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Probe current voltage pin 420*4450 head diameter 5.0 over current current and voltage pin

inductance

In order to measure the large current flowing in the high-voltage AC circuit, the current transformer is usually used to change the large current into a small current by using the ratio of the transformer, so that the measuring instrument does not need to be directly connected to the line to be tested, and the secondary circuit can be Connect any form of wiring diagram as needed to meet the requirements of metering, relay protection, automatic control, etc.
The current transformer operates as a current source, and the magnitude of the primary current is virtually independent of the secondary load, since the secondary load is converted to the primary side and is negligible compared to the system impedance. The current transformer and the load Zfh=Rfh+jXfh connected to the secondary side can be represented by the equivalent diagram of FIG. In the picture:

The primary current of the I1-current transformer is converted to the value of the secondary side;
I2-secondary current of the current transformer;
The excitation current of the I0-current transformer is converted to the value of the secondary side;
Z1=R1+jX1- resistance and leakage reactance of the primary winding of the current transformer (converted to the secondary side);
Z2=R2+jX2- resistance and leakage reactance of the secondary winding of the current transformer;
Z0=R0+jX0-current transformer excitation resistance and reactance;
Zfh=Rfh+jXfh-load resistance and reactance connected to the secondary side of the current transformer.

2. Error Current Transformer The current transformer is mainly composed of three parts: iron core, primary coil and secondary coil. Due to the existence of core magnetoresistance, the current transformer must consume a small amount of current for excitation during the process of transmitting current, so that the core is magnetized, so that the induced potential and the secondary current are generated in the secondary coil, and the current transformer error is Caused by the excitation current consumed by the core. Due to the presence of excitation current and iron loss, the difference between the primary current and the secondary current of the current transformer is a vector, and the error includes the ratio difference and the phase angle difference. The numerical error of the primary current and the secondary current is expressed as a percentage in relative error mode, which is called the ratio difference. The ratio is defined according to the national standard:

(1)

Where: I1 - the primary current flowing on the line;
I2-current in the secondary circuit of the current transformer;
Ke-current transformer rated current ratio;
It can be seen from the formula defined by the difference that the difference has positive and negative values. When the ratio is positive, the current reading of the measuring instrument connected to the transformer is multiplied by the ratio Ke and is greater than the actual current on the line; when the ratio is negative, the meaning is just the opposite. The difference in the ratio of the same current transformer to different currents and loads may be positive or negative.
In addition, there is a difference in phase angle between the primary current and the secondary current. The angle between the primary current vector and the secondary current vector after the 180o inversion is called the phase angle difference or simply the angular difference. The angular difference also has positive and negative points: When the secondary current vector leads the current vector before the inversion, the angular difference is specified as positive, and conversely, it is negative when it lags behind the primary current vector.
As a current transformer for measurement, the ratio of the difference and the angular difference directly affects the correctness of the measurement result. Therefore, the ratio difference and the angular difference are the most important characteristics of the current transformer for measurement. However, the requirements for the current transformer for relay protection and the current transformer for measurement are different. The current transformer for measurement only needs to be accurate during normal operation, while the current and current transformer for relay protection requires short circuit. The accuracy is accurate, that is, the transformer can be accurately converted between the steady-state short-circuit current and the transient short-circuit current.
Since the current transformer core has a gradual saturation characteristic, under the short-circuit current, the core of the current transformer tends to be saturated, the excitation current rises sharply, and the proportion of the excitation current in the primary current increases greatly, so that the ratio gradually shifts toward Negative values ​​and increase rapidly. When the current is increased to make the ratio difference exactly equal to -10%, the ratio of this current to the rated current (I1/I1e) is called the saturation factor. Since the operating current of the relay is generally several times larger than the rated current, the current transformer used as relay protection should ensure that the relay can operate reliably under a short-circuit current several times larger than the rated current. Therefore, the main characteristics of the current transformer for relay protection are not the ratio difference and the angular difference but the saturation factor.

