SVS Based FACTS Device

Among the SVS based Facts devises are the STATCOM, the SSSC and the UPFC.

Synchronous Voltage Source (SVS)

Controllable solid-state synchronous voltage sources are employed for compensating the dynamic and controlling real-time the power flow in transmission systems. This method, when compared to conventional compensation approaches employing thyristor switched capacitors and thyristor-controlled reactors, saves vastly premium performance characteristics and regular applicability for transmission voltage, reactance, and angle ability. It also gives the powerful tool to direct exchange active power with the AC grid, in addition to the independently controllable reactive power compensation, thereby giving a powerful new option for the counteraction of dynamic disturbances.

A functional model of the solid-state synchronous voltage source is shown in Figure

2.10. Reference signals Qref and Pref define the output voltage amplitude with its phase angle of the generated voltage and also the reactive and real power flow between the solid mention voltage source and the grid. If the goal of dynamic true power flow is not achieved, Pref = 0, the SVS gets a self-sufficient reactive power source as an ideal synchronous condenser, and the external energy storage device can be disposed of.

Various switching power converters can implement the solid-state synchronous voltage source, although of the switching converter mentioned as the voltage-sourced inverter. Thisparticular DC to AC switching power converter, which is based on gate turn-off (GTO) thyristors in appropriate multi-pulse circuit configurations, is presently applied in the most practical for high power utility applications. The functional and operating characteristics of this type of inverter, which saves the basic functional building block for the comprehensive compensation and power flow control approach, are explained below. An elementary, six-pulse, voltage-sourced inverter is shown in Figure 2.11. It consists of six self-commutated semiconductor (GTO) switches, each of which is shunted by a reverse-parallel connected diode. It should be noted that in a high power inverter, each solid-state switch consists of a number of series-connected GTO thyristor/diode pairs. With a DC voltage source (which may be a charged capacitor), the inverter can produce a balanced set of three quasi-square voltage wave-forms of a given frequency, as illustrated in Figure 2.12, by connecting the DC source sequentially to the three output terminals via the appropriate inverter switches.

There is exchange between the reactive power of the inverter and the AC system, which can be controlled by varying the magnitude of the produced three-phase output voltage.

When the amplitude of the output voltage is raised over the system voltage, then the current flows via the reactance from the inverter to the AC system and the inverter produces capacitive power for the AC grid. If the output voltage amplitude is decreased under that of the AC grid, then the reactive current flows from the AC system to the inverter and the inverter draws inductive power. When the output voltage is balanced with the AC grid voltage, the reactive power flow becomes zero.

In the same way, the real power transfer between the SVS and the AC grid may be controlled by shifter of the phase voltage of the inverter related to the AC system voltage. That is, the inverter from its DC energy storage provides real power to the AC network if the voltage of the inverter is leading the corresponding AC network voltage. This is because this phase advancement results in a real component of current through the tie reactance that is in anti-phase with the AC network voltage. By the same way, the inverter draws real power from the AC grid for DC energy storage, when the voltage of the inverter is lagging the AC network voltage. The real component of current flowing through the tie reactor is now in-phase with the AC system voltage. The mechanism by which the inverter internally generates reactive power can be explained simply by considering the relationship between the output and input powers of the inverter. The base of the explanation depends on the physical rule that the process of energy transfer through the inverter, consisting of nothing but arrays of solid-state switches, is absolutely direct. Thus, it is clear that the resultant power at the AC output ports are always equal to the net resultant power at the DC input terminals when neglecting the losses.

Assume that the inverter is operated to supply only reactive output power. In this case, the active input power provided by the DC source has to be zero. Furthermore, where reactive power, at frequency equals zero, by basics will be zero, the DC source generates no input power and therefore it clearly has no part in the supplying of the reactive output power. In another meaning, the inverter connects internally the three output terminals, like a method, which the reactive currents may move easily between them. Concerning this with the terminals of the network, it could be seen that the inverter produces an exchanged circulating power among the phases.

Although reactive power is inherently produced by the action of the solid-state switches, it is still essential to have a relatively small DC capacitor connected across the input terminals of the inverter.

The importance for the DC capacitor is primarily requested to satisfy the above stipulated the balance input power and output power. The waveform of the inverter output voltage is not a pure sine wave. It is a staircase approximation of a sine wave. However, the multi-pulse inverter absorbs a smooth, almost sinusoidal current from the network through the tie reactance. As a result, the resultant three-phase instantaneous apparent power (VA) at the output terminals of the inverter slightly fluctuates. Thus, for not violating the balance between of the real input power and output power, the inverter must draw a ripple current from the DC capacitor that keeps a regulated terminal voltage at the input. The existence of ripple part of input current is mainly due to the ripple components of the output voltage, which depend on the used technique in the output waveform fabrication.

In a high power inverter, using a sufficiently high pulse number, the output voltage distortion and, thereby, capacitor ripple current can be mainly decreased to any desired degree.

