Interconnected system load-frequency and Q-V control

An interconnected power system links many generating stations and areas by tie-lines so that they operate in synchronism. Its two main control loops are the load-frequency (P–f) loop and the reactive power–voltage (Q–V) loop.

Advantages

• Higher reliability; a failed unit's load is picked up by others.
• Less reserve capacity, as reserve is shared.
• Load diversity gives lower installed capacity and better load factor.
• Economic dispatch of the cheapest units; power exchange between areas.
• Larger combined inertia gives better frequency regulation.

Disadvantages

• Higher fault levels, so higher-rated breakers are needed.
• A fault can cascade and black out a large area.
• Stability and protection coordination become more complex.
• Costly tie-lines and communication/SCADA systems.

Control schematic

 P-f loop (frequency)
                   Speed changer (AGC set point)
                            |
 Speed/frequency -> [Governor] -> [Valve] -> [Turbine] --Pm--> [Generator]
        ^                                                          |
        +------------- speed (frequency) feedback ----------------+

 Q-V loop (voltage)
 Vref --> (+/-) --> [AVR] --> [Exciter] --If--> [Generator field]
            ^                                          |
            +-------- measured terminal voltage V -----+

Load / frequency control (P–f loop)

1. A load increase makes $P_e > P_m$. The deficit is taken from rotor kinetic energy, so speed and frequency fall.
2. The speed governor senses $\Delta f$ and opens the steam/water valve, increasing $P_m$. With droop $R$, the steady-state change is $\Delta f = -\Delta P_L / (\sum 1/R + D)$; a small error remains.
3. Automatic Generation Control (AGC) adds an integral of the area control error $ACE = \Delta P_{tie} + B\Delta f$ to the speed-changer set point, bringing $\Delta f$ and tie-line deviation to zero.

Reactive power / voltage control (Q–V loop)

1. The terminal voltage is measured, rectified and compared with $V_{ref}$.
2. The error is amplified by the AVR and drives the exciter, which changes the field current $I_f$ and hence the internal emf $E_f$.
3. Higher $E_f$ makes the generator supply more Q and raises the bus voltage; lower $E_f$ absorbs Q and lowers it.
4. In the network, voltage is also held by shunt capacitors/reactors, synchronous condensers, SVC/STATCOM and tap-changing transformers.

Because $X \gg R$, P depends mainly on $\delta$ and Q mainly on $|V|$, so the two loops can be designed and operated independently.