Papers in the Special Issue:
11 papers were accepted for this special issue covering a wide range of
subject matter expertise encompassing academia and research
organisations, networks owners, system operators, original equipment
manufacturers and consultancies. Of the proposed topics in the call for
paper the following were addressed by the authors.
- Grid-forming inverter design philosophies including virtual
synchronous machine/generator, droop, virtual oscillator control
- System strength support
- Black start provision in bulk transmission networks
- Angular stability, including both the small-signal and large-signal
(transient stability) impact
- Unintended consequences
- Modelling and model validation
Below provides a description of each of the accepted papers.
An Overview of Stability Challenges for Power-Electronic-Dominated
Power Systems: The Grid-Forming Approach (Shakerighadi et al.) focuses
on power system stability aspects brought by the introduction of IBR and
in particular the evolution from GFL to GFM control concepts. In doing
so, the authors not only discuss the possible advantages of GFMs but
also the concerns raised by several transmission system operators
(TSOs). The paper includes a summary of Class 3 GFM control strategy and
discusses the suitability of four main GFM control schemes to align with
those requirements. Reduction of system inertia holds the first position
in the ranking of stability challenges identified by various European
TSOs followed by a few other technical challenges such as resonances due
to cables and reduction of transient stability margins. Some of the
developments to address these challenges are discussed. This is followed
by several simulation results from different GFM control strategies
where the use of the exact system frequency information is shown to play
a crucial role in improving GFM performance.
Grid Forming Inverter and its Applications to Support System
Strength – A Case Study (Zhou et al.) proposes hardware-in-the-loop
testing of a GFM with a virtual synchronous generator (VSG) control with
configurable inertia contribution. The objective is to investigate its
capability to actively suppress sub-synchronous oscillations and to
provide an inertial response. The effectiveness of GFM controls with
battery energy storage systems (BESS) in improving system strength and
transient stability have been demonstrated using electromagnetic
transient (EMT) modelling of the transmission network in the Queensland,
Australia. The study shows that with proper tuning and coordination with
other generation in the area, the GFM BESS can address low system
strength related stability issues including sub-synchronous
oscillations, voltage stability and transient stability.
”Improving grid strength in a wide-area transmission system with
grid forming inverters” (Mayer et al.) presents GFM as a viable option
for system strength reinforcement using a wide-area EMT model of
Australia’s National Electricity Market (NEM) power system. This model
is already used for practical decision makings on power system
operation, planning and generator connection. Vendor-specific GFM models
are then integrated into the wide-area model and their performance is
compared to that achieved from already connected synchronous condensers.
Besides indicating promising results, the paper also highlights some
limitations with respect to GFM current limitations and oscillatory
control characteristics. Studies demonstrate the stabilising properties
of centralised and decentralised or spatial application of the GFM as a
credible technical alternative to synchronous condensers in mitigating
sub-synchronous control interactions previously experienced due to the
interaction of multiple GFLs under low system strength conditions.
An Adaptive Multi-Mode Switching Control Strategy to Improve the
Stability of Virtual Synchronous Generator with Wide Power Grid
Strengths Variation (Liu et al.) proposes an adaptive multi-mode
switching control strategy for use in a VSG-type GFM. The shortcomings
of the two existing types of implementations for VSGs are discussed. The
authors point out that the power support approach has good dynamic
performance in strong grids but poor performance in weak grids, and the
voltage support type has good stability in weak grids but with
oscillatory power output in strong grids. The shortcomings of the single
control structure are therefore attributed to grid strength variations.
An adaptive control with switching between the two operating modes is
suggested to address this limitation. Identifying the grid impedance is
the first important task. To avoid frequent switching between the two
modes, a hysteretic boundary characteristic is added. The performance of
the proposed adaptive switching algorithm is then assessed using
simulation studies indicating a good response in power control mode for
a strong grid and in voltage control mode for a weak grid.
An Application of Four-Wire Grid-Forming Power Inverter in
Unbalanced Distribution Network (Döhler et al.) presents a detailed
model for a GFM control based on two loops: an inner current control
with a proportional integral (PI) controller and an outer voltage loop
with a proportional (P) controller. The topology suggested is a
three-phase four-leg four-wire topology, capable of operating in
grid-connected or islanded mode if there is a need to provide the
reference voltage and frequency for the AC bus. The model used includes
three grid-connected photovoltaic (PV) systems connected via GFL
inverters operating in parallel with the electric utility grid to
emulate the dynamics of a power system with high share of distributed
generation. Simulation studies indicate stable operation of the GFM with
a sinusoidal voltage and frequency. Furthermore, the proposed GFM
strategy exhibits effective voltage balance control where during the
islanded mode of operation the unbalance factor (the ratio of negative
to positive phase sequence voltage) is well below the 2% acceptable
level.
