CONTROL IN POWER SYSTEM

 

CONTROL CIRCUITS

A.           INTRODUCTION

During power system faults, devices are used for fast isolation of affected equipment to save them from damage.  Special circuits called control circuits are used to realize the above objective.  Control circuits are used for other functions besides switching on or off of circuit breakers and isolators as enumerated below:

1. Voltage raise or lower in tap changer device of power transformers.
2. Frequency regulation and load control.
3. Power system monitoring such as power factor control.
4. Alarm and indication control.
5. Circuit supervision.
6. Audio/visual annunciation.

B.      CONTROL SYMBOLS AND ALPHABETS

In order to make for easy identification, symbols and alphabets are used for various devices in control circuits.  This method helps to simplify the control drawings.  Control symbols and alphabets generally used are as shown in Table 1.  A clear knowledge of these facilitates the understanding of the control drawings.

CONTROL CIRCUIT SUPPLIES

To effect operation of control circuits, external auxiliary power supplies are used.  Two major sources of supplies are most common namely:

·         D.C. supply

·         A.C. supply

D. C. SUPPLY

The major source of D. C. supply is from a storage battery.  The storage battery types commonly used are:

(a)          Lead Acid Accumulator type

(b)         Nickel Cadmium type.

Auxiliary D.C. supply has standard voltage ratings of 24V, 30V, 36V, 48V, 50V, 60V, 72V, 110V, 220V and 250V.  Generally 110V is used for Trip/Close control.  In some cases a combination of 50V and 110V D.C. are used.  In this case the relay coil energizes an auxiliary interposing relay whose contacts make to energize an 110V D.C. breaker trip/close coil which in turn opens/closes the contacts of a breaker.

Standard ampere-hour ratings of auxiliary D.C. supply are 45, 60, 100, 250, 500 and 1000AH.

The voltage rating and the Ampere-Hour rating are decided by:

(i)           The size and capacity of the generating station and or substations.

(ii)          The busbar switching arrangement, which decides the number of circuit breakers and isolators.

(iii)        The location of the control equipment in regard to the location of the controlled apparatus i.e. the distance from the control room to the controlled apparatus.

In most 11KV, 33KV and 132KV substations  110V DC batteries are installed.  In 330KV substations, both 50V DC and 110V DC batteries are used for control circuits.

The ampere-hour rating range between 100 and 250 AH.

A D. C. distribution panel is generally associated with a D. C storage battery.  The size of the panel depends upon the number of individual circuits it serves.  A Non-fused breaker usually protects each sub-circuit of the distribution panel, which trips as soon as a fault exists along the circuit being protected.

To protect the D. C. circuits from ground fault, a ground fault relay is installed which usually flags whenever there is a ground fault within any of the poles of the D.C circuits.  For example, if there is a fault within the positive pole of the

D. C. circuits, the D.C. ground positive target of the ground fault relay will operate.  The relay will not reset except the source of the fault is cleared.  In some cases, the fault signal is wired to a visual alarm, which will indicate the actual pole that is faulty.  In some installations, a switch is used to monitor the amount of voltage leaking to ground.  Under normal conditions P-E and N-E voltages are equal.  But a pole loses the voltages to ground if faulty.

A.C. SUPPLY

The A.C. supply for the control circuits is obtained from a station auxiliary transformer.  This, in the case of generating units, may be directly connected to the generator terminals as unit auxiliary transformers.

A standby A.C generator is also used as an alternate source of A.C. supply for control circuits.  In stations where A.C. supply is to be reliable, there could be two sources from which auxiliary supply is obtained with an automatic change – over switch.  In this case, if supply from one source fails then, supply from the other source is readily available.  The alternative source could be another auxiliary transformer from a separate source, D.C. motor, A.C. generator set, or battery inverter circuit.

In control circuits, A.C. supply could serve the following purposes:

(a)  Control panel illumination

(b)  Control panel heater

(c)  Breaker spring operating motor.

(d)  Breaker control panel heater and illumination.

(e)  Control panel indication lamps

(f)   Audio/visual annunciation

(g)  OLTC gear motor operation in power transformers

(h)  Position indication for tap – changer progress.

TRIP CIRCUIT

The control circuit for the opening of switchgear during normal operation or on fault is usually known as Trip Circuit.

To ensure that this circuit does not fail whenever a signal is sent to operate the breaker/disconnect switch, it is being monitored continuously by a relay known as Trip Circuit Supervision relay.  The relay is wired in such a way that the relay coil is energized as long as the trip circuit is healthy.  If for any reason there is a fault within the trip circuit causing a loss of D.C. supply, this relay de-energises causing the mechanical target to flag, which will indicate, “Trip circuit faulty”.  This relay is usually a self-reset relay, which resets itself as soon as the D.C. supply is restored.  D.C. supply can also be lost if the battery charger is faulty or the D.C. fuse gets ruptured as a result of a short-circuit fault within the D.C. circuit.