FACTS devices — SVC / STATCOM / TCSC / UPFC

Reference notes

FACTS — Flexible AC Transmission Systems — is the family of power-electronic devices that provide fast (millisecond-scale) control of transmission system parameters: voltage, reactive power, real-power flow, and phase angle. Use Next → to walk through the four major device families (SVC, STATCOM, TCSC, UPFC), their topologies, and modern applications.

The four FACTS families

SVC — Static Var Compensator

• First-generation FACTS (1970s onward).
• TCR (Thyristor-Controlled Reactor) — smoothly variable inductive Q via thyristor firing-angle control. Each half cycle, the reactor conducts for a controllable fraction → variable fundamental reactive current.
• TSC (Thyristor-Switched Capacitor) banks — discrete capacitive Q steps. Switched in/out by thyristors.
• Net output: smoothly variable from full inductive (TCR max + TSCs off) to full capacitive (TCR off + all TSCs on).
• Pros: mature, cheapest FACTS per Mvar.
• Cons: TCR generates 5th, 7th, 11th, 13th harmonics → large filter banks (30-50 % of footprint). Output Q ∝ V² → reactive support COLLAPSES during deep voltage dips, exactly when most needed.
• Applications: arc-furnace flicker compensation, long-line voltage support, fast Q reserve.

STATCOM — Static Compensator

• Modern second-generation FACTS shunt technology.
• VSC (Voltage-Source Converter) with DC link capacitor, built with IGBTs or modern MMC topology.
• Synthesizes an AC voltage with controllable magnitude / phase. Q exchange determined by V_VSC vs V_bus:
  • V_VSC > V_bus → STATCOM exports Q (capacitive).
  • V_VSC < V_bus → STATCOM absorbs Q (inductive).

• Pros over SVC: ~10× faster response (< 10 ms); near-CONSTANT current at depressed bus voltage (Q ∝ V instead of V²); no harmonic filters (MMC synthesizes near-sinusoidal directly); smaller footprint.
• Cons: higher capital cost (IGBTs + DC caps); higher converter losses than SVC.
• Modern applications: HVDC inverter station support, wind/solar plant grid connection, urban substations.

TCSC — Thyristor-Controlled Series Capacitor

• Series FACTS device for power-flow control.
• Architecture: fixed series capacitor (rated voltage V_cap sized for full load current × series reactance) in parallel with a TCR. Thyristor firing angle controls the parallel-combination apparent reactance.
• Net effect: variable series reactance inserted in the line → controls power flow via P = (V²/X) · sin δ.
• Additional capabilities:
  • Damping inter-area oscillations (0.1-2 Hz modulation of reactance).
  • Subsynchronous Resonance (SSR) mitigation — suppresses the SSR mode that destroyed Mohave Power Plant turbine-generator shafts (1970-71).
• Applications: long heavily-loaded transmission lines. Brazil, China, India, Sweden, Russia.
• Notable installations: Stode-Midskog 400 kV (Sweden), Imperatriz-Serra da Mesa 500 kV (Brazil), Itaipu transmission.

UPFC — Unified Power Flow Controller

• Most capable FACTS device.
• Architecture: TWO VSCs sharing a common DC link.
  • SHUNT converter acts like a STATCOM — exchanges Q with the bus, supplies P to the DC link.
  • SERIES converter injects a controlled voltage (magnitude + phase) in series with the transmission line.
• Capability: simultaneous and INDEPENDENT control of line P, line Q, and bus V.
• Cost: $50-100M for a 200 MVAR UPFC. Justified only where flexibility benefits exceed capital.
• Notable installations: AEP Inez 138 kV UPFC (West Virginia, 1998 — world's first); Korea Electric Power's Kangjin 154 kV UPFC; multiple Chinese transmission UPFCs since 2015.

Modern trends

• MMC topology — Modular Multi-Level Converter. Each phase arm = many small IGBT sub-modules in series, each with a DC capacitor. By selectively inserting/bypassing sub-modules, the converter synthesizes near-sinusoidal voltage without external filters. Standard in modern STATCOM and UPFC. Vendors: ABB, Siemens, GE, Hitachi, Mitsubishi, plus Chinese RXPE and NR Electric.
• IBR-grid support — high-renewable grids lose synchronous inertia and fault current. Best practice: STATCOM + synchronous condenser pair at major substations. STATCOM for fast voltage transients (< 10 ms); sync condenser provides rotor inertia plus fault MVA driven by its excitation system (~6× rated current for 3-5 cycles).
• Multiple Australian, US (ERCOT west Texas), and European IBR-heavy transmission projects use this combination.

Decision matrix

• Need cheapest reactive support, no voltage-collapse concern → SVC.
• Need fast, no-filter reactive support, voltage stability margin tight → STATCOM.
• Need power-flow redirection on long ties + SSR damping → TCSC.
• Need full independent P / Q / V control on a critical corridor → UPFC (if budget allows).
• Plus synchronous condenser for inertia + fault MVA on low-inertia grids.