Role of interconnection and reactive power control

Role of interconnection

Interconnection ties generating stations and areas together by tie-lines so they run in synchronism and share load. Its roles are:

• Reliability: loss of a unit is covered by other plants.
• Reserve sharing: less spinning reserve per area.
• Economy: cheapest generation is used first; surplus power can be exchanged between areas or countries.
• Load diversity: lower total installed capacity.
• Frequency stability: large combined inertia limits frequency swings.

Control of reactive power

Bus voltage depends mainly on reactive power ($\Delta V \approx QX/V$), so Q is controlled to keep voltage within limits (about ±5%). Methods:

1. Generator excitation (AVR): raising field current increases $E_f$, so the generator supplies Q (over-excited); lowering it absorbs Q (under-excited), within the capability-curve limits.
2. Shunt capacitors: supply Q at heavy load, raising voltage; switched in steps.
3. Shunt reactors: absorb Q at light load (e.g. on long EHV lines) to limit the Ferranti rise.
4. Synchronous condensers: unloaded synchronous motors that give or absorb Q smoothly by excitation control.
5. Static VAR compensators (SVC) and STATCOM: thyristor or converter-based devices with fast, continuous Q control.
6. Tap-changing transformers (OLTC): redistribute Q flow and adjust secondary voltage.
7. Series capacitors: cancel part of line reactance, reducing the reactive drop $IX$.