Interconnection importance, P-f and Q-V balance

Interconnection means tying several generating stations and control areas together by transmission lines so that they work as one synchronous system and jointly supply the total load.

Importance of interconnection

1. Reliability: if a generator or plant trips, others supply its load through tie-lines.
2. Reduced reserve: reserve capacity is shared, so less idle plant is needed.
3. Load diversity: different peak times mean lower total installed capacity and higher load factor.
4. Economy: economic dispatch uses the cheapest sources first; surplus energy can be exchanged (e.g. Nepal exporting wet-season hydro power to India).
5. Large efficient units become practical.
6. Frequency stability: large total inertia keeps frequency deviations small.
7. Maintenance can be planned without load shedding.

Real power – frequency balance

At constant frequency, total generation equals total demand plus losses: $\sum P_G = \sum P_D + P_{loss}$.

• A sudden load increase is first met from the kinetic energy of all rotating machines. By the swing equation $\frac{2H}{\omega_s}\frac{d^2\delta}{dt^2} = P_m - P_e$, they decelerate and frequency falls.
• Primary control: speed governors increase turbine input. With droop $R$ and load damping $D$, the steady-state deviation is $\Delta f = -\Delta P_L/(\sum 1/R_i + D)$. All connected units share the change.
• Secondary control (AGC): an integral controller acting on $ACE = \Delta P_{tie} + B\,\Delta f$ adjusts the speed changers until frequency returns to 50 Hz and tie-line flows return to schedule.

So, real power mismatch → frequency change, and frequency is a system-wide quantity.

Reactive power – voltage balance

For a line of reactance $X$:

$$Q_{12} \approx \frac{V_1(V_1 - V_2\cos\delta)}{X} \approx \frac{V_1\,\Delta V}{X}$$

• The voltage difference between two buses depends mainly on the reactive power flowing between them.
• If reactive demand at a bus exceeds local supply, Q must flow in over the line reactance, and the bus voltage falls. Surplus Q (light load, line charging) makes the voltage rise.
• Voltage is a local quantity; Q cannot be transmitted far because of high $I^2X$ losses, so it is balanced locally.
• Means of control: generator excitation with AVR, shunt capacitors and reactors, synchronous condensers, SVC/STATCOM, and on-load tap changers.

So, reactive power mismatch → voltage change.