FACTS devices

In short

FACTS (Flexible AC Transmission Systems) are power-electronic controllers that rapidly adjust the voltage, line impedance or phase angle of an AC transmission line so that power flow, voltage profile and stability can be controlled without changing the network itself. They let an existing line carry more power up to its thermal limit instead of being limited by voltage drop or stability margins. The topic is examined in Power Systems and Power Electronics papers and appears regularly in GATE EE (SVC, STATCOM, TCSC characteristics) and in interviews for POWERGRID, state transcos, NTPC, Siemens Energy, Hitachi Energy (formerly ABB), GE Vernova and Toshiba T&D. India has large installed FACTS capacity — POWERGRID's SVCs and STATCOMs on the 400 kV network and TCSCs on long inter-regional corridors — so field and design roles expect working knowledge.

Explained

Why FACTS: the limits of an AC line

• Power transferred over a lossless line: P = (V₁V₂/X) sin δ. Only three quantities can be manipulated — voltage magnitude, line reactance X and phase angle δ. Every FACTS device acts on one or more of these.
• Loadability limits: short lines (< 80 km) are limited by thermal rating; medium lines (80–300 km) by voltage drop/regulation; long lines (> 300 km) by steady-state and transient stability. FACTS relieve the voltage and stability limits so the conductor's thermal limit becomes the binding one.
• Surge impedance loading (SIL) = V²/Zs, with Zs = √(L/C) ≈ 350–400 Ω for an overhead line. Below SIL the line produces net reactive power (Ferranti rise at light load); above SIL it absorbs reactive power and voltage sags.
• Conventional compensation (fixed/switched shunt capacitors and reactors, fixed series capacitors, mechanically switched tap changers) is slow (cycles to seconds, limited operations), coarse and cannot damp oscillations. FACTS are continuous, sub-cycle and repeatable.
• Reactive power cannot be transported economically — it must be generated locally, which is why compensation is placed at load centres and at line midpoints.

Classification of FACTS controllers

• By connection:
  • Shunt controllers: SVC, STATCOM — inject/absorb reactive current at a bus, so they control voltage; TCBR is also shunt-connected but absorbs real power.
  • Series controllers: TCSC, TSSC, SSSC, TCSR (thyristor-controlled series reactor) — insert a voltage in series with the line, so they control effective reactance and hence power flow.
  • Combined series–shunt: UPFC, TCPST/TCPAR (phase-shifting transformer / phase-angle regulator), which control voltage, impedance and angle together.
  • Combined series–series: IPFC, which transfers real power between several lines of a corridor.
• By technology (generation):
  • First generation — thyristor (line-commutated, variable impedance type): SVC (TCR + TSC), TCSC, TCPAR. Thyristors act as switches; the device behaves like a controllable susceptance/reactance.
  • Second generation — voltage source converter (VSC) based, with self-commutated switches (IGBT/GTO) and PWM: STATCOM, SSSC, UPFC, IPFC. These behave like controllable voltage sources and can exchange both reactive and (with a real source or a common DC link) real power.
• Typical response times: mechanical switching 100 ms–several seconds; thyristor devices 1–3 cycles (20–60 ms); VSC devices 1–2 ms to a few ms.

