LOW VOLTAGE MCC – STUDY REPORT ON UNWANTED BREAKER OFF PROBLEM DURING EXTERNAL FAULTS

LOCATION : RO PLANT KHORFAKKAN

EQUIPMENT DETALIS : RO PLANT MCC & SEA WATER MCC

Report Prepared By:

Mr.Rajendrakumar.A.P

Electrical Supervisor

SEWA RO plant Khorfakkan

THE PROBLEM AND ITS HISTORY

Since the beginning of the plant in 2009, it has been observed that during any external faults i.e., faults in distribution network in khorfakkan RO plant MCC as well as Sea Water intake MCC incomer breakers are getting off and the system shows no indication of faults. The faults in the distribution networks are quite common especially during summer due to cable faults, which results in unwanted tripping of the RO trains.

PRELIMINARY OBSERVATIONS

When referred to the drawing (figure No.1Breaker open control circuit) I concluded the possibilities to the following two points.

1. OFF command from DCS

2. Breaker opens through under/over voltage relay contact(12UVR1)

The first point was ruled out when checked the DCS history. In the DCS history there was no OFF command to the breakers. The second point needed to be confirmed. The settings of the relay (time delay) found to be disturbed and were in the higher side. On delay found as high as 10 sec and it was reduced to 5 sec. So, with the permission of Plant Manager the breaker opening contact of the under/over voltage relay kept open in the sea water intake incomer # 2 which is having relatively un important loads(up-stream system gives back up protection for the same). During another external fault it was found that sea water incomer breaker #2 remained ON and all other three breakers got OFF. Further it was decided to keep sea water incomer breaker # 1 OFF contact from the under/over voltage relay to keep open. During the next external faults sea water incomer breakers did not get OFF. So it was almost confirmed that the problem is resulted from the under/over voltage relay.

FIGURE 1: BREAKER OPEN CONTROL CIRUIT FIGURE 2: UNDER/OVER VOLTAGE RELAY

Then a genuine question arised, how four relays can malfunction simultaneously or behave similar to a fault. The chance of all relays getting faulty at the same time is a remote possibility, so it was necessary to find out the root cause. Downloaded the relay manual from internet and studied. The following points about the relay characteristic found worth noting.

1. DRV3 is immune to voltage dips < 40 ms.

2. When the voltage is < 0.7 Ue for a time longer than 60 ms, the output relay de-energizes instantaneously.

CONCLUSION

A fault can result in the second condition and instantaneously de-energise the output relay of the under/over voltage relay resulting in opening of the breaker.

SOLUTION

Introducing a timer relay in the control circuit will be a cost effective and permanent solution to the problem. The modified circuit diagram is shown .Figure.3.

The required timer delay details:

On Delay timer

Input: 110 V DC (if universal 24 to 240 Volt AC/DC)

Time Range: 1 to 10 sec (0.5to 10 sec)

Aux Contacts: 1 NO & 1 NC.

Compact DIN rail mount, SPDT

Submitting for necessary approval for successful implementation.

Below given is a content from given web address which shows study of similar problem but the report is not accessible to non members or need to be paid.

http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=5638901&filter%3DAND%28p_IS_Number%3A5638851%29

Currently, under-voltage trip devices are widely installed in circuit breakers of low-voltage distribution networks and end users' sides. These protection devices can prevent equipments from damage when voltage sag or power supply interruption occurs. Although relay protection devices can act correctly and clear the fault as soon as possible when a ground or inter phase fault occurs in the nearby high-voltage transmission network, instantaneous action of under-voltage trip devices may lead to widespread loss of load, fluctuation of frequency, even worse, cascading failure and collapse of the power system. The widespread load loss accidents arising from under-voltage trip devices happened in recent years are summarized in this paper. Two typical load loss accidents happened in Beijiao substation are introduced. The effects of under-voltage trip devices on power system are discussed base on EMS accident recall, PMU, fault recorder and other tools. Finally, suggestions on management of under-voltage trip devices and accident precautions are proposed.

-THANKS –