Grid Resonance: A novel early warning system for transmission networks
04/29/2016 - 11.34 am
A new sub-synchronous oscillation monitoring system provides warning of the early stages of resonance—which could otherwise lead to protection tripping or system damage—across a far wider range of frequencies than previously existing systems.
[4] The electric grid that is powering the transition to a low-carbon economy will rely increasingly on generation from renewable sources, on HVDC links and on Flexible AC Transmission System (FACTS) plant1 to provide extra capacity and flexibility. However, increased deployment of these technologies to the grid, including series compensation and power electronic converters, has implications on system dynamic performance. It presents new challenges for stability and reliability, in particular, to avoid instability in the frequency range up to the nominal frequency of the grid, termed Sub-Synchronous Oscillation (SSO). Instability can result in large oscillations that can trip or damage plants, and an early warning system helps utilities to mitigate the risks.
SSO can take three main forms. The first of these, Sub-Synchronous Resonance (SSR), has been widely studied since the 1970s when it was identified as the cause of turbine shaft failures at the Mohave generating station in Nevada, USA. This involved interaction of shaft torsional modes with an electrical “LC” natural frequency2 created by capacitors in series with inductive transmission lines. The second, Sub-Synchronous Control Interaction (SSCI), is due to the interaction of power electronic converters, such as those contained in wind turbines and HVDC links, with the LC natural frequency of series compensated lines. Third, Sub-Synchronous Torsional Interaction (SSTI) arises from the interaction of power electronic converters with generator shaft torsional modes.
In energy management systems for transmissions networks, wide area monitoring systems (WAMS) utilize synchronized phasor measurements transmitted via the well-known IEEE C37.118 protocol at up to 50/60 frames per second (fps) for a practical observable range of up to about 10 Hz. But as Stuart Clark, WAMS Power Systems Engineer with GE Grid Solutions explains, “with SSO we’re dealing with much higher frequencies, 4-46 Hz, that typical WAMS measurements will not record.”
A new Waveform Measurement Unit (WMU) was therefore developed to provide synchronized voltage and current point-on-wave measurements at 200 samples per second, streamed in real time via the C37.118 protocol as “analog” type values. Additionally, speed measurements from turbine shaft transducers can also be sent. This approach accurately represents oscillatory components in the 4-46 Hz range, and also gives visibility of the 54-96 Hz range so as to differentiate modulations such as torsional modes from added components such as LC grid modes. Note that, although not a typical implementation of the C37.118 standard, this approach is fully compliant and rates such as 200 fps are encouraged by the standard.
The WMU is implemented on a multifunction recorder that already incorporates Phasor Measurement Unit (PMU) capability. The 200 fps voltage and current waveforms from each WMU stream are then collated, processed and stored by a central or regional WAMS server, where real-time analysis, monitoring and presentation of SSO information is performed.
1 FACTS plant describes a range of devices, often power-electronic based, that provide flexible control of the AC transmission system. Examples include Series Capacitors, the STATCOM (Static Synchronous Compensator) and the SVC (Static Var Compensator).
2 LC: Inductance (L) – Capacitance (C)
