FACTS Devices

FACTS devices (Flexible AC Transmission Systems) control bus voltage, branch flow and the sharing of power between parallel paths. In a steady-state power flow each device is a controllable network parameter (a reactive injection, a shunt susceptance, a series reactance, or a converter power) that the outer control loop around runpf! drives toward a target. Hands-on: chapter 4 of the advanced workshop tour, examples/others/exp_facts_limit_modes.jl (the three limit characteristics on one weak corridor, the SSSC window on a loop network) and examples/others/exp_facts_base_impedance.jl (a full UPFC on a meshed corridor, then short circuit and MATPOWER export on the base impedance).

Device family and where each one lives

All controllers report through ControlRunResult, controllableElements (element, device, actuator with live range, target, status/converged/at_limit) and the per-controller summary printers.

The limit characteristic is the device

In range, every shunt compensator holds a voltage target by injecting reactive power. The devices differ at their limit, where depressed voltage puts them.

Synchronous machine (constant-Q box). A fixed reactive capability $Q \in [Q_{min}, Q_{max}]$, independent of the terminal voltage. At the limit the machine is a fixed injection: the MachineVoltageControl default (limit_mode = :constant_q), the outer-loop analogue of PV to PQ switching.

SVC (constant-B limit). A continuous susceptance $B$ within $[B_{min}, B_{max}]$. At the limit the susceptance clamps and the delivered reactive power follows the voltage quadratically through the Y-bus stamp:

\[Q_{SVC} = V^2 \, B_{lim}\]

At $V = 0.9$ pu a fully switched-in SVC delivers 81 percent of its rating; ShuntVoltageControl needs no extra modeling, the clamped susceptance stays stamped in the Y-bus.

STATCOM (constant-current limit). A voltage-source converter bounded by its converter current, so the deliverable reactive power scales linearly with the terminal voltage:

\[Q_{STATCOM} = V \, I_{max} \quad\text{, in Sparlectra: } Q_{lim} = V \cdot S_{max}\]

with $S_{max}$ the rating at 1.0 pu; at $V = 0.9$ pu the STATCOM still delivers 90 percent. addMachineVoltageControl! with s_max_mva (or i_max_ka, converted via $\sqrt{3}\,U_n I_{max}$) selects this mode (Remote Voltage Control); the bound $\pm V \cdot S_{max}$ is a live bound (below).

Delivered reactive power at the capacitive limit, relative to the rating:

The machine column is idealized (a real machine derates through its capability curve).

Discrete banks: MSC/MSR

A switched bank shares the shunt physics above and lives in the same controller. With step_mvar the susceptance moves in whole blocks (e.g. four times 10 MVAr): the secant proposal is truncated toward the target to whole steps, so the bank never overshoots and cannot hunt between two adjacent blocks. When no whole block improves the voltage further, the controller parks on the reached step (status = :parked, the last step before crossing, so under-compensation rather than overvoltage) and releases itself when another controller moves the operating point enough for a block to help. At the outermost block the constant-B limit applies: delivered Q follows $V^2$ with the last block connected. A bank behind the OLTC of its bus switches only when the tap stands in its band or at a tap limit, see Controller priority.

Series side: fixed window versus voltage-bounded window

A TCSC moves the branch reactance within a hardware window $[x_{min}, x_{max}]$ independent of loading; at a window end the branch is a fixed compensated line. An SSSC injects a voltage in quadrature with the line current, so its reactance window $x_{base} \pm V_{inj,max}/|I|$ shrinks with loading: at high transfer the SSSC saturates while the TCSC keeps its full window. Both are addSeriesReactanceControl! (x_min_pu/x_max_pu or v_inj_max_pu) with the resonance guard eps_z; formula and window handling: Series Compensation.

Live bounds in the outer loop

STATCOM and SSSC share the live bound: the actuator range follows the solved operating point and is refreshed at the start of every outer iteration, before the secant step is clamped against it. A parked controller whose bound still moves is released and keeps adjusting; it reports at_limit only once its bound has settled. controllableElements and the report rows show the bounds of the last evaluated operating point, so actuator_min/actuator_max are the currently deliverable range, not the nameplate. In range, both modes behave like their fixed-limit counterparts.

Usage

Programmatic (full keyword sets in the docstrings):

# SVC: continuous susceptance, quadratic limit collapse
addShuntVoltageControl!(net; bus = "B", target_vm_pu = 1.0,
                        bs_min_mvar = -60.0, bs_max_mvar = 60.0)

# MSC/MSR: the same controller as a switched bank, whole 10-MVAr blocks
addShuntVoltageControl!(net; bus = "B", target_vm_pu = 1.0,
                        bs_min_mvar = -40.0, bs_max_mvar = 40.0,
                        step_mvar = 10.0)

# STATCOM: current-based limit, linear in V
addMachineVoltageControl!(net; bus = "B", target_bus = "C",
                          target_vm_pu = 1.0, s_max_mva = 25.0)

