FACTS Controller Interactions
Ø Controller interactions can occur in the following combinations:
1. Multiple FACTS controllers of a similar kind.
2. Multiple FACTS controllers of a dissimilar kind.
3. Multiple FACTS controllers and HVDC converter controllers.
Ø Because of the many combinations that are possible, an urgent need arises for power systems to have the ontrols of their various dynamic devices coordinated.The term coordinated implies that the controllers have been tuned simultaneously to effect an overall positive improvement of the control scheme.
Ø The frequency ranges of the different control interactions have been classified as follows: o 0 Hz for steady-state interactions
o 0–3/ 5 Hz for electromechanical oscillations
o 2–15 Hz for small-signal or control oscillations
o 10–50/ 60 Hz for subsynchronous resonance (SSR) interactions
o >15 Hz for electromagnetic transients, high-frequency resonance or harmonic
resonance interactions, and network-resonance interactions
1. Steady – State Interactions
Ø Steady-state interactions between different controllers (FACTS–FACTS or FACTS– HVDC) occur between their system-related controls.
Ø They are steady state in nature and do not involve any controller dynamics. These interactions are related to issues such as the stability limits of steady-state voltage and steady-state power; included are evaluations of the adequacy of reactive-power support at buses, system strength, and so on.
Ø An example of such control coordination may be that which occurs between the steady-state voltage control of FACTS equipment and the HVDC supplementary control for ac voltage regulation.
Ø Load-flow and stability programs with appropriate models of FACTS equipment and HVDC links are generally employed to investigate the foregoing control interactions.
Ø Steady-state indices, such as voltage-stability factors (VSF),are commonly used. Centralized controls and a combination of local and centralized controls of participating controllers are recommended for ensuring the desired coordinated performance.
2. Electromechanical – Oscillation Interactions
Ø Electromechanical-oscillation interactions between FACTS controllers also involve synchronous generators, compensator machines, and associated powersystem stabilizer controls .
Ø The oscillations include local mode oscillations, typically in the range of 0.8–2 Hz, and inter-area mode oscillations, typically in the range of 0.2–0.8 Hz.
Ø The local mode is contributed by synchronous generators in a plant or several generators located in close vicinity; the inter-area mode results from the power exchange between tightly coupled generators in two areas linked by weak transmission lines.
Ø Although FACTS controllers are used primarily for other objectives, such as voltage regulation, they can be used gainfully for the damping of electromechanical oscillations.
Ø In a coordinated operation of different FACTS controllers, the task of damping different electromechanical modes may be assumed by separate controllers.
Ø Alternatively, the FACTS controllers can act concertedly to damp the critical modes without any adverse interaction.
Ø Eigenvalue analysis programs are employed for determining the frequency and damping of sensitive modes.
3. Control or Small – Signal oscillations
Ø Control interactions between individual FACTS controllers and the network or between FACTS controllers and HVDC links may lead to the onset of oscillations in the range of 2–15 Hz (the range may even extend to 30 Hz).
Ø These oscillations are largely dependent on the network strength and the choice of FACTS controller parameters, and they are known to result from the interaction between voltage controllers of multiple SVCs, the resonance between series capacitors and shunt reactors in the frequency range of 4–15 Hz ,and so forth. The emergence of these oscillations significantly influences the tuning of controller gains.
Ø Analysis of these relatively higher frequency oscillations is made possible by frequency-scanning programs, electromagnetic-transient programs (EMTPs), and physical simulators (analog or digital).
Ø Eigenvalue analysis programs with modeling capabilities extended to analyze higher-frequency modes as well may be used .
4. Sub Synchronous resonance (SSr) Interactions
Ø Subsynchronous oscillations may be caused by the interaction between the generator torsional system and the series-compensated-transmission lines, the HVDC converter controls, the generator excitation controls, or even the SVCs.These oscillations, usually in the frequency range of 10–50/ 60 Hz, can potentially damage generator shafts.
Ø Subsynchronous damping controls have been designed for individual SVCs and HVDC links.
Ø In power systems with multiple FACTS controllers together with HVDC converters, a coordinated control can be more effective in curbing these torsional oscillations.
5. High – Frequency Interactions
Ø High-frequency oscillations in excess of 15 Hz are caused by large nonlinear disturbances, such as the switching of capacitors, reactors, or transformers, for which reason they are classified as electromagnetic transients.
Ø Control coordination for obviating such interactions may be necessary if the FACTS and HVDC controllers are located within a distance of about three major buses. Instabilities of harmonics (those ranging from the 2nd to the 5th) are likely to occur in power systems because of the amplification of harmonics in FACTS controller loops.
Ø Harmonic instabilities may also occur from synchronization or voltage-measurement systems, transformer energization, or transformer saturation caused by geomagnetically induced currents (GICs).
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