Discussion on Harmonic Measurement Method Using Capacitive Voltage Transformer

Capacitive voltage transformer (CVT) is divided by series capacitors, and then stepped down and isolated by electromagnetic transformer. It is used as a voltage transformer for meters and relay protection. Capacitive voltage transformer can also convert carrier Frequency coupling to transmission lines for long-distance communication, remote measurement, selective line high-frequency protection, remote control, teletype, etc. Therefore, compared with conventional electromagnetic voltage transformers, capacitive voltage transformers have many advantages in economy and safety in addition to preventing ferromagnetic resonance caused by saturation of the iron core of the voltage transformer.

1. Introduction to Capacitive Voltage Transformer

In addition to traditional harmonic sources such as electric arc furnaces and frequency converters in the power system, nonlinear loads such as new energy access and charging piles may generate a large number of harmonics. In order to prevent the further impact of harmonics on the power grid, it is necessary to accurately monitor and timely control the harmonic level in the power grid, and the correct measurement of the harmonic content in the power grid is the basis of monitoring and governance.

2. Composition of Capacitive Voltage Transformer

Capacitive voltage transformers are mainly composed of capacitive voltage dividers and medium voltage transformers. The capacitive voltage divider is composed of a porcelain sleeve and several series capacitors installed in it. The porcelain sleeve is filled with insulating oil to maintain a positive pressure of 0.1MPa, and steel bellows are used to balance different environments to maintain oil pressure. The capacitor voltage divider can be used as a coupling The capacitor is connected to the carrier device. The medium-voltage transformer consists of a transformer, compensation reactor, lightning arrester and damping device installed in a sealed oil tank, and the space on the top of the oil tank is filled with nitrogen. The primary winding consists of the main winding and the trimming winding, and a low-loss reactor is connected in series between the primary side and the primary winding. Since the nonlinear impedance and inherent capacitance of the capacitor voltage transformer sometimes cause ferromagnetic resonance in the capacitor voltage transformer, a damping device is used to suppress the resonance. The damping device is composed of a resistor and a reactor and is connected across the secondary winding. In general, the damping device has a high impedance under normal circumstances. When ferromagnetic resonance causes overvoltage, the reactor is saturated before the medium-voltage transformer is affected, and only the resistance load remains, so that the oscillation energy is quickly reduced.

3. Judgment of common abnormalities of capacitor voltage transformers

3.1 Secondary voltage fluctuation
The secondary connection is loose, the low-voltage terminal of the voltage divider is not grounded or the carrier coil is not connected; if the damper is a fast saturated reactor, the parameters may be improperly matched.

3.2 Low secondary voltage
The secondary connection is bad, the electromagnetic unit is faulty or the capacitor unit C2 is damaged.

3.3 High secondary voltage
The capacitor unit C1 is damaged, and the ground terminal of the voltage dividing capacitor is not grounded.

3.4 The oil level of the electromagnetic unit is too high
The capacitor unit in the lower section leaks oil or the electromagnetic unit enters water.

3.5 Abnormal sound when put into operation
The bolts of the reactor or medium voltage rheostat in the electromagnetic unit are loose.

4. Equivalent circuit model of capacitive voltage indicator

Under steady-state conditions, the entire capacitive voltage transformer equivalent circuit can be regarded as a linear system, and the stray capacitance C of the reactor is compensated. And the stray capacitance C of the primary side of the intermediate transformer: the influence at high frequencies cannot be ignored.

The iron core of the capacitive voltage transformer intermediate transformer can be regarded as working in the linear segment of the magnetization curve, ignoring the excitation inductance of the iron core, and the leakage reactance of the primary and secondary sides of the intermediate transformer is attributed to the compensation reactor.

5. Discussion on Harmonic Measurement Method Using Capacitive Voltage Transformer

Harmonic measurement by capacitive voltage transformer, there are large errors in the amplitude and phase of the measurement results. At some frequencies, the amplitude may reach more than twice the actual value at most, and the minimum may be only about 10% of the actual value; There is also a problem with nearly 1200 mutations in the phase.

