Interconnection advantages and real power-frequency balance

An interconnected power system is one in which several generating stations and areas are tied together by transmission lines (tie-lines), so that they all run in synchronism and share load. Compared with isolated systems, it is cheaper and more reliable, and its frequency is held by keeping total generation equal to total load.

Advantages over an isolated system

1. Better reliability: if a unit or plant trips, the others pick up its load through tie-lines, so supply continues.
2. Less reserve capacity needed: spinning and standby reserve is shared by all areas, so each area keeps less idle capacity.
3. Economic operation: the cheapest units (e.g. run-of-river hydro in Nepal) are loaded first and power is sent to where it is needed (economic dispatch across areas).
4. Use of diversity: peak demands of different areas occur at different times, so the total installed capacity required is lower; load factor and plant factor improve.
5. Larger, more efficient units can be installed because the large system can absorb them.
6. Better use of energy resources: surplus hydro energy in the wet season can be exported (e.g. Nepal–India power trade) and imported in the dry season.
7. Better frequency stability: the combined inertia is large, so a load change produces only a small frequency change.
8. Easier maintenance scheduling, since units can be taken out while others supply load.

Real power–frequency balance

At steady state, total mechanical input equals total load plus losses, and frequency is constant. When load changes by $\Delta P_L$:

1. Inertia response: the deficit is supplied from the kinetic energy of all rotors, so speed and frequency fall (swing equation $\frac{2H}{\omega_s}\frac{d^2\delta}{dt^2} = P_m - P_e$).
2. Primary (governor) control: each governor with droop $R$ raises mechanical power by $\Delta P_{m,i} = -\Delta f / R_i$. Load also falls a little with frequency (damping $D$). Steady-state deviation:

$$\Delta f = \frac{-\Delta P_L}{\sum_i \dfrac{1}{R_i} + D}$$

All interconnected units share the load change in inverse proportion to their droop. 3. Secondary control (AGC): in each area an integral controller acts on the Area Control Error

$$ACE = \Delta P_{tie} + B\,\Delta f$$

and moves the speed-changer set points until $ACE = 0$. Then frequency returns to the nominal value (50 Hz in Nepal) and tie-line flows return to schedule, so each area finally supplies its own load change. 4. Tertiary control: economic dispatch resets the unit outputs for minimum cost.

 Load change -> f falls -> Governor opens valve
      ^                         |
      |                         v
   P_gen = P_load  <-  AGC resets speed changer