Mastering DC supply selection schemes for HV control and protection panels

DC Supply Selection Schemes

In this article, we will explore various aspects related to DC supply selection schemes, the rationale behind their necessity, and the importance of redundancy and reliability in maintaining uninterrupted power to control and protection panels. Additionally, we will delve into the concept of pumping action in circuit breakers and the role of anti-pumping circuits in mitigating its adverse effects.

Through understanding these concepts, we can appreciate the significance of effective supply selection and anti-pumping mechanisms in safeguarding electrical systems against faults and ensuring continuous operation.

The effective operation of control and protection panels in electrical systems relies heavily on the selection and management of power supplies. Whether it’s in substations, power plants, or other critical installations, the choice of power supply and the implementation of robust selection schemes are paramount for ensuring system reliability and integrity.

Ok, let’s get into the details!

Table of Contents:

1. Introduction to DC Supply Selection Scheme:
   1. Importance of Supply Selection in Control and Protection Panels
   2. Why Supply Changeover Scheme is Necessary?
   3. Rationale Behind DC Supply instead of AC Supply
   4. Redundancy and Reliability
2. DC Supply Selection Scheme in the Protection Panel:
   1. Dual Power Option for Distance and Backup Relays
   2. Duplicate Protection Schemes
   3. Operation with Both DC Supplies Healthy
   4. Failure of DC Supply 1
   5. Restoration of DC Supply 1
   6. Scenario: DC Supply 1 Healthy, DC Supply 2 Failed
3. Switching Time Vs, the Relay or Device Reset Time
4. Concept of DC Mixing and why It Must be Avoided
5. Anti Pumping Circuit in Circuit Breakers:
   1. Pumping Action
   2. Continuous Cycling / Pumping Action
   3. Implications
   4. Anti-Pumping Action
   5. Understanding the Anti pumping Circuit
   6. Components of High Voltage Circuit Breaker
   7. Function of Anti-Pumping Relay
   8. Effectiveness of Anti-Pumping Relay
   9. Simulation of Anti-Pumping Relay
   10. Conclusion
6. Understanding Switches and Rotary Changeover Switches in Electrical Circuits:
   1. Varieties of Rotary Changeover Switches
   2. Range of Options
   3. Contact Material and Design
   4. Gold Plating Option
7. BONUS! Download Complete Drawings of 132kV GIS Line Relay Panel and GIS Control Cubicle (PDF)

1. Introduction to DC Supply Selection Scheme

1.1 Importance of Supply Selection in Control and Protection Panels

Supply selection is a critical feature in control and protection panels within substations, power plants, and various electrical installations. This feature addresses two fundamental questions:

1. The necessity of a supply changeover scheme, and
2. The rationale behind selecting DC supply for control and protection operations.

1.2 Why Supply Changeover Scheme is Necessary?

In substation and power plant environments, the primary functions include voltage transformation, switching operations, voltage and reactive power control, and most importantly, protection of transmission lines and associated equipment. However, during fault conditions, the voltages drop while the currents surge.

If alternating current (AC) were utilized for control and protection devices, the supply voltages would also dip during faults.

This scenario compromises the performance of control and protection systems, potentially leading to delayed or erroneous responses, jeopardizing the stability and integrity of the electrical grid.

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1.3 Rationale Behind DC Supply instead of AC Supply

DC supply is favored for control and protection operations due to its inherent stability and resilience during fault conditions. Unlike AC, DC supply remains unaffected by fluctuations in voltage and current, providing a consistent and reliable power source for control and protection devices.

In substations, AC supply is typically converted to DC through rectifiers known as DC chargers. These chargers not only supply power to control and protection devices but also charge the battery banks simultaneously.

In the event of AC supply failure or voltage dips, the batteries serve as backup power sources, ensuring uninterrupted operation of control and protection systems.

Figure 1 – AC supply distribution for 132 kV line relay panel (click to zoom)

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1.4 Redundancy and Reliability

Substations are equipped with redundant systems to enhance reliability and minimize downtime. This redundancy is exemplified by the provision of two battery banks and chargers. In the event of a failure in one DC system, the second system can seamlessly take over, ensuring continuous operation of control and protection systems.

Furthermore, circuit breakers at high voltage levels are equipped with redundant trip coils. These coils are vital for the swift and reliable tripping of circuit breakers during fault conditions. Delayed tripping can lead to excessive thermal and dynamic stresses on power system components, potentially causing damage or even cascading failures.

By utilizing DC supply and incorporating redundancy features, substations and power plants can effectively mitigate risks associated with voltage fluctuations and equipment failures, ensuring continuous and safe operation of critical infrastructure.

Figure 2 – DC supply distribution for 132 kV line relay panel (click to zoom)

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2. DC Supply Selection Scheme in the Protection Panel

In the realm of electrical power distribution, ensuring the reliability of protection systems is paramount. The schematics of a 132kV Line protection panel often incorporate sophisticated mechanisms to safeguard critical components such as distance relays and backup overcurrent relays.

One common strategy involves implementing a dual DC power selection scheme, as depicted in Figure 3. This scheme ensures that if one DC supply fails, the other supply automatically takes over, thus maintaining uninterrupted operation of the protection panel.