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Unified power flow controller

The Unified Power Flow Controller (UPFC) is a versatile Flexible AC Transmission System (FACTS) device that enables real-time control of active and reactive power flows, voltage magnitude, line impedance, and phase angle in high-voltage transmission networks by combining shunt and series compensation capabilities. It consists of two voltage-sourced converters—a shunt-connected Static Synchronous Compensator (STATCOM) for reactive power support and bus voltage regulation, and a series-connected Static Synchronous Series Compensator (SSSC) for injecting a controllable voltage in series with the transmission line—interlinked via a common DC capacitor bus that facilitates bidirectional active power exchange without requiring an external source. This structure allows the UPFC to dynamically adjust transmission parameters, enhancing overall system flexibility, reliability, and efficiency compared to standalone FACTS controllers like STATCOM or SSSC. In operation, the UPFC's shunt converter (STATCOM) regulates the sending-end voltage and maintains DC link stability using proportional-integral (PI) control loops for current and voltage, while the series converter (SSSC) modulates the injected voltage's magnitude (typically 0–0.2 ) and phase angle to independently control power flow, forming an elliptical controllable region in the active-reactive power plane. The device supports multiple control modes, including automatic power flow regulation, manual voltage injection, and damping of subsynchronous or electromechanical oscillations through supplementary lead-lag compensators. By providing both series and shunt compensation, the UPFC can mitigate voltage instability, significantly increase power transfer capacity on existing lines, and improve transient and dynamic stability in interconnected grids. The UPFC was first conceptualized in the early as the most advanced FACTS controller for comprehensive management, with the world's initial commercial installation completed in 1998 by (AEP) at the Inez substation in —a ±160 MVA unit on a 138 kV, 60 Hz system that demonstrated enhanced loop power flow control and voltage support. Subsequent deployments have focused on applications such as congestion relief and of sources, underscoring the UPFC's role in modernizing aging for higher efficiency and resilience. As of 2025, UPFCs continue to support the of into power grids, with the global market valued at approximately USD 0.59 billion in 2024 and projected to grow to USD 0.87 billion by 2033.

Overview

Definition and purpose

The Unified Power Flow Controller (UPFC) is a multifunctional device in the Flexible AC Transmission Systems (FACTS) , consisting of a shunt-connected (STATCOM) and a series-connected Static Synchronous Series Compensator (SSSC) that share a common DC link. This configuration allows the UPFC to function as a synchronous capable of injecting or absorbing controllable real and reactive power into the . The primary purpose of the UPFC is to enable independent and simultaneous control of real power (P), reactive power (Q), bus voltage magnitude, and impedance in high-voltage networks. By dynamically adjusting these parameters, the UPFC optimizes power flow distribution, mitigates congestion, enhances transient and dynamic stability, and improves overall grid reliability without requiring physical modifications to the infrastructure. Within the FACTS family, the UPFC stands out as the most versatile controller due to its combined series and shunt compensation capabilities, which provide comprehensive regulation of all basic power system quantities affecting performance. Key capabilities include real-time adjustment of active and reactive power flows to maintain desired levels under varying load conditions and effective through shunt compensation, thereby supporting efficient utilization of existing assets.

Historical development

The concept of the Unified Power Flow Controller (UPFC) emerged in the early as an advanced component within the broader Flexible AC Transmission Systems (FACTS) framework, spearheaded by the (EPRI) to enhance control. Laszlo Gyugyi, working with EPRI and , proposed the UPFC in 1991 as a versatile device combining shunt and series compensation capabilities, building on earlier FACTS innovations attributed to Narain G. Hingorani, often called the "father of FACTS," who emphasized for grid flexibility in the late 1980s. Theoretical foundations were detailed in a seminal 1995 IEEE paper by Gyugyi, C.D. Schauder, and colleagues, which introduced the UPFC's ability to independently real and reactive power flows through voltage injection, marking a significant advancement over prior FACTS devices like STATCOM and SSSC. This publication laid the groundwork for practical designs, emphasizing the UPFC's role in dynamic system compensation using voltage-source converters (VSCs) based on insulated-gate bipolar transistors (IGBTs) for efficient, high-power operation. The first prototype UPFC was commissioned in June 1998 at the substation in eastern by (AEP), in collaboration with (formerly ) and EPRI support, demonstrating real-time power flow regulation on a 138 kV line serving a 2,000 MW load area. This installation, rated at 320 MVA (two 160 MVA inverters), validated the technology's performance in voltage support and oscillation damping, paving the way for full-scale deployments in the 2000s across North American and international grids, including upgrades for congestion relief. Over the subsequent decades, UPFC technology evolved with advancements in , transitioning from traditional multi-pulse VSCs with IGBTs to modular multilevel converters (MMCs) post-2010, enabling higher voltage ratings, reduced harmonics, and scalability for large-scale applications without bulky transformers. MMC-based UPFCs, first proposed around , offer improved efficiency and reliability for ultra-high-voltage transmission, with prototypes tested in simulations and lab settings by the mid-2010s. Key milestones include the development of IEEE guidelines for UPFC implementation, with the IEEE 2745 series (initiated around 2018 and published in 2019–2021) providing functional specifications, terminology, and technical requirements for MMC-based systems to standardize design, testing, and commissioning. In the , UPFC adoption has accelerated in renewable-heavy grids to manage variability from and integration; for instance, studies and simulations demonstrate its use in enhancing stability for grid-tied photovoltaic systems, optimizing power quality and fault ride-through in hybrid renewable setups.

