1。 order

With the rapid development of power electronics technology, more and more new types of electrical equipment come out and put into use, and the influence of higher harmonics is becoming more and more serious. After the power system is polluted by harmonics, it will affect the operating efficiency of the system, even damage the equipment, and even endanger the safe operation of the power system. In the past, the main indexes to evaluate the power quality of power system were voltage amplitude and frequency. At present, all countries in the world regard the limit value of positive harmonic distortion rate of power grid voltage as one of the indexes to evaluate power quality, and correctly understanding harmonics has become one of the important tasks of power workers. Therefore, it is of great practical significance to study and analyze the causes and harms of harmonics and the measures to suppress them.

1 causes of harmonics

The appearance of harmonics in power supply system is mainly caused by two main factors:

(1) The widespread use of thyristor rectifiers and voltage regulators, the widespread use of thyristors in a large number of household appliances and the increase of various nonlinear loads lead to waveform distortion.

(2) The change of equipment design concept. In the past, it tended to adopt a design that worked under rated conditions or had a large margin. Nowadays, in order to compete, the design of electrical equipment tends to be critical. For example, in some designs, in order to save materials, the magnetic material works in the deep saturation section of the magnetization curve, which will lead to serious distortion of the waveform of the excitation material. Figure 1 is the principle wiring diagram of three-phase six-pulse rectifier. At this time, the Fourier series expansion of AC side current is:

It can be seen that the AC side current contains harmonics, the harmonic order is (6k 1), and the harmonic content of each order is.

Harm of Harmonics to Power System

Harmonic pollution to power system is becoming more and more serious. The injection of harmonic source increases the harmonic current and voltage of the power grid, and its harm spreads to the whole network, which has different degrees of influence and harm to various electrical equipment. The hazard analysis of specific equipment is as follows:

(1) alternator. Synchronous motors and induction motors will generate additional heat losses in stator windings and rotor windings. In addition to the copper loss I2nR of harmonic current, due to the skin effect of current, the heat loss will generate additional losses, thus increasing the heat loss of the rotor. For large turbo-generators, if the harmonic current exceeds 25% of the rated current for many times, the rotor may be damaged due to the above reasons. For the transformer, the iron core produces heat loss, especially eddy current loss, and there is harmonic current in the transformer winding, which induces magnetic flux in the iron core and produces iron loss.

(2) The harmonic current of overhead lines produces heat loss, and the larger harmonic current component can significantly delay the extinction of the secondary power supply current, resulting in the failure of single-phase reclosing. Harmonic current in cable will produce heat loss, increase dielectric loss and temperature rise of cable.

(3) Harmonic current of power capacitor will cause extra dielectric loss and accelerate insulation aging of power capacitor. The resonance of harmonic voltage or current in the system will cause overvoltage and overcurrent, damage the insulation of electrical equipment, and cause noise and vibration.

(4) The electronic computer will be distorted by harmonic interference; The function of industrial electronic equipment will be destroyed because of it.

(5) Interference and misoperation of relay protection, automatic control device and computer, resulting in electric energy metering error.

(6) Harmonic current flowing on high-voltage overhead lines will not only increase line loss, but also interfere with adjacent communication lines.

3 power system measures to suppress harmonics

In order to limit the interference (pollution) of harmonics to the power system within the acceptable range of the system, China and other countries in the world have promulgated the Interim Provisions on Harmonic Management of Power System and IEC standards respectively, and defined the limits of harmonics generated by various harmonic sources.

The main measures to suppress power system harmonics are:

(1) A group of reactors are connected in series in the compensation capacitor circuit.

As shown In Figure 2, before adding Xc, the resistor is omitted, and the harmonic voltage of the harmonic source in bus is UN = xsn? In; After the compensation capacitors are connected in parallel, the input harmonic reactance of the harmonic source is: harmonic voltage at this time, harmonic current injected into the system UN, ISN >；; In other words, the shunt capacitor amplifies the harmonics of the system. If a certain harmonic corresponds to Xsn-Xcn=0, the harmonic current and voltage tend to infinity. In order to get rid of this resonance point, a reactor is usually connected in series in the capacitor branch, and its inductance value is selected so that the total reactance of the capacitor circuit is inductive reactance rather than capacitive reactance at any possible harmonic, which fundamentally eliminates the possibility of harmonic generation.

(2) Install a single tuning filter and a high-pass filter consisting of capacitors, inductors and resistors.

A single-tuned filter is designed for a certain harmonic, and a high-pass filter is designed for absorbing several higher harmonics. The type and number of filters to be installed and their tuning frequency (filtering times) can be determined by specific calculation.

For example, electric locomotive is a high-power single-phase rectifier, which has harmonic problems. According to the measured data, the harmonic content of Shaoshan-1 locomotive current is roughly as shown in Table 1, and there are many factors that affect the harmonic current injected into the power system by electric locomotives. As shown in Figure 3, catenary is one of the influencing factors. In the figure, Zsn is the harmonic impedance of the power system, the total length of the power supply arm is L, and there is only one electric locomotive on the arm, which is located at the substation L.

If the impedance and admittance of the nth harmonic per unit length of catenary are Zn and Yn, respectively, the characteristic impedance Zcn and propagation constant γn of the nth harmonic can be obtained.

It can be seen that the general catenary can amplify the harmonic current of the locomotive, and the amplification effect is the greatest when the locomotive is at the end of the power supply arm (L = L). The general method to solve the harmonic problem of traction locomotive is to install filters 3, 5 and 7 in traction substation. In recent years, some electric locomotives newly put into operation have been replaced by on-board classified filters, which have a good effect of filtering out the third and fifth harmonics.

(3) Increase the phase number of rectifier.

Harmonic current is closely related to the number of rectified phases, that is, with the increase of phase number, the lowest order of harmonic becomes higher and the amplitude of harmonic current becomes smaller. Generally speaking, silicon controlled rectifier devices are mostly 6-phase. In order to reduce the higher harmonic current, 12 phase or 36 phase can be used instead. When using 12 phase rectifier, the high-order harmonic current only accounts for about 1% of the total current, and the harm is greatly reduced.

(4) When more than two rectifier transformers are powered by the same bus, the primary windings of the rectifier transformers can be alternately connected into Y-shape and△-shape respectively, so that the 5th and 7th harmonics cancel each other out, and only the influence of 1 1 and13rd harmonics needs to be considered. Because of high frequency and small amplitude, the harm is reduced.

4 conclusion

The appearance of (1) subharmonic affects the environment of the whole power system. For example, the appearance of harmonic noise in communication will reduce the call quality, make the control and protection equipment malfunction, overload the power equipment and system, and do harm to the normal operation of the power system.

(2) Harmonic control is usually to formulate the voltage harmonic content limit standard of user connection points, that is, to formulate relevant standards and take corresponding measures to strictly control and purify the power system environment.

(3) Pay attention to the accuracy of PT and CT when measuring harmonics, otherwise the error will be great. It is accurate and convenient to measure the harmonic voltage of the system with the partial voltage of CT terminal screen. It has been widely used in harmonic voltage test of EHV system.