CB Control Schematics
This technical article deals with schematics of close and charging motor control circuits for a medium-voltage circuit breaker. The proper functioning of MV switchgear depends on control circuits. For the switchgear to operate properly, the integrity of these control circuits is crucial, hence records of commissioning and maintenance activities are crucial for troubleshooting procedures.
In order to gain the skills of troubleshooting and following the sequence of operations, one must also be able to understand and interpret control circuits.
Despite having numerous control circuits, all of them ultimately boil down to the CB close/trip coils. It’s worth mentioning that switchgear designs must incorporate interlock systems in order to prevent unintentional closures that could compromise the security of both people and equipment.
Protection relays for medium-voltage circuit breakers are not built within the circuit breaker like those for low-voltage breakers, nor are they powered by the primary circuit’s current. The safety relays are offered externally. Because of this, medium voltage circuit breakers rely on control power to precisely and consistently trip or open the breaker in the case of a malfunction.
Since the availability of control power is critical to the protective function of a medium-voltage circuit breaker, the control power source is extremely reliable.
While the protective relay in medium voltage applications requires control power, the typical medium voltage breaker is closed and opened via mechanical springs in the breaker and there is a manual close and trip button on the face of the breaker along with a flag indicating breaker status.
The operating mechanism is a stored-energy mechanism. The closing spring is charged either electrically or manually. It latches tight at the end of the charging process and serves as an energy store. The force is transmitted from the operating mechanism to the pole assemblies via operating levers.
To close the breaker, the closing spring can be unlatched either mechanically by means of the local “ON” pushbutton or electrically by remote control. The closing spring charges the opening or contact pressure springs as the breaker closes. The now discharged closing spring will be charged again automatically by the mechanism motor or manually.
Then the operating sequence OPEN-CLOSE-OPEN is stored in the springs. The charging state of the closing spring can be checked electrically by means of a position switch.
Figure 1 shows the manual close button, the manual trip button, the flag indicating breaker open or closed, the flag indicating charging spring charged or discharged, and other elements of a circuit breaker.
Figure 1 – Circuit Breaker Spring-Charging Mechanism
- Closing Spring
- Latch Check Switch (To Rear of Motor Cutoff Switch)
- Motor Cutoff Switch
- Closing cam
- Spring Release Assembly
- Shunt Trip Assembly
- Closing Spring
- Reset/Opening Spring
- Manual Charge Socket
- Ratchet Wheel
- Operations Counter
- Charging Motor
Figure 2 shows the typical close and charging motor control circuit for a power circuit breaker. Table 1 defines some of the functions of the contacts in the control schematics of Figures 2 and 3 (see below).
The function of the charging motor (M) is to compress the main closing spring which is the mechanical stored energy mechanism. The energy required to trip or open the circuit breaker is provided by the tripping spring, while the energy required to close the circuit breaker is supplied by the closing spring.
The control circuit’s logic is served by the anti-pump relay (Y), which prevents a continuous electrical close signal from causing the circuit breaker to repeatedly close after receiving a trip signal. Solenoids are used to power the breaker’s electrical operation. Depending on which solenoid is triggered, the close and trip springs will either close or open the breaker.
Learn much more about how anti-pump relay works.
Table 1 – Device descriptions for Figures 5 and 6
|Device Designation||Device Description|
|LS||Spring charge limit switch shown with breaker closing springs discharged|
|M||Breaker closing springs charging motor|
|52/a||Breaker normally closed auxiliary contact|
|52/b||Breaker normally open auxiliary contact|
|LCS||Latch check switch|
|CS/C||Control switch close contact|
|CS/T||Control switch open contact|
With the charging spring discharged, the spring charge limit switch (LS) is closed between the charging motor (M) and the secondary stab pin 9. This applies DC voltage to the charging motor and runs the charging motor until the closing springs become charged.
The LS contact will become active as the closing springs charge. This allows the charging motor and secondary stab pin 9 to make contact, deactivating the spring charging motor (M). The closing spring will discharge as soon as the breaker is tripped and then reset, and the LS contact between the secondary stab pin 9 and the charging motor will automatically close, recharging the closing spring.
