ACB Circuits with Protection Relays

This segment introduces the application and functioning of protection relays in electrical systems, using an ACB (Air Circuit Breaker) incomer feeder as an example. It explains ANSI codes, the role of protection CTs, and differences between electromechanical and numerical relays. Emphasis is placed on modern relay features, including communication capabilities and their importance in smart grid systems.

This section introduces the CDG31 electromechanical overcurrent relay (Alstom make), explaining its construction, model variants (3DH and 3DV), and typical connection logic using its pin diagram. It highlights how current input is fed into the relay and how the normally open (NO) output contacts are used in parallel, to trip circuit breakers or provide fault indications. The importance of datasheets in understanding pin configurations is emphasized for accurate schematic implementation.

This part focuses on the vertical variant of the CDG31 relay (3DV model), describing its internal contact configuration and current input coil terminals. It compares electromechanical relays like CDG31 with numerical relays such as the Siemens 7SJ60, emphasizing their capabilities. The section also introduces the CDG11 single-pole relay, typically used for standby earth fault protection. Concepts such as “2 O/C + 1 E/F” connection configuration and neutral CT usage for backup protection are briefly explained.

The CAG14 relay provides high impedance restricted earth fault (REF) protection for generators, transformers, reactors, and bus bars. It uses a stabilizing resistor connected in series with the relay to prevent false trips and ensure sensitivity only to internal faults. The resistor value depends on CT rating and system parameters for example, 470Ω for 0.5A CT, 220Ω for 1A CT, and 47Ω for 5A CT.

Under voltage protection relays, like the VAGM22 (Alstom) and 7UG (Siemens), monitor power supply voltage levels to detect under voltage conditions. VAGM22 is an instantaneous under voltage relay rated for 110V AC, so a potential transformer (PT) is required to step down from 415V line voltage. It monitors two phases (e.g., R-Y), so at least 2 relays are needed to cover a 3-phase system. 7UG relays can be directly connected to 415V and change state when voltage drops below a threshold (typically 80% of rated voltage).

A Master Trip Relay is used when multiple protection relays monitor a feeder. All output contacts from the protection relays are connected in parallel to energize the master trip relay coil (e.g., VAJH13). Once energized, its NO contact initiates tripping of the breaker, and its NC contact ensures the breaker can't close during a fault.

There are two types:

· Hand reset type – must be manually reset after the fault is cleared (used in lockout applications, like 86 relays).

· Self-reset type – resets automatically after the fault is cleared.

The VAJH13 has terminals 9 & 10 for the DC voltage input coil and offers 1NC + 3NO output contacts.

Trip Circuit Supervision (TCS) relays are used to monitor the health of the trip coil in a circuit breaker. If the trip coil fails, the relay detects this condition and prevents the breaker from closing as a failed trip coil would stop the breaker from operating during a fault.

Key types:

· VAX21 (Areva): Has 1 coil, monitors the trip circuit only when breaker is closed.

· VAX31 (Areva): Has 2 coils, monitors trip circuit in both open and closed conditions of the breaker.

A healthy trip coil allows an indicating lamp to glow. If unhealthy, the relay changes state, turning off the lamp and interlocking the breaker closing circuit.

In this lecture we are going to study the connection of CT shorting Terminals.
1.Closed Type CT Terminal

2.Open Type CT Terminal

and its methodology

This section explains how overcurrent (O/C) and earth fault (E/F) protections are implemented in scheme drawings using the CDG31 relay. The relay is current-based and receives input from protection-class CTs. The section also explains CT wiring techniques, particularly the widely-used 2 O/C + 1 E/F configuration, in which the R and B phase CTs are used for overcurrent detection and the Y phase for earth fault detection. Differences in coil connections between phases are highlighted, emphasizing practical wiring schemes in protection logic.

