Basic Control Circuits

5.1       AUXILIARY SWITCHES

The names of various automatic switches are:

1. Level switch
2. Flow switch
3. Position or limit switch
4. Pressure switch
5. Temperature switch
6. Speed switch (centrifugal switch)

IMPORTANT LETTERS

Letters ‘a’, ‘b’, ‘aa’ and ‘bb’ are used in diagrams to represent switches or auxiliary switches.

a-                  It is closed when main device (Circuit Breaker, Isolator or Contactor/Relay) is closed /energized and it is opened when main device is open or de-energized. Sometimes, ‘a’ is also called normally open contact of a device.

b-                  It is closed when main device is opened and vice versa. ‘b’ is also called normally closed contact of a device.

aa-       It is always open. It only closes for a very short time when the driving force; (air pressure, hydraulic pressure or spring) operating the main device is in action. It returns to its original position when driving force ceases.

bb-       It is opposite to ‘aa’ i.e it is always closed. It opens for a very short time when driving force operating the main device is in action. It re-closes or re-sets when driving force is ceased

5.2       DEVICE FUNCTION NUMBERS

Function No.	Type of Relay
2	Time delay relay
3	Interlocking relay
21	Distance relay
25	Check synchronizing relay
27	Under voltage relay
30	Enunciator relay
32	Directional power (Reverse power) relay
37	Low forward power relay
40	Field failure (loss of excitation) relay
46	Negative phase sequence relay
49	Machine or Transformer Thermal relay
50	Instantaneous Over current relay
51	A.C IDMT over current relay
52	Circuit breaker
52a	Circuit breaker Auxiliary switch “Normally open” (‘a’ contact)
52b	Circuit breaker Auxiliary switch “Normally closed” (‘b’ contact)
55	Power Factor relay
56	Field Application relay
59	Overvoltage relay
60	Voltage or current balance relay
64	Earth fault relay
67	Directional relay
68	Locking relay
74	Alarm relay
76	D.C Over current relay
78	Phase angle measuring or out of step relay
79	AC Auto reclose relay
81	Frequency relay
81U	under frequency relay
81O	over frequency relay
83	Automatic selective control or transfer relay
85	Carrier or pilot wire receive relay
86	Tripping Relay
87	Differential relay
87G	Generator differential relay
87GT	overall differential relay
87U	UAT differential relay
87NT	Restricted earth fault relay (provided on HV side of Generator transformer)
95	Trip circuit supervision relay
99	Over flux relay
186A	Auto reclose lockout relay
186B	Auto recluse lockout relay

5.3       BASIC REQUIRMENTS OF CONTROL CIRCUITS

For circuit breakers there are a number of basic requirements which are desirable in the control circuit. These features can be found in the motor, solenoid, spring (stored energy) and pneumatically-operated Circuit Breakers.

A good understanding in these control circuit features will allow an intelligent approach to trouble-shooting. Examination of the circuit diagram of the modern breakers will relatively complicated network of switches, contactors and coils, the correct functioning of which is essential. Each individual component of a circuit has a definite function to perform, thus removing any one element will cause some type of faulty operation.

When the maintenance electrician knows the function of each component, he also knows what type of faulty operation to expect when that component is inoperative. Conversely when mal-operation of a breaker is found, the likely component or components at fault will be known.

5.3.1        CONTROL THE CLOSING

The closing circuit must do more than merely close the breaker, it must control this closing. To do this, the following features are necessary.

Initiate The Closing Stroke: Means must be provided in the circuit to energize the closing device, for example, the solenoid of the solenoid-operated breaker or the motor of a motor-operated breaker.

Cut-Off: The closing power must be cut-off or disconnected automatically at the end of the closing stroke. This is necessary to prevent overheating of the closing device. Solenoid coils used on circuit breakers have only a short time rating, thus if a closing coil is left energized for more than 15 seconds. It will overheat and suffer damage to the insulation. For this reason it cannot be left to the operator to decide when to cut off the closing power since it left on too long, damage will result. The alternative where the closing power could be left on for too short a time is covered in Seal-In.

Seal-In: It is desirable to have the control circuit ensure that the breaker will fully close each time that closing operation is initiated, if the breaker is closed by a simple switch. Simply speaking; it completes the operation automatically started by us manually so as not to hold the push button all the times.

5.3.2   CONTROL THE TRIPPING

Initiate Trip Stroke: Means must be provided to trip the breaker. This may involve energizing a solenoid coil to trip a latch or in case of air blast breakers, to admit air to the blast valves and contacts.

Cut-Off: For the reasons noted in above, means must be provided to automatically disconnect the trip coil.

5.3.3        TRIP FREE FEATURE

When closing a breaker, the closing device (for example, the solenoid in a solenoid-operated breaker) is energized and the plunger operates through the linkage to close the breaker contacts. At the end of the closing stroke, appreciable time is required to de-energize the solenoid coil. In the event that the breaker has been close on a faulted circuit, it must be reopened as quickly as possible. If a breaker can trip automatically upon receiving a trip signal before closing operation is complete, it is said to be “trip free”.

Various arrangements are provided to obtained trip-free action. Solenoid-operated breakers are sometimes provided with action. Solenoid-operated breakers are sometimes provided with a collapsible linkage. Pneumatically-operated breakers may be equipped with two latches, one of which is unlatched during a normal trip operation and the other is only unlatched for a trip signal while the breaker is closing. Other breakers use a large dump valve to quickly exhaust the air under the closing piston. Many motor-operated breakers obtain a fast trip-free action by use of a relay energized from the trip circuit to open the closing circuit. These methods would be known as mechanically trip free, pneumatically trip free and electrically trip free respectively.

