With the energy landscape changing, the legacy grid infrastructure, which has been in place for decades, urgently needs upgrading. Gone are the days of distributed generation at large fossil-fuelled power plants. The modern generation is a hybrid of these legacy stations with an ever-increasing volume of renewable energy and local generation. To meet these demands, the grid needs to be resilient, modern and digital. The glue that holds this network together is the high-voltage (HV) substation, that through systemic lack of investment, is in urgent need of refurbishment.
Maintaining security of supply whilst integrating large quantities of renewable energy is the main challenge for the future European power grid, but questions remain as to whether the current infrastructure is in a fit state to accomplish that. Much of Europe's grid is aging, unlike more recently developed regions of the world, where electrification has been relatively current and allowed them to use the latest technological advancements.
More than 10 million km of power lines are snaking across Europe to form the transmission and distribution grid, according to Eureletric, the EU electric trade association. This is enough cable to stretch to the moon and back 13 times. An integral part of that network is comprised of the thousands of high-voltage substations that act as node points of today’s complex power transmission infrastructure. They play a key role in meeting safety and reliably requirements for the power grid.
These substations comprise high-voltage switchgear, medium-voltage switchgear, major components such as high-voltage equipment and transformers, as well as all ancillary equipment. The problem is that much of this infrastructure was installed in the middle of the last century and is in urgent need of repair or upgrade. As the network moves towards smart control and integration of an increasing volume of renewable generation, it is imperative that these are upgraded.
A Reliable Meshed Network
The role that these substations play cannot be underestimated. They are crucial hubs in the power system where power can be pooled from generating sources, distributed and transformed, and delivered to the load points. Substations are interconnected with each other so that the power system becomes a meshed network. This increases the reliability of the power supply system by providing alternate paths for flow of power to take care of any contingency so that power delivery to the loads is maintained and the generators do not face any outage.
A lack of investment over past decades has exacerbated this problem and it is now a pressing issue. This lack of investment is really a side effect of the change in ownership from public organizations to private companies of the transmission and distribution network in Europe. But whatever the reason, it has left a backlog of upgrades that are required.
An increasing number of aging HV substations need to be refurbished to extend lifetime and performance. Adapting the original installation to current network requirements and upgrading protection, control and monitoring systems to the latest state of the art technology will enhance the overall reliability of your power supply system, resulting in reduced downtimes and increased network performance. It will improve the safety of your operating personnel and your substation will meet the latest environmental standards.
Delivering Turnkey Solutions
Every substation is a unique challenge that requires a turnkey engineered solution, but one challenge remains constant and that is time. Vital as these substations are, the time that they are offline must be kept to a minimum. To create as much standardization as possible Siemens has developed a basic concept for these substation upgrades that the system designers can use as a template to build on utilizing the company’s extensive experience.
The process of handling such a turnkey installation starts with the preparation of a proposal and proceeds through clarification of the order, design, manufacture and supply. All these high-voltage installations have in common their high standard of engineering, which covers all system aspects such as power systems, steel structures, civil engineering, fire precautions, environmental protection and control systems.
In designing outdoor substations, it is not only the electrical parameters but also the environmental conditions to which the substation is exposed that needs to be under consideration. Based on the customer’s specifications, the planning and design will ensure that the substation fits even the most challenging surroundings.
Meeting your demands requires the right balance of economy and reliability. Selecting the best-qualified circuit configuration and the appropriate switchgear layout is especially important. Siemens engineers can perform all studies needed for the construction of a high-voltage substation, including earthing, short-circuit, thermal, and mechanical calculations.
At the evaluation and concept planning stage, a site evaluation and network calculation are incorporated. It is always advantageous to conduct a site visit, but it is possible to assess the scope of work using Google Maps. Siemens can also create a 3D-scan using information from a scanner or a drone. With the resulting point-cloud image, the team can integrate a plan for the refurbishment into the current site arrangement to deliver an optimal solution. The scope of work will vary on each project, but careful attention is paid to the structure design, foundations and any civil work required. Depending on the wishes of the operator some of the existing equipment may be retained, this often includes the cabling.
Every aspect of technology and each work stage is handled by experienced engineers. All the planning documentation is produced on modern CAD/CAE systems; data exchange with other CAD systems is possible via interfaces.
Selecting the Right Switchgear Technology
Probably the most significant piece of equipment to be upgraded is the switchgear. When making plans, several factors help decide which route to take. What is the function and location within the power supply system? What are the climatic and environmental conditions? Are there specific requirements regarding locations? Are there space/cost restrictions? The plot size and topography clearly provide boundary conditions but can still offer potential depending on switchgear technology, HSE regulations and environmental aspects. Armed with the answers to these questions, either AIS or GIS can be the right choice, or even a compact or hybrid solution.