Components and Configuration

Shunt converter (STATCOM)

The shunt converter, also known as the (STATCOM) in the context of the Unified Power Flow Controller (UPFC), is a voltage-sourced converter (VSC) connected in parallel to the via a . This design allows it to function as a synchronous , injecting or absorbing reactive power into the bus to dynamically support voltage levels. The VSC typically employs insulated-gate bipolar transistors (IGBTs) or gate turn-off thyristors (GTOs) in a multi-level to handle high voltages, with the providing and between the converter output and the grid. In terms of operation, the shunt converter delivers dynamic compensation by generating a controllable voltage behind its , enabling independent of the bus voltage . It can operate in capacitive mode to supply reactive power during voltage sags or inductive mode to absorb excess reactive power during overvoltages, thereby maintaining system stability. Additionally, the converter exchanges active power bidirectionally with the shared link, which supports charging or discharging the and facilitates power transfer to or from the series branch without relying on external sources. () techniques are employed to synthesize a nearly sinusoidal output voltage at , minimizing harmonics and improving efficiency. The rating of the shunt converter is typically 20-50% of the transmission line's capacity to provide sufficient compensation without excessive overdesign, depending on the system's voltage profile needs and fault levels. For example, in a 138 kV network, a UPFC's shunt converter might be rated to inject up to 100 MVAR for voltage support during contingencies, as demonstrated in projects like the SDG&E Talega installation where a comparable STATCOM configuration achieved this capability. Within the UPFC framework, the shunt converter integrates seamlessly with the series converter through the common link, enabling energy exchange between the shunt and series branches to achieve unified of active and reactive flows. This shared DC storage, often a capacitor bank rated for 1.2-1.5 per unit voltage, ensures balanced operation and enhances the overall responsiveness of the UPFC.

Series converter (SSSC)

The series converter, also known as the Static Synchronous Series Compensator (SSSC), in the Unified Power Flow Controller (UPFC) consists of a voltage source converter (VSC) connected in series with the via a coupling . This configuration enables the injection of a controllable voltage in series with the sending-end voltage of the line, where the magnitude and phase angle of the injected voltage can be independently regulated. The VSC, typically based on gate-turn-off () or (IGBT) switches, generates the required voltage through techniques to produce a sinusoidal output synchronized with the line frequency. The SSSC functions by superimposing a series voltage onto the , which effectively alters the line's voltage profile and impedance characteristics. This allows for precise of active and reactive flows without dependence on the line current magnitude, unlike traditional series capacitors. By injecting voltage in with the current, the SSSC emulates variable capacitive or inductive ; in-phase injection directly influences the voltage magnitude difference across the line, thereby routing as needed. Key operational parameters of the SSSC include the injected voltage magnitude, which is generally limited to 10-20% of the nominal line-to-line voltage to ensure compatibility with converter ratings and avoid excessive losses. The coupling transformer provides and voltage matching, with a typical turns ratio adjusted to the desired injection level, while the converter handles real power exchange through the common link to maintain energy balance and manage any circulating currents arising from differences. These parameters ensure the SSSC operates within and limits of high-voltage systems. Within the UPFC framework, the SSSC integrates as the series branch, sharing the DC link with the shunt converter to enable unified control of all power flow parameters, including independent regulation of real power, reactive power, and bus voltage. This setup allows the SSSC to function standalone by bypassing the shunt converter, reverting to pure series compensation mode while retaining full voltage injection capability. The with shunt compensation briefly enhances overall system flexibility, but the SSSC's primary role remains series power routing. A representative application demonstrates the SSSC's impact: in a 230 transmission line, injecting a 20 series voltage (about 10% of line-to-line voltage) can shift active power flow by 50 MW, optimizing loading without altering line hardware.

Common DC link

The common link in a Unified Power Flow Controller (UPFC) serves as a capacitor-based element that interconnects the shunt converter (STATCOM) and the series converter (SSSC), enabling coordinated operation without requiring an external power source for steady-state functioning. This design typically employs a storage capacitor with values ranging from 0.5 mF to 12 mF or higher, scaled according to the UPFC's and dynamic requirements to support efficient energy buffering. The operates at voltage levels generally between 1.2 for medium-voltage applications and up to 40 for high-voltage transmission systems, ensuring compatibility with the converters' switching capabilities. The primary function of the common DC link is to facilitate bidirectional active power exchange between the shunt and series converters, allowing real power to flow from one to the other as needed for compensation tasks. This exchange maintains DC voltage stability essential for the reliable operation of both voltage-source converters, with the shunt converter typically regulating the voltage by injecting or absorbing active power from the AC system. During startup, the shunt converter charges the capacitor from the AC grid to establish the nominal DC voltage, after which the system achieves energy self-sufficiency through balanced power flows. Key aspects of the DC link include its role in ensuring energy balance, where the net power into the capacitor equals zero under steady-state conditions to prevent losses or drift, described by the relation P_{sh} + P_{se} = C \frac{dV_{dc}}{dt}, with P_{sh} and P_{se} as shunt and series powers, C as , and V_{dc} as DC voltage. The is specifically sized to limit voltage ripple to below 5% under full load, minimizing distortions and supporting stable converter . This integration of the common DC link underscores the UPFC's unified nature, decoupling the independent control of shunt reactive and series voltage injection while dynamically linking them through shared for versatile power flow management.