Shunt devices: SVC and STATCOM

• SVC (Static VAR Compensator): a shunt-connected variable susceptance built from
  • TCR (thyristor-controlled reactor): reactor in series with back-to-back thyristors fired at angle α (90°–180°); conduction is partial, so the fundamental current — and hence the absorbed VAR — varies continuously from full to zero. Generates odd harmonics (3rd, 5th, 7th), suppressed by delta connection and filters.
  • TSC (thyristor-switched capacitor): capacitor banks switched in discrete steps, fired when the voltage across the valve is zero (capacitor pre-charged to the system voltage peak) to avoid inrush; harmonic-free but stepped.
  • A practical SVC = TCR + TSC + harmonic filters + coupling transformer, giving smooth control from full inductive to full capacitive. FC–TCR (fixed capacitor + TCR) is the common simpler scheme.
• SVC V–I characteristic: inside its control range the SVC holds the bus voltage on a sloped reference line (slope 1–5% on rating, deliberately added for load sharing and to limit duty). Beyond the limits it degenerates into a fixed capacitor (capacitive limit) or a fixed reactor (inductive limit) — so its reactive output falls with V², exactly when support is needed most.
• STATCOM (Static Synchronous Compensator): a VSC with a DC capacitor behind a coupling reactance/transformer, acting as a controllable AC voltage source E behind reactance X.
  • If E > V → capacitive (supplies VAR); E < V → inductive (absorbs VAR); E = V → no exchange. Q ≈ V(E − V)/X.
  • Key advantage: maximum reactive current is maintained even at low voltage, so Q falls only linearly with V (against V² for an SVC) — far better low-voltage support and fault ride-through.
  • Smaller footprint (no large capacitor/reactor banks), faster (1–2 ms), lower harmonics with multilevel/MMC topologies, and can be made to supply active power or balance/filter if given storage or extra rating.
  • Cost per MVAr is higher than an SVC and converter losses and switch stress are higher — SVCs are still chosen where plain steady-state voltage support suffices.

Series devices: TCSC and SSSC

• Series capacitor compensation basics: inserting Xc in series reduces effective reactance to X_eff = X_L − X_C. Degree of compensation k = X_C/X_L, normally 20–70%. Transfer capability rises as P = V₁V₂ sin δ/(X_L(1 − k)).
• TCSC (Thyristor-Controlled Series Capacitor): a capacitor in parallel with a TCR, i.e. a thyristor-controlled reactor shunting the capacitor; plus MOV and bypass breaker for protection.
  • Operating modes: blocking (thyristors off → plain capacitor), bypass (thyristors fully on → net small inductive), capacitive vernier (α between the resonance angle and 180° → boosted X_C), inductive vernier (α between 90° and resonance).
  • Effective reactance X_TCSC(α) = X_C·X_L(α)/(X_L(α) − X_C); near the parallel-resonance firing angle the magnitude is very large, so operation is kept away from resonance.
  • Main uses: power-flow scheduling between parallel paths, damping of power oscillations by modulating reactance, and mitigation of subsynchronous resonance (SSR) — the thyristor control makes the device appear resistive/inductive at subsynchronous frequencies, which fixed series capacitors cannot do.
• TSSC uses the same hardware but switches capacitor segments in/out in steps (no vernier).
• SSSC (Static Synchronous Series Compensator): a VSC injecting a voltage V_inj in quadrature with the line current, independent of the current magnitude.
  • Behaves as a controllable series voltage source, not a reactance, so compensation does not collapse at low current and there is no series resonance with the line inductance → immune to classic SSR.
  • Can emulate capacitive or inductive reactance, and even negative/positive resistance for damping, within rating.

Combined controllers: UPFC, IPFC and phase shifters

• UPFC (Unified Power Flow Controller): two VSCs sharing a common DC capacitor — a shunt converter (like a STATCOM, at the sending bus) and a series converter (like an SSSC, in the line).
  • The series converter injects a voltage of controllable magnitude (0–V_max) and any phase angle (0–360°), so it independently and simultaneously controls line active power, reactive power and bus voltage — the most versatile FACTS device.
  • The shunt converter supplies/absorbs the real power demanded by the series converter through the DC link, and independently provides reactive support.
  • Functions as voltage regulator (in-phase injection), series compensator (quadrature injection) and phase shifter (angle injection) in one device; cost and control complexity are the highest.
• IPFC (Interline Power Flow Controller): several series converters on different lines on a common DC bus; real power is shifted from an over-loaded line to an under-loaded one in the same corridor.
• TCPST / TCPAR (thyristor-controlled phase-shifting transformer / phase-angle regulator): injects a quadrature voltage from an exciting transformer through thyristor taps to change δ rapidly — power-flow control by angle rather than by shunt reactive injection.
• TCBR (thyristor-controlled braking resistor) and TCVL (voltage limiter): transient-stability aids that dump surplus generator power or clamp overvoltage for a few cycles.
• Supporting devices often grouped with FACTS: fault current limiters, superconducting magnetic energy storage (SMES), BESS-coupled STATCOM (STATCOM with storage, now common for renewable-rich grids), and HVDC back-to-back links (not FACTS, but a comparable power-electronic power-flow controller).

