LESSON 6: A GUIDE FOR POWER SYSTEM PROTECTION SELECTIVITY AND COORDINATION (MHO TYPE AND REACTANCE TYPE DISTANCE RELAY) THE MHO RELAY IS THE IMPROVE TYPE OF DISTANCE RELAY OR IMPEDANCE RELAY (21). UNLIKE THE IMPEDANCE RELAY, MHO TYPE IS DIRECTIONAL. THIS USES A CYLINDER TYPICALLY SAME WITH THE DISTANCE OVERCURRENT ELEMENT. COILS ARE WOUND AROUND IN ELECTROMAGNET AND IT IS FED BY THE CT’S AND VT’S OF THE PRIMARY CIRCUIT.

THE RELAY CHARACTERISTIC SHOWN MEANS THAT ANY IMPEDANCE FALLS INSIDE THIS CIRCLE AREA WILL BE SENSE BY THE RELAY. THE IMPORTANT FEATURE OF THIS RELAY IS THAT THE MAXIMUM CLOSING FORCE OF THE RELAY IS IN SPECIFIC PHASE ANGLE OF ABOUT 60DEG SAME AS THE PHASE ANGLE OF LINE CONDUCTOR IMPEDANCE. IF THE PHASE ANGLE CHANGES AND IMPEDANCE BECOMES SMALLER THEN THE UNIT WILL OPERATE ONLY IN LOW LEVEL OF IMPEDANCE, THAT IS THE HIGH LEVEL OF CURRENT. THAT MEANS IT IS LESS SUSCEPTIBLE TO INCORRECT OPERATION DUE TO LOAD IMPEDANCE WHICH CAN CAUSE THE RELAY IMPEDANCE TO HAVE A PHASE ANGLE FOR ABOUT 20 DEG. REMEMBER THAT THIS RELAY WILL ONLY OPERATE FOR FAULTS WITHIN THE CIRCLE. HOWEVER IN SOME CASES WHEN THEY ACTUALLY WANT THE RELAY TO OPERATE AS BACKUP WHEN FAULT IS UPSTREAM. IN THIS CASE THE CHARACTERISTIC CAN BE OFFSET LIKE IN PIC 1, SO IT WILL OPERATE IN VERY LOW IMPEDANCE IN THE REVERSE DIRECTION.

PIC 1

FOR EXAMPLE IF THE FAULT IS CLOSE AT THE BUS SO PROBABLY THE IMPEDANCE WILL FALL IN THIRD QUARTER AT VERY LOW VALUE AND SO TRIP THE BREAKER SHOWN IN PIC 2.

PIC 2

PIC 3

SECOND ZONE OF PROTECTION, THE SECOND ELEMENT CAN BE OFFSET LIKE THIS. AGAIN BECAUSE OF THE RELATIVELY NARROW BAND IT IS WELL OUTSIDE OF THE LOAD IMPEDANCE SHOWN IN PIC 3.

GROUND FAULTS SPECIALLY TO SHORT LINES PROVIDE A PARTICULAR PROBLEM IN MHO RELAY. THE ARC OFTEN CONTAIN THE CONSIDERABLE RESISTANCE SO REDUCING PHASE ANGLE OF THE MEASURED IMPEDANCE. IT ALSO INCREASES THE VALUE OF IMPEDANCE AS SHOWN IN PIC 4.

PIC 4

PIC 5

CONSEQUENTLY, EVEN THE GROUND FAULT IS HALFWAY OF THE LINE. THE IMPEDANCE STILL FALLS OUTSIDE OF THE OPERATING CIRCLE AS SHOWN IN PIC 5.

ONE COMMON METHOD TO ANSWER THIS PROBLEM IS TO INSTALL A RELAY WHICH WILL MEASURE REACTANCE ONLY. THE OPERATING CHARACTERISTIC IS SHOWN BY WHITE HORIZONTAL LINE SHOWN IN PIC 6. DURING NORMAL OPERATION, THE REACTANCE OF BOTH THE TRANSMISSION LINE AND THE LOAD WILL BE ABOVE THE OPERATING SET POINT AS SHOWN IN PIC 7. THIS IS 90% OF 86.6 OHMS AS PER EXAMPLE IN LESSON 5 WHICH IS 78 OHMS. 90% IS BECAUSE OF THE 10% MARGIN ERROR OF EVERY RELAY THAT WE ARE PREVENTING THE RELAY FROM OVERREACH. IF THE GROUND FAULT OCCURS HALFWAY OF THE LINE, THEN THE RELAY WILL DETECT DECREASE OF VALUE IN REACTANCE IN ABOUT 43OHMS SHOWN IN PIC 8 AND OPERATE THE TRIPPING CIRCUIT. IT WOULD NOT AFFECTED BY THE RESISTANCE BY THE ARC.