Operating Principles

Basic mechanism of power control

The Unified Power Flow Controller (UPFC) achieves flexible control over active and reactive power flows in transmission lines primarily through the coordinated operation of its series and shunt converters, enabling precise manipulation of line voltage parameters. The series converter injects a controllable AC voltage in series with the transmission line, with independently adjustable magnitude (typically from 0 to a maximum value, such as 0.16 p.u. in practical implementations) and phase angle (ranging from 0° to 360°). This injection modifies the effective voltage difference across the line, thereby altering the power flow according to the fundamental transmission equation P = \frac{V_s V_r}{X} \sin \delta, where V_s and V_r are the sending- and receiving-end voltages, X is the line reactance, and \delta is the phase angle between them. By superimposing the injected voltage V_{pq} (with phase angle \rho relative to V_s), the active power flow becomes P = \frac{V_s V_r}{X} \sin \delta + \frac{V_s V_{pq}}{X} \sin (\delta - \rho), allowing the UPFC to increase, decrease, or even reverse power direction beyond conventional limits. In representation, the UPFC series branch is equivalent to an inserted synchronous connected through a small coupling reactance, where the injected phasor \vec{V}_{pq} is added vectorially to the sending-end to form an effective \vec{V}_{seff} = \vec{V}_s + \vec{V}_{pq}. This configuration enables the line current to flow through the controllable source, effectively emulating variable series impedance or shifting without physical components. The shunt converter, functioning as a (STATCOM), interacts with the series branch by maintaining the bus magnitude (e.g., at 1.0 p.u.) through reactive exchange, while also supplying or absorbing real via the common DC link to support the series injection. This shunt action decouples the control of active P (primarily influenced by the series injection's ) from reactive Q (adjusted via shunt compensation), as expressed in the reactive Q_r = \frac{V_s V_r}{X} (\cos \delta - 1) - \frac{V_s V_{pq}}{X} \cos (\delta - \rho) at the receiving end. The power decoupling capability of the UPFC distinguishes it from other (FACTS) devices, such as phase angle regulators or Static VAR Compensators (SVCs), which typically couple P and Q control or limit operation to specific quadrants (e.g., lagging compensation only). In contrast, the UPFC provides independent, bidirectional regulation of both parameters across a wide operating range—for instance, adjusting P from 70 MW to 240 MW while maintaining Q from -30 MVAR to +100 MVAR in example systems—without constraints from the transmission angle \delta or line impedance. This comprehensive control stems from the synchronous voltage source model's ability to fully utilize the injected voltage's magnitude and for versatile power flow optimization.

Injection of voltage and current

The Unified Power Flow Controller (UPFC) operates by injecting a controllable series voltage vector through its series converter and a controllable shunt current through its shunt converter, enabling precise of parameters. The series voltage injection, denoted as \vec{V}_{se}, is synthesized by the voltage-source converter (VSC) and added in series with the line voltage, allowing independent of both real and reactive power flows. This injection can be decomposed into relative to the receiving-end voltage : the in-phase component emulates resistive effects to adjust voltage magnitude and reactive power, while the quadrature component emulates reactive effects to modify the effective line impedance and real power transfer. The phasor form of the series injected voltage is expressed as \vec{V}_{se} = \frac{m_{se} V_{dc}}{2} e^{j \theta_{se}}, where m_{se} is the of the series VSC (typically ranging from 0 to 1), V_{dc} is the voltage across the common link, and \theta_{se} is the controllable phase angle determining the injection's orientation. Phasor diagrams illustrate this process: the pre-injection receiving-end voltage \vec{V}_r is shifted by \vec{V}_{se} to yield the post-injection phasor \vec{V}_r + \vec{V}_{se}, resulting in magnitude variations for and angular shifts for phase angle control, thereby altering the power angle \delta between sending- and receiving-end voltages. For instance, a injection primarily rotates the phasor to increase or decrease real power without significantly affecting magnitude, while an in-phase injection scales the phasor length to manage reactive power. The shunt converter injects a current phasor \vec{I}_{sh} at the sending-end bus, which sources or absorbs reactive power to maintain bus voltage profile and stability, while simultaneously providing the real power demanded by the series converter to balance the DC link. This shunt current injection ensures the UPFC functions as a versatile power conditioner, decoupling active and reactive power exchanges and supporting independent control of transmission parameters. The combined series voltage and shunt current injections model the UPFC as a synchronous voltage source for the line and a synchronous current source for the bus, offering full controllability over power flow. Practical operation constrains these injections based on converter ratings and system stability: the series voltage magnitude is typically limited to |\vec{V}_{se}| \leq 0.2 to avoid excessive heating or overvoltages, and the shunt to |\vec{I}_{sh}| \leq 1 to respect inverter current limits, with both bounds expressed on a common per-unit base relative to nominal line voltage and rated .

