PART 5 PROTECTIVE RELAYS AND CONTROL SYSTEMS
Chapter 5-1 Protective Relays up to 1kV
Article 407. Scope of Application
As stipulated in Technical Regulation.
Article 408. The Selection of Breaking Capacity
As stipulated in Technical Regulation.
Article 409. The Selection of Rated Current
As stipulated in Technical Regulation.
Article 410. Application of Fuses and Auto-Breakers
Regarding the protection of a neutral line, the reason for prohibition against use of automatic circuit breakers is as follows.
-Solidly earthed neutral: The fault current can't flow after interruption of a neutral line
-Isolated neutral: if a neutral line is interrupted and the load between the supply lines and the neutral
line is unbalanced in the single-phase three-wire distribution system, the load equipment may breakdown or ignite at worst due to the overvoltage which rises after interruption of the neutral line.
For electrical equipment with voltage up to 1kV in power system which has directly grounded neutral conductor, in order to ensure automatic disconnection of faulted line section, phase conductor and protective neutral conductor shall be so selected that in case if earth fault of cover or protective neutral conductor happens, short circuit current shall be not lower than:
-3 times of nominal current of nearby fuse.
-3 times of nominal current of no-adjustable breaker or calibrated current of adjustable switch of circuit breaker with reverse-current relay.
In case that protecting power network by automatic circuit breaker with only magnetic interrupter (high speed overflow), the above-mentioned conductor shall maintain the current not lower than starting calibrated current multiplied with dispersed/decay factor (according to data of manufacturer) and reserve factor is 1.1. In case that data of manufacturer are not available, as for automatic circuitbreaker with nominal current to 100A, multiple of short circuit current in comparison to calibrated current shall be not lower than 1.4; For automatic circuit-breaker with nominal current over 100A, the above-mentioned multiple shall be not lower than 1.25. In all cases, full conductance of productive neutral conductor shall not be lower than 50% of conductance of phase conductor
If the required multiple is not met in case earth fault of cover or protective neutral conductor, interruption of this short circuit fault shall be performed by special protective device.
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Article 411. Display of Nominal Current
As stipulated in Technical Regulation.
Article 412. Short Circuit Fault Protection
Regarding the protection of short circuit, it shall be carried outwith minimum breaking time and selective breaking.
It shall be always operated if the following faults occur at the end of the protected area:
-Singleand multi-phase for solidly earthed neutral grid
-Twoand 3-phase for isolating neutral grids.
If ratio between minimum calculated short circuit current and nominal current of fuse or automatic circuit-breaker is not lower than the value stipulated inArticle 410, a breaking operation is ensured.
In the power grid, only short circuit protection is required. It is not necessary to calculate the multiple of short circuit current as required in Article410, if the following conditions are satisfied: in case of comparison with allowable long-term currents, protective devices have multiple not higher than:
-3 times of nominal current of fuse wire.
-4.5 times of calibrated current of automatic circuit breaker which has -breaquick, overflow interrupter.
-1.0 time of nominal current of automatic circuit breaker with nonadjustable- reverse-current relay (not dependent on whether quick interrupter is available or not).
-1.25 times of starting current of built-in interrupter in automatic circuit breaker with adjustable reserve current. In case that high speed interrupter is available in automatic circuit breaker, the multiple of starting current of quick interrupter is not limited.
Article 413. Overload Protection
In circuits with overload protection, selection of conductors should be based on calculated current and ensuring conditions as specified for permissible current values in Chapter2-2-Part II and the protection device shall have the multiple not higher than:
- 80% of nominal current of fuse wire or calibrated current of automatic circuit breaker, which only has high speed interrupter (for conductors covered with synthetic plastic, rubber or insulatorwith the similar thermal characteristic). This figure is allowed to reach 100% for conductors which are placed in inflammable and not-explosive spaces.
-100% of nominal current of fuse wire or calibrated current of automatic circuit breaker which has only quick-interrupter (for power cables covered with insulating paper).
-100% of nominal current of automatic circuit breaker with nonadjustablereserve current, not depending on whether quick interrupter is available or not (for all types of conductor).
-100% of starting current of automatic circuit breaker with adjustable reserve current for conductors covered with synthetic plastic, rubber or insulators with the similar thermal characteristics.
- 125% of starting current of automatic circuit breaker thwi adjustable reserve current for power cables covered with insulating paper and vulcanized polyethylene insulator.
Permissible long-term load current of conductor to motors with squirrel cage rotor shall be not lower than:
- 100% of nominal current of rotors in areas without fire and explosion risks.
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- 125% of nominal current of rotors in areas with fire and explosion risks.
Relationship between permissible long-term load current of conductor connected with squirrel cage rotor and calibrated current of protective devices, in any case, shall be not higher than values specified in Article 412.
If permissible long-term load currents of conductor determined according tothe values specified in the above mentioned andArticle 412 are not satisfying values mentioned in Tables of permissible currents presented in Chapter2-2-Part II, it is allowed to select the conductors with smaller cross
section area closed to values in the Tables but not higher than values determined according to calculated currents.
Article 414. Installation Location
It is necessary to install protective devices in places where their conductor’s cross section area downsizes (towards load side) in electric circuit, or at points where it needs ensuring sensitivity and selectivity (refer to Article415, 416).
Article 415. Placement of Protective Devices
The protective device shall be placed right at the point of connecting protected component to feeding line. In case of necessity, it is allowed to use up to 6m length of branching section between protective device and feeding line. Cross section area of this line section can be less than that of feeding line but not less than cross section area of conductor after protective device.
For branches arranged at inconvenient places (e.g. at too high point), it is allowed to use upto--30m- long section connected to protective devices for facilitating operation (e.g. input line of switchyard, starters of electrical equipment, etc.). In this case, cross section area of branch shall be not smaller than cross section area determined according to calculated current, and make sure not smaller than 10% loading capability of protected main line. The above-mentioned branching conductors (6m or 30m) shall have insulating sheath and be going inside refractory conduits. For other cases (except underground power cables), in inflammable and explosive areas, such conductors can be laid out open
on structures which are prevented from mechanical damages.
In case that fuses are used for protecting power grid, they shall be installed on all poles or normal phase (ungrounded phase).
It is prohibited to install the fuses on neutral wires.
In case automatic circuit breaker is used for directly protecting grounded neutral power network, its interrupter shall be installed on each ungrounded conductor.
In case that automatic circuit breaker is used for protecting power network with 3-phase-3-lines, or 1- phase-2-lines neutral wire, or DC power grid, it is necessary to install interrupter of automatic circuit breaker on 2 phases (for 3-line power grid) and on 1 phase (pole) (for 2-line power grid). Note: in one power grid, protective devices should be installed on phases (poles) which have the same name.
It is only allowed to install interrupter of automatic circuit breaker on neutral conductor if when it actuates all live conductors shall be switched off at the same time.
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