An important area of the energy industry is the improvement of supply and distribution networks. Currently, all over Ukraine, distribution electric networks are in serious condition, due to:
- a high degree of physical and moral deterioration of electrical equipment (the age of equipment often reaches 50 years and above);
- high losses of electric energy
- low level of automation (low level of telealarm and telecontrol)
Along with this, growing demands on energy distribution companies on:
- reduce power shortages
- increase the transmitted power
- increase in transformer power
The increase in electrical loads is noted not only in the places of new development, but also in areas with already existing network infrastructure, where the construction of new substations is extremely difficult. The growth of electrical loads, especially in the centers of regional cities, requires cardinal decisions. The experience of European countries such as Poland, France and the Federal Republic of Germany shows the possibility of a solution by creating a new electric network with an increased voltage level. In such countries, an electrical network with a voltage level of 20 kV has been introduced and exists.
- improving the reliability of power supply;
- improving the quality of power supply;
- reduction of electric energy losses;
- increased line throughput;
- reduction of short circuit currents;
The purpose of the appearance of an electrical network of 20 kV is:
Undoubtedly, the creation of a 20 kV power supply network in Ukraine, primarily in regional centers in places with a high density of electrical loads, raises a number of serious and complex financial, organizational, technical, legal issues, but under current conditions there is no worthy alternative to this solution.
Based on the goals, an important component of the 20 kV electric network should be the construction of relay protection and automation, telemechanization and dispatch control on principles and approaches different from the 10 kV network.
Kyivpribor company at the Elcom2017 exhibition presented a new fourth generation of protection series MRZS. The fourth generation is distinguished by its modular design and the ability to increase hardware resources with the help of additional easily installable blocks, a significant improvement in application software and the expansion of the range of supported communication protocols.
Among the presented devices of the MRZS series, the differential protection of the MRZS-D line is especially notable, the first among domestic devices that fully meets the requirements for relay protection systems in 20 kV networks.
The device can be used in the design of new networks and the reconstruction of existing substations and transformer points.
MRZS-D is a microprocessor-based device for phase-by-side two-sided differential protection and line control designed to protect overhead lines and power cables in power grids of industrial companies and power grids of industry, including radial, ring and closed distribution networks with or without distributed generation.
MRZS-D is adapted to work in networks with isolated, grounded through active resistance, compensated (grounding through impedance), including networks with longitudinal compensation and grounded neutral.
The MRZS series is distinguished by the implementation of the IEC 61850 standard in terms of communication and interoperability of substation automation equipment.
MRZS-D can be used to protect feeders in ring line networks. The differential protection devices of the MRZS-D line allow selectively disconnecting the damaged part of the network and ensuring the distribution of electricity in the serviceable part of the network.
The MRZS-D has several different standard configurations that allow you to optimally use the available capabilities.
Depending on the configuration, the following functions can be implemented in MRZS-D devices:
- phase differential line protection, maximum;
- distance protection;
- directional overcurrent protection;
- non-directional overcurrent protection;
- directional earth fault protection;
- non-directional earth fault protection;
- earth fault protection based on integrated conductivity monitoring;
- earth fault protection based on active power control;
- earth fault protection based on the control of higher harmonics;
- phase failure protection;
- reverse sequence current protection with power direction control device;
- transient/intermittent earth fault protection;
- zero sequence overvoltage protection;
- protection based on phase voltage and frequency control;
- function of three-phase multiple automatic reclosure of overhead lines, including with synchronism capture;
- redundant fail-safe function (CBD).
The MRZS-D device can also be used for differential protection of systems with a transformer in the protected area.
The device provides basic protection with absolute selectivity of overhead lines and cable feeders in distribution networks.
The MRZS-D also contains current-based monitoring functions for long-range redundancy, as well as local redundant protections in addition to the basic line differential protection.
The line differential protection function includes 2 protection levels:
- sensitive stage with braking;
- high-speed coarse protection level (cut-off).
Sensitive stage with braking provides sensitive differential protection and maintains stability, for example, when the current transformer is saturated. To inhibit the sensitive stage, the second harmonic detection can be used if the power transformer is to be switched on outside the protected zone.
If the power transformer is located in the protected zone, when it is turned on, the occurrence of magnetization currents of considerable magnitude may occur.
The content of the second harmonic in the differential current exceeding the selected threshold value blocks the trip from protection. Since the braking function during inrush currents of the magnetization current works in phase, the protection is fully operational when the transformer is switched on for single-phase damage, when the magnetization current can flow in one of the undamaged phases.
The blocking signal of the protection operation is transmitted to all devices, thereby ensuring the failure of devices located at the other ends of the protected object.
Compensation of the capacitive current due to the capacity of the overhead or cable line, in turn, allows for greater sensitivity.
The connection of voltage circuits from the voltage transformer of the connection allows you to automatically compensate for capacitive currents.
Based on the nominal data of current transformers and the magnitude of the measured currents, the errors of the current transformers are calculated from the possible errors, which affects the calculation of the necessary amount of self-braking.
Thanks to the self-braking function, the differential protection always works with the highest possible sensitivity, since the braking values automatically change according to the levels of available errors. Thus, even damage arising through a large transient resistance, under the conditions of the flow of significant load currents, can be effectively detected.
