Theodor Borsche

Impact of Low Rotational Inertia on Power System Stability and Operation

Abstract Large-scale deployment of Renewable Energy Sources (RES) has led to significant generation shares of variable RES in power systems worldwide. RES units, notably inverter-connected wind turbines and photovoltaics (PV) that as such do not provide rotational inertia, are effectively displacing conventional generators and their rotating machinery. The traditional assumption that grid inertia is sufficiently high with only small variations over time is thus not valid for power systems with high RES shares. This has implications for frequency dynamics and power system stability and operation. Frequency dynamics are faster in power systems with low rotational inertia, making frequency control and power system operation more challenging. This paper investigates the impact of low rotational inertia on power system stability and operation, contributes new analysis insights and offers impact mitigation options.

Power and energy capacity requirements of storages providing frequency control reserves

Due to their fast response time and high ramp rates, storage systems are capable of providing frequency control reserves. However, the limit in energy capacity poses difficulties as frequency control signals are not unbiased. We describe a scheme to recharge or discharge the storage without impeding the quality of the provided service, and formulate an analyzing method to investigate the resulting size of the storage. We show that even small storage sizes are sufficient to provide continuous and reliable primary and secondary frequency control reserves to the grid.

Defining a degradation cost function for optimal control of a battery energy storage system

Optimal control of Battery Energy Storage Systems (BESSs) is challenging because it needs to consider benefits arising in power system operation as well as cost induced from BESS commitment. The presented approach relies on the methodology of Model Predictive Control (MPC) for optimal BESS operation. Variable and strongly usage dependent battery degradation costs constitute the bulk of the marginal costs for BESS operation. Battery degradation is usually modeled with nonlinear functional dependencies or an implicit cycle counting approach unsuited for an MPC implementation. In this paper an explicit cost function considering battery degradation is developed, which sufficiently captures the nonlinearities and is applicable for arbitrary battery load patterns. The resulting piece-wise affine cost function leads to a mixed-integer quadratic programming problem allowing a standard hybrid MPC formulation. As proof-of-concept, a peak shaving algorithm relying on the proposed cost function and on adaptive soft limits is developed and implemented on the Zurich 1MW BESS demonstration project, owned and operated by the utility of the Canton of Zurich (EKZ).

A framework for and assessment of demand response and energy storage in power systems

The shift in the electricity industry from regulated monopolies to competitive markets as well as the widespread introduction of fluctuating renewable energy sources bring new challenges to power systems. Some of these challenges can be mitigated by using demand response (DR) and energy storage to provide power system services. The aim of this paper is to provide a unified framework that allows us to assess different types of DR and energy storage resources and determine which resources are best suited to which services. We focus on four resources: batteries, plug-in electric vehicles, commercial buildings, and thermostatically controlled loads. We define generic power system services in order to assess the resources. The contribution of the paper is threefold: (i) the development of a framework for assessing DR and energy storage resources; (ii) a detailed analysis of the four resources in terms of ability for providing power system services, and (iii) a comparison of the resources, including an example case for Switzerland. We find that the ability of resources to provide power system services varies largely and also depends on the implementation scenario. Generally, there is large potential to use DR and energy storage for providing power system services, but there are also challenges to be addressed, for example, adequate compensation, privacy, guaranteeing costumer service, etc.

Minimizing communication cost for demand response using state estimation

Demand Response (DR) is widely considered to be an integral part of future energy systems with high renewable energy penetration. Fast and reliable DR schemes require appropriate communication infrastructure. By limiting both the data sent and the measurements taken, the marginal cost of adding a load unit to the DR scheme can be reduced so that it becomes economically viable to utilize small loads such as hot-water boilers for DR. This paper presents an estimation and control topology for DR relying only on measurements of aggregated power and one-way communication. By using a state observer based on particle filtering and by switching individual loads based on comparing a drawn random number to a broadcast switching signal, accurate tracking of a reference signal can be achieved. In a case study it is shown that employing the proposed algorithms on a basic Smart Meter infrastructure enables the utility to significantly reduce overall cost by avoiding consumption of balancing energy.

Effects of rotational Inertia on power system damping and frequency transients

Rotational Inertia is an integral part of any electric power system. Due to the increased use of power electronics-both to connect Renewable Energy Sources (RES) and as drives for electric motors-inertia levels are generally reduced and become time dependent. The same power electronic technologies can also be used to actively provide inertia to the power system, raising the question what effect changes in inertia has on power system stability, and how to best place devices providing virtual inertia. We propose an optimization program that answers these question, and analytically derive the sensitivities of transient frequency overshoot and damping ratio to inertia and damping. An example shows how damping ratio can be improved while transient frequency limits are respected.

Impact of Frequency Control Reserve Provision by Storage Systems on Power System Operation

Abstract Batteries can be used to provide ancillary services, such as primary frequency response. However, their energy capacity is limited. Therefore, set-point adjustments are necessary and the energy for this has still to be provided by power-plants that do not face energy constraints. This paper investigates various aspects of and potential benefits for power system operation and stability, if energy-constraint units are allowed to participate in the ancillary service markets.

A new algorithm for primary frequency control with cooling appliances

Primary frequency control, also known as primary frequency response or droop, is one of the essential ancillary services guaranteeing stable operation of any power system. Provision of primary and secondary frequency control reserves creates high costs to the system operator. Demand Response, that is an automatic contribution by flexible loads, may provide some of the reserves needed for grid operation at lower cost than traditional power plants. An improved algorithm for primary frequency control provision with refrigerators is described. This algorithm (1) prevents synchronization of participating loads, and (2) guarantees continuous frequency support. A simulation based on a dynamic two-area system resembling the central European grid shows the performance of the algorithm, compliance with refrigerator temperature limits and improvements in power system frequency response after a contingency.

A new frequency control reserve framework based on energy-constrained units

Frequency control reserves are an essential ancillary service in any electric power system, guaranteeing that generation and demand of active power are balanced at all times. Traditionally, conventional power plants are used for frequency reserves. There are economical and technical benefits of also using energy-constrained units such as storage systems and demand response, but so far they have not been widely adopted as their energy constraints prevent them from following traditional regulation signals, which sometimes are biased over long time-spans. This paper proposes a frequency control framework that splits the control signals according to their frequency content. This guarantees that all control signals are zero-mean over well-defined time-periods, which is a crucial requirement for the usage of energy-constrained units such as batteries. A case-study presents a possible implementation, and shows how different technologies with widely varying characteristics can all participate in frequency control reserve provision, while guaranteeing that their energy constraints are always fulfilled.

Balance group optimization by scenario-based day-ahead scheduling and demand response

Balance Groups (BGs) have to provide a day-ahead consumption schedule and buy Balancing Energy (BE) to cover deviations. Demand Response (DR) can be used to reduce BE consumption and associated costs. However, this flexibility to mitigate deviations is limited and depends on the dispatch of the loads participating in DR. In this paper, a scenario-based approach and stochastic programming are used to optimize the dispatch of the BG. We assess the influence on BG cost of different formulations: 1) optimization of BG schedule taking into account the varying prices and uncertainty, 2) optimization of schedule of flexible loads taking into account the possibility to react during the day, 3) combination of 1) and 2). These formulations are compared to a benchmark representing current practice. We find a clear improvement compared to the benchmark, but the relative difference between 1) to 3) to be rather small.