Power-system protection
Power protection is a term used in electrical engineering, describes the technical arrangements which allow the electric power transmission network to be protected from the effects of faults (short circuit, earth) in individual power supplies. The network protection devices measure the current on the current transformer and / or voltage converters, the voltage, they differ from normal operation in case of error. If an error is detected, the associated power switch is turned off and thus separated from the rest of the failed power supply network segment. This is therefore protected from the effects of the error.
In Germany distance protection devices or definite time protection devices are used in medium and high voltage networks usually. For existing communication paths and line differential protection devices are used. These are in contrast to the distance and definite protective devices strictly selective ( protected area between the current transformers on both sides ), but can not be used as backup protection outside the protected area.
Fuse
The fuse is the simplest protection of all. However, their limits when it comes to the shutdown power. In addition to the breaking capacity fuse has the disadvantage that it is applicable only once and must be replaced after triggering. Unlike the system described below, however, it does not need power supply.
Definite time protection
At a definite time protection ( independent maximum current protection) is when a set amount of current, eg 400 A is exceeded, given a signal to turn off the circuit breaker after the associated delay time. The delay time is independent of the actual current, that is, it does not matter if, for example, now 450 A or 4500 A flow. (English definite time-delay overcurrent protection).
Several definite time relays which are connected in series, can be extended to a multi-layered protection and thus achieve a higher selectivity with the help of a graduation trip time. The disadvantage of such a grading plan is that a short circuit is located directly at the feed has the highest delay time, however, there is also the highest short-circuit current is to be expected.
DMT -R Protection
With this protection in addition to power, the mains voltage is evaluated and a possible network error now gets one direction. This fault in the forward direction and in the reverse direction relative can be distinguished on the relay location. These errors can then be switched with different times from the net. This can be achieved by good selective behavior with simple protective devices in just meshed networks.
IDMT protection
The dependent maximum time overcurrent protection operates according to the operating current exceeds a preset. After crossing the tripping time is a function of the actual fault current flowing. In today's digital relays can be there to set different trigger characteristics. In a comparison with fuses comes the IDMT protection of this principle certainly the next. Again, the resulting tripping time depends on the fault current and there are also different characteristics. Its application it is mainly used in motors (large low voltage motors / high voltage motors), because they require a very high inrush current in their characteristics. In addition, one finds the inverse time overcurrent protection even with transformers. A graduation of IDMT relay as in DMT is not easy to implement due to the nonlinear tripping characteristic, as these would have to be adjusted at each network change.
Distance protection
The distance protection is also needed for detecting flaws current and voltage. From these two quantities are constantly the impedance ( impedance ) is calculated.
If a short circuit breaking example the voltage collapses, a high current flows. This has by a small impedance result.
(The ideal short circuit U → 0)
A range of impedance ( impedance zone ) is a release time assigned (eg 0-2 Ω → 0.2s | 2-4 Ω → 2.5 s ). A distance relay thus offers several staggered release times. Errors that are closer to the point of measurement of the distance protection, have a lower impedance ( because the impedance is essentially determined only by the piece of overhead line or cable to the fault ) and are usually switched off faster than more distant errors. Again, the direction of the error can be recognized and thus, an error in the forward direction, for example, 0.12 Ω with a time of 0.05 s can be switched out of the network, while at the same amount- impedance of 0.12 Ω in the reverse direction of the error with 1.5 s is switched from the net.
Transformer differential protection
For the protection of power transformers transformer differential protection devices are used. In a transformer differential protection, the currents of the upper voltage and the low voltage side can be determined. The currents are then converted to a reference side of the transformer. Now you should - considering the translation factor - after the first cherry- hoff 's law, the sum of the incoming currents to be equal to the sum of the currents flowing. If this basic requirement is not met, triggers the protection.
Due to the error of the current transformer is always a low metrological fault current available. In order to respond safely and selectively to errors outside the transformer area on the error, some calculations are in the protective devices employed ( form of stabilization and differential currents, determination of switch- based harmonics, etc.). In addition, as backup protection for the transformer are also distance and / or DMT - protection devices on the transformer with in use.
Line differential protection
The line differential protection operates on the same basic principle as the transformer differential protection. However, here a communication path is used to transmit the measured value of the current from one side of the line to the other side. Thus, both protective devices know one's own and the power of the remote site. Here, a difference is detected, the line is switched off via the associated power switch.
