Protective relay

Relay protection - protection system of chains and consumers against overloads and failure through the use of various relays. According to the classification of the subject under consideration it is part of the electrical automation, relieving operating personnel, saving man-hours.

basic definitions

Lesions are called minor problems, not accompanied by the collapse of the system, but causing current leakage and as a result, the voltage drops. At the same time consumption is growing. The increased current causes overheating of certain parts of the network that is able to cause a serious accident. Undervoltage serves as a limiting factor, causing disabling customers, disrupting work schedule system.

Abnormal modes are accompanied by symptoms described as circuit parameters leave a valid regulations, guidelines range. Each case is part of a system failure, causing deviations and failures. Abnormal mode causes damage, although not directly recognized breakage condition or failure of anything.

System performance is often restored insulation broken section of chain, has saved other subscribers from unsolicited instructions on operating conditions. Locating the short circuit, relay protection saves energy at the same time indicating the technical staff of the need to repair the equipment. Effective method of prevention is to bring the parameter back to normal. Transmission Protection provide the adjustment system will adjust the voltage, current, returning the operation.

Lack fuses applied before is obvious: it is necessary to wait patiently for the start of operation of the system. Relay as the disappearance of destructive factors independently adjusts operation of equipment, easily recovering its original state. This protection is characterized by constant attention to the object of protection, completely eliminated the human factor. A second advantage of getting the speed - the equipment immediately derived from the danger zone.

Actuator systems are considered contactors. Relays controlling keys form the correct power distribution between users. Protection is usually operates in conjunction with machine:

1. Reclosing (reclosure).
2. Frequency discharge (ACR).
3. Backup equipment and power (ABP).

Why protect equipment

The most frequent damage becomes short circuiting between phases either on the ground. Less closed coil windings of the transformer or motor, what causes a sharp portion overheating addition float away from the nominal parameters. It is described by the term insulation failure. The reasons are:

• Aging, for example depolymerization cambric.
• Exceeding the network settings of the maximum permissible.
• Mechanical factors, including signs of strain.
• Operational personnel errors.
• Constructive imperfection of equipment or poor-quality assembly.

These factors are the cause of the abnormal condition or damaged. Short circuits are often seen in the literature, considered the risk of breaking. This fault is often accompanied by a decrease in voltage, which is a separate hazard in three-phase circuits, potentially becoming a prerequisite for equipment failure. This is especially true of the compressors. Voltage decreases most strongly at the point of short circuit, as the distance from the localization accident parameter restores value.

Powerful substations are so severe that they can not see a short-circuit. At the same time the energy provider bears serious losses, and strangers, and staff at risk. Statistics known death repairmen power lines and ordinary citizens, who happened to be at the scene. Dies periodically cattle. The major danger is the latter step voltage. Nebezobiden case of short-circuit and substation: transformers begin to function in a negative mode.

Extremely disadvantageous case becomes arcing. And for man and machine. Then a short circuit causes burns and creates a fire hearths. As for the sagging voltage, the negative effect of detrimental effect on the operation of induction motors. Moment of shaft movement is dependent on the square of the voltage, which greatly increases the sensitivity of the equipment. Pull is enough modes, accompanied by the constant change in the voltage, the operation unprofitable.

Undervoltage causes reduction shaft speed induction motors. Because of this slip value capable surpass critical. Running the engine becomes unstable. Stall speed and a full stop followed by an increase in consumption of reactive power. Its deficit in the network reduces the avalanche voltage, exacerbating the already unfavorable situation for the equipment. Accidents of this kind permanently deprive consumers of energy supply.

the voltage drop in a branch of the three-phase network disrupting the operation of generators in parallel included - change the direction of current. As a result, the generators are switched off. When the voltage falls to zero moment counteracting the rotation of the turbine shaft. The result is a dramatic increase in speed. Regulator having a large inertia, can not cope with the problem of maintaining a predetermined speed. Given the size of the impeller, the options are expected unpleasant consequences. For this reason, the generator is switched off without notice of consumers.

