АНГ 2 Relay protection / Individual_Task3

1. What is the secondary current?

2. How many times fuse can be used?

3. Explain the construction and purpose of an electrical fuse?

4. Why do we use relays in the power systems?

5. What is pick-up current? How can it be selected regarding to fault current and load current?

6. How many types of overcurrent relays do you know?

7. What is the purpose of using time delay in the overcurrent relays?

8. What is the tripping characteristic curve?

9. How many inverse time curves is mentioned in the video?

1. The secondary current is the current that is converted to a smaller value by the current transformer and that is fed to the relay, this current. If this current exceeds the trip value for a certain period of time, the relay will issue a trip command because the circuit breakers interrupt the current path.

2. The fuse, when the specified current value is exceeded, the conductor, which is made of copper, zinc, lead or silver, is heated to a temperature at which it melts. This breaks the chain. Fuses are designed for only one operation. If a fuse fails, it must be replaced.

There is also a bimetallic fuse that does not need to be replaced with another one because the current passing through the bimetallic plate heats it up, and when the current in the circuit in which the fuse is connected rises above the permissible value, its heating increases greatly, and the bimetallic plate bends, opening the contacts. After the circuit breaks, the bimetallic plate cools down and returns to its original position, closing the contacts.

Also a reusable fuse is a self-resetting fuse, which If too much current begins to flow through the fuse, it starts to heat up, and at some point in time the polymer passes into an amorphous state, increasing in size. Because of this increase, the carbon chains begin to break, which causes an increase in resistance, and the fuse heats up even faster. Eventually, the resistance of the fuse increases so much that it begins to noticeably limit the flowing current, thus protecting the external circuit. After the device cools down, the crystallization process takes place and the fuse again becomes an excellent conductor.

3. The purpose of the electrical fuse is to protect the elements of electrical energy distribution systems and equipment from overcurrents: short circuit currents and overload currents.

The main parts of any fuse are:

fusible insert;

an element used to place (fasten) the fusible link and create conditions for extinguishing the arc when the fusible link burns out;

fuse base in the form of a rack or cartridge, depending on the type of fuse, with a clamp for connecting to an electric current circuit.

The base of the fuse and the element used to accommodate the fusible link are provided with appropriate contact devices. With the help of contact devices, the element is fixed in the base of the fuse, and reliable inclusion of the fusible link in the protective current circuit is also ensured.

Some fuses are equipped with additional devices: clips to prevent the fuses from falling out during vibration, handles for convenient and safe removal of the removable fuse element from the switchgear.

A fuse connected in series with the protected element blows when the current of the protected circuit exceeds by a certain amount the rated current of the fuse-link. In this case, the fuse automatically turns off the damaged section of the network. The fuse does not respond to any other deviations from the normal operation of the network. To restore power to the network section when the fuse-link burns out, it is necessary to replace the burnt-out fuse-link with a new one.

4. Relays have certain technical characteristics and operational qualities. These device parameters are determined by their intended purpose. There are three types of relays:

Management.

protection.

Alarms.

Control relays are primary devices that are installed directly in the electrical circuit. This type of KU is necessary to turn on and off certain components of the circuit. Such relays are used as an independent element of the circuit or are components of low-voltage complete devices:

boxes;

panels;

cabinets.

The switching protection device is switched on and off by network elements with thermal contacts. Such equipment includes electric motors and fans. If the temperature rises, the thermal contacts open. Over time, when the temperature regime reaches operating values, the operation of the equipment is restored.

Alarm relays are an important element of security systems installed on vehicles, at enterprises and adjacent territories. The switching device generates a signal when the set value of the controlled parameter is reached. These characteristics may include:

current;

voltage;

frequency;

pressure;

temperature;

acoustic parameters.

5. Tripping current is the smallest secondary current taken from the current transformer that exceeds the allowable value for a certain period of time, which corresponds to issuing a trip command to the relay.

The selection of the operation current according to the load current is carried out from such a condition that the relay operation current is greater than the maximum load current , or this can be expressed by the following formula: The relay operation current is equal to the product of the reliability factor by the maximum load current, where the reliability factor takes a value from one and a half units to two.

The selection of the relay operation current according to the short circuit current is carried out so that the relay operation current is greater than or equal to the product of the safety factor and the three-phase short circuit current at the end of the protected line, in the maximum mode.

6. Current relays are divided into primary and secondary. Primary current relays are built directly into the circuit breaker drive, being its integral part. They are mainly used in networks with voltage up to 1 kV.

Secondary relays are connected through a current transformer installed directly on the power bus or the core of the power cable. Secondary overcurrent relays are further divided into several subgroups. These are electromagnetic relays, induction relays, differential relays, relays on integrated circuits.

Electromagnetic relays work in such a way that an armature with magnetic properties is attracted to the core. As a result, the contact group opens or closes. When the current drops, the return spring returns the movable element to its original state.

The induction relay consists of a fixed magnetic circuit with coils (windings) and a movable part, made in the form of a metal disk or cylinder, located on the axis. When alternating currents are supplied to the relay windings, alternating magnetic fluxes arise, which induce currents in the moving part of the relay. As a result of the interaction between the indicated magnetic fluxes and currents, a torque arises in the movable part of the relay, under the influence of which the movable part can rotate or turn at a certain angle. From the above it can be seen that induction relays can only operate on alternating current.

The differential current relay compares the currents in a certain area. With an external short circuit, their difference is zero and the relay does not work, and with an internal short circuit, the currents in the controlled area add up and the relay works.

Relays on microcircuits (integrated electronic). Such types are made on the basis of semiconductor elements.

7. The purpose of using a time delay in an overcurrent relay is to avoid false positives. For example, in an asynchronous motor, starting currents are large, and the workers are five times less than them. And so that the protection does not work when the engine is started (since it is designed for operating current with a margin), a time delay is introduced. The inrush currents drop quickly and the relay does not operate.

8. The tripping characteristic curve is the dependence of the tripping time of a circuit breaker by a release on the strength of the current flowing through it.

9. There are two reverse time curves mentioned in the video, namely reverse time curve and inverse time curve.

The inverse trip time curve indicates that the relay will trip faster and faster due to the higher overcurrent.

Inverse curve other than inverse time curves Definite time delay curve sign Same delay time for all overcurrents that exceed the pickup value in our example curve, which means that the tripping time is the same for ten times overcurrent and hundred times overcurrent overload current, that is, the relay operates with maximum tripping speed after exceeding the tripping current, which means that the relay trips as quickly as possible for very high currents.