# TCSC: fixed reactance window
addSeriesReactanceControl!(net; fromBus = "A", toBus = "B",
                           p_target_mw = 35.0, x_min_pu = 0.02, x_max_pu = 0.30)

# SSSC: injected-voltage window, shrinks with loading
addSeriesReactanceControl!(net; fromBus = "A", toBus = "B",
                           p_target_mw = 35.0, v_inj_max_pu = 0.05)

Declarative, under control.controllers (Control Framework):

control:
  enabled: true
  controllers:
    statcom_c:
      type: machine_voltage
      bus: B
      target_bus: C
      target_vm_pu: 1.0
      s_max_mva: 25.0
    sssc_ab:
      type: series_reactance
      from_bus: A
      to_bus: B
      p_target_mw: 35.0
      v_inj_max_pu: 0.05

UPFC: the stationary quadrature composite

The unified power flow controller combines a STATCOM (shunt side, bus voltage) and an SSSC (series side, branch flow) behind one DC link. Restricting the injected series voltage to quadrature with the line current makes the series converter exchange (approximately) no active power with the line, and what remains is an SSSC on the branch plus a STATCOM at the bus. addUpfcControl! (YAML type upfc) registers the pair as one named device: the series controller steers the branch active power inside v_inj_max_pu, the shunt controller holds a remote bus voltage inside s_max_mva (or i_max_ka). Registration is all-or-nothing, and the composite equals the manually registered pair to machine precision. The result table keeps one row per actuator with at_limit per converter side; both rows carry the device string UPFC series/shunt (VSC pair, stationary quadrature model). The composite has no series active-power injection; independent P and Q steering needs the full model below. The quadrature restriction removes the DC-link coupling between the two converters, so each side runs as the existing single-actuator controller.

control:
  enabled: true
  controllers:
    upfc_main:
      type: upfc                 # model: quadrature is the default
      from_bus: A
      to_bus: B
      shunt_bus: B
      target_bus: LOAD
      target_vm_pu: 1.0
      p_target_mw: 35.0
      v_inj_max_pu: 0.05
      s_max_mva: 25.0

UPFC: the full DC-link-coupled model

model = :full (YAML model: full) adds the phase-shifter degree of freedom: a series voltage V_se of arbitrary phase, so the line carries independent active and reactive targets at once. V_se is a voltage drop in the direction of the line current, so the active power the series converter delivers into the line is P_se = -Re(V_se·conj(I_s)) = -|I_s|^2 Re(z_add); it flows through the DC link and the shunt converter draws it from the bus (P_sh = -P_se). In quadrature it is zero and the device collapses onto the composite above:

        Im (quadrature to I_s: reactance, NO DC power)
         ^
         |      V_se
         |     /
         |    /  in-phase part -> P_se -> DC link -> shunt   (the UPFC DOF)
         |   /
         +--------------->  Re, aligned with the line current I_s

The series source is an equivalent series impedance z_add = V_se / I_s on the branch (Re(z_add) < 0 when the converter injects active power), so the line stays an ordinary branch. With the terminal voltages frozen each outer iteration the from-end flow is affine in V_se, so the series step is an exact 2x2 solve; the coupled iteration uses an adaptive damping line search.

• one call, one controller: model = :full steers the from-end line flow to p_target_mw and q_target_mvar;
• the shunt converter provides the DC-link balance plus a reactive setpoint q_shunt_mvar, inside the current-based rating with headroom Q_max = sqrt((V·s_max)^2 - P_sh^2);
• the result row carries V_se magnitude and angle, P_se, P_sh, the shunt Q, and the DC-link residual |P_se + P_sh| (a convergence quantity: the balance holds by construction only at the frozen state);
• series_phase = :quadrature forces P_se = 0 and reproduces the composite.

control:
  enabled: true
  controllers:
    upfc_full:
      type: upfc
      model: full
      from_bus: I
      to_bus: J
      shunt_bus: I
      p_target_mw: 40.0
      q_target_mvar: 10.0
      q_shunt_mvar: 0.0
      v_inj_max_pu: 0.20
      s_max_mva: 120.0

Limitations

• Stationary model: no dynamics; IPFC (a shared DC bus across several lines) is out of scope.
• The shunt side takes a reactive setpoint, not a closed-loop voltage: a shunt-voltage secant coupled with the line reactive-flow control does not converge in the sequential outer loop.
• No explicit series current limit: only |V_se| <= v_inj_max_pu is clamped, not the series-converter current |I_s| <= i_max.
• The branch impedance is modified in place (like SSSC/TCSC): z_add stays on the branch after the control run, with a negative resistance part for the full model. Every branch therefore carries its physical base impedance (r_base_pu/x_base_pu) next to the live value, and runShortCircuit! and the CGMES/MATPOWER exports read the base. restoreBaseImpedances!(net) returns the live field to the base, clearUpfcFullControllers!(net) does the same and drops the controller.
• Low line current: z_add = V_se / I_s is floored at a minimum current, so a dead line stays finite and keeps its base impedance.
• Convergence regime: feasible, moderate flow targets converge; aggressive targets near the injectable-voltage limit may not.