Distribution System Synchrophasor-based Control Systems
- Written by Jay Giri and Douglas Wilson
Demonstration projects in the UK are evaluating the use of phasor measurement units in wind generation control and microgrid management. But the potential of PMUs in distribution systems does not end there. They also will find application in phase balancing using three-phase measurement, improved state estimation, locating faults in distribution systems with generation, short circuit capacity identification, and improved modelling and efficient reinforcement planning, among other things.
Synchrophasor Measurement Units (PMUs) are being increasingly deployed across transmission power grids worldwide. With each PMU capturing 12-16 measurements, up to 60 times each second, with precise time-tags, operators will be armed with a degree of actionable visibility that is unprecedented in the history of grid management. PMUs produce sub-second high-resolution grid measurements, which augment the traditional 2-4 seconds SCADA measurements. For the first time in history, grid operators will be provided with a time-synchronized view of grid conditions.
Today transmission control centers are deploying PMU measurement-based analytics that augment the traditional model-based energy management analytics and pave the way for us to monitor, analyze and control grid behavior at a sub-second rate. An advanced visualization framework synthesizes information from the various analytics to provide operators with not just improved situational awareness, but more importantly, actionable information. Operators want to fix problems, not just know about them.
Operational benefits of adding synchrophasor applications at the transmission control center include maximizing utilization of existing transmission capacity by operating the grid closer to its true operating limit; providing early warning of grid disturbances; monitoring for un-desirable grid dynamics and oscillations; identifying islanding conditions; and enabling efficient forensic post-disturbance analysis to find out what just happened, where and why.
Managing the smart grid of the future will require that we add intelligent solutions not just to the high-voltage transmission but also to lower-voltage distribution systems. Thus, ways are being developed of using synchronised measurement technologies to improve the capabilities of the distribution system to accommodate sustainable energy resources and maintain or improve security of supply.
Active network management is being increasingly used to facilitate connection of more renewable generation to distribution grids. Direct control is needed especially to limit generation to enable the network to be loaded beyond the present security limits. This involves constraining the active plant connected to the network, so that the network is not loaded beyond its safe capability. Generation, loads, tap changers and storage devices can be candidates for active control to maintain system operation within its safe operating boundaries, such as: thermal constraints, voltage limits, fault level limits and for power reversals in transformers.
So far, the most active network management schemes have used steady-state measurements at all possible constraining boundaries and define the output limits for the participating devices. A great many measurement points are required to capture network limits, which implies a high dependency on measurement and communication from many locations. What is more, such schemes are generally defined for intact networks and are not easily reconfigured for maintenance schedules.
By using synchrophasor measurements, it is possible to capture key operating conditions of the system without detailed monitoring. Synchrophasor measurements provide a better representation of the loading conditions between the measured locations. Limits based on angle difference between synchrophasors, or other values derived from the synchrophasor measurements, enable constraint enforcement using fewer measured variables and therefore simpler reliable control schemes.
Furthermore, the speed of synchrophasor measurement captures the dynamic behavior of the power system and therefore can be used to provide a fast response to a fault or system reconfiguration. Another advantage is that PMU-based systems can be designed to accommodate maintenance scenarios with one or more outages.
The concept of using synchrophasor measurements in wind generation control in a 33kV network is being demonstrated in the Scottish Power Manweb network in the UK. In the network where the scheme is applied, a total of 18 potential constraint locations can be accommodated with four remote measurements. In the part of the network where the project is applied, the capacity for “fit-and-forget” wind connections is fully used, with no direct control of generation; and any new connections require either expensive network reinforcement or active control.
Applying a conventional control approach would require many monitoring points, as relieving one constraint through a measurement fed into a controller would result in another unobserved network segment becoming the constraining factor, and so on.
In a relatively complex 33kV network, many line currents, voltage levels and transformer loads need to be monitored and included in an active control mechanism. By contrast, synchrophasor measurement provides angle differences between key points of the network that summarise the loading within sections of the network with many components. The phasor measurements can differentiate the high generation / low load scenarios where generation should be constrained, with only a few measurements.
Synchrophasor information also can be important in a microgrid where generation resources are dispersed and require remote measurement and communication to manage the changeover between grid-connected and autonomous operation and ensure supply to connected loads.
In a demonstration project in the UK, the use of synchrophasors is being tested on the Isles of Scilly network on geographic islands off the coast of England. The intention of this project is to show that a network of phasor measurements can be used to:
- Identify the separation and the connections between distributed generators. Instead of shutting down and restarting generation that has separated from the bulk transmission system, it is possible to continue to supply load using the local distributed generation. Separation from the transmission system must be detectable, and the control scheme of the island must be adapted to the emergency scenario.
- Signal to the generators which mode of operation to deploy: grid-connected, speed setting, or speed following. Grid-connected, the generator will operate in constant power mode, without responding to frequency. When the distribution subsystem is separated from the bulk grid, the local generation must maintain a stable frequency. One generator should be designated “speed setting” for the network and run in a frequency control mode. Other generators connected to the same subsystem will operate with a droop characteristic. However, there can be different topologies and generation connection, so the control mode must be decided when the fault occurs.
- Align the generator angle to a remote angle in the bulk grid. This is done through the same mechanism that the generator uses for synchronising to the grid, but with the difference that the governor control and alignment is applied to a remote measurement in the bulk grid rather than the grid-side of its own breaker.
- Enable or block the resynchronisation of the island network with the bulk grid.
The advantage of this application is to ride through loss of connection between the geographic islands and the bulk grid, and the reconnection, without loss of supply to the customers.
In considering the overall benefits of incorporating phasor measurement units in distribution system design, bear in mind that PMUs—in contrast to traditional SCADA measurements—measure all three phases of voltages and currents. Therefore they are ideally suited to monitor unbalanced distribution systems and have immense promise for the intelligent management of distribution networks. There are opportunities for improved observability and control that can improve power quality and grid resilience; better planning decisions also will result.
The cases described in this paper illustrate only two applications of synchrophasor measurements, in renewable generation connection and microgrid management. But there are many more applications that can be explored through similar pilot projects and later rolled out to widespread application. For example, the technology can be applied to phase balancing using three-phase measurement; significantly improved state estimation, as most distribution networks are not fully observed; managing open points for running in a closed loop configuration or for ease of switching to move the open point; locating faults in distribution systems with generation; short circuit capacity identification; and improved modelling and efficient reinforcement planning.
Having said that, there is a need for many more demonstration projects to be applied to a variety of different systems in order to gain practical experience, evaluate benefits and standardize the design and deployment processes. These project experiences will be invaluable towards making these solutions available for beneficial use in distribution systems worldwide.