Correction relays

Approximate length of wire for each feeder up to the power factor correction relay (PFCR) 35 m. 

The p.f. correction relays are normally available in three versions, i.e. 

Figure 23.38 Typical front views of automatic power factor correction relays...

Determine the discharge device for the discharge of a three-phase 6.6 kV, 50 Hz, 1000 kVAr, yconnected capacitor bank, connected in units of 10 x 100 kVAr each, through an automatic p.f. correction relay, having a closing cycle of 10 seconds. Data available from the capacitor manufacturer,... 

The direct way of determining the spectrum corrections relays on composing the total Hamiltonian as... 

Any of the above reasons tnay result in noise and an increase in temperature and must be corrected. Critical installations such as a refinery, a petrochemical plant, a chemical plant or a petroleum pipeline may require special precautions and control to avert any excessive heating of the bearings, which may become fire hazards. For these installations, bearing temperature detectors with a relay and alarm facility may also be installed in the control circuit of the switching device to give warning or trip the motor if the temperature of the bearing exceeds the preset safe value. 

Note If a power capacitor is connected after the relay, say across the motor terminals, to improve the p.f. of the machine, then only the corrected value of the current musi be used. For example, if a capacitor bank of 130 kVAr is used for an individual correction of the machine then... 

It is a relaying unit to receive actual parameters from the field and relay out corrective signals... 

Polarity test to check the connections through the potential transformer to ensure that the connections between the transformer and the meters or relays have a correct relative polarity. Otherwise the meters would show erratic readings, while the relays would transmit wrong signals. This test may also be conducted with a low-voltage source of 10 V by ob.serving the deflection of the instruments. 

To perform all the duties of an auto-synchronizing relay and monitor and correct AV, Af and A6. 

Here the relay assesses the kVAr requirement of the system and switches ON all the required capacitors simultaneously. Theoretically any size of capacitor units can he used. But when a capacitor is taken out for maintenance, this can create confusion in the p.f. correction. The relay will not be aware of this and act accordingly. The bank size may also be large and cause switching surges. 

This is a highly recommended method of capacitor switching for installations that are large and require very fine monitoring and correction of p.f. with the smallest number of banks. The entire reactive requirement is arranged in only a few steps yet a small correction up to the smallest capacitor unit is possible. The relay is sequenced so that through its binary counter the required switching is achieved in small steps, with just four or six sets of capacitor units or banks. The operation of the entire sequence can be illustrated as follows ... 

For loads with fast variations, this type of a modified version too may be sluggish, due to step-by-step tuning. In such cases it is possible to employ two relays, one for coarse correction, such as through a serial relay (FIFO) which can quickly switch the large banks, and the other for fine tuning, which may be a binary relay. These two relays may be combined into one hybrid unit. The binary scheme can thus be modified to suit any required duty with prompt and finer corrections and least strain on a particular unit or on the system. 

As soon as the system s actual p.f. deviates from the pre-set limits, the relay becomes activated and switches in or switches out capacitor units one by one, until the corrected p.f. falls within the sensitivity limit of the relay. 

After every correction the relay again carries out the measurements on the capacitor units (periodically, say, in 24 hours in supervisory mode), to inonitor their health and to detect for any deterioration. When the capacitance of a unit falls below the acceptable level it can emit an alarm. 

At the start of the correction cycle the relay calculates the required kVAr, examines the available capacitor values stored in its memory, takes a decision to switch ON or OFF the appropriate capacitors and switches them in rapid succession to reach the target p.f. in the least possible time. Now it uses the CTs provided in the main incoming circuit (Figure 23.39). 

For more than one capacitor bank one RVT for each bank will be necessary for the correct discharge of each bank, (Figure 26.5). An RVT, if provided with an additional star-connected tertiary winding rated for 110 V line to line, can aiso check the discharge condition and its duration and if it meets the design requirements. This can even be monitored remotely through a relay and control panel. 

Protection is the branch of electric power engineering concerned with the principles of design and operation of equipment (called relays nr protective relays ) which detect abnormal power system conditions and initiate corrective action as quickly as possible in order to return the power system to its normal state. The quickness of response is an essential element of protective relaying systems—response times of the order of a few milliseconds are often required. Consequently, human intervention in the protection of system operation is not possible. The response must be automatic, quick, and should cause a minimum amount of disruption to the power system. 

Reliability is generally understood to measure the degree of certainty that a piece of equipment will perform as intended. Relays, in contrast with most other equipment, have two alternative ways in which they can be unreliable. They may tail to operate when they are expected to, or they may operate when they are not expected to. This leads to the two-pronged definition of dependability, the measure of certainty that the relays will operate correctly for all faults for which they are designed to operate and security, the measure of certainty that the relays will not operate incorrectly for any fault. 

Tests carried out on protection systems comprise factory tests, on-site commissioning tests and maintenance checks. Those made on individual relays will demonstrate the compliance of the equipment with specification and the verification of its operation under simulated conditions, while tests carried out on-site prior to the equipment being put into service ensure that the full protection scheme and associated equipment operate correctly. These on-site tests must be comprehensive and should include ... 

Capacitors as bulk units can be connected to the supply busbar via a fuse switch, molded-case circuit breaker of air circuit breaker. In this type of installation control is purely manual, and in cases of a reasonably constant load and where the amount of power factor correction is limited such a manually controlled system is perfectly adequate. The supply authority may, however, require to be informed that a capacitor bank is permanently connected to the supply. Capacitors are more generally connected either in banks controlled from a VAR sensitive relay or across individual loads (e.g. motors). 

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