Network protection
Grid protection is a term from electrical power engineering that describes the technical precautions with which the power grid is protected from the effects of faults ( short circuit , earth fault ) in individual power supplies. The network protection devices measure the current via current transformers and / or the voltage via voltage transformers ; they differentiate between faults and normal operation. If the fault is detected, the associated circuit breaker is switched off and the faulty network segment is thus separated from the rest of the supply network. In this way, it remains protected from the effects of the error.
In Germany, distance protection devices or UMZ protection devices are usually used in medium and high voltage networks. Line differential protection devices are also used with existing message paths. In contrast to the distance and UMZ protection devices, these are strictly selective (protection area between the current transformers on both sides), but cannot be used as backup protection outside the protection area.
The protection devices and their functions are identified according to the ANSI device numbers .
Fuse
The fuse is the simplest protection of all. However, there are limits when it comes to breaking capacity. In addition to the breaking capacity, the fuse has the disadvantage that it can only be used once and has to be replaced after it has been triggered. Unlike the systems described below, however, it does not require any auxiliary energy.
Overcurrent protection
With overcurrent protection, the measured current is used as the triggering criterion. In special cases such as directional overcurrent protection, the measured voltage is also used to determine the direction of the fault. The overcurrent protection function is mostly used for simple applications or as a reserve function (for example if the measuring voltage on the distance protection fails).
UMZ protection (ANSI 50)
At a definite time protection ( u nabhängiger M aximalstrom for eitschutz), also known as overcurrent-time protection, is on exceeding a set current amount for. B. 400 A, issued a signal to turn off the circuit breaker after the associated delay time . The delay time is independent of the current actually flowing, that is, it does not matter whether z. B. now 450 A or 4,500 A flow. ( English definite time-delay overcurrent protection ).
However, a distinction is made between overcurrent and high-current stages, which can differ in the tripping times.
Several UMZ relays, which are connected in series, can be extended to a multi-level protection with the help of a graduation of the tripping time and thus an increased selectivity can be achieved. The disadvantage of such a staggered plan is that a short-circuit that is directly on the infeed has the highest tripping time, but the highest short-circuit current can also be expected there.
UMZ-R protection (ANSI 67)
In this protection also will be added to the current line voltage evaluated and a possible power failure now gets an R ichtung. In this way, errors can be distinguished in the forward direction and in the reverse direction based on the relay installation location. These errors can then be switched off the network with different times. This allows good, selective behavior to be achieved in simply meshed networks with simple protective devices.
AMZ protection (ANSI 51)
Of a bhängige M aximalstrom z eitschutz, also known as dependent overcurrent protection, operates according to the exceeding of a set operating current. Once exceeded, the tripping time is a function of the fault current actually flowing. With today's digital relays, various triggering characteristics can be set there . In a comparison with fuses, the AMZ protection certainly comes closest to this. Here, too, the resulting tripping time depends on the fault current and there are also different characteristics. It is mainly used in motors (large low-voltage motors / high-voltage motors) because their characteristics require a very high inrush current. In addition, AMZ protection is also found on transformers. A grading of AMZ relays as with the UMZ is not easy to implement due to the non-linear tripping characteristic, as this would have to be adjusted with every network change.
Distance protection (ANSI 21)
The distance protection also requires current and voltage to detect errors. The impedance (apparent resistance) is continuously calculated from these two quantities .
In the event of a short circuit, z. B. the voltage together, it flows a high current. This results in a small impedance.
(With an ideal short circuit U = 0)
A tripping time is assigned to an impedance range (impedance zone) (e.g. 0 - 2 Ω → 0.2 s | 2 - 4 Ω → 2.5 s). A distance protection relay thus offers several, staggered tripping times. Faults that are closer to the measuring point of the distance protection have a lower impedance (since the impedance is essentially only determined by the piece of overhead line or cable to the fault location) and are usually switched off faster than faults that are further away. Here, too, the direction of the error can be seen and an error can be detected in the forward direction with z. B. 0.12 Ω can be switched from the mains with a time of 0.05 s, while with the same absolute impedance of 0.12 Ω in the reverse direction, the error is switched from the mains with 1.5 s.
Differential protection (ANSI 87)
The differential protection uses the current sum as a triggering criterion according to Kirchhoff's node point theorem . All currents flowing into or out of the protected object are measured and added up. The differential current (= current sum) for the protected object is formed from this. If the differential current is not equal to 0, tripping occurs. The differential protection is simple (no further measured variables such as voltages are required), fast and 100% selective in the protection area.
Transformer differential protection (ANSI 87T)
Transformer differential protection devices are used to protect power transformers. With transformer differential protection, the currents on the high-voltage and low-voltage sides are determined. The currents are then converted to a reference side of the transformer. Now - taking into account the translation factor - according to the first Kirchhoff law, the sum of the incoming currents should be equal to the sum of the outgoing currents. If this basic requirement is not met, the protection is triggered.
Due to the faults of the current transformers, there is always a low measurement fault current. In order to be able to react safely and selectively to the fault even in the event of faults outside the transformer area, some calculations are made in the protective devices (formation of stabilizing and differential currents, determination of switch-on processes based on harmonics, etc.). In addition, as backup protection for the transformer, distance and / or UMZ protective devices are also used on the transformer.
Line differential protection (ANSI 87L)
The line differential protection works on the same basic principle as the transformer differential protection. However, a message path is used here to transmit the measured value of the current from one side of the line to the other. This means that both protection devices know their own current and that of the remote station. If a difference is found here, the line is switched off via the assigned circuit breaker.