With the breaker open, the contact 52/b is closed. The 52/b contact is an auxiliary contact that simply mirrors breaker status. When the breaker is open, the 52/b contact is closed, and when the breaker is closed, the 52/b contact is open. The normally open spring-charged limit switch (LS) contact below the 52/b contact is closed when the closing spring is charged.
This is a normally open contact off the LS mechanism. In order to ensure that there is mechanical force available to close the breaker, this contact is only closed when the closing spring is charged.
Downstream of the 52/b contact is the latch check switch (LCS). The circuit breaker can be used for instantaneous reclosing thanks to the latch check switch. Before allowing the instantaneous re-closure, the switch makes sure that the mechanical mechanism has been reset and is prepared for a reclose following a breaker trip.
Downstream of the latch check switch (LCS) is a normally closed contact from the anti-pump relay (Y). The anti-pump relay (Y) acts as a one-shot device.
Looking at Figure 2, you can see that the anti-pump relay is driven by the close signal on stab pin 11 and the position of the charging spring limit switch normally closed contact.
Figure 2 – Circuit breaker close circuit schematic
Before the breaker is closed, the anti-pump relay is not yet energized as the charging spring limit switch is open. Once the breaker closes, the closing spring discharges. This closes the normally closed charging motor limit switch LS which energizes the anti-pump relay coil (Y). The Y relay seals itself in with the Y relay normally open contact, in parallel with the LS normally closed contact.
The normally closed contact from the Y relay prevents the close coil from being re-energized until the anti-pump relay Y resets. What resets the Y relay is the removal of the close command from the control switch contact CS/C.
The normally closed contact from the Y relay is closed because the anti-pump relay is no longer powered. Stabil pin 11 receives voltage when the control switch close contact (CS/C) is closed. The close coil (CC) is energized if the 52/b contact, LS contact, LCS contact, and Y contact are all closed. The 52/b contact automatically opens when the breaker closes, cutting off power to the close coil.
Figure 3 shows the typical trip control circuit of a circuit breaker. Please consult the Table 1 (see above) as it defines the functions of the contacts used in the control schematics of Figure 3.
The trip coil of the breaker is connected in series with auxiliary 52/a breaker contacts so that it only energizes when the breaker is closed and needs to be opened or tripped. This prevents damaging of the trip coil should the trip signal remain on the breaker trip coil after the breaker opens and the trip coil no longer needs to be energized but the control switch or protective relay contact is still closed.
Figure 3 – Circuit breaker trip circuit schematic
Either the control switch trip contact (CS/T) closing or any closing protection relay contacts (PR) will energize the trip control coil. The green light is powered by the normally closed auxiliary breaker contact (52/b), so whenever the breaker is opened, the 52/b contact closes and the green light becomes active, signaling that the breaker is open.
Notice that the red light is not only fed from two 52/a contacts in series but is also fed through the breaker trip coil between the two 52/a contacts.
If you approach this breaker and observe the red light on and the green light off, you will know it is closed, have control power available to trip it, and continuity through the trip coil, which confirms the trip coil’s integrity.
If you approach the breaker and neither the red nor the green light is on, what does that mean?
One of two things is implied by this. Either the breaker is closed but the trip coil has failed and is open, or we have opened our fuse to the breaker’s trip circuit. It is crucial to be alert to this condition and raise the alarm so that the problem can be fixed. A contact closure from the protective relay (PR) or the control switch trip contact (CS/T) will NOT open or trip the breaker if the breaker is closed and the trip coil is open.
This indicates that we have lost any overcurrent, differential, or other protection that this breaker may have been providing, and this issue needs to be fixed right away.
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This is a good moment to explain a word of caution about the location of the red light! Notice that when the breaker is closed, the red light is illuminated by allowing current to flow through the trip coil. To prevent nuisance tripping of the breaker, the resistance that the red light presents must be much higher than the resistance that the trip coil presents to prevent this current from activating the trip coil.
