Dos and don’ts in creating operation logic diagrams for LV and MV switchgear feeders

Switchgear Operation Logic

Dealing with the switchgear operation logic diagrams is like setting the brain functions. It needs vigilance and patience from design engineer supported with hours and hours of testing and operation simulations. This technical article brings some light to the essence of creating operation logic for for low-voltage and medium-voltage switchgear feeders.

The creation of a logic diagram marks the beginning of the preliminary design work for both low voltage (LV) and medium voltage (MV) systems. The initial stage of the design process is referred to as the base design stage, and it is comprised of the phases that are listed below.

1. Preparation of single line diagram: In this diagram, major components of substations are confirmed, such as bus bar scheme, circuit breaker, isolator, short circuit rating, continuous current rating, voltage rating, transformer ratings, etc.

2. Preparation of Relay & Metering Diagram: After completion of the single-line diagram, the next step is to prepare the relay and meter single-line line diagram (R&M SLD). At R&M SLD protection and metering devices and scheme are selected.

Operational Logic For LV and MV Switchgear Feeders:

1. Keep It Simple
2. Standardized the Symbol and Function Code
3. Step-by-Step Guidance
4. Truth Table / Operational Logic Table
5. Presentation Using Logic Gates (AND, OR, NOT, NAND, and NOR Gates)
   1. Case Study #1
   2. Case Study #2
   3. Case Study #3
6. Device Characteristics You MUST Understand
7. Safety Requirements
8. Short Circuit and Rated Current
9. Understand the System Components for Designing the Operational Logic:
   1. Protection Relays
   2. Circuit Breaker
   3. Earthing Switch
   4. Isolator or Draw-Out Mechanism
   5. Local / Remote Operation and Authority Control
   6. Interlocking between Circuit Breaker, Isolator, and Earth Switch

Operational Logic Design

Safe and efficient operation is the key essence while designing the operational logic for LV & MV switchgear. A switchgear operation should be a foolproof design that means any false or wrong operation is prevented or blocked by the system which is referred to by the term “Interlocking”.

Operational aspects that should be considered while designing the system are as follows.

1. Keep it Simple

An ideal operational system should be as simple as possible, simple system design not only create ease of operation but also reduces the chances of mistake. The guideline is to use clear and concise instruction. An ideal system should involve less number of operations.

Figure 1 – Components of MV switchgear feeder

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2. Standardized the Symbol and Function Code

For a given network the right approach is to standardize the symbols and the functional codes for different devices used in the operational logic diagram.

An example of standardization is the American National Standards Institute (ANSI) organization that administrates and coordinates standards and conformity assessment systems. ANSI /IEEE Standard C37.2 Standard that deals with Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.

Following is the list of some device codes used as per ANSI / IEEE Standard:

• 50 – Instantaneous Overcurrent
• 51 – AC Inverse Time Overcurrent Relay
• 52 – AC Circuit Breaker
• 83 – Automatic Selective Control or Transfer Relay
• 86 – Lockout Relay
• 87 – Differential Protective Relay
• 89 – Line Switch

A complete list of ANSI codes is available in the following document: Protection Relay – ANSI Standards

Find below the list of some suffixes:

• B -Bus
• G – Ground or generator
• L -Line
• N – Neutral
• T – Transformer
• U – Unit

Figure 2 – LV Switchgear Single Line Diagram (click to zoom)

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3. Step-by-Step Guidance

To reduce the risk of a mall or wrong operation detailed operation sequences are also created. These sequences define step-by-step operations, authority level, and responsibility to carry out specific tasks. Step by step guide is also referred to as Standard Operating Procedure (SOP).

An SOP is specific to the operation of a given substation or switchgear, industry regulations, and laws. SOP minimizes operational mistakes or hazards, operation of switching devices involves several risks such as.

1. Closing of feeder Circuit Breaker onto earth switch closed (Temporary portable ground or permanent earth).
2. Operation of Isolator on load, i.e. when the circuit breaker is closed.
3. Rack in or rack out the trolley while the circuit breaker is in the close position.
4. Performing the operation without permit or energization without closing all permits.
5. Cancellation of the permit without removing the temporary portable earth.
6. Closing bus coupler without synchronization facility.
7. Allowing to give shutdown at cable compartment, while the cable is live or could be live from the other end and cable is not earthed.
8. Allowing operation of circuit breaker when trip coil or coils are failed.
   Therefore, once the circuit breaker is closed, it can never be opened by the operator or protection device.
9. Loading a transformer or generator more than its capacity.
10. Energization of the feeder without confirming the remote substation earth status.
11. Capacitive voltage indicators are provided in MV switchgear to check the status cable/feeder, an SOP should also define to check and confirm the status of feeder before any operation.
12. MV busbar is usually provided with the voltage transformer, and the LV busbar is provided with the voltmeter hence, to check the busbar voltages should also be incorporated in operational logic.
13. Wrong use of interlock bypass switch.
14. Not considering the addition of fault MVA while multiple sources are connected in parallel.