What is SSTI and why it shall be studied?
In this short post we provide a little introduction into what is SSTI and how it can be linked to variable frequency drives (VFD). The post also explains the motivation for studying such phenomenon.
What is SSTI?
SSTI stands for sub-synchronous torsional interaction. It is one specific type of sub-synchronous interaction (SSI). We can decode the term part by part.
- Sub-synchronous means that it happens at frequencies below the power frequency (less than 50/60 Hz).
- Torsional points out that it excites torsional mode of a rotor shaft system. In other words, it is an electro-mechanical interaction associated with torsional vibration.
Classical impact of a power drive system on the adjacent systems has the form of pulsating torque on the load side and harmonic distortion on the grid side.
Pulsating motor air gap torque can potentially excite torsional natural frequencies of the mechanical shaft system. Analogous, the harmonics that the VFD injects into the supply grid may excite an electrical resonance in the grid. However, besides the electrical interaction on the grid side there can also be an electromechanical interaction on the grid side as well.
SSTI affects the grid side and yet it is electrical-mechanical interaction. To be even more precise it is an interaction involving three domains:
- Electrical
- Mechanical
- Control
The involvement of the control part is substantial and SSTI applies for converters with active grid side control. Those include HVDC converter stations, STATCOMs, power electronic based energy storage systems, wind converters or large VFDs with active grid side control (LCI or AFE type of VSI).
Main concern is that the algorithms of active control, when not properly designed or parameterized, destabilize torsional modes of nearby rotating machines (such as turbine-generator shaft strings). Consequently, the torsional vibration of the shaft system increases and may reach a trip level or even lead to a severe damage on equipment (when not detected).
When is SSTI applicable?
Does it mean that every VFD with active front end (AFE) must be studied with regards to SSTI? Obviously not. Nonetheless, there are factors and parameters that indicate the risk of a possible SSTI interaction. Examples of such factors are:
- High VFD installed power with regards to grid short circuit power at the point of connection¹
- Local power generation with turbine-generator shaft lines
- Low natural damping of the torsional mode
A screening study is often performed to identify the risk of SSTI. While it is meanwhile a common practice for HVDC installations, the awareness of importance for specific VFD installations was realized just recently. First step can be obtained by calculating the UIF¹. If the screening indicates increased risk, more detailed studies are recommended.
In SSTI analysis, the electrical and mechanical damping is often analysed as key criterion for stability. Based on simplified transfer functions model the damping can be expressed as a real part of the corresponding transfer function.
De (ω) = Re {Ge (jω)}
De … electrical damping
Ge … transfer function of the electrical system
As can be seen from the expression, the damping is not a constant value but it is a function of frequency. Typically, the VFD has positive electrical damping in certain frequency range while the damping drops and eventually becomes negative in other frequency range. The effective damping is the sum of the “physical damping” (dissipative elements in the power hardware and consequent losses) and the damping of the control system. While the physical damping is always positive, control damping can be both positive or negative depending on the realization of the control loops and parameterization (gains, time constants, signal lagging).
Passivity based control aims to avoid electromechanical interaction and potential amplification inside the closed loop control.
¹ In HVDC world, a so called Unit Interaction Factor (UIF) is defined to provide an indication of SSTI risk. The empirical threshold is typically 0.1 and values above this threshold indicate higher risk. Further studies are recommended in such case. The approach using UIF was applied also for VFD installations – see ref. [2].
History of SSTI
Sub-synchronous torsional interaction as a phenomenon was initially experienced in relation with HVDC converter terminals. Square Butte² was the first HVDC station where SSTI was observed back in 1977 during system tests few months after the start of commercial operation [1]. The rectifier current control of the HVDC terminal was interacting with the first torsional mode of turbine-generator unit. Modifications on the control system solved the problem and led to a stable operation.
Square Butte initiated the awareness of SSTI. The learnings led to a development of so called sub-synchronous damping control. The goals are sufficient damping of sub-synchronous torsional oscillations within turbine-generator units and adequate gain margin. At the same time, the performance and transient response of the HVDC system shall not be compromised.
Few decades later, the same type of phenomenon was noticed in installations of high-power VFDs (e.g. compressor drives) that also have a nearby turbine-generator unit. In this case the VFD can have torsional interaction at each side: motor-compressor shaft train on one end and turbine-generator shaft line on the other end. That results in multiple frequencies of concern. SSTI in VFD world does not refer to continuous duty applications only but may include also large soft starters.
Analogy between HVDC and VFD
A typical VFD has similar topology like an HVDC transmission: rectifier, DC link and inverter. The only difference is that typical HVDC has a DC link of several hundred kilometres while the VFD has a back-to-back arrangement (however, back-to-back HVDC configuration exists as well as part of grid interties). A classical thyristor based HVDC has clear analogy with the load-commutated inverter (LCI) as shown in [2]. Similarly, voltage source converter type of HVDC [3] is analogous to a typical voltage source inverter (VSI).
² Square Butte project with ± 250 kV, 500 MW HVDC link connecting power generation in North Dakota with consumers in northeast Minnesota.
Growing importance of SSTI
In near future we will likely see growing importance of SSTI analysis in specific VFD projects. The unit VFD power is steadily growing. On top, there is a clear trend to replace gas turbines with all-electric drive trains along with many other electrification initiatives. That brings significant increase of high-power drives (unit rating but also total installed power). Additionally, the necessity of energy saving technology will be reinforced and VFDs remain highly attractive means to reduce emissions and to reduce the energy consumption. In parallel with those trends the power grids will see further increase of renewable sources and decline of available short circuit power.
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References
[1] M. Bahrman, E.V. Larsen, R.J. Piwko, H.S. Patel, Experience with HVDC – Turbine-Generator Torsional Interaction at Square Butte, IEEE Trans. on Power Apparatus and Systems, Vol. PAS-99, No. 3, May/June 1980
[2] P. Jörg, A. Tresch, M. Bruha, A model based approach to optimize controls of a large LCI VSD for minimal grid-side sub-synchronous torsional interaction, PCIC Europe, Istanbul, 2013
[3] HVDC Light ® – Hitachi Energy (official website),
https://www.hitachienergy.com/products-and-solutions/hvdc/hvdc-light
