Flexibilidad en los procesos de manufactura

Since this device is to be employed in this project work to achieving the required compensation, it is worthwhile to analyse it in more pronounced details. Some of the main achievements that the SVC can impose on a power system when sited for compensation are as follows [1]:

 Reactive power flow control during power system steady state conditions:

- maintain required voltage profile on a power system - minimise system losses

 Monitor and control voltage change as a result of:

- synchronising power flow swings

- dynamic changes in high voltage DC conveters - daily load cycles

- repetitive impact loads causing voltage flicker (e.g arc furnances) - load shedding to maintain the system balance

 Enhancement of power system stability to:

- prevent transient instability

- prevent oscillatory dynamic instability

- prevent voltage instability or voltage collapse - maintain steady state power transfer capability

SVC plant comes in a number of design specifications to achieve a range of useful characteristics. In one design, the control output can be either completely passive or active (as in saturable reactors which are static components). Also, thyristors or conventional (non-solid state) switches can be employed to achieve switching [96]. Figure A.3 shows a typical design of an SVC.

Figure A.3: Typical SVC [1]

This device has voltage regulator feed-back control (active control) and more than one thyristor switch assemblies to control the output. Figure A.3 shows the SVC control characteristic.

Figure A.4: SVC control characteristic [1]

As shown by fig. A.4, the thyristors control the current through the SVC reactors and / or SVC capacitor banks when the device is operating in the linear region of the steady state control characteristic. Any range from minimum to maximum inductive and capacitive VAr output can be achieved, should it be required. Also, the fine tuning of the device‟s

respective thyristor firing angles can result in respective outputs being fine-tuned, as may be needed. Harmonic filtering is achieved by fixed capacitor assemblies.

There are a range of SVC configurations as outlined by [97, 98] and they are as follow:

 Thyristor-controlled reactor (TCR), fixed capacitor (FC)

Generally the TCR is rated larger than the total fixed capacitance to compensate this capacitance and offer net negative VArs. Two or more fixed capacitor banks supply positive VArs. This setup provides reasonable flexibility in control and up-rating terms. The setback with this assembly is that, there is high generation of high losses and, harmonics and therefore extensive filtering is required.

 Segmented TRC-FC

This arrangement is the same as the one above, but the single large TRC is replaced by two or more smaller TRC segments. The result is the reduced harmonics in the output but with increased cost and reduced efficiency.

 12-pulse TCR-FC

The twelve pulse setup results in further reduction of harmonics as compared to the segmented TRC-FC. Effective cancellation of all but twelfth harmonics is achieved by employing two coupling transformers, or one with secondary windings (one star connected and the other delta connected) and dividing the reactive elements between these windings, a 30˚ phase shift is created between the outputs of the two halves.

 High impedance thyristor controlled transformer (TCT)

The impedance of a specially designed transformer is employed in place of air core reactors to provide a controlled reactance. The transformer bears a leakage reactance of around 100% and a delta connected thyristor on the secondary winding controls the short circuit current flow through its impedance. This offers the whole setup a built-in overhead capacity, that is, during severe transient over-voltages the TCT has the capability for excessive short term VAr absorption. This setup is very costly.

 Thyristor switched capacitor (TSC), TCR

This device is similar compared to the TCR-FC fixed configuration, only the capacitor banks are in series with a solid state switch. The capacitor rating is only a portion of the total output and capacitance is varied in discrete steps in order to keep the operation of the reactor bank within its normal control range. This device offers reduced operating losses and improved performance during large system

perturbations especially when the demand for compensation exceeds the linear control range of the SVC. Fixed capacitor type compensators act as parallel LC circuits and oscillations can be established between the system and the LC circuit, and this arrangement helps during large disturbances. Oscillations can be avoided by switching in or out capacitor banks (TSC-TCR) rapidly as this minimises the perturbations. The setback is that this arrangement is costly and has a complex control circuitry.

 Mechanically switched capacitors (MSC), TCR

This device is similar to the TSC-TCR arrangement but only less expensive. In place of thyristors, conventional switches are employed to control current through the capacitors. The mechanical switching can be effected in four cycles as opposed to half and one cycle for thyristor switches. The life of mechanical switches and slower response discourages the use of this device for steady-state voltage regulation.

 Saturable reactor (SR)

This device does not use any solid state switches or active control. It responds to variations in its terminal voltage by self regulating itself. This regulation of the SR compensator is dependent on the natural saturation characteristics of the iron-core reactor. SR type compensators have the best harmonic character of any commercially available SVC. Series slope-correction capacitors are in place to change the voltage regulation characteristics and these slow down the time response to a state comparable to solid-state SVCs, and introduce harmonic effects. Control and uprating are less flexible. Load-tap changing coupling transformers are needed to change the voltage reference point (saturation point).

SRs cost more or less to solid-state devices and are associated with more losses than switched capacitor devices, as at zero net VAr output, losses are still observed in the coupling transformer of the reactor itself.