Short Answer:

A slack bus in power systems is a special type of bus used during load flow analysis to balance the total active and reactive power in the network. Since the actual power losses in the system are not known before the analysis, the slack bus supplies or absorbs the difference to maintain power balance.

At the slack bus, the voltage magnitude and phase angle are fixed, while the active and reactive powers are calculated. It acts as a reference for all other bus voltage angles and ensures the system remains stable and balanced during load flow calculation.

Detailed Explanation:

Slack bus in power systems

In power system analysis, especially during load flow studies, a slack bus (also called reference bus or swing bus) is an essential element. It is one of the three main types of buses used in a system, the other two being PV buses and PQ buses. The slack bus has a unique role that makes the entire calculation process possible and meaningful.

In a power system, the total generation must equal the total load plus system losses. However, these system losses are not known before performing load flow analysis. To handle this, one bus is chosen as the slack bus, which absorbs or supplies the extra power (both active and reactive) needed to maintain the power balance.

Characteristics of slack bus

1. Fixed voltage and angle:
   At the slack bus, the voltage magnitude (|V|) and voltage angle (δ) are known and fixed before starting the analysis. These values are usually set to 1.0 per unit and 0 degrees, respectively.
2. Unknown P and Q values:
   The active power (P) and reactive power (Q) are not specified for the slack bus. These are calculated as part of the load flow solution to balance the system.
3. Reference for phase angles:
   Since voltage angle differences are used in calculating power flows, the slack bus provides a reference point for all other buses in the system.
4. Balances power mismatch:
   Any mismatch between the scheduled generation and total system load is compensated by the slack bus. It adjusts its generation output automatically to ensure that Kirchhoff’s laws are satisfied.

Why slack bus is needed

During a power flow calculation, each bus must follow the principle:

Total Power Generation = Total Power Load + Power Losses

But losses are not known in advance. If we assign fixed values of power at all buses, the system may become mathematically unsolvable due to imbalance. So, one bus must be left flexible — that is the slack bus.

The slack bus fills in the unknowns by adjusting its generation to match the losses and maintain overall system balance. Without it, the system equations would be underdetermined, and the load flow problem could not be solved.

Selection of slack bus

• Usually, a bus connected to a large generating station is selected as the slack bus.
• It should have enough capacity to absorb or supply the extra power needed.
• In large systems with multiple generators, one is selected as slack while others are treated as PV buses.

Role in real systems

While the slack bus is a mathematical concept, it also reflects reality to some extent. In actual systems, automatic generation control (AGC) adjusts generator outputs in response to system demand changes, similar to the slack bus behavior.

However, in practice, the mismatch is often shared among multiple generators, but for simplicity, the load flow analysis assumes one slack bus to handle it.

Conclusion:

A slack bus in power systems is a specially chosen bus that balances the active and reactive power in the network during load flow studies. It has fixed voltage and angle values, and its power output is adjusted to match system losses. The slack bus plays a critical role in solving the power flow equations and maintaining the overall power balance of the system.