Shunt compensation: need and two methods

Shunt compensation means connecting reactive-power devices (capacitors, reactors, or controlled devices) in parallel with the line or at the buses, to supply or absorb reactive power.

Why shunt compensation is necessary

• Voltage control at heavy load: large lagging loads draw reactive current, which causes a large voltage drop ($\Delta V \approx QX/V$). Shunt capacitors supply this Q locally and hold up the voltage.
• Voltage control at light load: below SIL the line's charging Mvar exceeds its inductive absorption, so voltage rises (Ferranti effect). Shunt reactors absorb this excess.
• Reduced losses and current: supplying Q near the load reduces line current, $I^2R$ loss and heating.
• Better power factor and released capacity of generators, transformers and lines.
• Higher power transfer and voltage stability: holding the mid-point or receiving voltage up raises the transfer limit $P = \frac{V_SV_R}{X}\sin\delta$ and helps prevent voltage collapse.

Method 1: Shunt capacitors and shunt reactors (fixed or switched)

   Bus  ---+-------------+----- line
           |             |
          === C        (((L)))
           |             |
          ---           ---
       capacitor      reactor

• Shunt capacitor banks at substations and load centres supply lagging Mvar; they are switched in steps as load rises. They are cheap, low-loss and simple.
• Shunt reactors at the ends of long EHV lines absorb charging Mvar at light load and limit the Ferranti rise and switching overvoltages.
• Limitation: control is in steps, and capacitor output falls as $V^2$, just when support is most needed.

Method 2: Static VAR compensator (SVC)

   Bus ----+-----------+-----------+
           |           |           |
         [TCR]       [TSC]       [Filter /
       thyristor-  thyristor-    fixed C]
       controlled   switched
        reactor    capacitor

• A thyristor-controlled reactor (TCR) varies the absorbed Q continuously by controlling the firing angle.
• Thyristor-switched capacitors (TSC) or fixed capacitors provide the capacitive range.
• Together they act as a fast, continuously variable susceptance, from capacitive to inductive, responding within a few cycles.
• Used for dynamic voltage control, damping oscillations and improving transient stability.

Other shunt methods: synchronous condensers (over- or under-excited synchronous machines giving smooth Q control) and STATCOM (a voltage-source converter; it is faster than an SVC and keeps up its reactive current output at low voltage).