The measured frequency characteristic curves of the capacitive voltage transformers of various manufacturers all have peaks and valleys within a certain frequency range, and the phase changes suddenly at the peak and valley frequencies. The frequencies of peak and valley values in the capacitive voltage transformer harmonic measurement of different manufacturers are different, and the measurement errors of the true amplitude and phase at the peak and valley values are also different. The capacitive voltage transformer harmonic measurement error results of the same model of the same manufacturer are basically the same.

Therefore, the harmonic data obtained through capacitive voltage transformer measurement in the power system is already the result of the distortion of the above-mentioned frequency characteristic curve, and cannot truly reflect the harmonic situation of the primary side system. In the case of a large number of capacitive voltage transformers used in the 110 kV and above power grids, the measurement accuracy of harmonic voltages needs to be solved. Aiming at this problem, the following improvement methods are suggested for reference.

5.1 When site conditions permit, capacitive voltage dividers should be used to measure harmonic voltages. Capacitive voltage dividers should be used to measure the site where the capacitor voltage divider has been installed. If there is no installation and the test point needs to know the harmonic voltage in detail, the capacitor voltage divider should be temporarily installed and then measured.

5.2 Connect two current sensors to the grounding circuit of the high and low voltage capacitors of the capacitive voltage transformer to measure the current of the capacitor circuit on the high and low voltage side of the capacitive voltage transformer, and calculate the voltage drop of the two capacitors respectively. After vector superposition, the voltage of the capacitive voltage transformer corresponding to the frequency of the next primary side can be obtained value. Existing devices based on this principle include PQ Sensor of ABB Company, etc. Using this device to simulate a test on a 150 kV capacitive voltage transformer, compared with parallel electromagnetic voltage transformers, the results have a good consistency. This device can be installed in On the capacitive voltage transformer that has been put into operation, the original performance of the capacitive voltage transformer will not be affected.

5.3 For newly-built or renovated substations, if more attention is paid to the harmonic measurement function, a special capacitive voltage transformer with harmonic measurement function can be used. The special capacitive voltage transformer with harmonic measurement function has the functions of voltage measurement and relay protection of conventional capacitive voltage transformer, and can also be applied to the measurement of medium and high voltage power grid harmonics. However, since the demand for harmonic measurement at voltage levels of 110 kV and above is still at a relatively low stage, and some scholars believe that this type of capacitive voltage transformer may have certain safety hazards, this type of capacitive voltage transformer is hardly used in the field.

5.4 For areas where power quality monitoring networks have been established, a large number of on-line power quality monitoring devices continuously send back on-site power quality data, and the harmonic data is the result of capacitive voltage transformer measurement distortion, which is not easy to achieve with the above methods. However, if the manufacturer provides the capacitive voltage transformer frequency characteristic curve when the capacitive voltage transformer leaves the factory, the harmonic measurement results can be corrected according to the curve, and the corrected results will have greater practical reference value.

6. Summary of Capacitive Voltage Transformer

6.1 It is demonstrated that the capacitive voltage transformer can be equivalent to a linear circuit under the condition of harmonics, and the cascaded hierarchical analysis method is used to analyze the harmonic transmission characteristics of each component and the whole of the capacitive voltage transformer, and the harmonic transmission characteristics of the capacitive voltage transformer circuit parameter changes are obtained. The quantitative influence law of the characteristic is clarified the influencing factors of harmonic voltage measurement by capacitive voltage transformer.

6.2 The amplitude-frequency characteristics show a "band-pass" characteristic in the frequency band of 50-1000 Hz, which conforms to the traditional understanding of the capacitive voltage transformer harmonic transmission characteristics. The damper parameters will affect the change rate of this "band-pass" characteristic and play a role in "smoothing". "The role of the amplitude-frequency peak value, so in the design of capacitive voltage transformer products, it is necessary to consider the coordination of the equivalent stray capacitance and the damper parameters.

6.3 Due to the different capacitive voltage transformer parameters of each voltage level, the parameters such as the range of excitation parameters of the intermediate transformer that do not affect the harmonic transfer characteristics of the capacitive voltage transformer may be different, so it is necessary to know the typical parameters of the capacitive voltage transformer of each voltage level to clarify the specific parameters of the above parameters The range is used to guide the design of capacitive voltage transformer products and realize the measurement of grid harmonic voltage through capacitive voltage transformer.