Digital protective devices can usually still be operated as UMZ protection if the communication path fails. However, the strict selectivity of the line differential protection is then missing. In addition, as backup protection for the line, distance and / or UMZ protective devices are often used on both sides of the line.
Busbar protection (ANSI 87B)
The busbar protection also works according to the measuring principle of the weighted current comparison from the differential protection. This protects busbars with extremely short tripping times. With digital busbar protection systems, an error is usually detected after 15 ms and the busbar area is switched off busbar-selectively.
In the area of medium-voltage systems , simple busbar protection systems can occasionally be found (rear locking). The protective devices of the line outlets are used to block a tripping stage on the transformer protection. This type of protection assumes that no short-circuit current flows from the network in the direction of the busbar.
Frequency protection
With frequency protection , the network frequency is measured and evaluated as a measured variable. (There is also frequency protection beyond energy technology in the HF range in the sense of protecting interference-free frequency use, see Frequency Protection Contribution Ordinance of May 13, 2004.)
Underfrequency protection (ANSI 81U)
If the grid frequency should drop due to a power deficit, individual regions are specifically and automatically switched off using electronic frequency relays according to a five-stage relief plan in order to restore a balance between the power generated and the power required.
Overfrequency protection (ANSI 81O)
If the grid frequency increases, the drawn grid power is less than the power currently being generated and fed in by the machine sets. Now the power is supplied to the rotational energy of the generators and these are accelerated. If a specified frequency is exceeded, a warning is issued or the machine set is switched off automatically. Here, too, the aim is to protect the machine with the shutdown and to restore a stable state in the network.
Types of stimuli
Overcurrent excitation
The most common type of pickup is overcurrent pickup. When a set current level is exceeded, the device picks up - a set time begins to run down. If the error persists, the assigned circuit breaker is triggered after the set time has elapsed .
Application: Mainly medium voltage; Operating current <short circuit current
Voltage-dependent overcurrent excitation
With this type of excitation, the voltage at the relay installation location is also taken into account in addition to the current. If the voltage falls below a set value, the excitation value of the current becomes more sensitive, e.g. B. over 45V with 2.5A under 45V with only 0.5A.
Application: medium, high and extra high voltage. The operating current in the load range (see also angle-dependent UI phi excitation or impedance excitation) can now be greater than the short-circuit current! As a rule, the short-circuit current in meshed networks is a multiple of the operating current.
Impedance excitation
The impedance excitation represents the high-end version of the excitation. The angle between current and voltage is also evaluated here. In this case, even the smallest fault currents can be recognized and the lines switched off, although the voltage has remained constant in relation to the time before the fault occurred.
Application: Mainly high and extra high voltage; largest operating current> smallest short-circuit current (since overcurrent excitation is not possible here)
Special functions
Automatic restart (ANSI 79)
With automatic reclosing, the tripped circuit breaker is automatically reclosed after the pause command time (single or three-pole). In the past, this process was also called short interruption (KU).
This function is used in overhead line networks. It is assumed that the error disappears during the shutdown, this means that the error path has been eliminated. In the event of atmospheric disturbances (thunderstorms, snow, etc.), the fault path is deionized and when it is switched on again, there is sufficient insulation from the surrounding air.
Various control options are specified in the devices for the control of the EVU and the behavior when switching to a still present fault (tree in overhead line).
Breaker failure protection (ANSI 50BF)
The breaker failure protection (SVS) is also called breaker reserve protection (SRS) or fall back / fall back protection. Here it is assumed that an error z. B. can not be switched off on a line because z. B. the circuit breaker on this line is defective. In this case, the switches of all the other lines are used (therefore also "backreach protection" or "backreach") that are connected to the same busbar.
"Current seeks the path of least resistance" - so if the switch on the faulty line does not trip, current flows from the other lines via the busbar to the fault. All that remains is to switch off all lines that can continue to supply the fault with power. The way it works is very simple: the connected protective relay (of whatever type) detects an error and issues an OFF command. This TRIP command is sent to the drive of the circuit breaker (via auxiliary relays, contactors). At the same time, a timer is started with this OFF command, so a preset time begins to run down. If the circuit breaker is OK, it trips after approx. 15 ms (after a TRIP command - not after pickup) → error is switched off, protective relay no longer measures an error → OFF signal is deactivated.
If the circuit breaker does not trip, the time relay runs through and actuates a contact after the set time (approx. 0.3 - 0.7 s) has elapsed. (With digital protective relays, this delay can also be set internally for a corresponding binary output.) This contact sends an OFF signal to a ring line. This signal is fed to all circuit breakers whose lines are on the same busbar via this ring line and a disconnector switch image (image of the switching status of the system, here you can see which line is connected to which busbars and much more). In this sense, the breaker failure protection is not an independent protection, at least it is not an independent protective relay. “Only” the functions of other protective devices are used differently.
See also
literature
- Adolf J. Schwab: electrical energy systems. Generation, transport, transmission and distribution of electrical energy. 3. Edition. Springer, Berlin 2012, ISBN 978-3-642-21957-3 .
- Walter Schossig, Thomas Schossig: Network protection technology. 5th edition. VDE-Verlag, Berlin 2016, ISBN 978-3-8007-3927-1 . (also: EW Medien und Kongress GmbH, Frankfurt am Main 2016, ISBN 978-3-8022-1137-9 )
- Walter Schossig: History of Protection Technology. VDE, ETG member information July 2014, pp. 31–36. ( online. Accessed December 28, 2015)
- YouTube: Video explaining how overcurrent protection works: https://www.youtube.com/watch?v=PJNGgn_2E9o