Control Strategies

Real and reactive power regulation

The Unified Power Flow Controller (UPFC) regulates real power flow primarily through the series converter, which injects a controllable voltage in series with the . By adjusting the phase angle of this injected voltage relative to the line voltage, the UPFC modifies the effective phase difference δ between the sending and receiving ends, thereby controlling the real power P according to the power flow P ≈ (V_s V_r / X) sin δ, where V_s and V_r are sending and receiving end voltages, and X is the line . Reactive power regulation is achieved via both converters, with the shunt converter providing primary VAR support by controlling the magnitude of the injected shunt current to maintain bus voltage magnitude. The series converter complements this by injecting a voltage component in quadrature to the line current, enabling line reactive power compensation and overall Q flow adjustment at the UPFC terminals. UPFC operates in various control modes, including constant real and reactive power (P/Q) regulation to enforce specified flow values, voltage regulation to stabilize bus magnitudes, and impedance emulation to mimic variable line reactance. These modes typically employ proportional-integral (PI) controllers in a cascaded structure for the shunt and series branches, ensuring accurate reference tracking by regulating DC-link voltage, AC currents, and injected voltages. The voltage source converter (VSC) architecture of the UPFC enables fast dynamic performance due to high-frequency switching that allows rapid adjustment of injected voltage and current phasors. This supports real-time power flow correction without relying on mechanical components. In practice, the UPFC can redirect nominal line power flow bidirectionally by appropriately setting the series voltage magnitude and , enhancing utilization while maintaining margins.

Stability enhancement techniques

The Unified Power Flow Controller (UPFC) enhances power system by providing dynamic control over voltage, impedance, and phase , enabling rapid responses to disturbances that could lead to . Unlike steady-state power flow regulation, these techniques focus on mitigating oscillations and preventing of synchronism through supplementary control loops that detect power swings and inject counteracting voltages or currents. This capability is particularly valuable in interconnected systems where electromechanical oscillations can propagate, and the UPFC's shunt and series converters work in tandem to absorb or inject energy as needed. For damping inter-area and local oscillations, UPFC employs supplementary power oscillation damping (POD) controllers that utilize wide-area measurement signals, such as phasor measurement unit (PMU) data, to generate modulating signals for the converters. These controllers detect low-frequency oscillations (typically 0.1–2 Hz) and inject voltages in phase opposition to the swing, thereby increasing the effective torque and shifting eigenvalues in small-signal analysis to more stable regions. In cases of subsynchronous (SSR), which arises from interactions between series-compensated lines and turbine-generators at frequencies below 60 Hz, UPFC damping controllers provide subsynchronous damping by modulating the series inverter to counteract torsional vibrations, preventing amplification. Eigenvalue analysis confirms that UPFC integration improves damping ratios from near-zero values (e.g., 0.03) to over 0.45 in single-machine infinite-bus systems under varying loads, enhancing small-signal margins. Transient stability is bolstered by the UPFC's ability to perform rapid active and reactive power (P/Q) adjustments during faults, such as three-phase short circuits, to arrest accelerating rotor angles and prevent angular instability. By coordinating with power system stabilizers (PSS), the UPFC injects supportive shunt reactive power and series voltage to maintain synchronism, while the common DC link absorbs excess energy during fault ride-through, stabilizing voltage dips and limiting transient excursions. This coordination ensures that the system returns to equilibrium faster, eliminating post-fault oscillations in multi-machine setups like the IEEE 9-bus system. Advanced techniques incorporate adaptive, , or AI-based controls to handle nonlinear responses under uncertain conditions, such as varying fault durations or load changes. POD controllers, for instance, use rotor angle deviation as input to dynamically adjust UPFC parameters like modulation indices and phase shifts, providing robust without fixed settings. These methods outperform conventional linear controls in wide operating ranges, further integrating with PSS for comprehensive stability support. In multi-machine systems, UPFC application has demonstrated significant reductions in rotor angle swings during severe disturbances, as evidenced in simulations with improved ratios under heavy loading, preventing cascading failures and maintaining inter-area synchronism.

