The relay protection of distribution transformer
1 Distribution transformer should be configuration of relay protection:
(1) low voltage side of transformer should be installed with short circuit protection and overload protection. Short circuit protection as main protection of busbar protection, main transformer, and as the backup protection of distribution line.
(2) the low voltage side of transformer main protection should be with high side and low voltage power distribution line protection has good selectivity, and ensure the system normal peak current (such as motor starting current) does not cause the protection misoperation.
(3) the low voltage side of transformer main protection can also double as single phase earthing protection, can be used with single-phase grounding protection of low voltage circuit breaker for the low voltage side of transformer main protection (e.g., DW16 type low voltage circuit breaker), such as when sensitivity is not enough, should add zero sequence protection.
(4) to get action time cooperate and line protection, the low voltage side of transformer short-circuit protection generally USES the instantaneous or short time delay tripping device moves to disconnect circuit breaker on the low voltage side, over load protection with long time delay tripping device to low-voltage circuit breaker or to send out alarm signals on duty personnel.
2 power distribution line main protection:
(1) low voltage power distribution lines shall be installed with short circuit protection, overload protection and earth fault protection. In the failure to cut off the power supply or signal alarm.
(2) installed with short circuit protection device of distribution circuit, at the end of the protected line: neutral directly grounded system of single phase short circuit, protection device must be action as soon as possible (for the action time of the fixed electrical equipment is not more than 5 s) removal of the fault.
3 the protection of the ac motor:
In order to protect the ac motor coil turn-to-turn short circuit, grounding and overload, ac motor should be installed and short circuit protection, and installed according to the specific situation ground-fault protection, overload protection, and phase failure protection. Electric hunger of short circuit protection, appropriate USES fuse or with snap or short time delay tripping device of low voltage circuit breaker; The important motor for individual capacity is bigger, also can be used over current relay on the low voltage circuit breaker. The points mentioned below.
(1) Short circuit protection:
Interphase short circuit occurred when the motor terminal locations and when one-phase ground fault in neutral directly grounded system, protection device should cut off the fault as soon as possible. When the normal starting of the motor and self-starting, short circuit protection device should not be wrong action. Therefore, the rated current of the fuse melts meter namely deviation should be based on the Ann s characteristic curve after slightly higher than the intersection of motor starting current and starting time to select, but shall not be less than the rated current of the motor. To snap the overcurrent trip or relay setting current preferable starting current of the motor cycle component of 1.7 to 2 times to avoid the influence of the aperiodic component when starting current. Consisting of low voltage circuit breaker and contactor electric main circuits, when the short circuit current is greater than the contactor "bounce off the current" (the contactor contact bounce off automatically by electric current), even if low voltage circuit breaker in the contactor disconnect before release, but in the contact arc time contact will bounce off due to the effect of short circuit current electric power so that a strong arc, burns contact, produce fusion welding, make the contactor can't continue to work. In this case, the operation is not frequent motor, can use rapid movements of low-voltage circuit breaker or in low voltage circuit breaker with electric operator instead of the protection circuit of low voltage circuit breaker and contactor; Motor for frequent operation, as far as possible put contactor in the end of the main circuit, to reduce the contactor after short-circuit current, make it less than "bounce off the current".
Ac motor short-circuit protection appliances should be installed by following regulation: every electric made respectively installed with short circuit protection, but when the total number of electric motor current is not more than 20 a is calculated and allow indiscriminately, cut off or on craft a set of motor is closely related to a short circuit protection appliances can be Shared. At this point, the short circuit protection should be with the lateral load control electrical appliances and motor overload protection appliances. Short circuit protection and make ground fault protection, should be in each earth-free climbs. Short circuit protection for phase short circuit protection, only fuse should be in each earth-free climbs, overcurrent trip or relay shall be installed at least two terms.
(2) the ground fault protection
Ac motor of earth fault protection shall be installed according to the following:
The power method is used to make indirect grounding protection, each electric made respectively installed ground fault protection, but when sharing a set of a set of motor short-circuit protection appliances or allow indiscriminately, to cut off the secondary end of the line, a set of several electric motor can be Shared a ground-fault protection appliances.
The setting of earth fault protection shall be the grounding system type and fault loop impedance values. When earthing fault occurring in addition to meet the requirements of the thermal stability, should also cut off the fault circuit quickly, make within the duration of the fault voltage danger will not cause personal casualty and electrical fire accident getting an electric shock.
(3) the overload protection
Ac motor overload protection shall be installed according to the following:
Easy to overload in the operation of the motor and starter or since starting harsh and calling for a restriction on the starting time of electric motor, should be installed over load protection, continuous operation of the electric made furnish overload protection. Overload protection generally delay action in power off.
Short-term job or intermittent cycle of the motor, can not installed over load protection, but may occur in the operation of the locked-rotor, shall furnish locked-rotor protection. Suddenly loses power will lead to more losses than a load motor, should not be installed over load protection or overload protection in the signal.
Overload protection appliances generally adopt thermal relay or over-current trip, over-current relay may be adopted. For a large ac motor when conditional still can use temperature protection, or other appropriate protection.
Overload protection action characteristic of electrical appliances should be made to match the motor over load characteristics. When the motor running, or the normal starting since the start, protection appliances should not be wrong action.
(4) open phase protection
Ac motor open phase protection, should be installed by following regulation: continuous operation of the three-phase motor, with the fuse protection should be installed when open phase protection, open phase protection with low voltage circuit breaker protection of installations. Short-term job or intermittent cycle power rating is not more than 3 kw motor, don't furnish phase failure protection. Phase failure protection appliances generally adopt open phase protection of thermal relay, current production of motor protection switch, mostly with phase failure protection device.
4 Common relay protection device
In the applications of low voltage power distribution system relay protection device is very much, have a low voltage circuit breaker, fuse and various protective relay. The following main analysis of low voltage circuit breaker and residual current protective device.
(1) low voltage circuit breakers
Circuit breaker for low-voltage distribution system, according to the installation category is divided into levels the power level of universal circuit breaker and power distribution circuit breaker, power distribution circuit breaker is divided into power distribution and motor protection type two kinds, the use of these two kinds of circuit breaker condition is different, its protective properties also have bigger difference. Choose the main technical parameters of distribution protection and motor protection circuit breaker: is the difference between the instantaneous current setting of the former is 10 times that of the latter is 12 times. Motor protection circuit breaker long time delay of the overcurrent trip in 7.2 times setting electricity flow action time should be greater than the actual starting time of motor. For the universal circuit breaker CCW1 technology parameter selection more convenient. Its current and time parameters after the setting values given are adjustable sizing, parameter selection is more accurate.
(2) The residual current protective device
(Residual current Protective Device(RCD), often referred as leakage protector or leakage switch. Installed in the low voltage power distribution system is the residual current protective device is one of the effective measures to prevent electric shock accidents, and prevent the damage caused by residual current electrical fire and electrical equipment the technical measures of the accident. Should install residual current protective device and other various technical measures to prevent electric shock, such as automatic cut off the circuit, grounding, equipotential connection, etc.) at the same time. To make selective leakage protection devices should be based on the principle of grading protection, make the end of the end and power leakage protection device characteristics. Generally, according to the specific circumstances, can be divided into two to four levels of protection. At the same time, the protection between the action value and time value at various levels shall cooperate with each other.
Unlike general protection, leakage protection using graded protection should be based on the terminal protection, power supply side chooses low sensitivity of current value, to protect the action time lag between higher and lower should not be less than 0.2 s.