Harmonics filter circuits

To reduce the no-load iron losses caused by such harmonics the machine core may be formed of thinner low-loss laminates (see also Section 1.6,2(A-iv)). When the machine has already been manufactured and there is a need to suppress these harmonics, filter circuits may be employed along the lines discussed in Section 23.9. 

When harmonic filter circuits are connected to a power system with saturated reactors (Figure 27.2(e)) resonance may occur between the reactor and the filter capacitors to give rise to overvoltages. 

Excessive harmonics may also make the protective devices behave erratically or render them inoperative. Filter circuits would suppress the harmonics and eliminate these effects. 

This can be defined by the most severe external fault at which the schetne will remain inoperative. It should also remain inoperative in healthy conditions. That is it should be immune to the momentary voltage or current transients and normal harmonic contents in the circulating current. Series LC-filter circuits are generally provided with the relay coil to suppress the harmonics and to detect the fault current more precisely. 

Such relays are normally instantaneous, highly sensitive and operate at low spill cuiTents. Since they detect the residual current of the system, the current may contain third-harmonic components (Section 23.6) and operate the highly sensitive relay in a healthy condition. To avoid operation of the relay under such conditions, it is a normal practice to supply the relay coil with a tuned filter, i.e. a series L-C circuit to filter out the third-harmonic components. The capacitance of the filter circuit may also tame a steep rising TRV (Section 17.10.3) during a momentary transient condition and protect the relay. 

Filter circuits suppress harmonics in a power network 23/745... 

Electronic appliances that are highly susceptible to such effects are, however, provided as standard practice with harmonic filters in their incoming circuits. 

Use of filter circuits in the power lines at suitable locations, to drain the excessive harmonic quantities of the system into the filter circuits. 

A filter circuit is a combination of capacitor and series reactance, tuned to a particular harmonic frequency (series resonance), to offer it the least impedance at that frequency and hence, filter it out. Say. for the fifth harmonic, =... 

In an HT system, either the star is not grounded or it is a delta-connected system and hence the third harmonic is mostly absent, while the content of the. second harmonic nuiy be too small to be of any significance. For this purpose, where harmonic analysis is not possible, or for a new installation where the content of harmonies is not known, it is common practice to use a series reactor of 6% of the reactive value of the capacitors installed. This will suppress most of the harmonics by making the circuit inductive, up to almost the fourth harmonic, as derived subsequently. Where, however, second harmonics are significant, the circuit may be tuned for just below the second harmonic. To arrive at a more accurate choice of filters, it is better to conduct a harmonic analysis of the system through a harmonic analyser and ascertain the actual harmonic quantities and their magnitudes present in the system, and provide a correct series or parallel filter-circuits for each harmonic. 

As noted from general experience, except for specific large inductive loads such as of furnace or rectifiers, the fundamental content of the load current is high compared to the individual harmonic contents. In all such cases, it is not necessary to provide a filter-circuit for each harmonic unless the current is required to be as close to a sinusoidal waveform as possible, to cater to certain critical loads or instruments and devices or protective schemes operating in the system, where a small amount of harmonics may lead to malfunctioning of such loads and devices. Otherwise only the p.f. needs be improved to the desired level. Also to eliminate a parallel resonance with the... 

If there are many large or small consumers that a distribution line is feeding, it is possible that the voltage of the network may be distorted beyond acceptable limits. In this case it is advisable to suppress these harmonics from the system before they damage the loads connected in the system. Preferable locations where the series inductor or the filter-circuits can be installed are ... 

The use of a reactor in series with the ctipacitors w ill reduce the harmonic effects in a power network, as well as their effect on other circuits in the vicinity, such as a telecommunication network (see also Section 23.1 1 and Example 23.4). The choice of reactance should be such that it W ill provide the required detuning by resonating below the required harmonic, to provide a least impedance path for that harmonic and filter it out from the circuit. The basic idea of a filter circuit is to make it respond to the current of one frequency and reject all other frequency components. At power frequency, the circuit should act as a capacitive load and improve the p.f. of the system. For the fifth harmonic, for instance, it should resonate below X 50 Hz for a 50 Hz system, say at around 200-220 Hz, to avoid excessive charging voltages w hich may lead to... 

It should be ensured that under no condition of system disturbance w ould the filter circuit become capacitive when it approaches near resonance. To achieve this, the filter circuits may be tuned to a little less than the defined harmonic frequency. Doing so will make the L and hence Xl always higher than Xc, since... 

