformers produced. In addition, in the set of discrete capacitance values of a capacitor bank, a value equal to the inductive component of the short circuit resistance of a particular power transformer under test may not be present. In this case, the condition for resonant tuning of the high-current circuit is violated. Then the forced component of the test short-circuit current will change with the frequency of the electrical network, and the free component due to the pre-charge of the capacitor bank will change with the frequency determined by the inductance of the short circuit of the tested power transformer and the installed capacitance of the capacitor bank. As a result, the shape of the test short circuit current is distorted, eliminating the possibility of conducting test tests of short circuit. Thus, the limited ability to adjust the capacitance of a capacitor bank to a resonant value limits the range of tested power transformers.

The purpose of the invention is the expansion of the range of tested power transformers.

This goal is achieved in that the capacitor bank is divided into N sections with a ratio of section capacities 2 ⁰ : 2 ¹ : 2 ² : ...: 2 ^N-1 , provide the possibility of currentless switching of individual sections, pre-charge the capacitor bank from the busbars of the electric network via high voltage a circuit galvanically isolated from the short circuit current flow during M periods of voltage of the electric network, the number of which is selected depending on the rated power S _{HOM of the} tested power transformer according to the criterion S _HOM / M = const for the entire range of tested power transformers, and the experience of short short circuits begin at the moment of maximum voltage on the busbars of the electric network of the opposite direction sign of the voltage of the pre-charge capacitor bank.

In FIG. 1 shows a diagram of a test bench that implements the proposed method for electrodynamic testing of power transformers.

The electrodynamic test bench is connected to the busbars 1 of the electric network 2. The electrodynamic test bench includes a high-current circuit 3 and a high-voltage charging circuit 4, each of which is connected to the busbars 1. The high-current circuit 3 is formed by a high-voltage thyristor switch 5, sectioned capacitor bank 6 s sectional disconnectors 7.1, 7.2, ..., 7.N, one of the phase high-voltage windings of the tested power transformer 8, the remaining windings of which are short-circuited and connected to the ground loop.

The high-voltage thyristor switch 5 with one of the terminals is connected to the busbars 1 and is made of series-connected thyristor cells, each of which is formed by counter-parallel connected power thyristors 5.1 and 5.2. Power thyristors 5.1 provide the flow of odd half-waves, and power thyristors 5.2 provide the flow of even half-waves of the test current during short circuit experiments.

The capacitor bank 6 contains N parallel connected sections 6.1, 6.2, ..., 6.N capacitances C ₁ , C ₂ , ..., C _N which are selected in the ratio C ₁ : C ₂ : ...: C _N = 2 ⁰ : 2 ¹ : 2 ² : ...: 2 ^N-1 . Disconnectors 7.1, 7.2, ..., 7.N provide current-free switching of sections 6.1, 6.2, ..., 6.N to set the resonant value of the capacitance of the capacitor bank 6 before conducting a short circuit test.

The high-voltage charging circuit 4 contains an adjustment transformer 9, a step-up transformer 10, the high voltage winding of which is connected to a capacitor bank 6 through a high-voltage diode block 11 and current limiting resistors 12, 13. The primary winding of the regulation transformer 9 is connected to the busbars 1 of the electric network 2 and the ground loop electrodynamic test bench.

The voltage transformer 14 is connected by a high voltage side to the busbars 1 of the electric network 2, and a low voltage side is connected to the “Synchronization” input of the control system 15 by a high-voltage thyristor switch 5. At the “Start” input of the control system 15 a short circuit test is sent. The output of the control system 15 is connected to the control electrodes of the power thyristors 5.1, 5.2 of the high-voltage thyristor switch 5. The high-voltage arm of the voltage divider 17 is connected to a common point of the high-voltage thyristor switch 5 and the capacitor bank 6, and the low-voltage arm is connected to the input “Charge level” of the control system 15.

The proposed method of electrodynamic testing is as follows.