Applications and Benefits

Power transmission optimization

The unified power flow controller (UPFC) optimizes by enabling precise control of active and reactive power flows in transmission lines, allowing operators to increase line loading up to their limits without exceeding margins. This capability is achieved through the UPFC's ability to inject controllable voltage and current components, which dynamically adjusts impedance and voltage profiles to maximize usable capacity while minimizing losses. In meshed networks, UPFC deployment can reduce loop flows, alleviating congestion in parallel paths and improving overall grid utilization. In the context of integration, UPFC provides dynamic support for active (P) and reactive (Q) power, mitigating the intermittency of and farms by stabilizing voltage and facilitating smoother grid connection. For instance, it regulates power fluctuations from variable renewable sources, enhancing transmission efficiency and enabling higher penetration levels without compromising stability. Practical case studies demonstrate these optimization benefits. The (AEP) installation at the Inez substation in 1998, the world's first commercial UPFC, increased the power transfer capacity of the 138 kV Big Sandy-Inez line from approximately 670 MW to 770 MW, an increase of about 100 MW, and relieved thermal overloads in the region. Similarly, (KEPCO) deployed an 80 MVA UPFC at the 154 kV Kangjin substation around 2003, with operational enhancements post-2005 supporting HVDC-AC interfacing by controlling bidirectional power flows and improving system reliability in interconnected grids. Economically, UPFC adoption yields significant savings by deferring or eliminating the need for new lines compared to traditional expansions. It also enhances available transfer capability (), allowing greater power dispatch flexibility and reducing operational costs through optimized congestion management.

Limitations and challenges

Despite its advanced capabilities, the unified power flow controller (UPFC) faces significant technical limitations, including high in schemes against faults in the link and converters, which can lead to operational vulnerabilities during transient events. While early converter-based UPFCs were limited to lower voltages, modular multilevel converter () topologies have enabled applications at 500 kV and potentially higher. Additionally, UPFC operation generates harmonics due to switching in the power electronics, necessitating dedicated filters or advanced techniques to maintain power quality. Economically, UPFC deployment is hindered by substantial capital and installation costs, often making it more expensive than traditional compensators like static VAR compensators, which restricts widespread adoption without careful economic justification. These costs, driven by high-rated voltage source inverters and transformers (e.g., 160 MVA units), can reach tens of millions per installation, with periods typically spanning 5-10 years through benefits like relief and reduced losses, though optimal placement is essential to maximize . Operationally, UPFCs require rigorous maintenance of components in harsh substation environments, including regular calibration of control algorithms to adapt to varying grid conditions, which adds to ongoing expenses and complexity. Early installations faced reliability concerns due to converter faults, but advancements post-2010 in insulated gate bipolar transistors and designs have improved reliability in modern systems. Coordination challenges in multi-controller setups further complicate operations, potentially leading to suboptimal performance during rapid network changes. Recent MMC-based UPFC deployments, such as the 500 kV installation in , (operational since around 2017), highlight advancements in scalability for high-voltage grids and support for integration, enhancing grid resilience as of 2025.

Modeling and Analysis

Mathematical models

The of the unified power flow controller (UPFC) represents it as two synchronous voltage sources: one in series with the and one connected in shunt to the bus. The series voltage source, V_{se}, has controllable magnitude and phase angle, allowing it to inject or absorb active and reactive power into the line. The shunt voltage source, V_{sh}, regulates the bus voltage and exchanges reactive power with the system. This model is derived from the components, neglecting higher-order harmonics from switching. Power balance equations are central to the steady-state analysis, where the active power from the shunt converter P_{sh} and series converter P_{se} satisfy P_{sh} + P_{se} = P_{losses}, with losses often assumed negligible in ideal cases, leading to P_{sh} + P_{se} = 0 to maintain constant DC-link voltage. Reactive power flows are decoupled, as Q_{sh} and Q_{se} can be independently controlled without affecting the DC link. These equations ensure across the converters sharing the common DC bus. For integration into power flow solvers like the Newton-Raphson method, the UPFC is incorporated via an admittance matrix that combines the series and shunt branches with the impedances. The overall bus admittance matrix Y_{bus} is modified to include the effects of V_{se} and V_{sh}, using power injection models where the UPFC nodes are treated as PQ or buses with specified magnitudes and angles. This formulation allows solving the nonlinear power flow equations iteratively, accounting for the controllable parameters. The dynamic model captures the transient behavior, particularly the DC-link voltage evolution, given by C_{dc} \frac{dV_{dc}}{dt} = I_{dc}, where I_{dc} is the net current into the capacitor from both converters. In terms of AC-side quantities, this expands to \frac{dV_{dc}}{dt} = \frac{3\sqrt{2}}{2 C_{dc} V_{dc}} (I_{sh} \cos \phi_{sh} + I_{se} \cos \phi_{se}), reflecting the average power transfer assuming sinusoidal approximations and balanced three-phase operation. Converter dynamics include inductor currents and capacitor voltages, modeled with resistances and inductances of the coupling transformers. State-space representations facilitate small-signal analysis, with the full-order model comprising states for line currents, shunt/series currents, voltage, and phase angles. The large-signal equations are L \frac{di}{dt} = -R i + v_{sys} - v_{conv}, coupled through the link. around an yields \Delta \dot{x} = A \Delta x + B \Delta u, where x includes d-q transformed variables, enabling eigenvalue analysis of system oscillations. Assumptions include negligible switching losses, balanced sinusoidal voltages, and ideal converter switching without delays.