Benefits, limitations and Indian practice

• Benefits: higher power transfer on existing rights-of-way; voltage regulation and reduced losses; improved steady-state, transient and small-signal (oscillatory) stability; load balancing between parallel circuits; damping of inter-area oscillations; reduced flicker and voltage unbalance (industrial SVCs for arc furnaces, rolling mills, traction); better renewable integration and reactive support during faults.
• Limitations: high capital cost (typically ₹ crores per MVAr class of equipment); harmonics from thyristor devices needing filters; converter losses (roughly 0.5–1.5% of rating for VSCs); SSR and protection-coordination issues with series capacitors; complex control tuning and interaction between multiple nearby devices; skilled O&M required.
• Indian codes and practice (verify the current amended version before citing in exams or work):
  • CEA (Technical Standards for Connectivity to the Grid) Regulations and CEA (Grid Standards) Regulations cover reactive capability and connectivity requirements.
  • CERC Indian Electricity Grid Code (IEGC) — the IEGC was comprehensively replaced by the IEGC 2023 — requires grid voltages to be held within the specified bands (approximately 380–420 kV for 400 kV, 198–245 kV for 220 kV; check the current schedule) and makes reactive power a system-operator-managed resource with reactive energy charges under the deviation/ancillary framework.
  • CEA (Measures relating to Safety and Electric Supply) Regulations, 2023 govern installation safety; relevant equipment standards include IS/IEC 60871 (shunt power capacitors), IEC 61954 (SVC thyristor valve testing) and IEEE Std 1031 (SVC functional specification guide).
  • Planning: CEA/CTU National Electricity Plan (Transmission) and POWERGRID reactive-compensation studies specify where line reactors, bus reactors, capacitor banks, SVCs and STATCOMs go; STATCOMs have been added at several 400 kV substations for dynamic reactive support and TCSCs on long east–west/north–south corridors.

Key points

• FACTS - power-electronic control of AC transmission parameters (V, X, δ) for power flow, voltage and stability control, defined by IEEE/CIGRE.
• P = (V₁V₂/X) sin δ - the master equation; every FACTS device changes V, X or δ in this expression.
• Shunt device - controls voltage/reactive power at a bus (SVC, STATCOM); series device - controls line reactance and power flow (TCSC, SSSC).
• TCR - continuously variable inductive VAR by firing-angle control, 90°–180°, generates odd harmonics.
• TSC - capacitors switched in discrete steps at zero voltage across the valve, harmonic-free, no continuous control.
• SVC - variable susceptance; Q ∝ V², so its support collapses during deep voltage dips; cheap and proven.
• STATCOM - VSC-based current source in effect; rated current held at low voltage, Q ∝ V, 1–2 ms response, small footprint.
• Slope (droop) of 1–5% is deliberately built into SVC/STATCOM voltage control for stable sharing and reduced duty.
• TCSC - thyristor-controlled series capacitor; capacitive/inductive vernier modes, power-flow scheduling, oscillation damping and SSR mitigation.
• Degree of series compensation k = X_C/X_L, typically 20–70%; P rises as 1/(1 − k).
• SSSC - injects a quadrature voltage independent of line current; no series resonance, so inherently SSR-free.
• UPFC - shunt + series VSCs on a common DC link; independent control of P, Q and bus voltage; most capable, most costly.
• IPFC - multiple series converters on one DC bus; redistributes real power among lines of a corridor.
• TCPAR/TCPST - rapid phase-angle control by quadrature voltage injection from a tapped exciting transformer.
• Line loadability - thermal limit (short), voltage-drop limit (medium), stability limit (long); FACTS push the limit towards thermal.
• SIL = V²/Zs - the loading at which a line neither absorbs nor generates reactive power.
• Indian application - POWERGRID 400 kV SVCs/STATCOMs for voltage support, TCSCs on long inter-regional lines; IEGC 2023 sets the voltage bands and reactive-power regime.

