Electric distribution companies can achieve huge savings in the new frontier of energy-efficiency improvement by maximizing energy delivery efficiency and optimizing peak demand. VVO will help achieve these objectives by optimizing reactive resources and voltage control capabilities continuously throughout the year.
The world has a huge appetite for electric energy, consuming thousands of billions of kilowatt-hours (kWh) annually, a figure that continues to climb as more countries become industrialized. The world’s electric consumption has increased by about 3.1 percent annually between 1980 and 2006, according to International Energy Annual 2006 by the US Energy Information Administration and is expected to grow to 33,300 billion kWh by 2030, according to World Net Electric Power Generation: 1990–2030, also by EIA. (Figure 1) The world’s electricity consumption for 2008 was 16,790 billion kWh so by 2030 the world demand for electricity is expected to have almost doubled.
Electric Energy Losses
Currently a significant amount (about 10 percent) of electric energy produced by power plants is lost during transmission and distribution to consumers. About 40 percent of this total loss occurs on the distribution network. In 2006 alone, the total energy losses and distribution losses were about 1,638 billion and 655 billion kWh, respectively. A modest 10 percent reduction in distribution losses would, therefore, save about 65 billion kWh of electricity. According to the 2009 CIA Online Factbook, that’s more electricity than Switzerland’s 7.5 million people consumed in 2008 and equates to 39 million metric tons of CO2 emissions from coal-fired power generation. As the demand for electricity grows, new power plants will have to be built to meet the highest peak demand with additional capacity to cover unforeseen events. The peak demand in a system usually lasts less than 5 percent of the time (i.e., just a few hundred hours a year). This means that some power plants are only needed during the peak load hours and their productive capacity is utilized only occasionally. By active demand management on the distribution system, through demand response and VVO, the peak demand on the whole electric grid can be reduced. This eliminates the need for expensive capital expenditure on the distribution, transmission, and the generation systems.
Even very modest reductions in peak demand would yield huge economic savings. For the United States in 2008, for example, the non-coincidental peak demand (i.e., the separate peak demands made on the electrical system recorded at different times of the day) was about 790 GW. Thus, with every one percent reduction in the peak demand there would be a reduced need to build a 7,900 MW power plant.
Distribution System Losses
The electric distribution network moves electricity from the substations and delivers it to consumers. The network includes medium-voltage (less than 50 kV) power lines, substation transformers, pole- or pad-mounted transformers, low-voltage distribution wiring and electric meters. The distribution system of an electric utility may have hundreds of substations and hundreds of thousands of components all managed by a distribution management system (DMS) as depicted in Figure 2, above.
Most of the energy loss occurring on the distribution system is the Ohmic loss resulting from the electric current flowing through conductors. The energy loss is due to the resistance in the conductor. The amount of loss is proportional to the product of the resistance and the square of the current magnitude. Therefore, losses can be reduced by reducing either the resistance the current magnitude, or both. The resistance of a conductor is determined by the resistivity of the material used to make it, by its cross-sectional area, and by its length, none of which can be changed easily in existing distribution networks. However, the current magnitude can be reduced by eliminating unnecessary current flows in the distribution network. (Figure 3)
For any conductor in a distribution network, the current flowing through it can be decomposed into two components – active and reactive. Reactive power does not do real work but uses the current carrying capacity of the distribution lines and equipments, and contributes to the power loss. Reactive power compensation devices are designed to reduce or eliminate the unproductive component of the current, reducing current magnitude – and thus energy losses.
The voltage profile (spatial distribution and voltage magnitudes) on the feeders (medium-voltage lines to distribute electric power from a substation to consumers or to smaller substations) can also affect the current distribution, depending on the types and mixture of loads in the system, although indirectly and to a smaller extent, thus affecting power loss.
Voltage and VAR Control Devices
Voltage regulating devices are usually installed at the substation and on the feeders. The substation transformers can have tap changers, which are devices that can adjust the feeder voltage at the substation, depending on the loading condition of the feeders.
Special transformers with tap changers called voltage regulators are also installed at various locations on the feeders, providing fine-tuning capability for voltage at specific points on the feeders.
Reactive compensation devices (i.e., capacitor banks) are used to reduce the reactive power flows throughout the distribution network. The capacitor banks may be located in the substation or on the feeders. Capacitor banks can be fixed or switched.
Traditional Control Versus VVO
Traditionally, the voltage and VAR control devices are regulated in accordance with locally available measurements of, for example, voltage or current. On a feeder with multiple voltage regulation and VAR compensation devices, each device is controlled independently, without regard for the resulting consequences of actions taken by other control devices. This practice often results in sensible control actions taken at the local level, which can have suboptimal effects at the broader level.
Ideally, information should be shared among all voltage and VAR control devices. Control strategies should be comprehensively evaluated so that the consequences of possible actions are consistent with optimized control objectives. This could be done centrally using a substation automation system or a distribution management system. This approach is commonly referred to as integrated VVO.
The accelerated adoption of substation automation (SA), feeder automation (FA) technology, and the widespread deployment of advanced metering infrastructure (AMI) over the last few years have laid the foundations for a centralized control approach, by providing the necessary sensor, actuator, and reliable two-way communications between the field and the distribution system control center.