3. Factors affecting the error 3.1 The internal parameters of the current transformer are the main factors affecting the current transformer error.
(1) Influence of secondary coil internal resistance R2 and leakage resistance X2 on error: When R2 increases, the ratio difference and angular difference increase; when X2 increases, the ratio difference increases, but the angular difference decreases. Therefore, to improve the error, R2 and the appropriate X2 value should be minimized. Since the secondary coil internal resistance R2 and the leakage resistance X2 are small compared with the secondary loads Rfh and Xfh, the change of R2 and X2 has little effect on the error, and only the influence on the small-capacity current transformer is significant.
(2) Influence of the cross section of the iron core on the error: The increase in the cross section of the iron core reduces the magnetic flux density of the iron core, and the excitation current decreases, thereby improving the ratio difference and the angular difference. The unbalanced current transformer has a small magnetic flux density under the rated condition, so reducing the magnetic flux density also reduces the magnetic permeability, so that the excitation current is not reduced much, and the smaller the magnetic flux density, the more effective the effect. difference.
(3) The influence of the number of turns on the error: Increasing the number of turns of the coil is to increase the ampoule. Increasing the number of turns can reduce the magnetic flux density, and the effect of improving the error is much more significant than increasing the cross section of the core. However, an increase in the number of turns of the coil causes an increase in the amount of copper, and at the same time causes a decrease in the dynamic stability factor and an increase in the saturation factor. In addition, for single-turn current transformers (such as a through-type or bushing type current transformer only one turn), the error cannot be increased by increasing the number of turns.
(4) Reduce core loss and increase magnetic permeability. Under the condition that the core flux density is constant, reducing the core excitation ampere and loss ampere will also improve the ratio difference and angular difference. Therefore, the magnetic permeability can be improved by using high-quality magnetic materials and adopting a suitable annealing process. Reduce the purpose of loss. The strength of the core magnetic core also affects the saturation factor, and the saturation ratio of the core is small when the magnetic properties are poor.

3.2 Current transformer error in operation When the current transformer has been finalized and its internal parameters are determined, its error magnitude will be affected by the secondary current (or primary current), secondary load, power factor and frequency. These factors are called external factors, and the error of the current transformer in operation is mainly affected by these four factors.
(1) The influence of the variation of current frequency on the error is more complicated. Generally, the frequency of the system changes very little, and its influence is negligible. If the frequency changes too much, for example, a current transformer with a rated frequency of 50 Hz is used in a 60 Hz system, the influence of the frequency should be considered, because the frequency variation not only affects the core loss, the magnetic flux density and the coil leakage reactance, but also affects The magnitude of the secondary side load reactance value.
(2) When the primary current decreases, the magnetic flux density decreases proportionally, but at low magnetic flux density, the reduction of the excitation ampoule is slower than the decrease of the magnetic flux density, so the absolute value of the difference and the angular difference is relatively increased.
(3) The current transformer error has the following characteristics: when the primary current changes within the specified range, the secondary current changes proportionally, and when the secondary load impedance changes within the specified range, the magnitude of the secondary current is not affected. Therefore, when the secondary load is reduced within the rated range, the magnetic flux density is also reduced, and since the secondary current is constant, the exciting current is reduced and the error is also reduced. The factory manual of the current transformer generally indicates the rated secondary load impedance value. In operation, the error should be checked according to the maximum secondary load impedance value in the given wiring mode.
(4) The power factor of the secondary load increases, that is, Rfh increases, Xfh decreases, the angular difference will increase, and the ratio will decrease. For the saturation factor, the saturation factor indicated in the transformer manufacturer's specification refers to the saturation factor when the power factor is 0.8. This value corresponds to the “minimum value” of the saturation factor, so the power factor increases or decreases, The saturation factor is increased.

3.3 Measures to reduce the error The excitation current is the main cause of the current transformer error, so reducing the excitation current can reduce the error:
(1) The material with high magnetic permeability is used as the iron core, because the magnetic properties of the core not only affect the ratio difference and angular difference, but also affect the saturation factor.
(2) Increase the cross section of the core, shorten the length of the magnetic circuit; increase the number of turns of the coil. Increasing or decreasing the core section or coil ampere will increase and decrease the saturation factor. When increasing the core section or coil ampule to improve the ratio difference and angular difference, the influence on the saturation factor must be considered.
(3) Limit the impact of secondary load. In the field, a method of increasing the effective cross section of the connecting wire is generally used, such as using a cable with a larger cross section, or using multiple cores in parallel to reduce the impedance value of the secondary load. It is also possible to use two current transformers of the same type and the same ratio in series, so that the load of each current transformer becomes half of the entire load.
(4) Appropriately increase the current transformer ratio. In the field operation, a transformer with a large ratio is selected.
In addition, there are fractional compensation of the secondary winding, secondary side capacitor shunt compensation, and the like.

4. The current transformer used for the calibration of the current transformer is generally not subjected to error check when it is selected, and the error test is only performed during operation. When designing, the secondary connection conductor cross section is selected according to its exact grade and secondary load, and only the ratio difference between the lower limit load (such as 10% rated current) and the upper limit load (such as 120% rated current) is checked during operation. There are many test methods for phase angle difference, ratio difference and angular difference, such as dual ammeter method, AC compensator method and so on.
For the current transformer for relay protection, the error check or the short-circuit current multiple and the secondary connection load are generally checked according to the 10% error curve (or volt-ampere characteristic curve). Transient error checking is required when the power system has transient requirements for the current transformer.