5.3.4    ANTI-PUMPING FEATURE

When a breaker is closed and a trip-free operation results, the close and trip stroke will be completed in a very short time for a modern pneumatically-operated high voltage breaker, the complete operation will take less than one-half of one second, thus it is quite likely that the operator will still have the control switch in the closed position. Means must therefore be provided to prevent the breaker from closing a second time, even though the operator is holding the control switch in the closed position. This is usually accomplished by the use of a sealed in relay which can only be released which in the closed position. This is usually accomplished by the use of a sealed in relay which can only be released by opening the closing control switch. When this feature is incorporated in the control circuit, the breaker is said to be “pump free” or “anti-pumping”. Following a trip-free operation of the breaker, the operator must release the control switch before a second attempt to close the breaker can be made.

5.3.5        ANTI-SLAM FEATURE

This feature prevents the energization of closing coil or tripping coil of an already closed breaker or tripped breaker respectively.

1.      In closing circuit this feature is mostly achieved through auxiliary switch b.

2.      In opening circuit, this feature is mostly achieved through auxiliary switch a.  

5.3.6        RELIABILITY

A circuit breaker is a protective device. It will be called upon to open faulty circuits infrequently, however while it may stand inoperative for long periods; it must be relied upon to operate correctly in time of trouble. Reliability for such a protective device is essential. For this reason a battery supply is always used to provide the tripping power and in most cases for closing.

The control circuits usually have a separate trip and close bus. This is to give extra reliability to the trip circuit. On 115 kV and above circuit breakers there are dual trip buses thus, if a closing control circuit fuse fails during a closing stroke, the trip circuit or circuits are not affected.

The above requirement of extra reliability during tripping is also seen in the size of fuses used in the trip and closed circuits, for example, on a solenoid-operated breaker the fuses in the closing circuit will be rated at slightly less than value of current obtained by dividing the voltage by resistance. The fuses must be so rated to provide a measure of protection for the short time rated closing coil. During a normal closing operation, the closing coil will be energized for less than one second. To have the closing fuses blow in approximately six seconds, it is necessary to use a size which is actually less than the maximum current that the closing coil will normally draw. Conversely, in the trip circuit the fuses will be rated at several times the current obtained by voltage to resistance ratio. If the trip coil is not automatically disconnected at the end of the trip stroke, the trip coil may carry current for a long period and being are short time rated coil, it will be damaged. The fuses in this circuit will open only due to some fault condition. In order to gain more reliable tripping, we do not protect the short time rated trip coil. The trip fuses are not put in the breaker but are located in the control building.

5.3.7    GENERAL MAINTENANCE OF BREAKER CONTROL RELAYS

Frequent reference is made in this reference material to relays. These are control relays located in the operating mechanism housing. They are concerned entirely with the sequence of the mechanism of the breaker. The control circuit relays are all located on the breaker side of the four-pole isolating switch. Such relays are a part of the breaker, being required for the breaker’s correct functioning as much as possible, for example, a trip coil or interrupter and as such are the responsibility of the maintenance electrician.

Other relays remote from the breaker determine under what system conditions the breaker will be tripped. These protective relays, together with the interposing relays where such are used, form the protective network and are the responsibility of the Meter and Relay Department. The dividing line between the breaker control circuit relays and protective relays is well defined and there should be not confusion in this regard.

5.3.8        ASA DEFINATIONS

RELAY: A relay is a device that is operative by a variation in the conditions of one electric circuit to effect the operations of other devices in the same or another electric circuit.

CONTROL RELAY: A control relay is a relay which functions to initiate or to permit the next desired operation in a control circuit or scheme.

PROTECTIVE RELAY: A protective relay is a relay, the principal function of which is to protect service from interruption by removing defective components or to prevent or limit damage to apparatus.

5.4      OVERLOAD PROTECTION

In order to avoid damage to motors etc. due to temperature rise because of overloading and defective bearings etc., overload protection features are incorporated in motor control circuits. It should be noted that over load relay or element is always incorporated in the power circuit but its contact is installed in the control circuit. Due to this, the life of contact increases as it breaks small current because in control circuit current is small. Over load relays are mostly operated thermally and may be of bimetallic strip type or solder pot type.

5.5       OVER CURRENT OR SHORT CIRCUIT PROTECTION

The function of over current protective devices is to protect motors and its circuit elements etc. from damage in case of phase-phase short circuits or phase-ground faults etc. The over-current device must be capable of carrying the starting current of motors. Mostly fuses and magnetic devices are used as over current protective devices. Rating of fuse should not exceed 300% of full load current.

5.6              CONTACTOR

Contractor is a device which is operated electrically and controlling the operation of other circuits magnetically. Contactor may also be called as an ON-OFF Switch. Contactor has two types of contacts:

1.      Main Contacts: These are used in power circuits and hence must be strong to carry the full load current of motor continuously without undue heating.

2.      Auxiliary Contacts: These are small and used in control circuits only. These may be NO or NC and are used as seal in contact (NO), interlocking contact (NC) and for indications etc.

5.7              MAINTENANCE OF CONTACTOR

Contactor maintenance mainly includes

1.      Removal of rust or deposits etc. from contacts with emery paper and dry cloth. Never file the contacts as it will remove the elkonite from the contacts.

2.      Checking of contacts alignment

3.      Free movement of moving contacts assembly with binding etc.

4.      Checking of connections at terminal points.