Until recently, however, a key technology has not been available that can take advantage of advanced sensing, communication, and remote actuation capabilities that can be used to continually optimize voltage and VAR. Prior generations of VVO technologies have been hindered by their inability to model large and complex utility systems, and by their unsatisfactory performance in solution quality, robustness and speed.
How Does VVO Work?
VVO is an advanced application that runs periodically or in response to operator demand, at the control center for distribution systems or in substation automation systems. Combined with two-way communication infrastructure and remote control capability for capacitor banks and voltage regulating transformers, VVO makes it possible to optimize the energy delivery efficiency on distribution systems using real-time information.
VVO attempts to minimize power loss or demand without causing voltage/current violations. Voltage/current violations refer to the undesirable excursion from normal operating range, e.g., current exceeding the maximum limit safe for a given conductor type, or voltage exceeding a limit unsafe for the consumer or falling short of a limit needed for normal operation for end users. VVO is designed to work in various system design and operating conditions. A distribution system could be meshed, supplied from multiple sources, unbalanced construction, and unbalanced loadings.
The control variables available to VVO are the control settings for switchable capacitors and tap changers of voltage regulating transformers. For a single switchable capacitor bank, the control variable is binary, with value zero and one corresponding to the switched out or in status. For a typical tap changer, the control variable is an integer that varies from -16 to +16. The capacitor and regulator controls can be either ganged (multiple phases operated in unison) or un-ganged (each phase -operated independently).
Main Benefits of VVO
The main benefits of VVO for distribution system operators are:
• Improved energy efficiency leading to reduced greenhouse gas emissions.
• Reduced peak demand and reduced peak demand cost for utilities
General Problem Definition for VVO
VVO must achieve the objective of minimize power loss or MW demand while maintaining acceptable voltage profiles on the distribution feeders. VVO can be formulated to minimize the weighted sum of energy loss + MW load
+ voltage violation + current violation, subject to a variety of engineering constraints:
• Power flow equations (for multi-phase, multi-source, unbalanced, meshed system)
• Voltage constraints (phase to neutral or phase to phase)
• Current constraints (cables, overhead lines, transformers, neutral, grounding resistance)
• Tap change constraints (operation ranges)
• Shunt capacitor change constraints (operation ranges)
The control variables for optimization include:
• Switchable shunts
• Controllable taps of transformer/voltage regulators
• Distributed generation
VVO in essence is a combinatorial optimization problem with the following characteristics:
• Integer decision variables – both the switching status of capacitor banks and the tap position of regulation transformers are integer variables.
• Nonlinear objective being an implicit function of decision variables – energy loss or peak demand are -implicit functions of the controls.
• High dimension nonlinear constraints – power flow equations numbering in the thousands in the multi-phase system model.
• Non-convex objective and solution set.
• High dimension search space – with un-ganged control, the number of control variables could double or triple.
People who are familiar with optimization problems will tell you that mixed-integer nonlinear, non-convex (MINLP-NC) problems are the worst kind to solve. The major challenge is to develop optimization algorithms that are efficient and robust for large problems. Since a certain amount of computation (i.e., CPU time) is needed to evaluate the loss and demand for a single specific control solution (a single functional evaluation), an algorithm that requires fewer functional evaluations to find the optimal solution is generally regarded as more efficient than one that requires more functional evaluations to achieve the same objective.
Next Generation VVO
A new-generation VVO capable of optimizing very large and complex networks with online application speed is emerging. An innovative solution methodology enables the detailed and accurate modeling of the distribution system components and connections. It rapidly identifies the optimal voltage and VAR operation strategy from millions, if not billions, of operation possibilities using advanced mixed-integer optimization algorithms.
A prototype has been developed, which has performed very well in the lab with distribution network models of a real utility system. Both the solution quality and speed robustness met or exceeded design criteria for online applications. This new generation VVO is capable of optimizing very large and complex networks with online application speed.
The following table is a summary comparison of between the new method and previous traditional ones.
To accurately model a distribution network’s behavior under different control strategies, VVO uses a detailed load flow model where individual phases of the system construction and loading are modeled explicitly. Loads or capacitor banks can be delta or wye connected.
Transformers can be connected in various delta/wye and secondary leading/lagging configurations with or without ground resistance, with primary or secondary regulation capability. Both voltage and VAR controls can be ganged or un-ganged. The method works on radial as well as meshed networks, with single or multiple power sources. Voltage constraints controls are enforced for each individual phase, using phase-to-ground or phase-to-phase voltage, depending on the connection type of the load.
One Smart Technology at a Time
With the accelerating deployment of advanced sensor network, smart metering infrastructure, and remote control capability, there is a growing need for smart applications like VVO that optimize the operation of the distribution system. The new generation of VVO technology is just of the many smart grid technologies that can help us to have efficient, reliable electric power while reducing energy and CO2 footprint.
About the Authors
Xiaoming Feng is based at the ABB Corporate Research Center in Raleigh, North Carolina. He can be reached at email@example.com.
William Peterson is based at ABB Power Systems in Raleigh, North Carolina. He can be reached at firstname.lastname@example.org.