Before conducting a short-circuit test, a combination of closed-open positions of disconnectors 7.1, 7.2, ..., 7.N is established, at which the capacitive resistance of the capacitor bank 6 becomes equal to the short-circuit resistance component of the tested power transformer 8. The necessary combination is determined by the expression

x _T is the inductive component of the short circuit resistance of the tested power transformer 8; x _KB - the total capacitive resistance of sections 6.1, 6.2, ..., 6.N of the capacitor bank 6, necessary for conducting a short circuit experiment; x _O - capacitance of section 6.1 of the capacitor bank 6; S _(7.1) , S _(7.2) , ..., S _(7.N) is a binary coefficient that takes into account the position of the disconnector 7.1, 7.2, ..., 7.N, respectively, and takes the value "1" when the disconnector is closed or the value "0" when open disconnector.

If all disconnectors are in the “closed” position, i.e.

then the capacitance of the capacitor bank 6 will take a minimum value

and the total capacity will be maximum

If only one 7.1 disconnector is installed in the closed position, i.e.

then the capacitance of the capacitor bank 6 will take the maximum value

and the capacity of the capacitor bank 6 will be minimal

To ensure electrodynamic tests of power transformers with a rated power of (25 ÷ 630) MVA and a rated voltage of (110 ÷ 500) kV, the capacitance of the capacitor bank 6 must be set in the range

The error in fulfilling the condition (1) will be determined by the number N of sections of the capacitor bank 6

Table 1 presents the Δx _KB values (%) depending on the number N of sections 6.1, 6.2, ..., 6.N of the capacitor bank 6.

According to table 1, the number of sections of the capacitor bank 6 is selected according to the required accuracy of setting the capacitive resistance and fulfilling the condition (1) of the resonant tuning of the high-current circuit 3.

As an example, table 2 presents the values of the control codes that are generated by various combinations of states of the disconnectors 7.1, 7.2, ..., 7.N "closed - open" at N = 6, and the parameters of the capacitor bank 6 corresponding to each code for the above range of tested power transformers. When N = 6, the capacitance of the capacitor bank 6 is set with an error of less than 1%.

After setting the required value of capacitance, the capacitor bank 6 is charged to voltage

where U _HOM is the rated voltage of the tested power transformer 8; K _UD - shock coefficient, which determines the magnitude of the shock current in the experience of short circuit.

The capacitor bank 6 is charged from the busbars 1 of the electric network 2 using a high-voltage charging circuit 4. The regulation transformer 9 and the step-up transformer 10 provide the required level of charging voltage corresponding to the rated voltage of the tested power transformer 8. In addition, the step-up transformer 10 provides galvanic isolation of the capacitor batteries 6 from busbars 1 of the electric network 2. By the time the charging process is over, energy is accumulated in the capacitor bank 6

where ω is the angular frequency of the voltage of the electric network 2.

Taking into account expression (3) and a pre-fulfilled condition for resonant tuning (x _T = x _KB ) of the high-current circuit 3, the value stored in the capacitor bank 6 can be expressed in terms of the parameters of the tested power transformer 8

The inductive component of the short circuit resistance of the tested transformer 8 is determined by the known expression

where u _K% is the passport value of the short circuit voltage of the tested power transformer 8; S _HOM - rated power of the tested power transformer 8.

Taking into account the last relation, the amount of energy stored in the capacitor bank 6 can be numerically correlated with the rated power of the tested power voltage 8

Current-limiting resistors 12, 13 limit the magnitude of the charging current and determine the charging time of the capacitor bank 6, the duration of which depends on the average charge power of the regulating transformer 9 and step-up transformer 10

where t ₃ is the charge time of the capacitor bank 6 to the voltage defined by expression (3).

If the charge of the capacitor bank 6 continues for M periods of voltage of the electric network 2, i.e.

then the average charge power of the regulating transformer 9 and step-up transformer 10 will be determined by the value

The short circuit voltage for the above range of power transformers is (10.5 ÷ 14)%, the shock factor in the test tests of short circuit should be provided at the level K _UD ≅1.8. Under these conditions, the ratio of the rated power of the tested power transformer 8 and the average charge power of the regulating transformer 9 and the step-up transformer 10 will be determined

the number of periods of voltage of the electrical network 2, during which the charge of the capacitor bank 6.

For a given range of tested power transformers at rated power

constant value

and correspondingly

испытуемого силового трансформатора 8.will provide unchanged and the value of the average charge power in the process of charging a capacitor bank 6 at any set resonant value of capacitive resistance. The choice of medium-charging power, which determines the mass-dimensional characteristics of the control transformer 9 and step-up transformer 10, it is advisable to carry out for the most powerful

power transformer under test 8.