Simulation approaches

Simulation of Unified Power Flow Controllers (UPFCs) employs various approaches to analyze their performance in power systems, ranging from transient dynamics to steady-state operations. Time-domain simulations, such as those using Electromagnetic Transients Program (EMTP) or Alternative Transients Program (ATP), are essential for capturing fast transients and switching behaviors in UPFC devices. These methods solve equations in the to model electromagnetic interactions accurately, particularly for fault conditions and converter switching. Phasor-domain simulations, on the other hand, focus on steady-state and slower electromechanical dynamics, using simplified sinusoidal representations to reduce computational complexity. Tools like / provide phasor-type UPFC blocks that include integrated subsystems for real and reactive power , enabling efficient analysis of power flow in transmission networks. PSCAD/EMTDC supports both and detailed time-domain models, allowing simulations that couple electromechanical interactions with electrical transients for comprehensive system studies. Validation of these simulations often involves comparing results with field data from UPFC installations, such as demonstration projects, to ensure model accuracy under real operating conditions. For instance, PSCAD/EMTDC models have been validated against measured data from dynamic voltage scenarios, confirming reliable prediction of power flow adjustments. Advanced simulation techniques include real-time digital simulation (RTDS) for hardware-in-the-loop testing, which replicates UPFC behavior in real time to evaluate strategies and interactions with physical components. Additionally, simulations incorporating modular multilevel converter () topologies for large-scale UPFCs model high-voltage applications, addressing challenges like voltage balancing and harmonic mitigation through detailed sub-module representations. In IEEE test systems, such simulations demonstrate that UPFCs significantly enhance oscillation damping ratios compared to uncompensated networks. Recent advances as of 2024 include AI-hybrid optimization for UPFC parameter tuning and modeling of advanced topologies like the advanced capacity-expansion-type UPFC (ACET-UPFC) for improved integration with sources.

