Diesel/gas static UPS systems for mission critical applications [Part 4/9: Main components]

After the introduction of diesel/gas static UPS systems, this article will discuss the main components of these devices.

The main components of diesel/gas static UPS systems can be divided between active and passive components. These main components are integrated into three systems, a single conversion static UPS system, an automatic transfer switch and a standby generator. The standby generator, typically a fossil fuel generator, could be a diesel generator, a gas generator or a dual fuel or bi-fuel generator. The combined operation of these three systems provides both short term protection against momentary power supply disturbances and longer term emergency power generation.

Depending on design, diesel/gas static UPS systems can use different types of switchgear and transformers to connect to the electric power system including electromechanical and power electronic switches, step-up transformers, and current and voltage transformers.

Single conversion static UPS systems are ideal for use with standby generators because they can accept input power with relatively wide variations in voltage and frequency. It is important that the standby generator is properly sized so it can absorb most voltage variations that might occur during system operation. To ensure proper operation of the system, the design can include an electronic governor on the standby generator and an in-phase monitor on the automatic transfer switch.

Different manufacturers have different construction approaches, but diesel/gas static UPS systems are usually formed by five sub-assemblies:

• Connection module: It contains a static switch that separates the device and the protected equipment or facility from the supply in case of an event in the supply occurs, and a fail-safe bypass switch to automatically bypass the device when a fault occurs. It also contains an automatic transfer switch to connect and disconnect the standby generator as needed. Current transformers are used to measure the supply side and the equipment side currents, and protection fuses on the supply and on the equipment sides. A step-up transformer is used if the device needs to be connected to a system over 690 V.
• Energy storage inverter: It produces the needed currents to take care of installation’s power quality problems and energy efficiency requirements. It also converts the energy storage’s DC energy to three-phase AC power and recharges the energy storage as needed after an event in the supply occurs.
• Energy storage: The selected energy storage media provides power to the protected equipment or facility when needed. To suit required autonomy, several types of energy storage media can be used.
• Control & protection system and HMI: It detects disturbances in the power supply side or in the equipment or facility side and handles accordingly the operation of the energy storage inverter and the energy storage media.
• Standby generator: Provides backup power for an almost unlimited period of time.

Energy storage inverter

One of the main components of the single conversion static UPS system that it is integrated into the diesel/gas static UPS system is the energy storage inverter (ESI for short).

Components

The main function of energy storage inverters in diesel/gas static UPS systems is to handle the bidirectional transmission of energy between the electric power system and the energy storage media. They are typically formed by four basic components:

• Power calculator: A signal processing function that forms a reference waveform, IREF, indicating the amount of power that the inverter needs to transfer.
• Inverter: A power inverter built with power electronics switches (IGBTs with antiparallel freewheeling diodes allowing the flow of bidirectional currents) and a coupling LCL output filter able to reproduce the reference waveform at suitable amplitude, IESI. It is equipped with DC capacitors that act as short-term energy storage reservoirs.
• Control system: A pulse-width modulator (PWM) and, in the case of a voltage source converter used to inject the current, a local current control loop that ensures that IESI tracks IREF.
• Synchroniser: A signal processing block that ensures that the injected waveforms are correctly synchronised to the system voltage.

Inverter topology

Most modern energy storage inverters are built on 3-level neutral-point clamped (NPC for short) inverter topology, which brings several benefits compared to inverters built on the conventional 2-level topology.

In 3-level topology, the switching frequency and voltage stress are distributed among the IGBT switches leading to a better spectral performance of the output voltage with an exceptionally low harmonic distortion. Reduced stress extends lifetime of the power electronics. Higher efficiency, smaller output current ripple, lower losses, lower noise levels and a more compact design with smaller layout are also achieved.

Standby generators

Protection against both short and long-term power supply interruptions is essential for many critical applications. Static UPS systems are not designed to deliver power indefinitely, because of that, standby generators are an ideal complement for UPS systems for applications that must remain operating continuously. UPS systems shield the protected equipment from brief anomalies, while also give standby generators time to start up and synchronise. Standby generators, once running, can continue operating for as long as necessary, subject to fuel availability.

Standby generators convert stored energy (fuel) into electrical energy. They are formed by the fuel itself, an engine to develop mechanical energy, and an alternator to convert the mechanical energy into electrical energy. Engine speed and supply frequency are set by a mechanical or electronic governor, which regulates the amount of fuel fed to the engine (more fuel increases both speed and frequency). Mechanical governors are less expensive but they are less responsive and provide less stable engine speed and voltage frequency regulation. Electronic governors are highly responsive and offer very stable speed and frequency regulation, which is essential for UPS synchronisation. Because of this, electronic governors are usually specified for any standby generator that has to be used with a static UPS system.

Standby generators typically used in power protection systems can be classified into three categories:

• Diesel generators (DG).
• Gas generators (GG): These generators can be further divided into natural gas generators (NGG) and propane gas generators.
• Dual fuel or bi-fuel generators.

Each standby generator comes with its own set of advantages and disadvantages. Natural gas generators can be slower to respond and may need to be sized larger than propane gas generators. Diesel is widely considered the best fuel for standby generators and is typically used for generators over 50 kW; however, it can be costly.

Proper standby generator sizing is essential to ensuring the safety of both the protected equipment, the facility and personnel during a lengthy interruption. They must be sized adequately for the connected static UPS system and being capable of handling its step load. They must have also sufficient capacity to recharge the energy storage and cover the conversion losses of the static UPS system. If the standby generator is not sized large enough, it will not be able to hold voltage and frequency within input tolerances when the static UPS system comes online.

The next article of this series will discuss other components of diesel/gas static UPS systems.

If you would like to receive any of my publications on the topic or to explore how #DieselGas static #UPSsystems can benefit your application, feel free to reach me at [email protected].

You are also welcome to join my running series of weekly #FreeWebinars for Asia-Pacific region on cutting edge #PowerElectronics solutions and their applications.

-----------------------

About the author:

Pedro Esteban is a versatile, multicultural and highly accomplished marketing, communications, sales and business development leader who holds since 2002 a broad global experience in sustainable energy transition including renewable energy, energy efficiency and energy storage. Author of over a hundred technical publications, he delivers numerous presentations each year at major international trade shows and conferences. He has been a leading expert at several management positions at General Electric, Alstom Grid and Areva T&D, and he is currently working at Merus Power Plc.