Formulas and examples

Example 1 — Series compensation raises transfer capability. A 400 kV line, X_L = 100 Ω, both ends held at 400 kV, operating at δ = 30°.

• Uncompensated: P = (400 × 400/100) × sin 30° = 1600 × 0.5 = 800 MW.
• With 50% series compensation: X_eff = 100(1 − 0.5) = 50 Ω → P = (160000/50) × 0.5 = 1600 MW (doubled).
• Maximum (δ = 90°) rises from 1600 MW to 3200 MW, so at 800 MW the stability margin improves greatly.

Example 2 — TCR susceptance and SVC output. A TCR has X_L = 20 Ω per phase (star equivalent referred to 33 kV), fired at α = 120° (= 2π/3 rad).

• 2π − 2α + sin 2α = 6.2832 − 4.1888 + sin 240° = 6.2832 − 4.1888 − 0.8660 = 1.2284.
• B_TCR = 1.2284/(π × 20) = 1.2284/62.832 = 0.01955 S (against 1/20 = 0.05 S at α = 90°).
• Inductive Q = V²B = (33 × 10³)² × 0.01955 = 1089 × 10⁶ × 0.01955 ≈ 21.3 MVAr (full-conduction value would be 54.5 MVAr).
• If a fixed capacitor of 54.5 MVAr is in parallel (FC–TCR), net output = 54.5 − 21.3 ≈ +33.2 MVAr capacitive.

Example 3 — SVC versus STATCOM during a voltage dip. Both rated 100 MVAr capacitive at 1.0 pu bus voltage. Bus voltage falls to 0.7 pu.

• SVC (fixed-capacitor behaviour at its limit): Q = 100 × (0.7)² = 49 MVAr.
• STATCOM (constant rated current): Q = 100 × 0.7 = 70 MVAr.
• The STATCOM delivers about 43% more reactive support exactly when the system is weakest — the standard interview point.

Example 4 — Shunt compensation sizing from short-circuit level. A 220 kV bus has a short-circuit level of 4000 MVA and sits at 0.96 pu; the target is 1.0 pu.

• ΔV/V = 0.04 → Q ≈ 0.04 × 4000 = 160 MVAr capacitive.
• Chosen as a 160 MVAr SVC: say ±160 MVAr with a 3% slope → at full capacitive output the regulated voltage settles about 3% below the no-load reference, which must be set accordingly.

Example 5 — Line charging and reactor sizing. A 400 km, 400 kV line with C = 0.0115 µF/km, f = 50 Hz.

• Total C = 0.0115 × 400 = 4.6 µF. ω C = 314.16 × 4.6 × 10⁻⁶ = 1.445 × 10⁻³ S.
• Charging MVAr = V²ωC = (400)² × 1.445 × 10⁻³ = 160000 × 1.445 × 10⁻³ ≈ 231 MVAr.
• A 125 MVAr line reactor (about 54%) absorbs part of this to limit the light-load Ferranti rise; the balance is handled by an SVC/STATCOM that must also absorb reactive power (inductive mode).

Where it is used

• Inter-regional transmission (POWERGRID / CTUIL): SVCs and STATCOMs at 400 kV substations for dynamic voltage control, and TCSCs with fixed series capacitors on long high-capacity corridors to schedule flow and damp inter-area oscillations.
• State transcos and SLDC/RLDC operation: reactive-power dispatch, voltage-profile management under IEGC bands, and switching of bus/line reactors and capacitor banks alongside FACTS controllers.
• Renewable integration: STATCOMs (increasingly with battery storage) at large solar and wind pooling stations in Rajasthan, Gujarat, Tamil Nadu and Karnataka for reactive support, low-voltage ride-through and flicker control; grid-forming converters are the emerging extension.
• Heavy industry: SVCs and STATCOMs on electric arc furnaces, rolling mills, mines and welding plants (steel plants, aluminium smelters) for flicker and unbalance mitigation; railway traction STATCOMs for unbalance and power-factor correction on 25 kV AC feeders.
• Job roles: transmission planning engineer (PSS/E, PSCAD, ETAP, DIgSILENT studies), FACTS/HVDC commissioning and control engineer at Hitachi Energy, Siemens Energy, GE Vernova, Toshiba T&D, BHEL and CPRI/ERDA test roles, substation O&M engineer, R&D at CPRI Bengaluru and NIWE.
• Academic and testing: CPRI and IIT laboratories for converter and compensator testing, RTDS/hardware-in-the-loop validation of FACTS controls.