This section introduces the Siemens 7SJ610 numerical protection relay, which provides both overcurrent (O/C) and earth fault (E/F) protection. The relay receives current inputs from protection-class CTs. It has designated terminals for phase currents (IA, IB, IC) and a separate input (310) for ground fault measurement via the star point or a neutral CT. The section explains that while schematics may differ in layout, the wiring logic remains consistent with electromechanical relays. Additionally, numerical relays offer multiple protections in one unit, improving system efficiency and cost-effectiveness.

1. Restricted Earth Fault (REF) protection using the CAG14 relay and Class PS CTs.

2. Standby Earth Fault (SBEF) protection using a numerical relay (7SJ6612).

In REF protection, Class PS CTs from each phase (T7 to T10) and from the transformer neutral are connected in parallel to the input of the CAG14 relay. A stabilizing resistor (R57) is also connected in series with the relay to ensure stability during external faults. In SBEF protection, the numerical relay 7SJ6612 uses terminals Q7 and Q8 to receive input from a neutral-mounted protection CT, offering backup protection in case primary earth fault protection fails.

This section discusses how voltage-based protection functions like:

· 27 (Under Voltage)

· 59 (Over Voltage)

are achieved using protection relays such as the VAGM22 (electromechanical) and P3U30 (numerical, by Schneider).

Both relay types require 110V AC voltage input to their voltage coils, typically from the protection core of a Potential Transformer (PT).

In the case of the P3U30 numerical relay, the configuration includes:

· 3 current coils for phase fault protection

· 1 ground fault current coil

· 3 voltage coils for standard voltage protections

· 1 extra voltage coil (V3) used for synchronization functions

This enables the relay to provide both current-based and voltage-based protections in one compact setup.

This section explains how under-voltage (U/V) protection is implemented using the VAGM22 relay and VTT11 timer: The VAGM22 relay offers instantaneous under-voltage protection, useful for quick actions like motor protection from single phasing. For Incomer ACB tripping, instantaneous action is not ideal as temporary voltage dips may cause false trips. A time delay is introduced using the VTT11 static timer (Alstom make) to differentiate between a momentary dip and a sustained under-voltage condition (typically 2–3 seconds). The relay logic ensures that tripping occurs only when:

o Both MPCB and MCB auxiliary contacts are closed (indicating the PT supply path is healthy).

o The U/V condition persists long enough for the timer to complete its cycle.

If voltage recovers during the timer countdown, the breaker is not tripped.

In an ACB trip circuit, output contacts from various protection relays (like overcurrent, earth fault) are connected in parallel to a master trip relay coil. When any relay detects a fault, its contact energizes the master trip relay, which then trips the ACB. Numerical relays can combine multiple protection outputs into one contact or use separate contacts for different faults. Some numerical relays have an 86 (lockout) function, whose output contact can directly trip the ACB and provide trip indication and interlocking signals.

Numerical relays provide multiple protections and can also supervise the trip circuit (Trip Circuit Supervision - TCS). Two coils of a TCS relay are wired in series with the breaker’s auxiliary contacts (NO and NC) and the trip coil. This setup ensures a conducting path through the trip coil whether the breaker is open or closed. If the trip coil fails, the path breaks, causing the TCS relay to change state. Its output contact signals the trip circuit status via an indicator lamp and interlocks the breaker closing coil to prevent closing when the trip coil is faulty.

In this lecture, we are going to study the circuit development of a 4-pole ACB incomer without releases. Based on the customer requirements, protections like Overcurrent, Earth Fault, Restricted E/F, Standby E/F, under voltage, Trip Circuit Supervision, and Lockout are provided. Numerical relays Siemens 7SJ600 and 7SJ610 are used for current-based protections, while electromechanical relays VAGM22 and VAJH13 are used for under voltage and lockout. The circuit includes CTs and PTs, feeds current to an APFC panel, and incorporates multifunction and digital KWH meters with a communication port.

