کتاب Investigating the use of distance relay structures in transmission lines

1-1-2-Selectivity
A. Selecting a relay means selecting its time-current characteristic, which is determined based on the following parameters:
• Relay regulatory current
• Type of relay characteristics (constant time, decreasing from standard decreasing type, or very decreasing type, or very high decreasing type)
• Time in constant time relays or time factor in decreasing relays
Note: If the selected relays in a network have the same characteristics, their coordination is better.
B. The relay regulatory current must be higher than the permissible overload current.
C. The time-current characteristic of the relay should be such that the permissible transient currents (such as the inrush current of transformers and the starting current of motors) do not cause the relay to operate.
The regulation of primary over current relays of distribution transformers is as follows:
• The regulatory current of the instantaneous element of the primary side of the distribution transformer must be at least 4 times the nominal current of the transformer.
• The regulatory current of the delay element on the primary side of the transformer must be at least equal to the nominal current of the transformer.
Network and protection types
Depending on the voltage level of the system, geographical conditions and the concentration or non-concentration of the consumption load, different types of distribution networks can be used to meet the needs of subscribers.
Open network (radial)
This network is fed from one side, in such a network one or more conductors are connected from the current source to the main distribution board. In the radial network, any consumer may feed directly from the main switchboard. In such a case, the reliability factor of the network is good. Because in case of a connection in one of the branches, only one consumer without current is important. This network, which is installed to feed large consumers, is used in factories and industrial facilities. In another situation of this network, several consumers are fed by a branch line from the main distribution board. Obviously, in this case, as soon as there is a connection or defect in the branch line, all the consumers of the conductor fed from this line will be without current. Such a network is used for household purposes (especially lighting). Another application of open networks is the distribution of electric energy in different areas of cities and villages. In this case, if the low pressure distribution line be aerial, fuses should be installed at the head of each beam from which the branch is taken. The simple radial network is only fed from an output feeder of the distribution substation, and only one input line and one output line are connected to each busbar. The compound radial network is also fed from only one output feeder of the distribution substation, but more than one output line is connected to some busbar. Radial networks are cheap and simple to operate. The cheapness of these networks is on the one hand due to the small number of equipment and on the other hand due to the relatively low short circuit and as a result the equipment is simpler and cheaper. Protecting these networks is also easy and inexpensive.At the same time, the losses and voltage drop of these networks are high and usually the maximum possible use of the capacity of network transformers is not possible. Also, when a fault occurs, the electricity of a number of subscribers will be cut off, so the reliability of the network is low.
Ring network
In places where accidental power outage are not allowed, it is better to feed the networks from two different substations in order to increase the reliability factor of the electrical networks. In this case, with the failure of one of the two feeder lines, the power needed for the energy distribution line of the consumers can be supplied from the other side. The operation of ring networks is important, like the operation of networks fed from both sides, with the difference that in a network, the initial ring and the end ring of the conductor line are connected to a feeding point (source). Such a network is mostly used to feed points with high consumption density. The protection of networks fed from both sides and ring networks requires sensitive and accurate protection devices such as directional over current relays. With the correct implementation of the protection system of networks fed from both sides and ring networks, when a short circuit occurs in one of the lines, all the transformer substations can continue to work without any problems.
Ring network with feeding from one side: This network, like the radial network, is only fed from one feeder of the super distribution substation, but each of the distribution substations in the network is connected to this feeder from both sides. Usually, in ring networks, a switch that is normally opened is used, and the network is operated radially. In case of a fault in a part of the network, after isolating the damaged part, the mentioned switch is closed and the consumers who cannot be supplied with electricity from the previous route are fed from the second route. Naturally, these networks are more expensive than radial networks, but instead they have higher reliability.