Interview questions

• Q: What exactly do FACTS devices control? A: They control the parameters of the power-transfer equation P = (V₁V₂/X) sin δ — bus voltage magnitude, line series reactance and phase angle — rapidly and continuously using power electronics.
• Q: Why is a STATCOM better than an SVC during a fault or deep voltage dip? A: An SVC at its limit behaves as a fixed capacitor, so its output falls with V², while a STATCOM holds rated current, so its output falls only linearly with V — it gives much stronger support when voltage is low.
• Q: What is the role of the DC capacitor in a STATCOM? A: It maintains the DC-link voltage that the converter modulates into AC; it exchanges only ripple energy in steady state, so no bulk energy storage is needed unless active power support is required.
• Q: Why is a slope or droop deliberately added to an SVC's voltage characteristic? A: To reduce the MVAr rating needed, prevent hunting with transformer tap changers, and let parallel compensators share reactive duty stably.
• Q: How does a TCSC mitigate subsynchronous resonance? A: Thyristor control makes the device present an apparent resistance/inductance rather than a pure capacitance at subsynchronous frequencies, so the torsional interaction with turbine-generator shafts is not reinforced.
• Q: Difference between TCSC and SSSC? A: A TCSC is a controllable reactance whose injected voltage is proportional to line current; an SSSC is a voltage-source converter injecting a controllable quadrature voltage independent of line current, and it cannot resonate with the line.
• Q: What makes a UPFC unique? A: Its series converter can inject a voltage of any magnitude and any phase angle, with the shunt converter supplying the real power through the DC link, so line P, line Q and bus voltage are controlled independently and simultaneously.
• Q: Can a STATCOM supply active power? A: Only if an energy source or storage (battery, SMES) is added on the DC side and the converter is rated for it; a plain STATCOM exchanges only reactive power apart from its small loss component.
• Q: Why are TCRs connected in delta and provided with filters? A: Firing-angle control makes the current non-sinusoidal; a delta connection traps triplen harmonics, and tuned filters remove the 5th and 7th, limiting injection into the grid.
• Q: What is the maximum practical degree of series compensation and why not higher? A: Usually 20–70%; beyond that the short-circuit current, SSR risk, protection and distance-relay mis-operation and voltage-control problems outweigh the gain.
• Q: Is HVDC a FACTS device? A: No — FACTS controllers remain within an AC network; HVDC converts to DC and back, though both are power-electronic power-flow controllers and VSC-HVDC terminals can provide STATCOM-like support.
• Q: How is reactive power handled commercially on the Indian grid? A: Through the IEGC (the 2023 code) and CERC regulations, which require users to stay within voltage bands and provide reactive capability, with reactive-energy drawal/injection charges and ancillary-service mechanisms administered by the system operator.

Common mistakes

• Treating an SVC and a STATCOM as interchangeable; the V² versus V output behaviour at low voltage is the whole point of the comparison.
• Writing a TCSC as a plain capacitor; it is a controllable reactance and must be kept away from the parallel-resonance firing angle.
• Confusing "degree of compensation 50%" with "reactance halved and power halved" — the power rises to 1/(1 − k) times, i.e. doubles.
• Forgetting that series compensation raises fault current and upsets distance-relay reach, so protection must be re-coordinated.
• Assuming a STATCOM can inject real power without storage, or that a UPFC series converter can supply its real power on its own.
• Using line-to-neutral voltage in P = V₁V₂ sin δ/X with a three-phase answer, or mixing per-phase and three-phase MVAr in SVC sizing.
• Claiming FACTS let a line exceed its thermal rating; they only remove the voltage and stability limits so the thermal limit can be approached.

Related topics

Transmission lines and cables · Power system stability · HVDC transmission · Power quality and harmonics · Smart grid and renewable integration · Power system operation, SCADA and EMS