One critical value microgrids brings to power systems is resilience, the capability of being able to island from the main grid under certain conditions and connect back when necessary. Once islanded, a microgrid must be synchronized to the main grid before reconnection to prevent severe consequences. In general, synchronization of a single machine with the grid can be easily achieved using a synchronizer. The problem becomes more challenging when it comes to a multi-bus microgrid with multiple distributed generators (DGs) and dispersed loads. All DGs need to be properly controlled in a coordinated way to achieve synchronization. This paper presents a novel bi-level distributed cooperative control framework for a multi-bus microgrid. In this framework, DGs work collaboratively in a distributed manner using minimum and sparse communication. The topology of the communication network can be flexible which supports the plug-and-play feature of microgrids. Fast and deterministic synchronization can be achieved with tolerance to communication latency. Experimental results obtained from hardware-in-the-loop simulation demonstrate the effectiveness of the proposed approach.

A Distributed Cooperative Control Framework for Synchronized Reconnection of a Multi-Bus Microgrid

This paper presents the numerical methods for modeling and simulation of fundamental continuous time-varying delays. Continuous time-varying delays are categorized as variable time delay and variable transport delay. These two fundamental delays are formulated and implemented as two Simulink® built-in blocks. The difference between these two delays are discussed. With these two fundamental time varying delays, physical system such as convey belt, control system with delayed control signals, delay differential equations, can be easily modeled and simulated.

Modeling and simulation of time-varying delays

A Networked Control System (NCS) is a feedback-driven control system wherein the control loops are closed through a real-time network. Control and feedback signals in an NCS are exchanged among the system’s components in the form of information packets via the network. Nowadays, wireless technologies such as IEEE802.11 are being introduced to modern NCSs as they offer better scalability, larger bandwidth and lower costs. However, this type of network is not designed for NCSs because it introduces a large amount of dropped data, and unpredictable and long transmission latencies due to the characteristics of wireless channels, which are not acceptable for real-time control systems. Real-time control is a class of time-critical application which requires lossless data transmission, small and deterministic delays and jitter. For a real-time control system, network-introduced problems may degrade the system’s performance significantly or even cause system instability. It is therefore important to develop solutions to satisfy real-time requirements in terms of delays, jitter and data losses, and guarantee high levels of performance for time-critical communications in Wireless Networked Control Systems (WNCSs). To improve or even guarantee real-time performance in wireless control systems, this thesis presents several network layout strategies and a new transport layer protocol. Firstly, real-time performances in regard to data transmission delays and reliability of IEEE 802.11b-based UDP/IP NCSs are evaluated through simulations. After analysis of the simulation results, some network layout strategies are presented to achieve relatively small and deterministic network-introduced latencies and reduce data loss rates. These are effective in providing better network performance without performance degradation of other services. After the investigation into the layout strategies, the thesis presents a new transport protocol which is more effcient than UDP and TCP for guaranteeing reliable and time-critical communications in WNCSs. From the networking perspective, introducing appropriate communication schemes, modifying existing network protocols and devising new protocols, have been the most effective and popular ways to improve or even guarantee real-time performance to a certain extent. Most previously proposed schemes and protocols were designed for real-time multimedia communication and they are not suitable for real-time control systems. Therefore, devising a new network protocol that is able to satisfy real-time requirements in WNCSs is the main objective of this research project. The Conditional Retransmission Enabled Transport Protocol (CRETP) is a new network protocol presented in this thesis. Retransmitting unacknowledged data packets is effective in compensating for data losses. However, every data packet in realtime control systems has a deadline and data is assumed invalid or even harmful when its deadline expires. CRETP performs data retransmission only in the case that data is still valid, which guarantees data timeliness and saves memory and network resources. A trade-off between delivery reliability, transmission latency and network resources can be achieved by the conditional retransmission mechanism. Evaluation of protocol performance was conducted through extensive simulations. Comparative studies between CRETP, UDP and TCP were also performed. These results showed that CRETP significantly: 1). improved reliability of communication, 2). guaranteed validity of received data, 3). reduced transmission latency to an acceptable value, and 4). made delays relatively deterministic and predictable. Furthermore, CRETP achieved the best overall performance in comparative studies which makes it the most suitable transport protocol among the three for real-time communications in a WNCS.

A transport protocol for real-time applications in wireless networked control systems

Ethernet is emerging for many distributed real-time applications in industrial environments It is also considered for substation automation in power systems with regard to IEC 61850 offering many advantages over conventional parallel wiring with analog signals Mission-critical for protection and control systems as well as other demanding applications is the end-to-end delay of messages Beside the frame duration of an Ethernet message on the physical layer, the overall delay is affected by processing times of communication stacks and applications significantly A timing analysis of Ethernet frames is possible with standard network interface cards (NICs) but a high-resolution is only given with specific hardware or professional network analyzers Furthermore, to determine stack processing delays, immediate signals are required from the application prior to the transmission and after retrieval additionally This paper proposes a method to measure delays of Ethernet communication with a common transceiver The achievable timing resolution is less than 01μs using a typical micro-controller unit (MCU)

Measuring delays of ethernet communication for distributed real-time applications using carrier sense

In this paper, the jitter correction method based on Markov chain model was achieved for Numerical Control Field-bus. To improve the accuracy of numerical control field-bus synchronization, transformation delays were sampled to obtain the state transition matrix. The mean of estimated values were accumulated based on Markov prediction estimates method. And the jitter of numerical control field bus was corrected. The test results showed that the worst jitter error can be reduced to 0.5µs. The requirement of numerical control field bus synchronization for NC high-speed and precise processing can be met.

The Study of Jitter Correction Method for Numerical Control Fieldbus Based on Markov Chain Model

In order to use an Ethernet communication network for signal transmission in a feedback control loop, variable transport delay introduced by the network must be considered. In this paper a stochastic model of transport delay is proposed that can be used as the base for synthesis of a control loop.

B. Dermanovic

Papers

Modeling of transport delay on Ethernet communication networks