STATCOM (Static Synchronous Compensator)It is a device connected in derivation, basically composed of a coupling transformer, that serves of linkbetween the electrical power system (EPS) and thevoltage synchronous controller (VSC), that generatesthe voltage wave comparing it to the one of the electricsystem to realize the exchange of reactive power. Thecontrol system of the STATCOM adjusts at eachmoment the inverse voltage so that the current injectedIn the network is in cuadrature to the network voltage, Donsiónin these conditions P=0 and Q=0.In its most general way, the STATCOM can be modeled as a regulated voltage source Vi connected toa voltage bar Vs through a transformer.STATCOM (Static Synchronous Compensator)STATCOM -A VSC interfaced in shunt to a transmission lineDonsiónThe STATic COMpensator(STATCOM)uses a VSC interfaced in shunt to atransmission line. In most cases the DC voltage support for the VSC will beprovided by the DC capacitor of relatively small energy storage capability -hence, in steady state operation, active power exchanged with the line has to bemaintained at zero, as shown symbolically in the Figure.STATCOM (Static Synchronous Compensator)STATCOM -A VSC interfaced in shunt to a transmission lineDonsiónWith the active power constraint imposed, the control of the STATCOM isreduced to one degree of freedom, which is used to control the amount ofreactive power exchanged with the line. Accordingly, a STATCOM is operatedas a functional equivalent of a static VAR compensator; it provides fastercontrol than an SVC and improved control range.STATCOM (Static Synchronous Compensator)Each GTO converter generates a voltage that is stepped up by a line-side-series-connected multi-stage converter transformer. The converter Donsióntransformer enables the build-up of a sine-wave voltage in bothmagnitude and phase. Because STATCOMs with multi-stage convertertransformers do not generate significant internal harmonics, theygenerally require minimal, or no, harmonic filtering. If the number offiring pulses for the GTOs is increased (i.e., pulse-width modulation(PWM) order), the harmonics are further decreased. High-side voltage isgenerally used as a controller input, as indicated in the figure.STATCOM (Static Synchronous Compensator)The figure shows theequivalent circuit of aSTATCOM system.TheGTO converter with adc voltage source andthe power system areillustrated as variable acvoltages in this figure.These two voltages are Donsiónconnected by areactance representingthe transformer leakageinductance.Equivalent circuit of a STATCOMSTATCOM (Static Synchronous Compensator)Using the classical equations that describe the active and reactive power flow in a line in terms of Vi and Vs, the transformer impedance (which canbe assumed as ideal) and the angle difference between both bars,we candefined P and Q.The angle between the Vs and Vi in the system is d. When the STATCOM operates with d=0 we can see how the active power send to the system devicebecomes zero while the reactive power will mainly depend on the voltagemodule. This operation condition means that the current that goes through thetransformer must have a +/-90º phase difference to Vs. In other words, if Vi isbigger than Vs, the reactive will be send to the STATCOM of the system(capacitive operation), originating a current flow in this direction. In the Donsióncontrary case, the reactive will be absorbed from the system through theSTATCOM(inductive operation) and the current will flow in the oppositedirection. Finally if the modules of Vs and Vi are equal, there won´t be norcurrent nor reactive flow in the system.Thus, we can say that in a stationary state Q only depends on the module difference between Vs and Vi voltages. The amount of the reactive power isproportional to the voltage difference between Vs and Vi.STATCOM (Static Synchronous Compensator)Principle of operation of a STATCOMThere can be a little active power exchange between the STATCOM and the EPS. The exchange between the inverter and the AC system can be controlled adjusting the output voltage angle from the inverter to the voltage angle of the AC system. This means that the inverter can not provide active power to the AC system form the DC accumulated energy if the output voltage of the inverter goes before the voltage of the AC system. On the other hand, the inverter can absorb the active power of the AC system if its voltage is delayed in respect to the AC system voltage.DonsiónSTATCOM (Static Synchronous Compensator)V-I characteristic of a STATCOMThe STATCOM smoothly and continuously controls voltage from V1 to V2. However, if the system voltage exceeds a low-voltage (V1) or high-voltage limit (V2), the STATCOM acts as a constant current source by controlling the converter voltage (Vi) appropriately.Thus, when operating at its voltage limits, the amount of reactive power compensation provided by the STATCOM is more than the most-commoncompeting FACTS controller, namely the Static Var Compensator (SVC).This is because at a low voltage limit, the reactive power drops off as thesquare of the voltage for the SVC, where Mvar=f(BV2), but drops offlinearly with the STATCOM, where Mvar=f(VI). This makes the reactivepower controllability of the STATCOM superior to that of the SVC,particularly during times of system distress.DonsiónSTATCOM (Static Synchronous Compensator)In addition the STATCOM has other advantages compared to an SVC,such as:•Quicker response time (A STATCOM has a step response of 8 ms to 30ms). This helps with compensation of negative phase current and with thereduction of voltage flicker.•Active power control is possible with a STATCOM (with optional energystorage on dc circuit). This could further help with system stability control.•No potential for creating a resonance point. This is because no capacitorbanks or reactors are required to generate the reactive power for aSTATCOM.•The STATCOM has a smaller installation space due to no capacitors orreactors required to generate Mvar, minimal or no filtering, and theavailability of high capacity power semiconductor devices. Designs of Donsiónsystems of equal dynamic ranges have shown the STATCOM to be as muchas 1/3 the area and 1/5 the volume of an SVC.•A modular design of the STATCOM allows for high availability (i.e., oneor more modules of the STATCOM can be out-of-service without the loss ofthe entire compensation system).STATCOM (Static Synchronous Compensator)DonsiónIn 1991, the world’s first commercial transmission system STATCOM(at the time known as SVG for Static Var Generator) was installed at theInuyama substation of The Kansai Electric Power Company in Japan, for theobjective of improving power system and voltage stabilization. It has beensuccessfully operating for nearly 9 years. The figure shows the one-linediagram of this 80 MVA STATCOM.STATCOM (Static Synchronous Compensator)Application of the Three-Phase STATCOM in Voltage StabilityVoltage stability is one of the biggest problems in power systems.Engineers and researchers have met with the purpose of discussing andtrying to consolidate a definition regarding to voltage stability, besidesproposing techniques and methodologies for their analysis. Most of thesetechniques are based on the search of the point in which the system’sJacobian becomes singular; this point is referred as the point of voltagecollapse or maximum load ability point.The series and shunt compensation are able to increase the maximum transfer capabilities of power network .Concerning to voltage stability, such Donsióncompensation has the purpose of injecting reactive power to maintain thevoltage magnitude in the nodes close to the nominal values, besides, toreduce line currents and therefore the total system losses. At the presenttime, thanks to the development in the power electronics devices, the voltagemagnitude in some node of the system can be adjusted through sophisticatedand versatile devices named FACTS. One of them is the static synchronouscompensator (STATCOM).STATCOM (Static Synchronous Compensator)DonsiónWith the help of the previous one-phase STATCOM formulation, it is easy to deduce the three-phase model. The shunt voltage source of the three-phaseSTATCOM may be represented by: EvρR = VvρR (cosδvρR + jsinδvρR )whereρindicates phase quantities, a, b and c.The equivalent circuit of the three-phaseSTATCOM is shown in Figure 4 in a wye configuration. This model is used toderive the steady state equations included into the three-phase power flowformulationSTATCOM (Static Synchronous Compensator)DonsiónThus, the three-phase STATCOM model is integrated into the steady state formulation. In the simulations, the STATCOM´s node where is connected, isrepresented as a PV type node. This node can change to PQ type when, duringthe process, one of the limits in the device’s voltage magnitude is violated.。