Air-insulated switchgear (AIS): AIS are favorably priced high-voltage substations for rated voltages up to 800 kV, which are popular wherever space restrictions and environmental circumstances are not severe. The individual electrical and mechanical components of an AIS installation are assembled on site. Air-insulated outdoor substations of open design are not completely safe to touch and are directly exposed to the effects of the climate and the environment.
Gas-insulated switchgear (GIS): The compact design and small dimensions of GIS make it possible to install substations of up to 550 kV right in the middle of highly populated urban or industrial areas. Each switchgear bay is factory-assembled and includes the full complement of disconnecting switches, earthing switches (regular or make-proof), instrument transformers, control and protection equipment, and interlocking and monitoring facilities commonly used for this type of installation. The earthed metal enclosures of GIS assure not only insensitivity to contamination but also safety from electric shock.
Mixed technology (compact/hybrid solutions): Besides the two conventional designs, there are also mixed technology or compact solutions available that can be achieved with air-insulated and/or gas-insulated components. Mixed technology solutions combine the advantages of the low maintenance efforts of GIS technology with the flexibility and easy installation of AIS components.
Optimizing System Design
While stranded conductors are enough for currents up to a certain level, solutions with aluminum tubes are required for higher-rated currents and high short-circuit currents. Such conductors can carry rated currents of up to 8,000 A and short-circuit currents of up to 80 kA without difficulty.
The switchyard design is not only influenced by the available space, but also by the location, the existing infrastructure and accessibility. The coordinates of incoming and outgoing overhead lines and the number and location of transformers and specified voltage levels are also fundamental for switchyard layout.
All these factors will be considered in the solution we propose. Almost all outdoor substation installations are unique, tailor-made solutions – especially when it comes to step-up systems in connection with power stations and large transformer substations in the extra-high voltage transmission system. HV/MV transformer substations for distribution purpose with similar substation schemes and requirements are more likely to be of standardized design. This allows establishing a modular concept for the substation arrangement which makes for an optimized execution process and short substation delivery times.
There is a multitude of designs including certain preferred layouts that are frequently requested and defined by the typical arrangement of the busbar disconnectors and circuit breakers such as H-arrangement, in-line longitudinal arrangement (Kiellinie) with center-break disconnectors, center-tower arrangement or diagonal layout with pantograph disconnectors breaker and-a-half layout.
A Sum of all its Parts
Selecting the correct switchgear and circuit configuration to meet the specific requirement of the substation are crucial. However, there are other high-voltage components within a substation package that each has their role to play, such as circuit breakers. Circuit-breakers are the central part of AIS and GIS switchgear that must fulfill several demanding requirements such as reliable opening and closing, consistently high quenching performance with rated and short-circuit currents even after many switching and high-performance, reliable, maintenance-free operating mechanisms.
Circuit breakers for air-insulated switchgear are individual components and are assembled on site with the individual electrical and mechanical components of an AIS installation.
The Siemens product range from 72.5 kV up to 800 kV includes high-voltage circuit-breakers with self-compression interrupter units – for optimum switching performance under every operating condition and for every voltage level.
Another essential part of electrical power substations are the disconnectors. They indicate a visible isolating distance in an air isolated gap. The center-break disconnector is the most frequently used disconnector type. The disconnector base supports the operating mechanism and two rotating porcelain support insulators. The current path arms that are fixed to the insulators open in the center. Each rotating unit comprises two high-quality ball bearings and is designed for high mechanical loads. They are lubricated and maintenance free for the entire service life.
Electrical instrument transformers transform high currents and voltages into standardized low and easily measurable values that are isolated from the high voltage. When used for metering purposes, instrument transformers provide voltage or current signals that are very accurate representations of the transmission line values in both magnitude and phase. These signals allow accurate determination of revenue billing.
When used for protection purposes, the instrument transformer outputs must accurately represent the transmission line values during both steady-state and transient conditions. These critical signals provide the basis for circuit-breaker operation under fault conditions, and as such are fundamental to network reliability and security. Instrument transformers used for network control supply important information for determining the state of the operating conditions of the network.
Keeping the power flowing
Outage planning is of prime importance to the operator. Factors such as required availability, execution time for modification work and permitted downtime need to be factored into any project. One consideration is whether a temporary solution can be implemented such as a power bypass or supplying a portable substation.
Given the scale of the upgrades required and the overriding requirements to deliver a reliable grid for the changing electricity distribution landscape, a scalable, rapid solution to HV substations refurbishment is essential.