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Energisation Studies for Offshore Interconnections

Introduction

Electrical inter-connectors linking offshore installations are becoming increasingly common for both new build as well as extensions to existing systems. Such systems are typically comprised of a subsea cable circuit and step-up and step‑down transformers between offshore platforms, or between an offshore platform and an onshore production facility.

During black-start or light load conditions, it is sometimes necessary to energise remote transformers against minimum generation with the inter-connector cable circuit in service. This network configuration can result in a low system resonant frequency that may coincide with one of the harmonic components present in the transformer inrush current. Such a harmonic component can excite the power system resonant frequency resulting in over-voltages that may last for several seconds depending on the system damping.

For energisation studies, such as predicting the transient performance of inter-connected offshore systems, a time domain calculation using an electromagnetic transient analysis program is required. PSCAD-EMTDC provides all the tools necessary to develop and complete a detailed inter-connected offshore energisation study. Mott MacDonald has used the software to perform a number of such studies. Based on the studies, recommendations concerning the online generation required and switching arrangements for the link can be made.

Building the model

Undertaking inter-connected offshore power system energisation studies requires modelling various elements of the system in detail. Using a full electromagnetic transient model allows the study to take account of circuit breaker point on wave switching, transformer non-linear magnetizing characteristics and magnetic remenance, the distributed nature of the impedances of the submarine cable and the response of the generator AVRs to the transients developed during an energisation event.

Subsea cable

Within the PSCAD model the subsea cables are represented using cascaded mutually coupled pi-sections. The number of pi‑sections required for the correct representation of a cable circuit depends mainly on the expected frequency of the transient oscillation. The highest frequency that can be attained by a pi-section representation for a particular cable is the natural frequency of one individual pi‑section element. The pi-section length for subsea cables used in energisation studies is calculated to give a maximum frequency of 20 kHz. This is considered satisfactory for transients investigated as part of these studies.

Transformers

When a transformer is energised, it may draw a high magnitude transient current from the supply. The magnitude and duration of the transformer inrush currents are highly dependent on the non-linear magnetizing characteristics of the transformer core, the residual flux linkage in the transformer core, the point on the voltage wave at the instant the transformer is energised (i.e. switching angle) and the impedance of the circuit supplying the transformer.

Magnetic core saturation is represented in the PSCAD-EMTDC transformer model by a current source connected across each LV winding. The asymptotic function relating the magnetizing current to the flux linkage is programmed internally within PSCAD, based on the magnetizing current at rated voltage, the position of the knee point on the characteristic and the air cored reactance of the winding.

For design studies, data on the transformer core and windings is not often available and so the air core saturated reactance or the peak inrush current cannot be calculated. In this case it is necessary to assume a maximum peak inrush current for the least favourable switching angle and residual flux linkage conditions, and to select the air cored reactance to replicate this current when energised against an ideal, zero impedance source. Residual flux linkage can be included in the model by inserting a dc current source in parallel with each LV transformer winding. The current is chosen to establish the desired level of residual flux linkage.

Harmonic analysis of transformer inrush current

Due to the non-symmetrical waveshape, the transformer inrush current contains all harmonic components i.e. fundamental, 2nd, 3rd, 4th, 5th, etc as well as a dc (zero frequency) component. As an example, using the transformer modelling methodology described above, the peak inrush current waveform experienced when energising a 4.5 MVA, 6/0.72 kV, Z=7.0%, 50 Hz, Dy11 transformer against an ideal zero impedance source was obtained using PSCAD. The harmonic content of the phase A transient inrush current was calculated using a Discrete Fourier Transform (DFT) with a moving window and is presented in Figure 1. In this case, the 2nd harmonic is by far the dominant one. The harmonic components decay as the inrush transient decays towards the normal steady state operating conditions.

Chart showing harmonic components present in transformer inrush currentFig.1 - Harmonic components present in transformer inrush current

Results

The results obtained from a PSCAD‑EMTDC study of a typical system configuration shown in Figure 2 [PDF 8KB] consisting of an onshore 11 kV plant linked to an offshore 6 kV platform are discussed. The platform is supplied via a 50 Hz inter-connector circuit consisting of an 11/35 kV, 25 MVA, 8.8% step-up transformer, 30 km subsea cable operating at 35 kV with a capacitive reactance of 15.3 MΩ/m and a 35/6.0 kV, 16 MVA, 6.4% step-down transformer. The onshore plant is supplied by up to four 11 kV, 30 MVA gas turbine generators (GTGs), each with a sub-transient reactance of 32%. The full PSCAD-EMTDC dynamic machine model was used to represent each generator as well as their AVRs via transfer function block diagrams.

Figure 3 shows the initial 11 kV system voltage waveshape after switching in the 4.5 MVA transformer at the offshore platform for least favourable point on wave switching against one GTG. The system is unloaded and the transformer initially had 80% residual flux linkage. High levels of distortion are present on the 11 kV voltage waveshape. The harmonic voltage components present in the 11 kV voltage waveform were calculated using a travelling sample window DFT. During the inrush transient the largest voltage components are at the 5th and 6th harmonics of the supply frequency.

Chart showing the 11kV system voltage when 4.5MVA transformer is energisedFig.3 - 11kV system voltage when 4.5MVA transformer is energised
The peak over-voltages are dependent on the amount of system damping present and the system impedance itself. Using the PSCAD-EMTDC harmonic impedance scanning tool, a plot of the variation of the system impedance for different generation line-ups looking into the 11 kV switchboard is given in Figure 4. This shows a parallel resonance close to the 5th harmonic when one GTG is connected. When the transformer at the remote installation is energised the 5th harmonic current component of the inrush current injected into the system excites the resonant frequency resulting in a sustained 5th harmonic over-voltage.

Chart showing the variation of system impedance with frequency seen at the 11 kV switchboard for different generation line-upsFig.4 - Variation of system impedance with frequency seen at the 11 kV switchboard for different generation line-ups
Figure 4 shows that as the number of online generators is increased, the system resonant frequency is increased. Therefore, increasing the number of GTGs connected can be beneficial in reducing the sustained over-voltages when one of the harmonic current components in the inrush current coincides with the system resonant frequency. As the system load increases the magnitude of the impedance peaks will tend to decrease. Therefore additional load is also beneficial in reducing harmonic over-voltages.

Conclusions

PSCAD-EMTDC has been used by Mott MacDonald to assess harmonic over-voltages due to transformer energisation in an offshore inter-connected system. Studies of this type can be used to recommend the generation and load line‑up required to ensure that both the voltage dip and harmonic over-voltages are within acceptable limits. PSCAD-EMTDC provides the tools necessary to perform this type of analysis and helps to identify possible problems and investigate practical engineering solutions.


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