Hybrid synchronous condenser system design and control for enhanced grid services

One of the major challenges for the future power systems is decommissioning of synchronous machine-based generation, which will result in reduced overall strength of the system. For example, as the number of synchronous machines in the generation mix subsides, the inertia in the system will reduce, making the frequency more susceptible to large swings in the face of disturbances, significantly affecting the voltage control, as well as the short-circuit power levels. The total renewable energy capacity of the world has increased by ca. 260 GW in 2020 as compared to the previous year, taking the total capacity to ca. 2800 GW. The share of the renewable energy in the energy mix is expected to continue to rise, making it a necessity to address the problems previously mentioned.

Phasing out of conventional synchronous machines means that other technologies will have to be able to provide and replicate their functions and capabilities in terms of voltage and frequency control, short-circuit power, inertia, oscillation damping etc. The most common technology is the power converter interfaced renewable energy. Flexibility of power converter control is one of the major advantages of this technology in bringing the necessary functions to the system and providing similar characteristics as a conventional generation. However, there are some obstacles that cannot be overcome by only power converter based renewable energy sources due to the physical and economical aspects of the technology. In that regard, battery systems and supercapacitors can introduce that additional active power and provide a range of beneficial functions. In addition, synchronous condensers have been identified as a technology that can contribute with high levels of short-circuit power and provide inertia that will inevitably reduce in converter dominated power systems.

This thesis proposes a hybrid synchronous condenser (HSC) solution consisting of a synchronous condenser, battery energy storage system (BESS), and a hybrid controller for the combined system. This system is designed with the goal to integrate and combine the benefits of both technologies with regards to provision of ancillary services. Furthermore, the capabilities of the different technologies and control strategies are analyzed and compared in order to quantify the contribution in terms of voltage and frequency support. The criteria for the comparison are based on the speed of the response, overloading capability, and performance in weak and strong grids. The idea was to test each technology and control strategy against voltage and frequency disturbance, voltage angle jump, and short circuit contribution.

Based on this, we propose a control design that can provide damping for torsional oscillations. The oscillation damping controller is integrated within the HSC system, where the active damping is provided by BESS, while the synchronous condenser contributes only passively. Each oscillating mode has a dedicated controller loop, and the interaction between different frequencies is minimal. The outputs are added together to form a single input to the current controller of the VSC. Each loop consists of a filter, gain, and phase compensator. The parameter tuning is performed by applying a simulation-based approach, where we ran simulations for each parameter set and evaluated the fitness function, utilizing a particle swarm method for optimization. The performance of the controller was first evaluated using a small signal perturbance on the mechanical torque and measuring the speed deviation and the electrical torque deviation to obtain the damping profile of the system.

At last a control method is proposed for maximizing the inertial contribution of the HSC system by using BESS to compensate the active power oscillation of HSC caused by the synchronous condenser. During a disturbance, the synchronous condenser will provide oscillating active power to the system, and periodically inject and absorb reactive power. By compensating the absorbed active power of the condenser with BESS, one can maximize the inertial contribution of the HSC system. The input signal for the controller is the measured active power output from the synchronous condenser, which means that we avoid a more complicated solution by using the rotor speed of the condenser. The controller loop consists of derivatives of the active power, gain, and the phase compensator. The analysis includes estimation of energy storage that is necessary relative to the size of the synchronous condenser, as well as performance evaluation in case of using a flywheel.

All the proposed controllers are validated by performing a hardware in the loop test and measuring the speed deviation of the affected generators. It is found from the thesis that the investigated HSC design can provide a wide range of services to the need of the stability of the power grids now and in the future, with several potentials yet to explore. However, in order to roll out the technology, it is also essential to further develop the current ancillary service market in order to quantify the value of the services and justify the investment in this technology, albeit this work is out of the scope of the thesis. From the technical standpoint, the aim of the project, which was to investigate and demonstrate the effect of HSC, has been actualized.