4.1 The 10% error curve (saturation factor) of the check ratio difference is related to the magnitude of the secondary load. When the secondary load increases, the ratio difference characteristic curve moves to a negative value, and the same current transformer reaches a difference of -10%. When the secondary load is large, the current ratio is smaller than (I1/I1e), the secondary load is small, and the current ratio (I1/I1e) is larger. If the secondary load is rated, this multiple is the saturation factor. The curve of the different secondary loads and the corresponding saturation multiples is the "10% error curve".

According to the operating experience of relay protection, under the actual operating conditions, the current error of the current transformer used by the protection device is not allowed to exceed 10%, and the angular error does not exceed 7 degrees. The 10% error curve given by the manufacturer is made as follows:
(1) Give the rated limit current multiple ne, and obtain the excitation ampere AW0=AWP/10 and the excitation ampere AW0/L of the unit magnetic path length from the corresponding primary ampere AWP.
(2) Check the BH curve of the corresponding core from AW0/L, and obtain the maximum magnetic density Bm, and obtain the corresponding secondary induced potential ESm according to the core and secondary coil parameters.
(3) The secondary load impedance ZS 求 is obtained from the ESm and the secondary rated current ISn and the allowable load impedance Zb is obtained from the secondary coil impedance.
(4) A series of short-circuit current multiples n are sequentially given, and the corresponding Zb value can be obtained, thereby making a curve n=f(Zb) which is a 10% error curve.
In practice, the volt-ampere characteristic method is often used to measure the volt-ampere characteristic curve of the current transformer. The test wiring is shown in Figure 2. During the test, the primary coil of the transformer is opened, and the voltage is applied to the secondary coil. From Figure 1 (the equivalent circuit diagram of the current transformer), this voltage is equivalent to the voltage U2, and the current meter is used to measure the current under the action of the voltage U2. The current I0 of the coil is obtained by the relationship between current and voltage. U2=f(I0) is the volt-ampere characteristic curve of the current transformer (Fig. 3).
In order to make the output voltage close to the sine wave in the test, the single-phase voltage regulator is used to adjust the current, and the electric or electromagnetic type meter is used to make the 10% error curve safer. Because the measured current contains the third harmonic component, the average value is smaller than the effective value, while the electric type and electromagnetic type meter reflect the effective value, and the rectifying type meter reflects the average value. From the equivalent circuit according to the current transformer:

When the impedance angles of Z0, Z2, and Zfh are the same, I1', I0, and I2 are in phase, and the ratio is the maximum value. When the ratio is -10%, the ratio difference formula (1) is obtained:

Since Z2 is small, the voltage drop on it is negligible, so U0≈U2, so the relationship U0≈U2=f(I0) can be obtained from the volt-ampere characteristic curve. Defined by current multiple:

(Note: The rated current on the secondary side of the current transformer is uniformly specified as 5 amps or 1 amp)

According to equations (7) and (9) and the volt-ampere characteristic curve U0≈f(I0), the relationship between the current multiple K and (Z2+Zfh) can be plotted, that is, the 10% error curve.
Assuming that the nominal transformation ratio Ke is equal to the turns ratio Kw, equations (7) and (9) can be simplified as:

Measuring the volt-ampere characteristic curve can also check whether the secondary coil has a turn-to-turn short circuit. Since the volt-ampere characteristic curves of the same type of current transformer are very similar, when there is a turn-to-turn short circuit, a circulating current will be generated in the short-circuited portion, which is equivalent to applying a short-circuit 匝 to the iron core, in the case where the applied voltage is the same. The current will increase a lot, causing the volt-ampere characteristic curve to move down significantly, and the volt-ampere characteristics when there is no short circuit between turns is very different.

5. Conclusions (1) The ratio difference and angular difference are the main characteristics of the current transformer for measurement, and the saturation coefficient is the main characteristic of the transformer for relay protection.
(2) The internal resistance and leakage resistance of the secondary coil, the number of turns of the secondary coil, the magnetic permeability of the iron core, and the cross section of the iron core are internal factors that affect the error of the current transformer; the magnitude of the secondary current and the secondary load And power factor and frequency are external factors that affect the current transformer error.
(3) CT for measurement is usually not subjected to error check during selection. Error calculation or error test is only performed during product design or operation; when design is selected, the secondary connection wire cross section is generally selected according to its accuracy level and secondary load. The protection CT generally performs error verification or short-circuit current multiple and secondary connection load verification according to a 10% error curve (or volt-ampere characteristic curve).