Ring network with feeding from both sides: The difference between this network and the previous type is in the feeding of the two adjacent super distribution substations. Because the possibility of simultaneous blackout of two super distribution substations is very low. This network will have high reliability. This network can also be fed from two different feeders in a super distribution substation. Radial distribution systems are usually protected by reclosers, fuses, and over current relays [3]. For such systems, IBDG often has negligible effects on protection coordination [4]. In contrast, with SBDG, the fuse may operate before the initial operation of the recloser, thus affecting the fuse saving strategy. Similarly, the SBDG can affect the coordination between the fuse and the over current relay, leading to unnecessary disconnection of the entire feeder [5]. Reducing such issues requires placing a fuse or adjusting the settings of the recloser or over current relays [6]-[10].
For power systems with fault currents in positive and negative directions, relays that can be useful differently for each direction [30] are used. Dual setting directional relays have been proposed by relay manufacturers in [31] and [32]. In [33], directional relays are equipped with two sets of settings for positive and negative directions for radial distribution system with DG. For dual setting relays, each over current relay element can be programmed with different settings for positive and negative directions [32]. The advantage of this feature is that one relay can only perform the function of two directional relays [31].
Protection coordination
Current and time adjustments of high current protective equipment in order to achieve the desired performance are called coordination of protective equipment. The goals of coordination of protective equipment are as follows:
A. It fixes the fault in the shortest possible time, so that it is less than the duration of the equipment tolerance against passing currents.
B. It minimizes the extent of the network affected by the fault.
In general, the following information should be collected in order to coordinate between protective equipment:
1. A single-line diagram of the distribution network in which the type of all protective devices and the conversion ratio and protective class of CTs are specified.
2. Impedance of all parts of the network or currents and short circuit powers at any point.
3. Time-current performance curves of protective devices and tolerances of their performance characteristic (specific fault percentage).
4. Maximum permanent load current, short-term permissible overloads of transient loads (such as starting current of motors, inrush current of transformers, etc.).
For ring distribution systems, the DG can continue to operate after fault isolation to feed power through other feeders. Protective equipments that can provide fast fault isolation are necessary so that the low voltage period remains short and therefore the fault passing through the DG is improved [13]. For ring systems, DOCRs become an attractive option due to the bidirectional nature of the fault currents. Such relays are optimally coordinated to minimize the total operating time of all relays [14]. In order to overcome the effect of DG, it is necessary to determine new optimal settings of the relay, taking into account the presence of DG. Different optimization methods, including conventional and innovative techniques, have been applied to optimally determine the time index and pickup current settings of DOCRs, which guarantee the minimum and coordinated relay operation times [15]-[23]. Other protection coordination strategies take advantage of capabilities of digital DOCRs to improve the performance of the protection system, especially with the presence of DG, by using different groups or modified relay settings and specifications [24]-[27], or by taking advantage of communication potentials in digital relays [28], [29].
Research structure
This thesis consists of 5 chapters. In the first chapter, the generalities related to this research and the initial definition of the issues related to the research topic are presented. In the second chapter, the theoretical foundations related to the research topic have been collected. The third chapter describes the research implementation method. Chapter 4 related findings and simulations are stated. In chapter 5, the discussion and conclusion along with suggestions and solutions related to the research topic are presented.

Summary of the first chapter:
In the first chapter, the generalities of the research were examined. Discussions were also raised in connection with distance relays. The desired problem was raised in the research and the necessity of conducting the research was stated. In this research, high current relay is used. Relays or protective equipment are devices that protect power system parts and devices from damage and connections by breaking the circuit and opening the power switches. Before the power switch can be opened, its actuator coil must be turned on. This is done by the relays. A relay is a device that is triggered by a change in an electrical quantity such as voltage and circulation or a physical quantity such as temperature and oil movement (in the Buchholts relay) and causes other devices to operate and at the end of the cycle is opened by the power switch (in the production system and transmission and distribution) or circuit breaker. Therefore, the faulty part of the network is isolated by the relay and it prevents other healthy parts of the network from continuing to work and the stability of the network is kept in the same state as before. Goods and devices are sheltered from damage and connections and the amount of internal damage is limited