Yalou Li

Power flow control devices in DC grids

A HVDC grid based on voltage source converters (VSC) is an attractive solution for the grid-integration of offshore renewable energy, interconnection of AC grids, facilitation of power markets, and energy emergency support. However, the power flow within the DC grids is determined by the line resistance, and cannot be fully controlled by the VSCs only. Thus, Power flow control of DC grids are needed not only to realize the economic operation and power markets, but also to prevent overload of DC lines in post-fault conditions. Three types of control devices, DC transformer, the variable resistor and series voltage source, are presented to achieve the power flow control. The topologies and modeling of these control devices are presented. Their control performance is evaluated through simulations using a 5-terminal DC grid. Their effectiveness, feasibility, and applications are discussed. Through the preliminary comparison, the series voltage source is considered as the best solution in terms of the rating, power losses, and control performance.

Power Flow and Power Reduction Control Using Variable Frequency of Offshore AC Grids

In the “integrated strategy” for connecting offshore wind farms to onshore grids, an offshore AC grid is formed by interconnecting offshore wind farms and point-to-point VSC-HVDC links using an AC network. The advantage of this strategy is that in the case of a DC fault, AC circuit breakers are used to isolate the faulted HVDC link without the need for DC circuit breakers. This paper presents a control system that regulates power sharing between the HVDC links by varying the operating frequency of the offshore AC grid. Primary and secondary power regulation controllers are designed to achieve automatic power flow coordination between HVDC links. Increase in the offshore AC grid frequency during onshore AC grid fault or DC fault signals power flow imbalance. This signal is used to suppress the transient DC over-voltage through the coordination control between the wind farm side converters of HVDC links and the power reduction control of wind turbines. Importantly, because the inputs of the controllers are from local frequency signals only, fast communication is not required under the proposed control strategy. Simulation results are presented to verify the functioning of the proposed control strategy and illustrate variable frequency operation of an offshore four-node AC grid.

Improved ADC Model of Voltage-Source Converters in DC Grids

Due to a large number of converters in dc grids, the simulation speed using traditional electromagnetic simulation tools becomes very slow. An associated discrete circuit (ADC) switch model can improve the simulation efficiency greatly by avoiding the modification of system matrix during switching. However, existing ADC switches induce virtual power losses due to simulation errors during switching transients. The mechanism of the virtual power loss is analyzed, and a power loss model is established. An improved ADC switch model is designed by adding compensation sources to mitigate the simulation errors. Theoretical analyses are carried out to prove this improvement. A fast algorithm to obtain the initial values of the compensation sources is proposed by utilizing the complementary operation of IGBTs. The improved ADC switch provides fast simulation speed and high accuracy. The modeling is particularly suitable for investigating long term system dynamics of dc grids with multiple converters and fast converter transients at the same time.

An improvement of neutral-point balancing algorithm of STATCOM

Utilizing User-Defined (UD) modeling function of Electromagnetic Transient Simulator for DC and AC Power System (ETSDAC) in Advanced Digital Power System Simulator (ADPSS), this paper establishes an STATic synchronous COMpensator (STATCOM) model based on three-level inverter, whose function is the same as the STATCOM model in Electro-Magnetic Transients Program (EMTP). Neutral-point balancing algorithm used in this STATCOM model is using two-level modulation scheme for some phase when unbalance exits. After pointing out disadvantages of this algorithm, namely: problems caused by switch devices in series, this paper proposes a new neutral-point balancing algorithm, three-level voltage deviation between two capacitors hysteresis-band control algorithm. The neutral-point will be balanced by PWM control according to the state of voltage deviation between two capacitors and the direction of neutral-point current. Using this algorithm, an STATCOM model in ADPSS is established and simulated. The simulation results indicate the improvement of neutral-point balancing algorithm of STATCOM is feasible and effective.

Concept and mechanism on full-process dynamic real-time simulation of power system with parallel-in-time-space

The development of modern power systems has been expanding the grid, and all kinds of new equipment and new technology are emerging, which has increased the complexity of power system. We can not make a very risky experimental research in actual power system because of the importance of power system safety, so only the simulation system can be used as a testing means. Therefore, it is increasingly concerned about the research on real-time simulation techniques of the electric power system. For these reasons, the article explores the definition of time in theory, the main differences between unconstrained simulation and real-time simulation, and time control and technique foundation of real-time simulation. At last, concept and framework on full-process dynamic real-time simulation with parallel-in-time-space is given.

Online decision-making and control of power system stability based on super-real-time simulation

During power grid cascading failure, the operating point changes frequently, resulting in a mismatch or inaccuracy of online pre-decision making emergency control strategy. To solve this problem, an online power system stability decisionmaking and control method based on super-real-time simulation is presented in this paper. A decision-making and control system is developed in which the three main operation modes, offline pre-decision-making, online pre-decision-making, and real-time control, are all coordinated. To verify the method, ADPSS (advanced digital power system simulator) is used to simulate a digital power grid with about 20,000 buses. The closed-loop operation of the digital power grid and the online decisionmaking and control system are implemented. Test results show that when online pre-decision making control strategies are mismatched, aided real-time control works well and can prevent power grid stability loss.

Time control method on full-process dynamic real-time simulation of power system with parallel-in-time-space

The full-process dynamic real-time simulation not only needs to research the hybrid simulation technology, but also uses information theory, control theory and other disciplines of theory and knowledge, and coordinate information exchange process between the different simulation devices, and realizes real-time dynamic simulation for information flow between various types of physical devices, which is a fast and slow hybrid dynamic process in time scale. This requires that a more reasonable and effective time control method should be used, realizing the time unified control during the entire process of simulation. The paper analyzes the demand to platform of real-time simulation computer operating system, the emphasis is on the research and exploration of time control method of full-process dynamic real-time simulation of power system with parallel-in-time-space through analyzing time control of real-time system, and its application expectation is also given.