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Scheme Of Harmonic Filtering

Harmonic source in industrial

1, Electric arc furnace

Harmonic current components of the electric arc furnace are mainly 2nd to 7th times, where 2nd and 3rd harmonics are the biggest, whose average content can reach 5% to 10% of fundamental current. The reason for these harmonic generation are the rapid changing resistance during arc. The resistance can even change within half a cycle introducing huge current distortion; Difference between graphite electrode and steel electrode make the resistance different for positive half-cycle and negative half-cycle, thus making the current asymmetric in a single cycle, resulting in even harmonics; Three-phase arc are typically unbalanced, leading to the 3rd harmonic; and a variety of harmonic sources in power supply system lead to the formation of various harmonics.

Ultra-high-power (UHP) electric arc furnace often produces sudden, strong current impact in operation, leading to rapid fluctuations in voltage and frequency, ranging from 0.1 ~ 30Hz. Far beyond what the GB 12326-2000 allows.

UHP electric arc furnace can drag the power factor to as low as 0.1 when working in the melting period, which will cause severe drop in bus voltage.

The current large-scale electric arc furnace are developed towards much higher power, often making the short-circuit capacity of power grid within 20 to 40 times or less of that of the capacity of electric arc furnace, so the asymmetric three-phase load of electric arc furnace will result in voltage asymmetric of power grid.

2, rolling mill

Nowadays, rolling mill’s capacities are increasing and its load could change dramatically, with repeated impact or cycling change. The active powers’ peak can reach 80% to 300% of its rated capacity and lasting about a few seconds. Correspondingly reactive power fluctuated with the same level. Fluctuations in reactive power can cause fluctuations in bus voltage, even affecting regional power system. Especially, the cold and hot rolling mill’s capacity is huge, whose active power’s impact can reach tens of thousands of kilowatts.

3, electrochemical equipment

In the non-ferrous metal production, usually aluminum, copper, zinc and other products are extracted by Electrolysis, where Power rectifier is the main instrument. Modern electrolytic rectifiers adopt high-power thyristors. The electrolytic rectifiers are gradually developing towards higher-voltage, higher-current, high-capacity and higher-current-density. Because electrolytic rectifier are working with great power and huge current, therefore producing a strong electro-magnetic field, which again introduce high-order harmonics, in turn drawing down power factor.

Scheme of Filtering compensation

For users with harmonic load at high-voltage 3 ~ 10kV side, it is recommended after consider the need for low-voltage compensation, Set the HFPC series of high-voltage filtering and compensation equipment.

Where HFPC suit for user whose harmonic currents at High-Voltage side is big and the need for reactive power change over time, HFPC allowing setting several branches of Single-Tuned narrow passband filter capacitors as well as several branches of compensation capacitors. Intelligent HPJK-A series compensation control and protection Device can automatically switch and protect the filtering and compensating capacitors, according to the power factor and reactive power composite analysis, achieving the goal of both filtering and power factor optimization.

For user whose loads it light at 3 ~ 10kV side, and large harmonic current exists at Low-Voltage side, Set the LFPC series of Low-Voltage filtering and compensation equipment or used in conjunction with APF Series of Active Power Filter.

LFPC suit for user whose low-voltage side harmonic is quite stable in frequency distribution. Under such condition, we can set several Single-Tuned narrow passband filter or "C"-type high-pass filter, which can filter out currents. at the same time, setting up compensation branches to achieving btter power factor. Intelligent LPJK Series of compensation control and protection Device can automatically switch the filtering and compensating capacitors, according to the power factor and reactive power composite analysis, achieving the goal of both filtering and power factor optimization.

For user whose harmonic current is diverse in frequency distribution and also in amplitude in each order, Setting up LFPC in conjunction with APF Series of Active Power Filter. APF adopt DQ transformation analyzing the grid and offset the harmonic on the grid by producing its own harmonic with exact contrary phases through inverter. Then, LFPC can be assembled to filter low-order harmonic (whose amplitude is typically big) and compensating reactive power, where APF will focus on filtering out other harmonics not filtered by the LFPC. This scheme can save APF's capacity for active filtering where the cost of APF rise sharply with its output rated current in today's market. If APF are require to filter out all the harmonic, then a greater rated current model is needed, whose cost in first investment and maintenance can't compete with the scheme of using in conjunction with LFPC.

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