Sequence Diagrams: A Guide for Power System Engineering Jobs in Bangladesh

 Sequence Diagrams: A Guide for Power System Engineering Jobs in Bangladesh

If you’re an electrical engineer in Bangladesh preparing for job interviews at companies like PDB, PGCB, DPDC, or DESCO, there’s one topic you absolutely must master: symmetrical components and sequence diagrams. Analyzing power systems during unbalanced conditions, like faults, is a critical skill, and employers expect you to know it.

This guide will break down the foundational concepts, from Fortescue’s Theorem to drawing the positive, negative, and zero sequence networks for key power system components. We’ll also embed our detailed video tutorial to give you a step-by-step visual walkthrough.

 

The Problem of Unbalanced Systems and Fortescue’s Solution

In a perfect world, three-phase power systems would always be balanced. But in reality, faults and asymmetrical loads create unbalanced conditions, making direct analysis incredibly complex.

This is where Fortescue’s Theorem comes to the rescue. In 1918, Charles LeGeyt Fortescue presented a method that simplifies this complexity. The theorem states that any set of unbalanced three-phase phasors (be it voltage or current) can be broken down into three sets of balanced, symmetrical components.

These components are:

1. Positive Sequence: Three phasors with equal magnitude, displaced by 120°, rotating in the same sequence as the original system (A-B-C). This represents the normal, balanced operation of the system.

2. Negative Sequence: Three phasors with equal magnitude, displaced by 120°, but rotating in the opposite sequence (A-C-B). The presence of negative sequence components is a clear indicator of an unbalanced system.

3. Zero Sequence: Three phasors with equal magnitude and zero phase displacement (they are in phase with each other). These components are crucial for analyzing faults that involve a connection to the ground.

By separating an unbalanced system into these three simpler, balanced networks, we can analyze each one independently and then superimpose the results to understand the complete picture.

Drawing the Sequence Network for Each Component

To perform a fault analysis, we must draw a per-phase equivalent circuit for each sequence component. These are called sequence networks. Let’s explore how to represent different power system elements in each network.

1. Positive Sequence Network

The positive sequence network represents the system during normal, balanced operation.

• Source (Generator): Represented by its internal induced EMF in series with its positive sequence impedance (Z₁). Since positive sequence currents are balanced and sum to zero at the neutral, the neutral impedance (Zn) does not appear in this network.

• Y-Connected Load: Represented by its per-phase positive sequence impedance (Z₁).

• Transformer: Represented by its per-phase leakage impedance, which is equal to its positive sequence impedance.

2. Negative Sequence Network

The negative sequence network is similar to the positive one, with one key difference.

• Source (Generator): Synchronous machines are designed to produce only positive sequence voltages. Therefore, there are no voltage sources in the negative sequence network. It is represented only by its negative sequence impedance (Z₂).

• Y-Connected Load: Represented by its per-phase negative sequence impedance (Z₂). For static loads and transmission lines, Z₂ is the same as Z₁.For rotating machines, Z₂ is generally different.

• Transformer: Represented by its leakage impedance, as Z₂ is equal to Z₁ for transformers.

3. Zero Sequence Network

The zero sequence network is the most distinct and often the trickiest to draw. It is defined by the path available for zero sequence currents, which are equal in magnitude and phase.

• Source (Generator): Represented only by its zero sequence impedance (Z₀). There is no voltage source.The connection to the neutral is critical; if the generator is grounded through an impedance Zn, an impedance of 3Zn appears in the zero sequence network.

• Y-Connected Load:

  • If the star point is not grounded, there is no return path for the sum of the currents. Therefore, the zero sequence network is an open circuit at the load.

  • If the star point is solidly grounded or grounded through an impedance (Zn), a path exists. The load is represented by its zero sequence impedance (Z₀) in series with 3Zn (if present).

• Three-Phase Transformer: The zero sequence network for a transformer depends entirely on its winding connections and grounding.

  • Grounded Star – Grounded Star (Yg-Yg): A path exists for zero sequence currents on both sides. The network shows a series connection of the transformer’s zero sequence impedance.

  • Grounded Star – Delta (Yg-Δ): Zero sequence currents can flow into the grounded star winding. These currents become trapped and circulate within the delta winding and do not appear on the delta-side transmission lines.The network shows the transformer’s impedance connected to the reference bus on the Yg side and open on the delta side.

  • Ungrounded Star – Any Connection: If a star winding is not grounded, zero sequence currents cannot flow into it. This creates an open circuit in the zero sequence network for that side.

Watch Our Step-by-Step Video Tutorial

Reading the theory is one thing, but seeing it in action makes all the difference. To help you fully grasp how to draw these diagrams for any combination of source, load, and transformer, we’ve created a detailed video tutorial.

 

Conclusion

Mastering the art of drawing positive, negative, and zero sequence diagrams is not just about passing an exam; it’s a fundamental skill for a career in power system protection and analysis. Understanding how currents and voltages behave during faults allows engineers to design robust, safe, and reliable electrical grids.

By understanding Fortescue’s Theorem and the rules for representing each component, you will be well-prepared to tackle complex interview questions and excel in your engineering career in Bangladesh.