31 August 2021

Distributed Energy Generation Connection Using Reclosers

Table of Content
  • What is Distributed Generation: Definition
  • Distributed Power Generation Types
  • Distributed Generation Connection Points: Examples of Applications
  • Customer Problems
  • Tavrida Electric Technologies for Distributed Energy Generation
  • Customer Advanced Benefits
  • Photovoltaic Plant in Romania National Grid Connection Case
  • What is Distributed Generation: Definition

    Distributed Generation (DG), also Distributed Energy definition, is a quickly developing segment of energy market and it presents a good alternative to the bulk conventional centralized power planning. The investments in distributed generation units are largely stimulated by the increasing demand of quality and effective power supply, social interest in Green Energy Resources, greenhouse gas emissions reduce, political aspects and technological developments. Finally, renewables market segment development is largely supported by numerous governmental programs and incentives. Advantages like environmental friendliness, expandability and flexibility made distributed power generation an attractive option for modern and future grids. In many countries with remote clusters of loads the concept of Microgrid appears to be a cost-effective alternative to building long transmission lines from the main power system.

    Despite all the benefits provided by the DG, there are a number of new challenges and technical issues for power engineers who are responsible for distribution network operation, therefore significant attention is paid to DG connection points.

    Distributed Power Generation Types

    • Solar power generation – Photovoltaic (PV) systems and power stations
    • Wind power generation – wind turbines and farms
    • Small hydro generation – river run through or conventional type small hydro plant (up to few tens of MVA)
    • Capacity generation – diesel and gas generators used for peak shaving and grid balancing
    • Energy storage systems – two way nodes that act either as load or source. Also used for power quality purposes (power factor, harmonics, phase balancing etc.)

    Distributed Generation Connection Points: Examples of Applications

    Connection Point Single Line Diagrams

    Typically DG connection points is represented by one (Figure 1, No1) or two (Figure 1, No2) circuit breakers. Sometimes multiple DG electricity sources are connected to a single bus and then connected to the main grid through additional circuit breaker (Figure 1, No3). 

    DG connection to the grid

    Figure 1: DG Connection to the Grid

    For redundancy purposes sometimes two interface breakers are used. In some cases (e.g. large wind generation farms) multiple DG sources are connected to the ring (Figure 1, No4) or meshed network which is then connected to the distribution grid through several connections points.

    Electrical Switchgear

    Switchgear type varies from region to region and depends on many factors:

    • Connection voltage
    • Size (capacity) of generation
    • Available budget

    Types of switchgear used for distributed generation connection:

    • Panel switchgear
    • Ring Main Unit (RMU)
    • Recloser

    In North and South America automatic circuit recloser is the most common switchgear for DG connection point. Most if not all IPPs, cooperatives and smaller Distribution Network Operators (DNO) in USA are using reclosers to connect distributed energy to their grid (some examples: Gehrlicher Solar, SunEdison, Progress Energy Carolinas etc.) The same goes for South Africa, Middle East and Australia. In the UK it is a mix of reclosers, RMUs and indoor panel switchboards used for this purpose.

    Power System Protection

    Due to bidirectional power flow in distributed energy generation power system, the protection typically shall be able to distinguish between forward and reverse power flow. Thus overcurrent and earth fault protection elements shall be directional. Protection settings engineering is further limited by:

    • Existing grid protection and margins available for time grading
    • DG sources are often interfaced with power electronic converters that naturally limit the overcurrent (that in turn makes it more difficult to distinguish between fault and load currents)

    To avoid stress on the system when connecting DG to the grid Sync check feature is often specified as a requirement for DG connection switchgear. Some customers also demand other sophisticated protections: high impedance earth fault, broken conductor, neutral voltage displacement etc.

    Customer Problem

    Targets

    System integrator

    • To make connections at minimum overall cost
    • To execute DG projects quickly (to free resource and deal with new projects)

    Independent Power Producers and Utilities

    • To execute DG projects quickly (to get the maximum benefit from present government incentives)
    • To execute projects at low cost
    • To ensure minimum disturbance to the network
    • To ensure network protection is not jeopardized
    • Quick disconnection of DG source in the event of fault (typically under 2 periods) to avoid damage to cables and plant due to very high (above rated withstand) short circuit currents
    • To provide high equipment availability regardless weather conditions

    DG Problems

    • Indoor panel switchgear projects have considerable execution time (more than half a year)
    • RMUs require a number of additional equipment (sophisticated protection relays, voltage transformers, communication etc.) – it leads to delays and problems with commissioning)
    • Lack of engineers for protection settings calculations. As overall rate of DG connections and DG connection inquiries is very high – many utilities are forced to outsource protection settings calculation from external consultancies)
    • Lack of resource for new switchgear and protection maintenance)
    • RMU equipment is prone to floods)
    • Conventional RMU is not capable to disconnect DG sources quickly (typical operation time will be over 100 ms – and damage to the network can’t be avoided))
    • Conventional RMUs have SF6 leakage problems – regular gas monitoring and costly maintenances are required)
    • Technical problems in the areas of power quality (voltage and frequency stability issues, harmonics) and reliability)

    Tavrida Electric Technologies for Distributed Energy Generation

    Tavrida Electric offers the perfect solution for DG sources grid integration - Rec15/25/35 series automatic circuit recloser 15 - 38 kV. It serves as a complete switchgear package with advanced protection and automation features. Additionally, Tavrida Electric provides a “Plug and play engineering” approach to the customers - reclosers are delivered on site pre-programmed (both protection and communication settings) and factory tested (including functionality testing). On request Tavrida Electric also calculates protection settings for the customer.

