Learn how to read and analyze control circuits of MV gas insulated switchgear (GIS)

Tripping, disconnector, ground-switch…

Previously, an introductory part of the GIS and its components were discussed to pave the way for the GIS control circuits. Moreover, the single-line diagram example for a double-bus and a single breaker clarifies several concepts that are for the GIS only. It is challenging to comprehend and grasp the control circuits due to the interrelated components and functions that impose additional interlock schemes and control functions.

Hence, a brief description of drawing tips and drawing examples was elaborated before delving into the actual article topics (control circuits). Broadly, control circuits are categorized into:

• AC & DC auxiliary circuits
• DC of secondary (close/trip) for CB, disconnector, and ground switches
• Indication and alarm circuits
• SCADA indication and alarm circuits
• Equipment coil contacts arrangements and components details
• AC of secondary (CT & PT)

The first group shares similar concepts with the MV switchgear circuits in previous articles, so they were mentioned briefly. The second group includes many circuits-namely, breaker closing circuit, breaker trip circuit, and disconnector/ground switch close/open circuit. The breaker closing circuit requires so many conditions fulfillment before closing.

As a result, it is expected to have a complex circuit as we experienced in the last article.

Of course, the MV switchgear two articles are to be covered first for easier understanding. You can consider it as a series of articles that tackle the MV switchgear and then move to the GIS, so the difficulty level increases steadily. However, once you master it, it becomes an invaluable tool for your practical expertise for design, maintenance, and troubleshooting.

Table of contents:

1. Circuit Breaker Tripping Circuit
2. Disconnector/Ground Switch
   1. Control Circuit
   2. Interlocking Circuit
   3. Motor Circuit
   4. Double Operation Circuit
3. Bay Control-Protection Unit (BCPU) and Alarm Circuit
   1. Indication Lamp Circuit
   2. SF6 Gas Pressure Alarm Circuit
   3. Annunciator Alarm Circuit
   4. Bay Control-Protection Unit (BCPU)
4. Secondary AC Circuits
5. Summary

1. Circuit Breaker Tripping Circuit

In the previous article, there was a general closing command that must be pressed along with the breaker selector pushbutton to close the breaker in the emergency mode so that any accidental activation of the breaker selector pushbutton does not result in a breaker close. The same philosophy is implemented in the trip circuit.

This is illustrated in Figure 1, where the S1 pushbutton represents the breaker tripping in the emergency mode and K302 is the contactor energized when the general trip command pushbutton is pressed. This is identical to what was witnessed in the closing circuit. Consequently, the coil K12 circuit is completed such that its contactor closes in which the DC supply reaches the trip coil (Y2).

Again, the SF6 condition plays a major role in the trip circuit, too.

The gas pressure low-stage 2 comes into play if the gas pressure is compromised as shown in the figure. The mechanism is exactly like what was in the closing circuit, where the off-delay timer activates the K80 coil in five seconds delay, thereby preventing the breaker tripping command.

Simply, their circuits go to the BCPU and wait for the trip command; then, it reaches the very same point as the emergency mode does at the K12 contactor.

Figure 1 – Trip Circuit Control

The breaker Q0 is a control element as well as a protective element. The control side was mentioned above, and the protective part is to be discussed next. The contacts that are in parallel inside the dashed-line box represent the breaker failure protection, busbar protection, and two relay panels for the transformer protection. Any function can trip the breaker open should the protection function condition fulfilled.

The breaker failure protection has two contacts: lockout relay (86) and self-reset relay (94).