Differential Relays Flashcards
differential relay schemes and equipment they protect
bus transformer breaker shunt caps reactors series caps
list the types of relays for differential relay schemes for the bus
EM : CO-9, CO-11, IAC, PJC
SS : INX-5
MP : REB-670, RED-521, RED-531, BE1-851, BE1-951, SEL-487B
list the types of relays for differential relay schemes for transformer/ reactors
EM : HU-1, HU-4, BDD, IJD,
SS : KBCH, MBCH
MP : RET-670, SEL-387, SEL-487
list the types if relays for differential relay schemes for shunt caps
MP : SEL-187V, SEL-287V
list the types of relays for differential relay schemes for breakers
MP : SEL-587, SEL-787
Slope
- a setting on the differential relay that allows a certain amount of current in the operating coil before it will send a trip output.
- is a percentage set between 20-40%
- Relays with a slope setting are referred to as Percentage Differential
CT Mismatch
-In order for differential relay to normally see balanced secondary current in and out of the transformer Zone of Protection, CT ratios are selected so that the secondary current of both CTs are normal equal.
Example: for a 2:1 transformer, CTs should be selected that have a turns ratio of 1000:5 on the high side and 2000:5 on the low side
-CT Mismatch occurs when exact turns ratios can’t be used, or tap changer effects then transformer turns ratio
CT Saturation
- A saturated CT will produce inaccurate secondary current
- This will cause differential relays to see a imbalance when there isn’t one
- CTs are designed to not saturate until 20 times their normal current rating
Magnetizing Inrush
- When a deenergized transformer is first energized there is an immediate inrush of magentizing current which may be many times greater then full load current
- This current flows only long enough to establish the magnetic field of the transformer
- During this time the differential relay will see current entering the Zone of Protection but not leaving it, this can cause a trip output
- Consists mainly of reactive power
Percentage Differential
- Is used to compensate for mismatched CTs, losses within the Zone of Protection, and differing CT lead losses
- Is achieved by having 3 windings (2 restraint in series and 1 operating in parallel) in the relay with different numbers of turns.
- The restraint coils have 5 turns each and the operating coil has 40, this requires much more current flow in the operating coil in order to output a trip
- The number of turns in each coil establishes the slope
Harmonic Restraint
- Uses tuned circuits to disable the operating coil of the differential relay during transformer energization
- One tuned circuit blocks harmonics is used with the operating coil
- Another tuned circuit blocks fundamental frequency is used with the restraint coil
- Because magnetizing inrush consists mainly of reactive power there is al large amount of harmonic frequency
- This allows most of the inrush to pass through restraint coil, preventing the disc from turning and initiating a trip
Under Load Tap Change
- Some transformers have the ability to change the voltage of the transformer by remotely adjust the turns ratio
- Because the turns ratio is changed an imbalance can be seen by the differential relays
- Using a percentage differential relay will help prevent false trip outputs due to ULTCs
Define the term “Overlapping Protection” for differential relays.
- Ensures that all equipment is protected.
- Achieved by overlapping the Zones of Protection of adjacent Diff relays.
- The Zone of Protection for a relay is set by the location of the CTs the relay gets inputs from.
- Considered ideal protection.
Basic concept of a differential relay
What goes into the Zone of Protection must come out
What is a Zone of Protection for a differential relay
Protects specific bus and equipment
Boundaries defined by the CTs
IEEE number for differential relays
87
What is typically used as backup protection for a transformer differential scheme
Phase and neutral overcurrent relays
Tertiary ground detector
Sudden pressure relays
When shall Bus Differential relays be cut out per WS-4-1
- Before relay maintenance is performed
- Before a CT source in the differential relay scheme is bypassed or removed, that could adversely affect the relay
When are Transformer differential relays cutout per WS-4-1
- Before relay maintenance is performed.
- Before a CT source in the differential relay scheme is bypassed or removed, that could adversely affect the relay.
When shall shunt reactor differential relays be cut out per WS-4-1
-Before relay maintenance is performed
When she’ll shunt capacitor differential relays be cut out per WS-4-1
- Before relay maintenance is performed
- Anytime a capacitor section is in service and a potential source is to be interrupted to the voltage differential relay
What makes differential relaying the most selective relaying principal
The selectivity of this type of relay resides largely in the connections of the current Transformers and their locations
How does a line differential relay scheme work in basic
- Relays compare the difference in current on the line via microprocessor relays and Communications paths.
- Measures the instantaneous value (charge) of each phase and residual current as often as once every 1/2 millisecond, the local and remote charges are compared checking polarity into account. If the net charge differs from zero by more than a small threshold level an internal fault is declared and the power circuit breaker trip signals are sent.
- Differentials are not commonly used on transmission lines though some exist, most for shorter lions like between a Powerhouse and substation.
What are some best practices when restoring from maintenance of a SEL relay and why.
Always press the reset button on an SEL relay following maintenance.
-this resets possible trip outputs that were not indicated on the faceplate LEDs preventing false tripping
What is an inherent relay trap when disabling and INX-5 differential relay
These realize trip to both the LOR and each PCB