Asking for Fast Terminal Voltage Control in Grid Following Plants
Could Provide Benefits of Grid Forming Behavior (Ramasubramanian et
al.) discusses that in IBR dominated power systems, it is possible to
observe GFM inverter behaviour from a conventional IBR with a control
structure based on a phase-locked-loop (PLL). This is demonstrated by
analysing the stability behaviour of a conventional IBR by switching its
control strategy from reactive power to voltage control. Analytical time
domain results are further shown with small signal analysis including
identification of participation factors. A key observation is the
efficacy of fast voltage control to ensure system stability. The
analysis also demonstrates similarities between various GFM control
strategies with regard to fast voltage control characteristics.
Impedance Modeling and Stability Analysis of MMC with Different
Types of Grid-Forming Control Schemes (Guo et al.) investigates the
impedance-based stability characteristics of two GFM control schemes for
modular multilevel converters (MMC): power synchronisation control (PSC)
and direct power control (DPC). Using the harmonic state space, the
equivalent impedance models of PSC-MMC and DPC-MMC are developed
accounting for all controllers and MMC internal dynamic characteristics.
Impedance shaping effects of main controllers are further analysed to
identify the frequency bands where the MMCs have negative resistive
impedance characteristics. Finally, potential instability phenomena in
interconnected power systems are discussed using simulation case
studies, and the virtual impedance method is introduced as a
stabilisation control scheme. EMT simulation results demonstrate the
accuracy of impedance models, and the positive impact of the
stabilisation control scheme.
An improved damping adaptive grid-forming control for black start
of PMSG wind turbines (Meng et al.) investigates an improved damping
method for GFM control in permanent magnet synchronous generator
(PMSG)-based wind turbines. The proposed control aims to suppress system
frequency fluctuations and achieve a smooth black start from the wind
turbine. The paper also presents detailed black start process in a
microgrid with two PMSG-based wind turbines and energy storage. It is
shown that, compared to the conventional GFM controls, the proposed
control scheme can not only suppress frequency fluctuations better, but
also achieve a smooth formation of the grid. Hardware-in-the-loop tests
are also carried out to validate the proposed control strategy for
achieving a smooth black start.
Damping torque coefficient
analysis of PMSG-based WT with VSG control considering wind turbine
dynamics (Qu et al.) uses the damping torque coefficient analysis
(DTCA) for studying the small-signal stability of PMSG-based wind
turbines under VSG control with consideration of wind turbine dynamics.
Authors
discuss that wind turbine dynamics can result in system instability, and
propose that the minimum value of VSG damping control parameter shall be
maintained to ensure system stability under all loading conditions below
the rated wind speed. Simulation results from a single PMSG connected to
a large grid with and without consideration of wind turbines dynamics
are compared to verify the accuracy of the proposed analytical models
and to evaluate the impact of control and system parameters on the
system stability.
Assessment and Enhancement
Control for Small-Signal Voltage Stability of VSC-HVDC Systems Supplying
Passive Industrial Loads (Xue et al.) conducts small-signal voltage
stability analysis for voltage source converter (VSC)-HVDC systems
supplying industrial induction motor loads. The authors also propose
stability enhancement control for the VSCs. Eigenvalue analysis is
carried out to determine key factors influencing voltage stability. The
analysis demonstrates that decreasing either the proportional gain of
the VSC’s PI controller or the electrical distance between the VSC and
induction motor is detrimental to voltage stability. The proposed
control strategy adopts virtual reactance and virtual admittance control
for the VSC to improve the voltage control capability. The effectiveness
and robustness of the proposed enhancement control is validated with
time-domain simulations in a multiple induction motor system supplied by
VSC-HVDC.
Hierarchical control scheme for proportional power sharing and
robust operation in multiple virtual synchronization-based DC/DC
converters (Ji et al.) proposes a hierarchical control scheme to
achieve higher efficiency and superior anti-disturbance ability for
multiple DC/DC converters with virtual synchronization-based control. In
the primary control layer, the power sharing among different converters
can be equalized with the additional regulation signals from
neighbouring weak communications to eliminate the influence of line
parameter mismatch among the converters. In the secondary control layer,
the extended disturbance-observer-based backstepping control is proposed
to eliminate external power fluctuations on the whole system. Two
control levels are integrated as the hierarchical control architecture
to realise different control objectives in different time scales.
Theoretical analysis and simulations are presented to show the benefits
of the proposed hierarchical control scheme.