If the charge time t ₃ = 20 s, respectively, M _max = 1000, then the total average charge power of the regulation transformer 9 and step-up transformer 10

MBA суммарная среднезарядная мощность P_cpΣ=(1,55÷2,1) MBA, а среднезарядная мощность регулировочного трансформатора 9 и повышающего трансформатора 10 составит Р_ср=(0,78÷1,05) МВА.three orders of magnitude less than the rated power of the tested power transformer 8. For example, when

MBA the total average _charge power P _cpΣ = (1.55 ÷ 2.1) MBA, and the average charge power of the regulating transformer 9 and step-up transformer 10 will be P _cf = (0.78 ÷ 1.05) MVA.

МВА, количество периодов напряжения электрической сети 2, необходимых для заряда, может быть уменьшено до M_min=40.Then for the capacitor bank 6, providing electrodynamic tests of the power transformer 8 rated power

MVA, the number of periods of the voltage of the electric network 2 required for charging can be reduced to M _min = 40.

The fulfillment of the condition S _HOM / M = const is ensured during the exponential charge of the capacitor bank 6 with the resistance value of current-limiting resistors 12, 13

where R _Σ is the total resistance of current-limiting resistors 12, 13.

обеспечивается выбором R_Σ≅17 кОм. В этом случае минимальное время заряда конденсаторной батареи 6 при C_min=16 мкФ составит t_3(min)=0.8 с, а максимальное время заряда при C_max=513 мкФ составит t_3(max)=20 с.For the range of tested power transformers (25 ÷ 630) MVA, the ratio

provided by the selection of R _Σ ≅17 kOhm. In this case, the minimum charge time of the capacitor bank 6 at C _min = 16 μF is t _{3 (min)} = 0.8 s, and the maximum charge time at C _max = 513 μF is t _{3 (max)} = 20 s.

Increasing the resistance of current-limiting resistors 12, 13 in excess of the above value will reduce the average charge power of the regulating transformer 9 and step-up transformer 10, but will lead to an increase in the pre-charge time of the capacitor bank 6 and, accordingly, the preparation time of the short circuit experiment. The limitation “from above” on the value of R _{Σ is} imposed by the ohmic voltage divider 16, the resistance R _{(16) of} which will not affect the charge level of the capacitor bank 6 under the condition R ₍₁₆₎ >> R _Σ .

Prior to the start of the short-circuit test, the high-voltage thyristor switch 5 must withstand the application of a voltage equal to the sum of the DC component due to the charge U _{CO of the} capacitor bank 6 and the sinusoidal component with the amplitude U _{m (2)} due to the electric network 2, i.e.

The end of the process of charging the capacitor bank 6 to a predetermined level U _CO is fixed by the control system 15 using an ohmic voltage divider 16. From this moment in time, a control signal is generated in the control system 15 for conducting a short circuit experiment.

By the “Start” command, the control system 15 generates a driving signal of the conductivity interval of the high-voltage thyristor switch 5, the beginning of which is synchronized using a voltage transformer 14 with a voltage on the busbars 1 of the electric network 2, and, if there is an enable signal, it supplies control pulses to the power thyristors 5.1, 5.2 . The first control pulse, when indicated in FIG. 1 of the polarity of the pre-charge voltage of the capacitor bank 6 is supplied to the power thyristors 5.1 and determines the moment of the beginning of the short-circuit experiment with respect to the voltage of the electric network 2 on the busbars 1. During the short-circuit experiment, the instantaneous values of the voltage u ₂ (t) of the electric network 2, voltage u _KB (t) capacitor bank 6 and test current i _{short circuit} (t) short circuit flowing in high-current circuit 3, are determined under the condition of resonant tuning by the expressions

where r _T is the active component of the short circuit resistance of the tested power transformer 8; δ = ω⋅r _T / 2x _T is the decrement of voltage and current fluctuations of the capacitor bank 6; ψ is the initial phase of the voltage of the electric network 2 at the time of the beginning of the short circuit experiment.