References

  1. [1]
    Unified Power Flow Controller - an overview | ScienceDirect Topics
    The unified power flow controller (UPFC) realizes real-time control over power flow in transmission lines by adjusting the line parameters, including node ...
  2. [2]
    Unified Power Flow Controller (Phasor Type) - Simulink - MathWorks
    The UPFC uses a combination of a shunt controller (STATCOM) and a series controller (SSSC) interconnected through a common DC bus as shown on the figure below.Description · Control System · Parameters · Power Tab
  3. [3]
    [PDF] UNIFIED POWER FLOW CONTROLLER CONTROL STRATEGIES ...
    The UPFC has control over all the parameters affecting power flow on the transmis- sion line. UPFC is capable of controlling the real and reac- tive power on ...
  4. [4]
    Power System Studies - Unified Power Flow Controller (UPFC)
    The main applications of the UPFC are active and reactive power control, voltage control, VAR compensation, dynamic and transient stability, and damping ...
  5. [5]
    Unified Power Flow Controller (UPFC) and Interline Power ... - EPRI
    Oct 25, 2023 · This report describes the operation of STATCOM, SSSC, UPFC, and IPFC and focuses on technical and operational comparison of the UPFC and IPFC technologies.
  6. [6]
    https://www.businessresearchinsights.com/market-reports/unified-power-flow-controller-market-108705
  7. [7]
    Understanding FACTS | Wiley Online Books
    Dec 10, 1999 · Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems. Author(s):. Narain G. Hingorani, Laszlo Gyugyi ...
  8. [8]
    [PDF] Transient Stability Enhancement using Unified Power Flow ... - IRJEAS
    The Unified Power Flow Controller (UPFC) was devised for the real-time control and dynamic compensation of ac transmission systems, providing multi-functional ...<|separator|>
  9. [9]
    The Influences of UPFC on the Performance of LOE ... - Zenodo
    UPFC is developed by STATCOM and SSSC and was invented by Gyugyi in 1991. UPFC exchanges both P and Q for AC as well as absorbing and generating Q ...
  10. [10]
    [PDF] Understanding-Facts-Narain-G-hingorani.p.pdf
    Hingorani, Narain G. Understanding FACTS : concepts and technology of flexible AC transmission systems / Narain G. Hingorani, Laszlo Gyugyi. ... (UPFC) and ...
  11. [11]
    [PDF] The unified power flow controller: a new approach to power ...
    Apr 1, 1995 · This paper shows that the unified power flow controller (UPFC) is ... Laszlo Gyugyi and Colin David Schauder and Seth L. Williams and ...
  12. [12]
    AEP UPFC project: installation, commissioning and operation of the ...
    American Electric Power (AEP) has selected its Inez Substation in eastern Kentucky for the location of the world's first Unified Power Flow Controller (UPFC) ...
  13. [13]
    UPFC Installation At American Electric Power Is Operational
    INEZ, Ky., May 1, 1998 -- It´s official. The world´s first Unified Power Flow Controller (UPFC) -- an advanced solid-state transmission system control ...Missing: Siemens | Show results with:Siemens
  14. [14]
    AEP unified power flow controller performance - NASA ADS
    The installation of the world's first unified power flow controller (UPFC) has been completed and a series of commissioning tests were conducted at the Inez ...
  15. [15]
    [PDF] Innovators - EPRI
    Installed in 1998, the UPFC has provided reliable dynamic voltage support and power flow control to the Inez load area. AEP sees highly automated transmission ...Missing: Siemens | Show results with:Siemens
  16. [16]
    Modular Multilevel Converter for Unified Power Flow Controller ...
    Abstract: This paper presents a novel unified power-flow controller (UPFC) topology based on modular multilevel converter (MMC).Missing: evolution | Show results with:evolution
  17. [17]
    Three-phase modular multilevel converter based unified power flow ...
    The key point in a UPFC design is the design of its voltage source converters (VSCs). VSC has evolved from few output voltage levels to the multi-level types, ...
  18. [18]
    A Unified Power Flow Controller With Nine-Arm Modular Multilevel ...
    This paper proposes an improved topology of unified power flow controller (UPFC) based on nine-arm modular multilevel converter which could realize the same ...Missing: evolution | Show results with:evolution
  19. [19]
    IEEE 2745.1-2019 - IEEE SA
    Nov 27, 2019 · This guide provides the functional requirements for UPFC project application, including UPFC system composition, function and performance requirements.
  20. [20]
    IEEE Guide for Technology of Unified Power Flow Controller Using ...
    Sep 1, 2021 · A glossary and definition of unified power flow controller (UPFC) ... (STATCOM), and static synchronous series compensator (SSSC). Scope ...
  21. [21]
    Performance Enhancement of Grid-Connected Renewable Energy ...
    May 27, 2023 · This paper uses a Unified Power Flow Controller (UPFC) to enhance the reliability and performance of grid-tied renewable energy systems.
  22. [22]
    [PDF] Integration of UPFC in Solar PV Systems for Enhanced Green ...
    The findings suggest for the broader adoption of UPFC in renewable power structures, emphasizing its efficacy in enhancing the stability, excellent, and ...
  23. [23]
    [PDF] Grid Enhancing Technologies - Idaho National Laboratory
    Mar 21, 2024 · These include significant delays preventing greenfield (new build) transmission expansion, electrification growth, increasing rates of electric ...
  24. [24]
    [PDF] The unified power flow controller - COLLEGE OF ENGINEERING
    The paper describes the basic concepts of the proposed generalized P and Q controller and compares it to the more conventional, but related power flow ...Missing: Narain | Show results with:Narain
  25. [25]
    (PDF) SDG&E Talega STATCOM project-system analysis, design ...
    and distribution system applications. This STATCOM system, rated ±100 MVA at 138 kV, uses. GCT thyristors ...
  26. [26]
    [PDF] Power Flow Control In A Transmission Line Using Unified ... - Journal
    The first FACTS installation was at the C. J. Slatt Substation in Northern Oregon. This is a 500kV, 3-phase. 60 Hz substation, and was developed by EPRI, the ...<|control11|><|separator|>
  27. [27]
    Transformer‐less unified power flow controller in medium voltage ...
    Mar 3, 2023 · A transformer-less unified power flow controller (UPFC) is a power electronic device consisting of series and shunt voltage source ...
  28. [28]
    Modified unified power flow controller for medium voltage ...
    Aug 13, 2022 · The modified UPFC model demonstrated its capability to control the power flow of distribution networks using partially rated converters. Future ...
  29. [29]
    (PDF) Stability Analysis of a Unified Power Flow Controller
    ... V dc α SH (4) UPFC m V SE = SE V dc α SE 2 ∠ (5) Fig. 3. UPFC equivalent circuit. Where, VS is the AC voltage and Vdc is the dc voltage, mSH and mSE are ...