Thus, a perfect inverter would produce sinusoidal output voltage and draw pure DC input current without harmonics. To achieve purely reactive output, the input current of the perfect inverter is zero. Because of system unbalance and other unbalances like economic considerations, those ideal conditions are not practical, but approximated satisfactorily inverters of sufficiently high pulse numbers (24 or higher).

COMPENSATION BY STATCOM (STATIC COMPENSATOR)

A STATCOM is a static synchronous generator operated as a shunt connected static var compensator whose capacitive or inductive output current can be controlled independent of the ac system voltage .

A STATCOM is a solid state switching converter capable of generating or absorbing independently controllable real and reactive power at its output terminals, when it is fed from an energy source or an energy storage device of appropriate rating. A STATCOM incorporate a voltage source inverter (VSI) that produces a set of three phase ac output voltages, each of which is in phase with, and coupled to the corresponding ac system voltage via a relatively small reactance. This small reactance is usually provided by the per phase leakage reactance of the coupling transformer. The VSI is driven by a dc storage capacitor. By regulating the magnitude of the output voltage produced, the reactive power exchange between STATCOM and the ac system can be controlled.

In principle, a Static Synchronous Compensator or STATCOM is a shunt-connected device which injects reactive current into the AC system. This leading or lagging current, which can be controlled independently of the AC system voltage, is supplied through a power electronics-based variable voltage source. The STATCOM does not employ capacitor or reactor banks to produce reactive power as the Static Var Compensators (SVC) do. In the STATCOM, the capacitor is used to maintain a constant DC voltage in order to allow the operation of the voltage-source converter. A STATCOM controller with ESS is similar to an ideal synchronous machine which generates a balanced set of (three) sinusoidal voltages at the fundamental frequency, with controllable amplitude and phase angle

If the energy storage is of suitable rating, the SVS can exchange both active andreactive power with the network. The active and reactive power, supplied or drawn by theSVS, can be controlled independently of each other, and any combination of active power generated or absorbed, with active power, generated or absorbed, is possible. The active power that the SVS exchanges at its network terminals with the grid must, of course, be supplied to, or absorbed from, its DC terminals by the energy storage unit. In other way, the reactive power flow is internally developed by the SVS, without the DC energy storage device playing any significant part in it. The bi-directional real power exchange capability of the SVS, that is, the ability to absorb energy from the AC system and deliver it to the DC energy storage device (large storage capacitor, battery, superconducting magnet) and to reverse this process and deliver power for the AC system from the energy storage device, makes complete, temporary system support possible. Specifically, this capability may be used to improve system efficiency and prevent power outages. In addition, in combination with fast reactive power control, dynamic active power exchange is considered as an extremely powerful method for transient and dynamic stability enhancement.

The semiconductor valves in a STATCOM respond almost instantaneously to a switching order. Therefore the limiting factor for the comple plant speed of response is determined by the time needed for voltage measurements and the control system data processing. A high gain controller can be used and a response time shorter than a quarter of a cycle is obtained. The high switching frequency used in the IGBT based STATCOM concept results in an inherent capability to produce voltages at frequencies well above the fundamental one. This property can be used for active filtering of harmonics already present in the network. The STATCOM then injects harmonic currents into the network with proper phase and amplitude to counteract the harmonic voltages. By adding storage capacity to the DC side of STATCOM, it becomes possible not only to control reactive power,but also active power. As storage facility, various kinds of battery cells can be used, depending on the requirements on the storage facility. The result, STATCOM with energy storage (Fig. 7), is expected to come into use in years to come as dynamic storage facility particularly of renewable energy (wind, solar).

POINTS...

· A static synchronous generator operated as a shunt-connected static var compensator whose capacitive or inductive output current can be controlled in- dependent of the AC system voltage.

· A STATCOM is a controlled reactive-power source. It provides voltage support bygenerating or absorbing reactive power at the point of common coupling without the need of large external reactors or capacitor banks.

· The charged capacitor Cdc provides a DC voltage to the converter, which producesa set of controllable three-phase output voltages with the frequency of the AC powe rsystem. By varying the amplitude of the output voltage U, the reactive power exchange between the converter and the AC system can be controlled. If the amplitude of the output voltage U is increased above that of the AC system UT , a leading current is produced, i.e. the STATCOM is seen as a conductor by the AC system and reactivepower is generated. Decreasing the amplitude of the output voltage below that of the AC system, a lagging current results and the STATCOM is seen as an inductor. In this case reactive power is absorbed. If the amplitudes are equal no power exchange takes place.

· A practical converter is not lossless. In the case of the DC capacitor, the energy stored in this capacitor would be consumed by the internal losses of the converter. By making the output voltages of the converter lag the AC system voltages by a small angle, the converter absorbs a small amount of active power from the AC system to balance the losses in the converter.

· The mechanism of phase angle adjustment can also be used to control the reactive power generation or absorption by increasing or decreasing the capacitor voltage Udc, and thereby the output voltage U. Instead of a capacitor also a battery can be used as DC energy. In this case the converter can control both reactive and active power exchange with the AC system. The capability of controlling active as well as reactive power exchange is a significant feature which can be used effectively in applications requiring power oscillation damping, to level peak power demand, and to provide uninterrupted power for critical load.