To stop the breaker from accidentally tripping, the input resistance of this DCS input must be significantly higher than the resistance of the trip coil. Additionally, it is standard procedure to connect two DCS voltage inputs that are in series with one another in parallel with the trip contact when the breaker is essential to the proper operation of the plant.
This is done so that, in the event one of the two cards fails shorted, it does not cause a nuisance trip of the breaker.
Some of the specifications for the switchgear assembly’s construction are defined by IEEE standards. The types of switchgear that are being considered are divided up into separate IEEE standards. According to IEEE standard C37.20.1, low voltage power circuit breakers must comply. For metal-clad (MC) switchgear IEEE C37.20.2 applies.
Figure 4 – Metal-clad (MC) switchgear
For metal-enclosed interrupter IEEE C37.20.3 applies. Unless the customer specifies other arrangements, the phase arrangement on a three-phase assembled switchgear bus and connection are set up as phase A, phase B, and phase C from front to back, or top to bottom, or left to right as viewed from the front of the switchgear.
When viewed from the front of the switchgear, the polarities on the buses and connections of DC assembled switchgear are set up as positive, neutral, or negative from front to back, or from top to bottom, or from left to right.
Below is a description of some of the auxiliary contacts in a breaker, starter, or cubicle. A form “a” contact is a normally open contact while a form “b” contact is a normally closed contact. A form “c” contact has a normally open and normally closed contact with one side being common.
Below are some specific standard nomenclatures for auxiliary contacts along with their description.
Auxiliary contact 52/a -It’s opened when the breaker is in the de-energized or non-operated position. This is an auxiliary contact mounted directly on the breaker indicating status of the breaker.
Auxiliary contact 52/b – It’s closed when the device is in the de-energized or non-operated position. This is an auxiliary contact mounted directly on the breaker indicating status of the breaker.
Auxiliary contact 52/aa – It’s opened when the operating mechanism of the main device is in the deenergized or non-operated position. This is an auxiliary contact mounted directly on the breaker indicating status of the breaker operating mechanism.
This is also referred to as an early out contact because it operates earlier than the 52/a contact because it is derived from the operating mechanism rather than the breaker status itself.
Auxiliary contact 52/bb – It’s closed when the operating mechanism of the main device is in the de-energized or non-operated position. This is an auxiliary contact mounted directly on the breaker indicating status of the breaker operating mechanism.
Auxiliary contact 52TOC/a – It’s opened when the circuit breaker is not in the connected position. The TOC switch is mounted in the cubicle, not on the circuit breaker. TOC stands for truck operated contact.
Auxiliary contact 52TOC/b – It’s closed when the circuit breaker is not in the connected position. The TOC switch is mounted in the cubicle, not on the circuit breaker. TOC stands for truck operated contact.
Figure 5 – Truck-Operated Contact Auxiliary Switch (TOC switch)
Auxiliary contact 52MOC/a – It’s open when the circuit breaker is open. The MOC switch is mounted in the cubicle, not on the circuit breaker. MOC stands for mechanism operated contact.
Auxiliary contact 52MOC/b – It’s closed when the circuit breaker is open. The MOC switch is mounted in the cubicle, not on the circuit breaker. MOC stands for mechanism operated contact.
Figure 6 – Mechanism-Operated Control Auxiliary Switches (MOC switch)
Figure 7 – Mechanism-Operated Control Auxiliary Switch actuator (MOC switch)
When coils on devices such as breakers and control relays are connected to a DC supply and de-energized and are not disconnected from both the positive and negative supply leads from the control power, these coils are arranged such that the positive supply is isolated from the relay leaving the negative supply connected.
This is done to minimize the possibility of corrosion of the relay over long term service. When coils on devices such as breakers and control relays are connected to an AC supply and de-energized and are not disconnected from both the hot and neutral supply leads from the control power, these coils are arranged such that the hot supply is isolated from the relay leaving the neutral supply connected.
This is done to prevent inadvertent energization of the coil should a ground occur in the control system.
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