VFD industry requirements: Power generation
In today’s post we will have a look at variable frequency drives (VFD) employed in power generation industry. We start with applications of drives in power generation and then discussing the requirements on those VFDs.
First let’s have a look where we can find VFDs within the power generation plants.
Thermal power plants
In conventional thermal power plants (coal fired, gas fired, nuclear) the drives are used mainly in the auxiliary systems. Despite calling them auxiliary, the power rating can reach 20 MW or more as unit power in a large power plant. Typical application is a feedwater pump that brings water into the boiler where it is heated up and transformed into steam. VFDs for the feedwater pumps are normally the largest auxiliary drives with mentioned rating up to 20 MW or more. Utilizing the variable speed helps to optimize the internal consumption of the power plant, improve the overall efficiency and get more useful power to the end user. Another common application of VFDs in thermal power plants are variable speed fans: induced draft (ID) fan and forced draft (FD) fan. Gas fired power plants might have a VFD directly at the heart of the power station:
- Gas turbine starter – popular solution with load-commutated inverter (LCI)
- Variable speed drive of the fuel gas compressor that supplies the gas turbine
The waste heat can be utilized to heat homes and industrial buildings nearby the power plant. District heating pumps are used for that purpose. These pumps also benefit from using VFDs and controlling the flow.
¹ Gas turbine cannot start on its own and needs a driver to bring the turbine to the minimum speed where it is fired.
Hydro power plants
The ‘run-of-river’ hydro power plants may also use VFDs in auxiliary services to achieve energy savings, especially at partial load. Different situation is in pumped storage hydro power plants. These power plants extensively use static frequency converters (SFC) or variable frequency drives (VFD) up to considerable power range:
- Soft starter to start-up the motor-generator
- AC excitation converter in combination with double fed induction machine (DFIM)
- Full converter (large VFD) to control the speed and torque of the pump-turbine
Since the power electronic converters in pumped storage power plants have already been discussed in previous articles (e.g. [3]), they are not described in detail here.
Offshore wind farms
Wind converters are typically classified as a separate category of power electronic converters. Nevertheless, their hardware and operational principle is very similar to VFD. While the motor drive converts fix grid frequency into a variable frequency supplied to the motor, wind converter is supplied from a wind generator operating at variable speed and converts the electric power such that it can be supplied into the power grid. Thus, wind converters are part of a power generation unit and the requirements of a grid code are fully applicable.
Requirements on VFD in power generation
This section addresses the specific requirements of the power generation industry.
Availability and reliability
In principle, VFDs in power generation shall have very high reliability and availability. This aspect becomes crucial if there is no redundancy on the drive system level. Unplanned shutdown would affect the performance of the power generation unit or would even lead to a complete downtime. Therefore, robust and reliable design shall be used. The VFD shall have good accessibility for service and maintenance to be able to fix any potential issue within short time. Remote service access, if allowed by the customer, is also helpful for predictive maintenance, diagnostics and troubleshooting.
High efficiency
VFDs in power generation can reach high rated power. The efficiency of the drive system is very important. Finally, every kW of saved power is an extra power delivered into the grid and sent to the consumers. Thus, variable speed drive system shall feature as high efficiency as possible. On VFD level the expected efficiency at nominal load is 98.5% or higher (over 97% in case of integrated transformer). Very large liquid cooled VFDs may utilize variable speed pumps in their internal cooling circuit to optimize own auxiliary consumption at partial load.
Fast start-up time
Another possible requirement is a fast start-up time. The motor driven pumps shall be brought to the minimum speed within defined period. Relevant is normally the total start-up time consisting of the preparation (pre-charging of VFD where applicable), magnetization of the motor and acceleration to the desired speed. When start-up time is critical, the VFD may feature a hot standby mode.
Wide range of short circuit power
Specific thing might be a very wide range of grid short circuit power that can have an impact on system design. In normal operation the grid is inherently very strong as the VFD is physically very close to the power generation node. In special case, such as black start, the short circuit power is in contrary extremely limited.
Increased immunity against grid voltage variation
VFDs in power plants shall withstand larger excursions of grid voltage. Besides the continuous voltage variation of -10…+10% the drive system shall manage temporary overvoltages (typically 125…130% of nominal voltage for 2-3 seconds) as well as undervoltages (e.g. short time dips down to 75…80% of nominal voltage). The requirement on wider voltage variation is quite common in Europe and partially contributed to the changing power grid with larger content of renewable sources and consequently less short circuit capacity and larger susceptibility to load changes.
Grid supporting functionality
Power converters transferring power into the grid usually need to comply with the grid code. It is a set of rules defining how the generation unit shall behave in normal and (especially) in abnormal conditions. The converter shall be capable to inject or draw certain amount of reactive power to or from the grid. Such capability is both static (in normal operation) and dynamic (during grid fault). it is essential that the generation unit coupled to the grid through the converter remains connected during severe undervoltages – so called low voltage ride through. The profile of the grid voltage versus time is defined in the corresponding grid code.
Additional control functionality
The extensive use of power electronic converters and transformation of the power grid at the same require additional features. This category includes black start capability, i.e. starting the power plant through the auxiliary system after a blackout. Besides the interconnected operation (mostly the default mode) the equipment shall support island operation as well. The new challenges trigger a shift from grid following control to grid forming control.
Summary
Common requirements on VFDs in power generation sector include superior availability, high efficiency or potential fast start-up time. Specific for generating unit is the compliance with grid code posing requirements on the hardware dimensioning as well as control performance.
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References
[1] Energy savings in pump systems: Case study, https://mb-drive-services.com/case-study-energy-savings-in-pump-systems/
[2] ABB Gas Turbine Starters, https://search.abb.com/library/Download.aspx?DocumentID=3BHT%20490%20536%20R0001&DocumentPartId=
[3] Power converters for pumped storage hydro power plants, https://mb-drive-services.com/power-converters-for-pumped-storage-hydro-power-plants-part-1/
[4] SFC as grid intertie, https://mb-drive-services.com/sfc-as-grid-intertie/
