Chokes for filtration

Chokes for passive higher harmonics filters.
The significant level of the higher harmonics in industrial and municipal power networks is caused by the fast growth in the number of exploited converters and non-linear receivers exploitation. The voltage sinusoid deformation leads to increase in losses and, in extreme situations, even to the disturbance in the machines and devices operation. In order to limit the unfavourable impact of the non-linear receivers and converters on electric and power networks, the higher harmonics filters systems are used on the machines supplied from them and on batteries of the capacitors connected with the network.
Our company produces protective chokes of EDLC type intended for operation in the systems of LC higher harmonics filters.

The tasks of the higher harmonics filters in power networks.
In the most common 3-phase bridge converter systems (6-pulse systems), the current run on the transformer primary side – assuming the symmetry of supply voltages, commutation impedance and delay angles of thyristors switching off – will include, in addition to the basic component, at least the following harmonics: 5, 7, 11, 13, whose numbers are defined by the general equation (1).

(1)
where: n – harmonic number, k – natural number, p – rectified voltage pulse number.

The values of the harmonics components amplitudes may be determined by using the equation (2).

(2)
where: A1 – amplitude of voltage basic harmonic, An – amplitude of nth harmonic.

Too high contents of the supply current higher harmonics may cause a considerable increase of power losses in devices and machines cooperating with the converter as a result of the current flow with increased frequency, or it may cause disturbances in the operation of the device by deforming the supply voltage. This particularly refers to capacitors batteries, operating in parallel with the converting system. Decreasing the capacitor’s impedances, combined with frequency growth, may cause damages to the battery resulting from overloading with the currents of the higher harmonics frequencies.
Moreover, the parallel resonance in the system is also a dangerous phenomenon. The harmonics produced by non-degree power transmission systems may be enforced as many as 10-15 times in the parallel resonance circuits, formed by capacitive reactance of the capacitors batteries and network induction. This phenomenon may result in damaging both the capacitors batteries and converters.
In unfavourable conditions, the component harmonics may pose a threat to the mechanical structures of electrical machines. The harmonic pairs, e.g. 5 and 7, may produce mechanical vibrations in the generator or motor at the frequency of 6th harmonic
These vibrations arise as a result of the turning moment fluctuations due to deforming the supply voltage curve. When the frequency of these vibrations converges with the mechanical resonance frequency, the machine’s mechanical structure will be susceptible to considerable overloads.
The strenuous effect of the noisy operation of the electrical machines, resulting from the phenomenon of magnetostriction, is additionally reinforced due to the relatively high frequencies of the current harmonic components. The currents, deformed by the higher harmonics content, also cause a significant and more intense heating of the power ducts and cables as a result of the skin effect phenomenon or vicinity effect.

The task of LC filters that comprise the ED3LC chokes is to limit the negative impact of the current higher harmonics on the power network and all the devices connected to it.
Figure 1 shows a typical system for compensating reactive power and harmonics filtration. There are three filtering branches here, adjusted to 5, 7 and 11th harmonic. The number of installed filtering branches depends on the reactive power necessary for the compensation and on measurements as well as the precise analysis of the respective harmonics content in the network.
 


Fig. 1 Simplified diagram for the reactive power compensation circuit and harmonics filtration.

Filters are LC resonance systems in series, switched in parallel into the converter supply circuit. Their function is twofold: they compensate the reactive power consumed through the power transmission system and prevent the higher harmonic penetration to the power network. Depending on the harmonic number, the filter reactance is as follows (3).

(3)
where: Lf ,Cf – inductance and capacity of circuit branch being a filter; n – harmonic number; ω – pulsation

When the induction values and capacities are properly selected for the basic harmonic and for the harmonics with numbers lower than nr (resonance frequency), the filter will create capacitive load and for all the harmonics with higher numbers it will create the induction load. For the resonance frequency, LC branch will have rather small impendence, in practice equal to the choke winding resistance. The current with the resonance frequency will close between the converter and filter, not permeating to the supply network. For the basic harmonic, the filter branches are always capacitive in their character, which – in practice – means the execution of the reactive power compensation (figure 2.)


Fig. 2. LC filter impedance characteristics.

There are many solutions in industrial applications. However, the LC passive LC filters are most common (Fig. 3).


Fig. 3. Example of LC filters systems.

While operating, the branches of LC filter, shown in Figure 3a, are under the network forward voltage. In connection therewith, the capacitors batteries and chokes will be considerably more expensive than in the systems, especially in the scope of medium voltages (Figure 3b,c). For this reason, the filter configuration (Figure 3a), is commonly used in the low-voltage systems. The drawback of this solution is the lack of the possibility to filter the triple harmonics. This is possible only in the star system with an earthed zero point.
In the system shown on Fig. 3b, the distribution of voltages at the respective filter phases depends on the capacity and induction of each branch. Due to the necessity to ensure a proper working voltage in all the three phases, a precise symmetry of capacity and induction is required. The systems (Fig. 3a,b) may be used in any three-phase network system. However, the system shown on Fig. 3c may not be applied in the network with an insulated zero point or a zero point earthed by an arc-suppression coil. In such a system, the filter branches operate practically under equal voltages  (Up/√3). case of short-circuit with the earth in one phase, the inter-conductor voltage  Up appears on the other branches. This voltage is √3 times higher than in the state of normal operation. The capacitors battery in this situation should be switched off as soon as possible (t ≤ 1min.). However, in the networks with an insulated zero point, usually only earth faults are signalled and last longer, which poses a serious threat to the filters systems.


ELHAND filtration choke, type ED3LC-0,5mH/250A.

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