The high-speed coarse step (cut-off) of the differential protection is less sensitive, but it works quickly at high fault currents. If there is a transformer in the protected zone, the vector group is automatically compensated taking into account the types of windings and the values of the parameters of the connection group.
The sensitive stage can have an independent or inverse time delay.
The combination of the potential of GOOSE messages on the station bus and the transmission of discrete signals through the communication channel of protection and the transmission of discrete signals opens up new applications compared to traditional differential line protection, including providing superior speed, selectivity and reliability. Direct tele-shutdown ensures that both ends are always shut off simultaneously, regardless of short-circuit power.
Standard signal configurations can be changed using the WisiNet 2 signal matrix.
In addition, the WisiNet 2 logic configuration program supports the creation of multi-level logic functions using various logic elements, including threshold elements, timers, and triggers. The combination of protection functions with logical function blocks allows you to adapt the device configuration to customer requirements depending on the specific application. The device is supplied from the manufacturer with the standard settings and parameters described in the instruction manual: the assignment of digital inputs, digital outputs, communications between functions and alarm LEDs fully complies with the connection diagrams and communication arrangements of the devices.
Data is exchanged between devices via a dedicated fiber channel. For differential line protection, a multimode or single mode fiber optic cable with ST connectors is used, designed for a wavelength of 1310 nm.
The channel is used to transfer phase current values between devices. Current vectors from two MRZS-D devices located at a considerable distance from each other must be time synchronized in order for the protection algorithm to properly process differential currents. For synchronization, the so-called echo method is used according to the IEEE 1588 standard. Therefore, no external devices, such as a GPS synchronizer, are required for transmitting differential protection data.
Below is an implementation of this principle for a double-end line.
Each of the devices measures phase-by-phase current together with its installation and transmits information about its magnitude and phase to the opposite end of the line. As a result, these currents can be summed and processed independently in each device.
In the case when the number of ends of the line is more than two, a data exchange network is built and each of the devices receives information about the sum of currents from the corresponding current transformers in the place of their installation, flowing into the protected zone.
The data transmission network can also be closed in a ring, which will provide redundant data transfer between devices: even if one of the communication channels fails, the differential protection system will continue to function correctly. Devices detect errors during data transfer and automatically switch to another available communication channel. It is also possible to disconnect one end of the line, for example, to test, or to disable the local protection device. In such cases, with the ring-shaped data exchange network connection scheme, the remaining MRZS-D devices continue to function correctly.
An alternative to the fiber optic channel can be a galvanic connection via a control cable, consisting of a twisted pair cable and modems at the ends of the communication line. Compared with traditional solutions of longitudinal differential protection of a line with the transmission of an analog signal between half-sets via control wires, the MRZS-D devices in combination with modems provide modern phase-by-phase longitudinal differential protection of a line using digital measurement signals (transmission of auxiliary digital signals is also possible) over the existing channel communication.
The modems have a Quality of Service (QoS) indication, and the MRZS-D continuously monitors the communication channel. The modem provides an isolation level of 5 kV (rms) between the control cable terminals and ground. Modems (master and slave) are galvanically connected to the ends of the control cable and optically connected to the device with short single-mode optical cables.
A control cable (twisted pair) with a cross section of 0.8 mm2 usually allows you to maintain communication at a distance of up to 8 km.
The communication channel can be used not only for the continuous exchange of protection data, but also for the transmission of discrete signals (BST), i.e. to transmit user digital information between MRZS series devices.
Monitoring of communication channels allows real-time information about each channel to be obtained independently. The line monitoring function of the differential protection continuously monitors the protection data exchange channel. In case of a steady failure of the communication of the protection functions, an alarm is triggered in the MRZS-D device and in the automatic control system. Thus, redundancy of the communication channel increases the safety of operational personnel and ensures that the operator has all the necessary information about the system, provided that the backup channel is operational.
Further, by default, two coarse stages of overcurrent protection are switched on.
Particular attention was paid to protection against unauthorized access.
To protect the MRZS-D from unauthorized access and to ensure the integrity of information, the device has a four-level role-based authentication system with separate passwords and a number of additional innovations.
MRZS-D, like the entire MRZS series, is designed to implement the IEC 61850 standard in terms of communication and the possibility of interaction between substation automation equipment.
The application of IEC 61850 provides support for all monitoring and control functions. In addition, with the help of IEC 61850, it is possible to set the settings, read out the waveform files and data of the emergency event recorder. Waveform files in COMTRADE format are supported by any Ethernet application, as well as the WisiNet_2 program.
In order to ensure interaction with devices of other manufacturers and the possibility of upgrading the architecture of relay protection and automation systems at sites, the ideology of IEC 61850 is implemented in MRZS devices.
MRZS-D can transmit discrete and analog signals to other devices using the GOOSE protocol (typical object-oriented substation event) of IEC 61850-8-1 standard. The transmission of discrete GOOSE messages can be used, for example, for protection schemes and operational locks.
The fourth-generation MRZS series uses the IEEE 1588 standard for high-precision time synchronization.
To ensure the accuracy of operational switching and to avoid personnel errors, the control is based on the principle of a two-stage “choice before execution” command.
For MRZS-D devices, standard solutions, binding schemes are developed, recommendations for calculating settings, recommendations for maintenance, etc. are prepared. The company’s specialists provide warranty and post-warranty technical support for customers, take part in installation and commissioning. Corresponding MRZS series training courses have been developed and are being conducted.