Digital protection can often be operated even in case of failure of the message path as a definite time protection. However, lacks the strict selectivity of the line differential protection. In addition, as a back-up protection for the line are often distance and / or definite time protective devices on both sides of the line in use.
Busbar protection
After the measurement principle of the rated current comparison of the differential protection and the busbar protection works. This protects busbars with extremely short shutter times. With digital busbar protection systems, an error is usually detected after 15 ms and off the busbar area busbar- selective.
In the field of medium voltage systems, one occasionally finds simple busbar protection systems before ( rear locking ). Here, the protective devices of the cable outlets are used to block tripping stage at Umspannerschutz. This type of protection requires that no short -circuit current flows from the grid in the direction of the busbar.
Underfrequency protection
In the under-frequency protection, the mains frequency is measured as a measure and evaluated. If the grid frequency drop due to a power deficit, individual regions according to a five-stage relief plan specifically switched off and automatically by means of electronic frequency relays to achieve a balance between the generated and the required power again.
Over-frequency protection
In this case, the removed power output is less than the currently generated and fed power of the units. Now, the power is supplied into the rotational energy of the generators and this can be accelerated. At a fixed frequency is exceeded, a warning or an automatic shutdown of the machine set. Again, it is the aim of the cut-off to protect the machine, and a stable condition in the network to re-establish.
Excitation types
Overcurrent pickup
The most common Anregeart is the overcurrent pickup. For a fixed current level is exceeded, the device suggests - a set time begins to run. If the error persists, is triggered after the set time of the associated circuit breaker.
Application: Mainly medium voltage; Operating Current < Short-circuit current
Voltage dependent overcurrent pickup
In this type of excitation in addition to the current and the voltage at the relay is considered. If the voltage drops below a set value, the starting value of the current sensitive eg: . Above 45V 2.5 A at 45V with only 0.5 A.
Application: medium, high and very high voltage. The operating current can thus be in the load range (see also angle-dependent UI phi excitation or excitation impedance ) is now larger than the short-circuit current! In general, the short-circuit current is in meshed networks, a multiple of the operating current.
Impedance fault
The impedance fault detection, the high-end version of the excitation dar. Here also the angle between current and voltage is evaluated. In this case, you can already see the smallest fault currents and switch off the lines, even though the voltage with respect to the time before the fault occurs has remained constant.
Application: Mainly high and extra high voltage short-circuit current Operating current <<
Special Functions
Automatic reclosing
In the automatic reclosing (AR ), the triggered circuit breakers after the break command speed is automatically turned back on (one or three pole). Previously, this process was also called short interruption ( KU).
This function is used in overhead line networks to bear. It is assumed that the error during the shutdown disappears, this means that the error path has been eliminated. At atmospheric disturbances ( storms, snow, etc. ), the error distance is deionized and reconnection sufficient insulation by the surrounding air is given again.
For the activation of the AR and the behavior when switching to a still existing bug ( tree in overhead line) various control options are specified in the devices.
Breaker failure protection
The breaker failure protection (CBF ) is also called switch -up protection (SRS ) or resorting / backhitching protection. Here is assumed the case where an error such as on a line can not be disabled because, for example the power switch of this line is defective. For this case reference is made to the switches of all the other wires (hence " backhitching protection" or "back- gripping " ) which are connected to the same busbar.
"Electricity seeks the path of least resistance " - that does not trigger the switch of the faulty line, so current flows from the other lines on the busbar to fault. It remains only to switch off all lines which can supply the error continues with electricity. The operation is quite simple: the connected relays ( any kind ) detects an error and outputs an OFF command. This command is OFF to the circuit-breaker operating sent ( via auxiliary relays, contactors ). At the same time a timer is started up with this OFF command, so it starts a preset time count down. If the circuit breaker is in order, it triggers after about 15 ms (after OFF command - not after excitation ) → error is off, relay protection measures no mistake more → OFF signal is deactivated.
Does not trigger the circuit breaker, so the timer runs by and operated after the set time (about 0,3 - 0,7 s ) a contact. ( For digital protective relay this delay for a corresponding binary output can also be adjusted internally. ) This contact is an OFF signal to a loop. About this loop and an isolator switch image ( image of the switching state of the system, here's to see which line is connected to which bus bars, etc. ), this signal is supplied to all power switches whose lines lie on the same busbar. In this sense, the breaker failure protection is not independent protection, at least it is not an independent protective relay. There are "only" the functions of other protective devices utilized differently.