PIC6

PIC 7

PIC 8

ONE PROBLEM OF THIS REACTANCE TYPE DISTANCE RELAY IS THAT IT IS NOT DIRECTIONAL RELAY.SO IF THERE IS FAULT IN THE UPSTREAM IT WILL OPERATE.

IN ORDER TO PREVENT THAT PROBLEM. THE MHO-SUPERVISED REACTANCE RELAY IS USED. IN HERE, THE USE OF REACTANCE RELAY IN CONJUNCTION WITH MHO ELEMENT. THE FIRST AND SECOND ZONE WILL BE PROTECTED BY REACTANCE ELEMENTS AND FOR THE THIRD ZONE WILL BE PROTECTED BY THE MHO RELAY WITH EXTERNAL TIMER TO PREVENT EARLY TRIPPING. BUT ADDITIONALLY, THE MHO ELEMENT IS HAVE PERMISSIVE CONTACT CONNECTED IN TRIPPING CIRCUIT OF THE REACTANCE RELAYS.

CHARACTERISTIC OF MHO-SUPERVISED REACTANCE RELAY

PIC9

PIC 10

THIS RESTRICT OPERATIONS TO THE REACTANCE RELAYS IS IN RED AREA SHOWN IN PIC 10. THE PEMISSIVE CONTACT WILL ONLY CLOSE IF IT FALLS WITHIN THE CIRCLE. IF THE FAULT IS UPSTREAM, THE REACTANCE RELAY WILL OPERATE BUT THE TRIPPING CIRCUIT IS STILL INCOMPLETE BECAUSE THE MHO RELAY WILL NOT OPERATE BECAUSE THE PERMISSIVE CONTACT IS STILL OPEN AS SHOWN IN PIC 11. BECAUSE OF THIS THE MHO UNIT IS REFERRED TO AS STARTING UNIT.

PIC 11

DISTANC RELAYS OF SOLID STATE TYPE ARE DESIGNED WITH MANY VARIATIONS IN OPERATING CHARACTERISTICS SHOWN ABOVE.

PIC 12 (ZONE OF PROTECTION IN TRANSMISSION LINE 1,2 & 3)

THE FIRST ELEMENT PROVIDES A PROTECTION WITHIN ZONE 1 AND IT WILL TRIP THE BREAKER A IN LINE 1.THE SECOND ELEMENT WILL PROVIDE LOCAL BACKUP TO FIRST ELEMENT IF IT FAILS TO OPERATE WHEN FAULT IS IN ZONE 1, THAT IS IT WILL PROVIDE TIME DELAY. IT WILL ALSO PROVIDE REMOTE BACKUP WHEN THE FAULT IS IN BUS “B” OR LINE 2 WITH LIMIT OF ZONE 2 AND IF THE PRIMARY PROTECTION AT BUS “B” FAILS TO OPERATE. SIMILARLY, FOR THE THIRD ELEMENT, IT WILL PROVIDE REMOTE BACKUP WHEN THE FAULT IS IN BUS “C” OR LINE 3 WITH LIMIT OF ZONE 3 AND IF THE PRIMARY PROTECTION AT BUS “C” FAILS TO OPERATE.

ALSO, FOR LINE 1, ANOTHER SET OF DISTANCE RELAY IS ADDED IN FORM OF RELAY B WHICH IS LOOKING TO BUS “A” DIRECTIONS. IF FAULT OCCURS WITHIN THE OVERLAPPING PROTECTED ZONE OF RELAY “A” & “B”,THEN THE INSTANTANEOUS ELEMENT OF RELAYS WILL TRIP BOTH ASSOCIATED BREAKERS. IF FAULT IS IN THE LAST 10% OF LINE 1 AS SHOWN IN PIC 13, THE RELAY “B” WILL CLEAR OUT AND TRIP THE BREAKER “B“ INSTANTANEOUSLY BUT IT WOULD HAVE TO TRIP THE BREAKER “A” BY ELEMENT 2.

IN THIS CONTEXT, USING VARIOUS COMMUNICATION CHANNEL COULD BE USED.

NOWADAYS, WHAT IS MORE EMPHASIZE IS BY THE USE OF LOCAL BACKUP. SEVERAL TYPES OF PROTECTION ARE PROVIDED ON EACH ZONE. FREQUENTLY, THE USED OF REDUNDANT IDENTICAL RELAYS ARE PROVIDED.IT IS FED BY SEPARATE CT’S AND DC SOURCE FOR THE TRIPPING CIRCUIT WHICH IN TURN WILL BE FED TO SEPARATE TRIPPING COILS OF BREAKER. USUALLY IN FAULT CONDITIONS, BOTH RELAY WILL OPERATE BOTH COILS OF BREAKER IN THE SAME TIME.

PIC 13