  30. [30]
    [PDF] Enhanced power transfer capability by using SSSC
    At ρ1pq= 30°;|Vinj| = 0.2 pu ; V1pq= Vinj ∠ (δ1+30°) = 0.2∠60° pu. Real (V1pq) = V1p = 0.2 cos 60° = 0.1 pu. Imag (V1pq) = V1q = 0.2 sin 60° = 0.1732 pu.
  31. [31]
    [PDF] Modelling of Unified Power Flow Controller into Power Systems ...
    Automatic Power flow Control mode: the reference inputs are values of P and Q to maintain on the transmission line despite system changes. In general the ...
  32. [32]
    [PDF] Modelling and control of the unified power flow controller (UPFC).
    With its unique capability to control simultaneously real and reactive power flows on a transmission line as well as to regulate voltage at the bus where it is ...
  33. [33]
    [PDF] An Overview of UPFC with PSS for Power Flow Control and System ...
    Unified power flow controller. (UPFC) is one of the distinctive FACTS devices that can give concurrent control of all parameters of power system (bus voltage, ...
  34. [34]
    [PDF] Power System Stability Enhancement By UPFC Based ... - IRJET
    UPFC can improve power oscillation damping effectively. The damping ... electromechanical mode eigenvalue and its damping ratio ... Variation in rotor angle with ...
  35. [35]
    Application of UPFC to mitigate SSR in series‐compensated wind ...
    Dec 18, 2018 · The results verify the capability of UPFC in attenuating SSR in wind farm integrations by utilising subsynchronous damping controllers ...
  36. [36]
    [PDF] Transient Stability Enhancement of Power System Using UPFC ...
    Unified Power Flow Controller (UPFC) is in principle the best effective and multipurpose FACT device as it can execute the function of both STATCOM and SSSC at ...
  37. [37]
    https://hal.science/hal-01484902/document
  38. [38]
    Optimal location of unified power flow controller by differential ...
    This paper presents a Differential Evolutionary (DE) algorithm for finding the optimal location and the best parameter setting of Unified Power Flow ...
  39. [39]
    Improvement of power flow and voltage stability using unified power ...
    In this paper the working of UPFC is in the field of control flow of power in transmission-line. This research regarding the 6-bus power system to control the ...
  40. [40]
    Evaluation of optimal UPFC allocation for improving transmission ...
    The simulation results show that the UPFC has a strong ability to improve transmission capacity, and its use is greatly advantageous.Missing: increase | Show results with:increase
  41. [41]
    Impacts and benefits of UPFC to wind power integration in unit ...
    This paper addresses how UPFC explores the transmission flexibility and facilitates the integration of uncertain and volatile wind power generation.Missing: solar | Show results with:solar
  42. [42]
    Power Flow Electronics Help Solve Transmission Line Load Problems
    Jul 1, 1998 · With the UPFC control, however, the line will be capable of transferring about 770 MW with well-controlled voltage, solving a significant ...
  43. [43]
    The operation experience of KEPCO UPFC - IEEE Xplore
    KEPCO (Korea Electric Power Corporation) has installed UPFC (unified power flow controller) ... Kangjin UPFC normally operates in UPFC mode. Kangjin UPFC ...Missing: deployment | Show results with:deployment
  44. [44]
    A cost/worth approach to evaluate UPFC impact on ATC
    Aug 8, 2025 · The UPFC application worth is considered as the maximum cost saving in enhancing the ATC of the paths due to the UPFC implementation. The cost ...
  45. [45]
    New approach for optimal UPFC placement using hybrid immune ...
    The unified power flow controller (UPFC) is one of the most promising flexible AC transmission systems (FACTS) devices for the load flow control.
  46. [46]
    A Literature Review on the Unified Power Flow Controller UPFC
    A unified power flow controller (UPFC) is a one of FACTS elements that can ... generate or absorb controllable reactive power [5]. In (1994) Laszlo Gyugyi ...
  47. [47]
    A novel architecture of the solid-state unified power flow controller ...
    This paper proposes a novel unified power flow controller (UPFC) architecture utilizing a modular multilevel converter topology based on solid-state technology ...
  48. [48]
    Unified power flow controllers in smart power systems: models, methods, and future research | IET Smart Grid
    ### Summary of Limitations, Challenges, Economic Aspects, and Technical Limits of UPFC in Smart Power Systems
  49. [49]
    [PDF] Unified Power Flow Controller (UPFC) Used to Relieve ... - IIETA
    Jun 25, 2024 · A unified power flow controller is a member of the FACTS device family and allows for simultaneous control of voltage magnitude in addition to ...<|control11|><|separator|>
  50. [50]
    https://digital-library.theiet.org/doi/10.1049/iet-stg.2018.0065
  51. [51]
    https://www.iieta.org/download/file/fid/134721
  52. [52]
    [PDF] Mathematical Model and Steady-State Operational Characteristics of ...
    Abstract. The paper presents a mathematical model and steady-state operational characteristics of a Unified Power. Flow Controller (UPFC).
  53. [53]
    (PDF) Newton-Raphson UPFC model for power flow solution of ...
    In this paper, a generalized and improved modeling of the unified power flow controller (UPFC) for Newton-Raphson power flow studies has been presented.
  54. [54]
    [PDF] Unified Power Flow Controller (UPFC) Modeling and ... - ICREPQs
    Apr 16, 2018 · V2=U/sqrt(3)*sqrt(2);. %Initial power flow in the line. P2=500*10^6*2/3;. Q2=-250*10^6*2/3;. A1=(-P2*(Rt1^2+(Xt1)^2)+Rt1*V2^2)/V2;. B1=(Q2*(Rt1^ ...
  55. [55]
    Control Power Flow Using UPFC and PST - MATLAB & Simulink
    A UPFC is used to control the power flow in a 500 kV /230 kV transmission system. The system, connected in a loop configuration, consists essentially of five ...Missing: SSSC 50<|separator|>
  56. [56]
    [PDF] Investigation of advanced control for unified power flow controller ...
    Jun 8, 2021 · This research includes modeling of UPFC in. PSCAD/EMTDC software. Also, modeling of series and shunt controllers of UPFC are discussed. The ...Missing: EMTP | Show results with:EMTP
  57. [57]
    (PDF) Modeling and simulation of UPFC for dynamic voltage control ...
    Feb 22, 2016 · The model of UPFC are studied and validated in power system computer aided design & electromagnetic transient direct current (PSCAD/EMTDC) ...
  58. [58]
    [PDF] A Novel Real-time Approach to Unified Power Flow Controller ...
    Oct 20, 2010 · during transients, the minimum required capacitance is given ... the dc link voltage. This control works well for slowly changing. (or ...<|control11|><|separator|>
  59. [59]
    Enhancing Oscillation Damping in an Interconnected Power System ...
    Jan 21, 2019 · From the results presented in Table 2, it is known that the oscillation damping ratio tended to increase with the installation of UPFC. 4.2.