Filter circuits to compensate the harmonics R = Resistance to limit the fault level... 

But w hen the third and/or second harmonics are also present in the system, at a certain fault level it is possible that there may occur a parallel resonanee between the capacitor circuit and the inductance of the system (source), resulting in very heavy third or second harmonic resonant currents, which may cause failure of the series reactor as well as the capacitors. In such cases, a 6% reactor will not be relevant and a harmonic analysis will be mandatory to provide more exacting filter circuits. 

It is pertinent to note that since a filter circuit will provide a low impedance path to a few harmonic currents in the circuit (in the vicinity of the harmonic, to which it has been tuned) it may also attract harmonic currents from neighbouring circuits which would otherwise circulate in those circuits. This may necessitate a slightly oversized filter circuit. This aspect must be borne in mind when designing a filter circuit for a larger distribution network having more than one load centre. 

A filter circuit to absorb the harmonic currents generated by TCRs and in certain conditions, w hen TCR is OFF. ... 

D Thyristor switched capacitors Filter circuit to absorb harmonic currents caused by TCR... 

These types of reactors can now be used as current limiting reactors and also as harmonic suppressors. They are also recommended for capacitor application due to their linear characteristic which will not disturb the tuning of the filter circuit. 

These are meant to be used with a capacitor to tune a filter circuit, with resonances in the audio frequency range for reducing and filtering the harmonics or communication frequencies. They provide a near short-circuit for the required harmonics to filter them out of circuit. They may be single-phase or three-phase and connected in series or parallel of the capacitor circuit and may have a fixed or variable reactance, rated continuously with saturated magnetic characteristics. They may incur heavy losses. 

Applying harmonic filters requires careful consideration. Series-tuned filters appear to be of low impedance to harmonic currents but they also form a parallel resonance circuit with the source impedance. In some instances, a situation can be created that is worse than the condition being corrected. It is imperative that computer simulations of the entire power system be performed prior to applying harmonic filters. As a first step in the computer simulation, the power system is modeled to indicate the locations of the harmonic sources, then hypothetical harmonic filters are placed in the model and the response of the power system to the filter is examined. If unacceptable results are obtained, the location and values of the filter parameters are changed until the results are satisfactory. When applying harmonic filters, the units are almost never tuned to the exact harmonic frequency. For example, the 5th harmonic frequency may be designed for resonance at the 4.7th harmonic frequency. 

By not creating a resonance circuit at precisely the 5th harmonic frequency, we can minimize the possibility of the filter resonating with other loads or the source, thus forming a parallel resonance circuit at the 5th harmonic. The 4.7th harmonic filter would still be effective in filtering out the 5th harmonic currents. This is evident from the series-tuned frequency vs. impedance curve shown in Figure 4.22. 

Sometimes, tuned filters are configured to provide power factor correction for a facility as well as harmonic current filtering. In such cases the filter would be designed to carry the resonant harmonic frequency current and also the normal frequency current at the fundamental frequency. In either case, a power system harmonic study is paramount to ensure that no ill effects would be produced by the application of the power factor correction/filter circuit. 

Tuned harmonic filters. Tuned Harmonic filters, as their name implies, are aimed at eliminating a specific constant frequency they are normally used to cope with low order harmonics (e.g., 5th or 7th ) originating from DSU based passive front end drives. This type of filter is usually a series resonant circuit. 

Harmonic oscillations can be reduced by employing a higher number of pulses of the converter or application of filter circuits. 

For a converter with a pulse number of 12, for instance, the 5th and 7th harmonic disappears. In many cases, however, the filtering circuits are the more economical solution (Figure 13.14). Filtering circuits are series resonance circuits with their frequencies adjusted exactly to those of the harmonic oscillation currents to be eliminated. Therefore they represent very low impedance for these harmonic oscillations and prevent their flowing into the power supply system. 

Filtering circuits are mostly employed for the 5th, 7th, 11th, and 13th harmonic. In many cases, however, a filtering circuit for the 5th harmonic is sufficient. 

The basic principle of this relay is the sensing of the phase displacement between the fundamental waveforms of the voltage and current waves of a power circuit. Harmonic quantities are filtered out when present in the... 

In this part the author provides all relevant aspects of a reactive control and carries out an exhaustive analysis of a system for the most appropriate control. Harmonic effects and inductive interferences as well as use of filter and blocking circuits are covered. Capacitor switching currents and surges and methods of dealing with these are also described. 

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