FACTS Devices or Controllers This training was made possible with support from the American people, delivered through the U.S. Agency for International Development (USAID).
FACTS Devices or Controllers •
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Flexible ac Transmission System (FACTS) devices constitute a not yet well-defined class of equipment that resulted from advances in the 80's and early 90's in the field of forced-commutated thyristors (gate turn-off (GTO), MOS-controlled transistors (MCT), and insulated gate bipolar transistors (IGBT)). As opposed to the conventional thyristors found in HVDC converters and Static Var Devices (SVCs), forced commutated thyristors allow switching at any point of the ac sine wave. Because FACTS devices are only starting to make their way into the power industry, their configuration and control is still a matter of debate. For this reason, the models presented in PSS/E are simplified versions of the actual equipment and are only applicable for feasibility or scoping studies. They do include, however, some of the main features and limitations that need to be considered by the system planner contemplating application of such devices STATCON: Static Condenser STATCOM: Static Compensator (ABB’s brand name) SSSC: Static Synchronous Series Compensator TCSC: Thyristor Controlled Series Compensator
STATCOM or STATCON •
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A static synchronous compensator (STATCOM), also known as a "static synchronous condenser" ("STATCON"), is a regulating device used on AC electricity transmission networks. It is based on a power electronics voltage-source converter and can act as either a source or sink of reactive AC Power to an electricity network. If connected to a source of power it can also provide active AC power. It is a member of the FACTS family of devices. A STATCOM is a voltage source converter (VSC)-based device, with the voltage source behind a reactor. The voltage source is created from a DC Capacitor and therefore a STATCOM has very little active power capability. However, its active power capability can be increased if a suitable energy storage device is connected across the DC capacitor.
The reactive power at the terminals of the STATCOM depends on the amplitude of the voltage source. For example, if the terminal voltage of the VSC is higher than the AC voltage at the point of connection, the STATCOM generates reactive current; on the other hand, when the amplitude of the voltage source is lower than the AC voltage, it absorbs reactive power. In very simplified terms, a STATCON (Static Condenser) consists of a dc capacitor shuntconnected to the ac network by means of high-speed electronic switches. Given a sufficient number of thyristor-bridges it is possible for a converter to synthesize a controlled ac voltage with acceptable harmonic content behind the converter transformer reactance
STATCOM or STATCON •
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For pure reactive control, this ac voltage is kept in phase with terminal voltage. Because ideally there is no net exchange of active power with the network, the dc capacitor remains charged at the desired level. In practice, converters have losses that tend to discharge the capacitor. In order to replenish the capacitor charging, the synthesized voltage is controlled to slightly lag the terminal voltage, and thus maintain a small flow of active power between network and condenser. By this same mechanism capacitor voltage is controlled, which in turn determines the magnitude of the internal voltage and thus determines the reactive exchange with the network. STATCOM is a constant current source once it reaches its limit while the SVC is a constant impedance device when it reaches its limit. Thus, the reactive power output of the STATCOM is proportional to voltage when it reaches limit. The main objective of the STATCOM is the voltage control of the specified bus. The reactive power at the terminals of the STATCOM depends on the amplitude of the voltage source. For example, if the terminal voltage of the VSC is higher than the AC voltage at the point of connection, the STATCOM generates reactive current; on the other hand, when the amplitude of the voltage source is lower than the AC voltage, it absorbs reactive power.
STATCOM or STATCON •
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The response time of a STATCOM is shorter than that of an SVC, mainly due to the fast switching times provided by the IGBTs of the voltage source converter. The STATCOM also provides better reactive power support at low AC voltages than an SVC, Since the reactive power from a STATCOM decreases linearly with the AC voltage (as the current can be maintained at the rated value even down to low AC voltage). The STATCOM also provides better reactive power support at low AC voltages than an SVC, Since the reactive power from a STATCOM decreases linearly with the AC voltage (as the current can be maintained at the rated value even down to low AC voltage).
STATCOM or STATCON •
Modeling the STATCON in PSS/E •
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CSTATT and CSTNT are dynamic models for modelling of STATCON in PSS/E Modal Library The dynamic models of CSTATT and CSTCNT are the same. The difference between CSTATT and CSTCNT is that, for power flow CSTATT is modelled as a synchronous condenser (i.e., with no active power output), while CSTCNT is modelled as a FACTS device. Care must be taken to insure that the power flow current output does not exceed CSTATT current limitations. In order to prevent contributions to shortcircuit, a high value of ZSORCE is recommended. In the CSTATT and CSTCNT model, the Condenser is modelled as a controlled voltage behind a transformer reactance. The ac voltage is kept in phase with terminal voltage; i.e., a lossless STATCON is assumed. The resulting STATCON current is limited to the temporary and steady-state overload limitations of GTO converters. Inputs to the model are voltage reference, and terminal voltage. Because CSTATT is modelled as a generator in power flow, this model can also accept power system stabilizer output (PSSout). Output of the model is STATCON reactive current. Positive current corresponds to STATCON performance as a capacitor.
Modeling the CSTATT for STATCON
Modeling the STATCON
Calculating K
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Where ∆Eterm/∆Eint is the sensitivity of the terminal voltage to the internal voltage. This is calculated as follows
Zthev is evaluated from Short Circuit Studies and is inverse of the minimum expected P.U. short circuit value
Calculating K •
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For example lets consider a 550 MVA STATCOM at NKLP 132 kV Bus First we evaluate Zthev from the short circuit study
Considering typical values of X t = 0.1, ∆Eterm/∆Eint = 0.046217 And therefore K = 540 Thus the parameters for the STATCOM are 4144 'CSTATT' 1 0.20000 0.20000 0.20000 0.20000 540.00 0.30000E-01 1.2000 0.0000 2.0000 1.2500 0.20000 1.2000 0.10000 0.50000
VAR L+1 is important to monitor in the simulation results
End of Presentation
