Extremely high power availability is a critical requirement for any power plant. However, this poses environmental and electrical challenges which can preclude the use of commercial uninterruptible power supplies. David Bond describes how these challenges can be overcome
Power plants not only generate electricity – they also need to consume it to function safely.
Control, monitoring, IT, security and communications functions all use sensitive equipment that requires secure, clean power, as do less obvious areas like warehouses and workshops.
The normal approach to providing secure power is to install uninterruptible power supplies (UPS) of sufficient capacity to satisfy current demand, with scalability to accommodate future growth. UPS of all sizes are available from a huge number of suppliers throughout the world.
However, most of these UPS will not be suitable for power station environments, due to a number of limitations. A perhaps extreme but nevertheless illustrative example of this is a DC power supply system recently developed by Benning for a gas power plant operated by SPE (Société de Production de l’Electricité Algeriénne) at their Boufarik site in Algeria.
As this is an earthquake-prone area, the power protection system had to be contained within specially designed housings that are approved to be earthquake-proof. The system is also built to operate at high ambient temperatures and is protected from dust and dripping water to an international standard EN 60529 ingress protection rating of IP41.
Apart from the site’s extremely challenging environmental protection requirement, the system had to deliver electrically protected high power AC and DC supplies of various voltages and power ratings using a 1750 A rectifier, 60 kVA inverter blocks and multiple DC-DC converters. Such a complex requirement eliminates nearly all commercial UPS suppliers, even those offering customized systems.
So how is it possible for power plant operators to achieve the reliable, clean, tightly conditioned power they want, while accommodating a suitable range of AC and DC inputs, outputs and power capacities? And can all this be delivered from equipment suitably ruggedized for environmental conditions prevailing within the plant – and supplied as a package that’s economical in terms of both price and space?
Benning is one company that has developed viable answers to these requirements, based on over 50 years’ experience with both conventional and nuclear power plant operators around the globe. Its approach is broadly defined by three key areas – modularity, ruggedization and support.
In the commercial UPS industry, ‘modularity’ usually refers to the concept of achieving a required UPS capacity by assembling a number of small, self-contained UPS modules into a rack. Load sizes can be accurately matched without wasted capacity, and the system is scalable by increments as the load grows. N+1 redundancy can also be achieved economically. While Benning UPS extend this approach to heavy industrial applications, the company’s power protection strategy is modular in another sense, too.
An innovative approach is that the UPS does not have to be an indivisible function. It can be, but it can also be considered as an assembly of key components – the rectifier for converting incoming AC into DC for battery charging, the inverter for converting DC back into AC for the client load, DC-DC converters to provide multiple DC outputs and the battery systems.
Invertronic modular 20-120 kVA inverter system with manual bypass switch, static bypass switch and door mounted MCU 2500 monitoring & control unit
By working at this component level, flexibility can be achieved, offering any combination of AC or DC input and AC or DC output. For example, the Tebechop rectifier could be used to operate from a single- or three-phase AC input to provide a DC output. These rectifiers’ low-ripple, temperature-compensated outputs can safely charge lead-acid or Ni-Cad batteries as well as supplying the load, allowing DC UPS functionality.
Many applications will require a protected AC as well as DC supply – if so, inverter modules such as the Invertronic Modular can be added to provide this capability, drawing their power either from the rectifier or, if the mains fails, the battery.
Critically, these modules can be mounted in the same rack as the rectifiers in an application-specific ‘mix and match’ configuration. Eliminating the need for separate inverter and rectifier racks allows great space and cost savings.
These savings are enhanced because the inverters and rectifiers are also modular in the more traditional sense, offering flexibility and scalability while eliminating the need for unnecessary capacity. Overall availability can easily be improved by adding an extra module for N+1 redundancy; further availability improvements arise because the modules are ‘hot swappable’. This means that a failed module can be removed and replaced with a good unit without interrupting power to the system, which minimizes mean time to repair (MTTR).
Power availability is a very high priority in power stations of all types. RWE, one of Europe’s leading electricity and gas companies, relies on a broad energy mix of gas, coal, nuclear power and renewable sources for power generation. In Hamm, Westphalia, the company is operating the most advanced plant of its kind, with an output of 1600 MW. During construction, RWE chose Benning, as their long mutual history and the power protection suppliers’ capability to develop bespoke solutions convinced them that they would receive a power system with the security they needed, complying with the availability levels critical to power plants of all types.
125 V/DC power plant system with 1750 A rectifier and 60 kVA inverter blocks
Tebechop 3000 DC 12 kW DC-DC converter with MCU 2500 monitoring & control unit
Accordingly, Benning delivered power supplies based on modern IGBT rectifiers in mono block design, modular DC-DC converters and modular inverters. These were configured as two systems that operate entirely independently of one another, to ensure high availability.
Additionally, each system included N+1 redundancy, so availability levels actually exceeded RWE’s stated requirements. Efficient integration with a minimal footprint was achieved by mixing different module types within racks as described above.
The load comprises PLCs, computers, network components, process control equipment and emergency lighting. Some applications required rectifiers with very low input current distortion (THDi) and unity power factor; Benning’s rectifier modules fully complied with this requirement.
Ruggedizing UPS systems
As discussed, availability is maximized by using N+1 redundancy and by minimizing MTTR, but these techniques work by building on the underlying reliability of the equipment itself – and in demanding power station environments, this must include adequate ruggedization.
Benning’s approach is to invest in high reliability hardware at both component and system level. Over-specified components are carefully laid out on PCBs to help prevent ambient contamination (such as dust and moisture) from, for example, creating short circuits across tracks. In more hostile ambient environments, these PCBs can also be coated to provide additional contamination protection.
The PCBs are then integrated into inverters, rectifiers and DC-DC converters, which are assembled into enclosures entirely designed and built by the provider.
These cabinets can be IP-rated in accordance with EN 60529 to levels of moisture, water, dust and vermin ingress protection as specified by the customer. The cabinets can also be designed to withstand heavy vibration or seismic disturbances by using diagonal braces or cruciform shapes. For more extreme environments, cabinet systems with welded designs are developed.
Ultimately, success in power protection projects depends on high levels of customer support as well as quality of product. Ideally the provider is often involved with the customer from the early stages of a project and, if appropriate, takes part in power system design extending well beyond simply supplying standard components. At Boufarik, for example, the complex system development included 2500 A DC switches and custom-built transformers and chokes.
David Bond is Managing Director at Benning UK. benninguk.com