Reactive Power Control in a Pulp and Paper Industry : An Evaluation of Synchronous Machine Excitation and Capacitor Bank Switching

Sammanfattning

The Swedish electrical production mix is changing from a system dominated by large synchronous generators to a system with a higher share of renewable energy that utilizes power electronic inverters. This transition complicates reactive power balance management, which is critical for maintaining grid voltage stability and reducing losses. Reactive power is managed regionally by the so-called distribution system operator (DSO) by, among other methods, imposing consumption limits on large reactive power users such as electricity intensive large industries. Industrial plants commonly operate induction machines, which are inductive loads and thus by definition consumers of reactive power. To compensate for these, many industries utilize capacitor banks which provide a capacitive load, which counteracts the induction machines' reactive power consumption. These banks, however, provide only discrete steps in reactive power management which may sometimes over or under-compensate. In the studied plant they are also manually controlled, leading to a limitation in response time. An alternative solution is the use of synchronous machines, which can be magnetized to both produce and consume reactive power independent of their active power output. Furthermore, an opportunity exists to operate a synchronous machine in idle solely for reactive power support, as a so called synchronous condenser. The objective of this thesis is to investigate how the total reactive power at a Swedish pulp and paper industry can be controlled to stay within the DSO-imp-osed limits. As these threshold over-steps are billed based on 60-minute averages, the control also needs to be fast enough. This leads to the two research questions. The first question relates to which methods can be used to control reactive power control and the potential these methods have, while the second relates to how automatic control strategies can be implemented to improve the response time of the controllers. The work consists of two main parts, supported by a comprehensive theory chapter. The first part analyzes reactive power measurements from the studied plant and how this data compares to the DSO limits. These measurements are also integral for the second part where simulations are conducted to show how the control of existing capacitor banks and the potential of synchronous machine excitation can reduce the level down to the limits. Furthermore, automatic control strategies are studied to analyze how fast and coordinated control can be achieved. The findings indicate that automatic, dynamic synchronous machine reactive power control provides a fast and continuous method which improves flexibility through both added capability and a fast response time. This is also the case for the synchronous condenser. The results also show that capacitor banks serve an important role in compensating high steady-state reactive power demand and therefore act as an important complement to synchronous machine excitation.