PHASE CURRENT COMPARISION(PCC)
The section comparison protection evaluates the section currents at each ends of the protected object. the 2 7SD8051 devices at the ends of the protected object communicate over their protection interfaces. The phase-specific comparison and therefore the ensuing call to trip the electrical fuse is created on an individual basis for every finish.
The digitalized currents are first filtered to suppress DC components and higher harmonics.
Phase comparison protection, determination of the input variables
These filtered values are offered to a sensitive dynamic component and a static component. By comparison the polarity of the currents at the 2 ends of the protected object, they recognize whether or not the fault is external or internal. an internal fault applies if the polarity of the fault currents is identical on each sides; an internal fault or a load step happens with totally different polarities.
If the comparison shows with none doubt that a fault is gift, the trip command is distributed. it’s maintained over a collection minimum command period.
The part comparison protection might trip only at one finish just in case of single-end infeed. The non-feeding finish also can be converted by suggests that of a transfer trip signal.
The dynamic filter algorithmic rule generates the worth idyn(t). It represents the present change of the filtered worth (fundamental component) over 2 cycles. If the current change exceeds the set threshold 87L Idyn>, the part comparison protection is started.
The polarity of the present amendment is transmitted to the device at the remote finish of the protected object.
The dynamic part operates terribly sensitively just in case of internal faults. just in case of external faults, the strategy is incredibly stable even throughout totally different|completely different} primary rated currents or different saturation of the present transformers at the 2 ends.
The static component Istat operates directly with the filtered elementary price. If the amplitude of the basic part exceeds the set threshold 87L Isteady>, the part comparison protection is started.
The polarity of the present is transmitted to the device at the remote finish of the protected object.
The static component is insensitive towards low fault currents.
The dynamic and the static element pick up severally of every different by selection for every section.
To prevent tripping throughout an energization, a separate dynamic switch-on threshold 87L Idyn close> is employed.
The pickup is maintained over a pair of mensuration cycles. when the two mensuration cycles have invalid, the dynamic sign comparison is blocked.
If the pickup is made, an internal pickup signal is transmitted to the opposite device.
The part Idyn is blocked if the frequency deviates by more than ten to stick out from the rated frequency.
The perform is blocked if the communication between 2|the 2} devices at the ends of the protected object fails for quite two mensuration cycles.
This perform can even be blocked via binary input “>87L block”.
- EARTH CURRENT DIFFERENTIAL (ECD)
Basic Principle / Influencing Variables
In healthy operation, each ends of a line carry identical current. This current flows into one side of the thought-about zone and leaves it once more on the opposite facet. A distinction in current may be a clear indication of a fault inside this line section.
If the particular current transformation ratios ar identical, the secondary windings of this transformers CT1 and CT2 at the road ends are often connected to make a closed circuit with a secondary current I; a activity part M that is connected to the electrical balance purpose remains at zero current in healthy operation.
When a fault happens within the zone restricted by the transformers, a current i1 + i2 that is proportional to the fault currents I1 + I2 flowing in from either side is fed to the activity part. As a result, the easy circuit ensures reliable tripping of the protection if the fault current flowing into the protected zone throughout a fault is high enough for the activity part M to respond.
Basic principle of the differential protection for a line with 2 ends
This principle only applies to the first system as long as quadrature-axis elements of current ar negligible. Quadrature-axis elements of current may be caused by line capacitances or excitation currents of transformers and parallel reactors.
The secondary currents that are applied to the devices via the present transformers, are subject to activity errors caused by the response characteristic of the current transformers and also the input circuits of the devices. Transmission errors like signal jitters may cause deviations of the measured quantities. As a results of all these influences, the total add of all currents processed within the devices in healthy operation isn’t specifically zero. the bottom current differential protection is stabilised against these influences.
Additional measuring errors which can arise within the device itself by hardware tolerances, calculation tolerances, deviations in time or thanks to the „quality“ of the measured quantities like harmonics and deviations in frequency, also are calculable by the device and increase the native self-restraining amount mechanically. Here, the permissible variations within the protection information transmission and process periods also are thought of.
For transient inflow currents the devices have a separate inflow current restraint feature.
Evaluation of Measured Values
The ground current differential protection in grounded systems evaluates the total of the bottom current phasors.
Each device calculates a ground current at every finish of the protected object (fundamental element of the bottom current) and transmits it to the partner device. The received and also the domestically measured ground current phasoris additional to the bottom differential current. the bottom differential current price equals the fault current that the differential protection system „sees“. within the ideal case, it equals the short-circuit current. In healthy operation, the differential current price is low and for lines concerning the same as the capacitive charging current.
The restraining amount counteracts the bottom differential current. it’s the full of the utmost measured errors at the ends of the protected object and is calculated from this measured quantities and facility parameters that were set. Therefore, the best potential error price of this transformers inside the nominal vary and/or the short-circuit current vary is increased with this flowing through every finish of the protected object. the full price, as well as the measured internal errors, is then transmitted to the opposite finish. this can be the reason why the restraint current could be a reproduction of the best potential measuring error of the whole differential protection system.
The pickup characteristic of the differential protection springs from the restraining characteristic Idiff = Irest (45° curve) that is stop below the setting price 87N L: I-DIFF>. It complies with the equation
Irest = 87N L: I-DIFF> + Σ (current electrical device errors and alternative measure errors).
If the calculated differential current exceeds the pickup threshold and also the greatest potential measuring error, the fault should be internal (grayed space within the illustration).
Pickup characteristic of the bottom differential protection
If it’s desired that an indoor fault ought to initiate a visit command and to boot an area current of a particular amount ought to exist, the worth of this current are often set at87N L: I>RELEAS. The default setting for this parameter is zero so this extra criterion doesn’t become effective.
The differential current and also the restraint current 3I0diff and 3I0restr are enclosed within the fault record.
Blocking / Interblocking
The ground current differential protection can be blocked via a binary input. The block at one finish of a protected object affects all ends via the communications link (interblocking). If the overcurrent protection is designed as AN emergency operate, all devices can mechanically switch to the present emergency operation mode