The first terms of expressions (6), (7) determine the forced components

the voltage of the capacitor bank 6 and the test current of the high-current circuit 3, due to the voltage of the electrical network 2.

The second terms of expressions (6), (7) determine the free components

the voltage of the capacitor bank 6 and the test current of the high-current circuit 3, due to the pre-charge voltage of the capacitor bank 6.

When ψ = π, the voltage of the electric network 2 and the voltage of the preliminary charge of the capacitor bank 6 at the time of the beginning of the short circuit experiment are directed in the opposite direction. Therefore, the forced and free components of the voltage of the capacitor bank 6

are subtracted, and the forced and free components of the test current of the high-current circuit 3

испытательного тока, но и частично рекуперируется в электрическую сеть 2. В результате к моменту окончания опыта короткого замыкания конденсаторная батарея 6 оказывается практически полностью разряженной.add up. As a result, the test current of the high-current circuit 3 increases, providing a normalized value of the shock coefficient K _UD = 1.8 in the test experiments of short circuit. In this case, the energy previously stored in the capacitor bank 6 is consumed not only to create a free component

test current, but also partially recovered in the electric network 2. As a result, by the time the short-circuit test ends, the capacitor bank 6 is almost completely discharged.

The condition ψ = π, when u ₂ (t) = - U _{m (2)} is the most favorable phase of the voltage of the electric network 2 to start the short circuit experiment, which ensures the maximum value of the test current and almost complete discharge of the capacitor bank 6, In all other cases when ψ ≠ π, the value of the test current decreases, and the residual voltage of the capacitor bank 6 increases.

At the end of the driving signal of the conductivity interval of the high-voltage thyristor switch 5, the control system 15 stops the supply of control pulses to the power thyristors 5.1, 5.2, and the last control pulse is supplied to the power thyristors 5.2. As a result, the normalized duration of the test test of the short circuit (0.2 s) with the same number of positive and negative half-waves of the test short circuit current is ensured.

Thus, sectioning a capacitor bank with a capacitance ratio of the sections as 2 ⁰ : 2 ¹ : 2 ² : ..., pre-charging the capacitor bank for M periods of voltage of the mains, the number of which is proportional to the rated power of the tested power transformer, conducting a short circuit test from the moment the maximum voltage of the electric network opposite to the direction of the charge voltage of the capacitor bank allows you to get a positive effect of the proposed method of electrodynamic testing, which consists in expanding the range of tested power transformers.

Sources of information used in the examination:

1. Kuvshinov A.A., Khrennikov A.Yu. High-voltage thyristor valve for electrodynamic testing of power transformers // ELECTRO. - 2014. No. 2. - S. 42-46.

2. Kuvshinov A.A., Khrennikov A.Yu. Electrodynamic tests of power transformers with reactive power compensation // Electrical Engineering. - 2017. - No. 11. - S. 80-87.

Claims (1)

A method for electrodynamic testing of a power transformer, the phase output of a high voltage winding of which is connected through a series-connected high-voltage thyristor switch and a capacitor battery to the busbars of the electrical network, and the neutral and short-circuited low voltage windings are grounded, namely, the capacitance of the capacitor bank is set equal to the inductive component of the resistance short circuit of the power transformer and pre-charge the capacitor bank to a voltage corresponding to the nominal value of the high voltage winding and the specified value of the shock current of the short circuit, form the driving signal of the conductivity interval of the high voltage thyristor switch, equal to the duration of the short circuit experiment with the same number of positive and negative half-waves of short current short circuits, synchronize the driving signal with the voltage of the electric network and apply to the high-voltage thyristor The key, characterized in that the capacitor bank is divided into N sections with a ratio of capacities in sections as 2 ⁰ : 2 ¹ : 2 ² : ...: 2 ^N-1 , provide the possibility of current-free switching of individual sections, pre-charge the capacitor bank from electric buses mains along a high-voltage circuit galvanically isolated from the short-circuit current circuit for M periods of voltage of the mains, the number of which is selected depending on the rated power S _{HOM of the} tested power transformer according to the criterion S _HOM / M = const for the entire range of tested power transformers, and the short circuit test begins at the moment of maximum voltage on the busbars of the electric network, opposite to the direction of charge of the capacitor bank.

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