Finally, the falling water HPP or vapor stream fed to the power station at the same speed. Yes, there are regulating valve, but they are characterized by a great inertia. Therefore, it is believed to disconnect the equipment until the accident happened. At the same time the speed of adjustment is not guaranteed to lead to the restoration of system operability: the consumer will still receive less stress. A logical decision circuit uyazvlonnoy seen disconnection from the power supply. So to be able to maintain the voltage of the subscribers.

Parallel running generator, if located far from the scene of the accident, does not feel the voltage drop. However, a short circuit overloads the network considerably, and alone can not cope source. This is reflected in the fact that the unsustainable weight of consumers shaft will slow down. Guaranteed to work out of sync in parallel included generators. Dissynchronization shaft rotation speed would cause disruption frequency of 50 Hz, to which GOST 13109 imposes special requirements.

abnormal modes

Relays operate for a couple with automation. Design features protective equipment is closely related to the specific manifestation of abnormal conditions:

1. Overcurrent leads to overheating of the electrical insulation of cables and wiring, causing increased sparking. In this case, the equipment immediately unloaded, often requires a complete stop. EXAMPLE: kitchen mincer. When injected into the bone knife the current consumption increases dramatically, a shaft jammed. Solution: Immediately turn off the power to get rid of a factor counteracting the rotation of the motor.
2. The increased voltage. voltage increase of only 10% reduced lifespan incandescent bulbs in half. This symptom is observed when a sharp discharge network. For example, when the voltage at the generator terminals drops. The response speed of the shaft increases the pressure of the water, steam or gas. Regulating throttles and shuts, handing out less power. But because of the inertia of the mechanism is called short-term overload voltage. With the relay in this case it is recommended to lower the generator excitation current.
3. Reduced frequency as shown above occurs when sharp overloading generators. Most often it occurs when a short-circuit or the conclusion of operation of the power source (the increased proportion of each in the network). Solution: turn immediately back-up generator or disconnect the part of consumers.

Requirements for relay protection

selectivity

The system is required to localize the problem to the critical damage. This quality is characterized by a selective system or selectivity. The greater the number of different types of sensors than prudent engineering composition of dotted, the more likely that only a small part of the circuit would be out of operation.

Especially important quality it becomes to protect power plants and substations. The specified location chain branching occurs. And each area contains a lot of consumers. If you turn off all, the situation will nature of the accident: loss of profits and supplier customers. Retail outlets, stores, offices providing public services will lose customers that day.

Sensitivity

On the powerful lines of an accident often goes unnoticed. The damage will be a substantial amount. And consumers will suffer. Therefore, a good protection relay designed to provide the necessary sensitivity in time to prevent backfire.

speed

The relay circuit operate faster, the faster the danger is eliminated. On the scale of the system is of the utmost importance. Machines will not stop, transportation will continue to move. For each system speed is selected based on the conditions becoming available. For example, the dominant factor is the post-fault voltage quantity. The smaller the residue, the faster relies disconnect subscribers.

From this point of view, the most dangerous is a short circuit in two or all three phases if a neutral dully grounded. In these cases, the maximum voltage decreases and the current through the cable reaches appreciable size. Relays, respectively, are intended to respond to emergency situations quickly as possible tripping.

The second parameter is considered to be the total voltage of the circuit:

1. From 6 to 10 kV - off time 1.5... 3 seconds.
2. From 110 to 220 kV - breaking time of 1.15... 0.3 sec.
3. From 300 to 500 kV - breaking time of 0.1... 0.12 sec.

As for home appliances, it is usually not protected by relays instead used machines and fuses. Response speed is high enough. Especially in differential automata. The criterion of adequacy circuit shutdown speed is residual stress. It is, of course, not full-scale experiments and calculations.

According to the EMP is required to determine the residual voltage (after an accident in a short-circuit) outputs on the bus all the supply lines: transformers, generators, substations. If the figure goes below 60% of the nominal value, it is believed to disconnect the circuit immediately. off time is made up of a list of elements:

1. Response speed switch (typical values ​​are found in the references).
2. Speed ​​circuit protection relay.

Accordingly, existing requirements specified above, and the known response speed determined speed switch circuit relaying. In the future, the parameter you want to grant the right way, picking up equipment. Protection, triggered in less than 0.2 seconds, it is considered fast. But modern systems show at least an order of magnitude better parameters, instantly cutting down food.