"In this lecture, we are placing phase indication lamps on the secondary side of the PTs to indicate voltage availability. Red, Yellow, and Blue lamps are used for R, Y, and B phases respectively, each connected between phase and neutral. For under voltage protection, we use two VAGM22 electromechanical relays — K66 and K67 — monitoring the RY and YB phases. These relays require a 110V AC supply, sourced from the PT secondary. We position the relay coils with cross-references and use their NO/NC contacts in the closing and tripping logic. At this stage, only input connections are completed; output connections will be discussed later."

"In this part of the lecture, we focus on providing current input to the APFC panel and configuring protection CTs. CTs T10 to T12 are used to feed current signals to the APFC relay. For restricted earth fault protection, Class PS CTs T7 to T9 are connected to the Siemens 7SJ610 numerical relay, designated K55. The relay’s coil connections include stabilizing resistance and a varistor to protect the CTs and wiring from voltage surges. Additionally, CTs T4 to T6 provide inputs to the same relay for overcurrent and earth fault protection in a 2 overcurrent plus 1 earth fault configuration."

"In this lecture, we cover the stand-by earth fault protection using the Siemens 7SJ600 numerical relay, designated K56, which receives current input from a neutral CT of the upstream transformer. The circuit also feeds a space heater bus by tapping the R phase and neutral to supply 240V AC through a control fuse. The ACB control circuit operates on an external 110V DC supply, except for the spring charging motor, which runs on 240V AC. Control supplies are protected and distributed via MCBs and control buses. Additionally, electromechanical relay VAJH13 is used for the master trip lockout function, and a VTT11 timer block is included for true under voltage protection."

"In this lecture, we discuss the practical layout of the schematic developed so far. While component placement in the physical setup may vary, maintaining correct electrical connections is essential. Metering CTs and meters are placed on Sheet 1, PTs on Sheet 2, with voltage input connected via page connectors whose cross references are assigned for easy navigation. Metering CTs are located closest to the ACB, protection CTs farthest, and PTs outside the CT zone for proper line-side tapping. Relay blocks are grouped on Sheets 3 and 4, with relay coils cross-referenced on other sheets to maintain clarity and organization in the schematic.

"In this lecture, we will learn how relay output contacts are used in logic development for closing and tripping circuits. Normally Open (NO) contacts from relays K66 and K67 are referenced in the closing circuit, while Normally Closed (NC) contacts are used in the tripping circuit. Spare contacts are wired to terminals for future use. Numerical relays K55 and K56 feature configurable relay outputs and binary inputs, with some outputs assigned for trip circuit supervision and upstream breaker tripping. The coil and contacts of the master trip lockout relay K21 are also cross-referenced for tripping logic. Unused relay contacts are wired as spares. The ACB closing, tripping, and indication circuit logic will be addressed in upcoming lectures."

In this lecture, we are going to study the control wiring connections for the ACB spring charging motor, closing coil, and tripping coil.

The spring charging motor is powered by a 240V AC external supply via page connectors with proper fusing. The closing coil operates on 110V DC and includes conditions for healthy line voltage monitored by NO contacts of under voltage relays K66 and K67. The tripping coil also uses 110V DC and incorporates true under voltage protection through the timer relay K31 (VTT11), which delays tripping on under voltage conditions.

We also cover trip coil supervision using binary inputs B11 and B12 of the K55 relay, monitoring pre-close and post-close states. Current-based fault protections are provided by relays K55 and K56, whose outputs trigger the master trip lockout relay. Additionally, the VAJH13 relay K21 output is integrated into the trip circuit for lockout functions.

The ACB auxiliary contact block is powered by 110V DC via page connectors. Indication lamps for On, Off, and Trip states are connected to the ACB auxiliary contacts and the K21 VAJH13 relay output. Multipliers like K3S, K8, and K2 manage the ACB service and ON positions. Relay output B02 from K55 shows trip circuit supervision status. The schematic also includes fault and trip circuit healthy conditions, completing the 4-pole ACB incomer control circuit without releases.