    Tavrida Rec15/25/35 series automatic circuit recloser 15 - 38 kV for Distributed Energy Generation Systems

    Figure 2: Rec15 automatic circuit recloser

    The Rec series recloser employs a sophisticated measurement system and special protection elements to ensure the correct operation of DG unit. For example, Tavrida Electric was one of the first recloser manufacturers who had implemented an undervoltage / overvoltage (27/59) and underfrequency / overfrequency (81U/81O) protection functions. Furthermore, special elements such as Voltage Reclosing Control (VRC), Close Condition Verifier (CCV) and Synchronization indicator (SI) prevent potentially hazardous conditions, while Rate of Change of Frequency (ROCOF) and Voltage Vector Shift (VVS) prevent islanding and other potentially hazardous conditions.

    All Tavrida Electric products are designed with capability of reliable communication for remote access and supervision since this is the fundamental part of modern electricity networks. Rec series recloser has the embedded remote telecommunication unit that provides simple integration with SCADA over DNP3, Modbus or IEC 60870-5-104 communication protocols with no additional expenses. Moreover, it comes with a free user software package Tavrida Electric Automated Relay Manager (TELARM) and TELARM® Dispatcher system that works as a standalone SCADA and / or in parallel with an existing one.

    Sectionalizing various parts of electrical network with the help of a recloser is possible not only between network utility company and DG power plant but also inside a large power plant. For example, it is possible to combine several fields of solar panels and wind farms into a single power system. In the event of an emergency or planned shutdown of one field of distributed energy generation, it will not turn off the rest of the generating capacity, and distributed power plant will continue to operate.

    Customer Advanced Benefits

    Why Recloser?

    • Recloser is the most cost effective way of DG connection. The recloser is available at fraction of cost of indoor switchgear panel type (typically 20-30% when considering civils cost)
    • Recloser can be installed and commissioned at a much higher pace (the whole installation, including civils and earthing can be done in few days, whereas indoor switchgear will require minimum 6 months to execute)
    • Sophisticate protection and automation functionality is needed (including directional protection, sync check, communication etc.) and the recloser already has all of it embedded in a single device, whereas for RMU it is required to purchase and integrate additional modules
    • Recloser can be installed high above the ground, this installation is immune to floods without expensive flood protection

    Why Tavrida Electric?

    • Light weight for quicker and cheaper installation
    • Maintenance free equipment – zero operational expenses
    • Plug and play engineering ensures straightforward and hassle-free commissioning
    • Tavrida Electric provides consultancy services on request (protection settings calculation) Compatible with 3rd parties SCADA and provided with proprietary SCADA solution free of charge Fastest breaker on the market1, no need to use additional fault limiting solutions (current limiters, reactors, network reinforcement etc.)
    • Compatible with SEL-65IR control module
    • More than 60000 outdoor reclosers installed worldwide since 1997
    Recloser Costs comparison

    Figure 3: Costs comparison

    Photovoltaic plant in Romania National Grid Connection Case

    Case Description of Applying Tavrida Recloser for Solar PV Plants

    SC Energobit Tavrida SRL provided consultancy and equipment (reclosers Rec25) for the grid connection of PV plants placed in the fields of Transylvania, or southern Romania. These plants were placed on slopes with a southerly exposure, where the light from the sun is optimum, transforming solar energy into electrical energy. The customer is the SPG Energy Company. It produces, distributes and markets renewable and alternative energy resources. Tavrida Electric’s goal was to provide a reliable solution for renewable energy investor that meets high technological standards. This application should ensure that the power plant is interfaced with the grid, meeting all of the high-quality requirements of utility standards for produced energy. It should also provide solutions to the National Grid Dispatcher: the co-coordinating authority that manages the National Electric Network.

    photovoltaic plant in Romania grid connection case

    The renewable energy field is one of the most attractive new businesses with some of the fastest development in recent years, and Tavrida Electric appreciates it as an excellent business opportunity. It is also very challenging, a real test of technical and engineering expertise, in order to meet the highest technological standards.Tavrida Electric was the first in this market, in Romania, providing reclosers for DG connection. The biggest challenge here was to meet the client’s requirements (more energy production) with National Dispatcher requirements, whose primary concern is the security of the system. Our company needed to offer a complete and reliable hardware solution that could meet these two criteria, maintain freedom of choice to the client and offer a kind of “flexible” protection in the control cubicle.

    Result and Benefits

    Tavrida Electric’s solution provided a secure and reliable answer to the customer’s needs that exactly meets the imposed utility requirements on energy production. As a result, the customer experienced the following benefits:

    • One of the lowest overall costs for DG switchgear
    • Very fast project implementation (each recloser project has been completed within a month)
    • Maintenance-free and environmentally friendly equipment
    • Reclosers were integrated into utility’s SCADA without any problems
    • Sophisticated control and protection system with directional elements



    Vladimir Kirienko
    Key Account Director of Tavrida Electric Global