The specified quantities of load bus are

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$P$ and $Q$

Explanation: In power system load flow analysis, buses are categorized based on the known (specified) variables. A Load Bus (also known as a PQ Bus) is defined as a bus where the real power ($P$) and reactive power ($Q$) are specified, while the voltage magnitude ($|V|$) and voltage phase angle ($\delta$) are unknown and need to be calculated.

Working Principle: Load buses represent consumer load centers where the power demand is fixed but the bus voltage magnitude and phase vary depending on the network performance. Newton-Raphson or Gauss-Seidel load flow methods use the specified $P$ and $Q$ to iteratively solve for $V$ and $\delta$ by balancing the power injection equations at each node.

Key Formulas: $P_i = |V_i| \sum_{j=1}^{n} |V_j| |Y_{ij}| \cos(	heta_{ij} - \delta_i + \delta_j)$ — Real power balance equation; $Q_i = -|V_i| \sum_{j=1}^{n} |V_j| |Y_{ij}| \sin(	heta_{ij} - \delta_i + \delta_j)$ — Reactive power balance equation

Key Points: Load buses constitute the majority of buses in a typical power system network.; The number of equations to be solved at a PQ bus is two, matching the two unknown variables $|V|$ and $\delta$.; Slack bus (Swing bus) has $|V|$ and $\delta$ as specified, while Generator buses (PV buses) have $P$ and $|V|$ as specified.

Advantages: Reflects real-world consumer behavior where demand is predefined.; Simplifies the mathematical structure of the Y-bus matrix solution.

Disadvantages: Voltage stability issues can arise if reactive power support is insufficient at the load bus.

Applications: Distribution network modeling.; Power system steady-state load flow analysis.

Additional Info: Standard power system convention lists 'Slack Bus' (V, $\delta$), 'Generator Bus' (P, V), and 'Load Bus' (P, Q).. Option D is mathematically identical to Option B in many textbooks, but B is the standard convention for representing the P and Q quantities.

Answer: B is correct — A Load Bus is defined by specifying the real power (P) and reactive power (Q) injections, requiring the solution of voltage magnitude and phase angle.

Exam Tip: Remember the 'P-Q' mnemonic for Load Bus: 'Power and Quantities' (P & Q) are known, and 'Voltage Vector' (V & $\delta$) is unknown.

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