DC Line Protection for Multi-Terminal High Voltage DC (HVDC) Transmission Systems

The projected global energy shortage and concerns about greenhouse emissions have led to the significant developments in offshore wind farm projects around the globe. It is also envisaged that in the near future, a number of existing onshore converter stations and offshore stations will be interconnected to form a Multi-terminal (MT) HVDC systems, whereas protection issues remains a major challenge. This is largely due to the low inductance in DC network compared to AC interconnection which usually results in a sudden collapse in the DC voltage and rapid rise in the fault current thus reaching damaging levels in few milliseconds. Therefore faults in MT-HVDC system must be detected and cleared quickly before it reaches a damaging level; typically 4 – 6ms (including circuit breaker opening time) following the inception of the fault. For this reason, transient based protection techniques are ideal candidates if the protection scheme must be reliable and dependable. Transient based protection algorithms utilises the higher frequency components of the fault generated signal to detect a fault, therefore making it possible to detect the fault while the fault current is still rising and well before the steady state. The traditional protection algorithms developed for conventional high voltage AC (HVAC) systems such as distance protection are steady state based and as such not suitable for the protection of MT-HVDC systems. Another major issue is selectivity as only the faulty section must be isolated in the event of a fault. This constitutes a major challenge considering the anticipated lengths of the cables. Traditional protection techniques developed for two-terminal HVDC systems are also not suitable for MT-HVDC since it will de-energise the entire network and other sub-grids connected to the main network. DC line protection devices which will operate at a sufficient speed and which will isolate only the faulty section in the event of a fault are therefore required to avoid a total system failure during short circuit. It is anticipated that it will be achieved by the use of HVDC breakers, whereas the implementation and realisation of such circuit breakers still remain a major issue considering speed, complexity, losses and cost. However, two major vendors have proposed prototypes and hopefully these will be commercially available in the near future. The key issue still remains the development of a fast DC line fault detection algorithm; and it is on these premise that this research was undertaken. The work reported in this thesis is a novel time domain protection technique for application to HVDC grids.
The protection principle developed utilises the “power” and “energy” accompanying the associated travelling wave following the occurrence of a fault to distinguish between internal and external fault. Generally, either the “power” or “energy” can provide full discrimination between internal and external faults. For an internal fault, the associated forward and backward travelling wave power; or the forward and backward wave energy must exceed a pre-determined setting otherwise the fault is regarded as external. This characteristic differences is largely due to the DC inductor located at the boundaries which provides attenuation for the high frequency transient resulting from an external fault, hence making the power and energy for an internal fault to be significantly larger than that for external fault. The ratio between the forward and backward travelling wave power; or between the forward and backward travelling wave energy provides directional discrimination. For a forward directional fault (FDF) with respect to a local relay, this ratio must be less than unity. However, the ratio is greater than unity for reverse directional faults (RDF).
The resulting wave shape of the “travelling wave power” (TWP) components also led to the formulation of a novel protection algorithm utilising the wave shape concavity. For an internal fault, the second derivative of the resulting polynomial formed by the TWP must be negative, thereby indicating a “concave-upwards” parabola. However, for an external fault, the second derivative of the resulting polynomial formed by the TWP components must be positive indicating a “concave-downwards” parabola.
The developed and proposed protection techniques and principles were validated against a full scale Modular Multi-level Converter (MMC) – based HVDC grid, and thereafter the protection algorithm was implemented in MATLAB. Wider cases of fault scenarios were considered including long distance remote internal fault and a 500Ω high resistance remote internal fault. In all cases, both the pole-pole (P-P) and pole-ground (P-G) faults were investigated. The simulation results presented shows the suitability of the protection technique as the discrimination between internal and external faults was made within 1ms following the application of the fault.
Following this, the protection algorithm was implemented on both a low-cost experimental platform utilising an Arduino UNO ATmega328 Microcontroller and on a Compact RIO FPGA-based experimental platform utilising LAB-View. The experimental results obtained were consistent with those obtained by simulations.
An advantage of the proposed technique is that it is non-unit based and as such no communication delays are incurred. Furthermore, as it is time domain - based, it does not require complex mathematical computation and burden / DSP techniques; hence can easily be implemented since it will require less hardware resources which ultimately will result in minimal cost.