Table of Contents
General
The harmonics in an SVC are generated by the TCR. Neither TSC nor TSR (Thyristor-Switched Reactor) generate harmonics. The current harmonics generated by TCR can be classified into two categories:
- Characteristic harmonics are the harmonics present under ideal conditions such as sinusoidal, balanced AC voltages, equidistant firing pulses, and identical values of impedances in the three phases.
- Uncharacteristic harmonics are the result of non-ideal conditions such as:
(a) Unbalanced AC voltages, also containing harmonics
(b) Tolerances of short circuit impedances of the step-down transformer
(c) Tolerances of reactors
(d) Asymmetry of the firing angles in the phases
(e) Asymmetry of the firing angle for positive and negative half cycles
of one phase.
The non-characteristic harmonics are usually less than 1% of the characteristic harmonics for ∆L = 5%, ∆α = 1° (∆L is the tolerance in the reactors, ∆α is the deviation in the firing angle from equidistant pulse control). However, these non-characteristic harmonics cannot be neglected. For example, a small DC component present in the SVC current (due to the firing angle asymmetry) can drive the connecting transformer into saturation resulting in large harmonic voltages at the SVC bus.
Harmonic Interactions
A major concern is the possibility of harmonic interactions with the AC network (Note that this is a separate phenomenon from what was discussed in the previous section regarding control instability). The problem of harmonic magnification (of low order such as second or third) has been viewed by some as ‘harmonic instability’ in the past. The harmonic interactions with the network can arise from the following factors.
(a) Direct: Effects due to the harmonics present in the bus voltage. These harmonics influence the harmonics in the TCR current which in turn affects the harmonic distortion in the bus voltage. Normally such effects are secondary unless the impedance seen by the TCR current is very high.
(b) Through voltage regulation loop: The modulation of the TCR admittance at frequency fc, results in the frequencies, fo ± fc at the SVC bus which are picked up by the voltage measuring circuit and may
be amplified. This problem can be solved by providing notch filters in the control circuit. To eliminate the possibility of magnification of (positive sequence) second and third harmonics, filters at the fundamental frequency (f0) and second harmonic (2f0) are provided. The voltage unbalance can result in second harmonics on the DC side and third harmonics on the AC side.
(c) Through gate pulse synchronization: Although the gate pulses are central in a normal steady state, during network faults, there can be firing angle asymmetries that result in harmonics. In general, gate pulse synchronization should be insensitive to harmonics in the bus voltage.
Harmonic Filters
The presence of harmonics (both current and voltage) is viewed as ‘pollution’ affecting the operation of power systems. The injection of current harmonics by nonlinear loads also results in the distortion of the voltage waveforms due to the system impedances. The adverse effects of the harmonics include
- Harmonic overvoltages, particularly during a transient can also be sustained due to inrush current phenomena in transformers.
- Increased losses in shunt capacitor banks and rotating machines whose impedances are much lower at harmonic frequencies than at system frequency.
- Telephone interference.
The single-tuned shunt filters are commonly applied to eliminate harmonic currents injected by SVCs. The configuration of the single-tuned filter is shown in Fig.(a) and its impedance characteristics are shown in Fig. (b). The normalized impedance ( Z/Xr ) magnitude and its phase angle (ⲫ) are given by:
In FC-TCR type SVC, the fixed capacitor branches can also be designed to operate as tuned filters by inserting a reactor with an appropriate Q factor. Double-tuned and damped high-pass filters are also used as filters.
Frequently Asked Questions (FAQs)
What do you mean by harmonics?
In physics, acoustics, and telecommunications, a harmonic is a type of wave with a frequency that is a whole number multiple of the basic, or fundamental, frequency of a repeating signal. The fundamental frequency is called the 1st harmonic, and the other harmonics, which have higher frequencies, are called higher harmonics.
Why reduce harmonics?
Reducing harmonics helps prevent equipment damage, improves energy efficiency, and ensures stable operation of electrical systems.
Why do harmonics occur?
Harmonics are caused by devices that convert AC to DC, like variable frequency drives, computer power supplies, and energy-efficient lighting, which disrupt electrical systems.
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