Volt and Var Control and Optimisation
Objectives of voltage control
The primary purpose of voltage control is to maintain acceptable voltage (120 volts plus or minus 5%) at the service entrance of all customers served by the feeder under all possible operating conditions. There are many factors that influence the voltage at each customer service entrance, including:
- The voltage on the transmission or subtransmission supply line: Under normal and abnormal situations, the supply line voltage fluctuates by a small amount. Voltage regulating devices must adjust the distribution primary voltage so that supply line voltage fluctuations do not cause the distribution primary voltage to drift out of the acceptable range.
- The feeder loading: During peak load conditions, the current (ampere) flowing on the feeder increases, and the voltage drop along the feeder due to the current flow increases. This results in decreased voltage for customers that are further from the substation end of the feeder. The voltage regulating devices must automatically raise the line voltage under peak load conditions to account for increased voltage drop. Conversely, when the feeder is lightly loaded, the voltage drop is lower, so the voltage regulating devices must lower the voltage to avoid possible high voltage conditions
Electric utilities traditionally maintain distribution system voltage within the acceptable range using transformers with moveable taps that permit voltage adjustments under load. Voltage regulators located in substations and out on the lines and substation transformers with Tap Changing Under Load are commonly used for voltage control purposes (Load Tap Changer or LTC). These transformers are equipped with a voltage regulating controller that determines whether to raise or lower the transformer tap settings or leave the tap setting unchanged based on “local” voltage and load measurements.
The optimal strategy for distribution feeder design and operation is to establish acceptable voltage conditions for all customers while being as efficient as possible. The voltage profile along the distribution feeder and the flow of reactive power (VARs) on the feeder are typically maintained by a combination of voltage regulators and switched capacitor banks installed at various locations on the feeder and in its associated substation. Each voltage regulator includes a controller that raises or lowers the voltage regulator tap position in response to local (at the device) current and voltage measurements. Similarly, each capacitor bank includes a controller that switches the bank on or off in response to its local measurements.
Power flow to the feeder includes “real” power flow and “reactive” power flow. Real power flow (measured in kilowatts) represents the actual electrical power that is consumed by the customer plus the electrical losses consumed in heating the wires and other electrical equipment. Real power is best illustrated by the current flowing to an incandescent light, resistive electric stove element, or other primarily “resistive” device. Reactive power (measured in volt-ampere reactive VARs) is the electrical energy that is needed to energize the portions of the power system that behave like:
- Capacitors (e.g., the overhead conductors themselves) that are continuously charged and discharged by the alternating current (AC) waveform
- Inductors (e.g., electric motors and transformers) which store a considerable amount of energy in magnetic fields that are essential for device operation.
By reducing the amount of reactive power (VARs) flowing on the distribution feeder, the electric utility can reduce electrical losses and improve the voltage profile along the feeder. Electric utilities address the issue of reactive power flow by installing capacitor banks at strategic points along the feeder. These capacitor banks serve as a source of reactive power that the electric utility can position at any point on the feeder. Installing capacitor banks reduces the amount of reactive power supplied by the transmission system, so that the reactive power drawn from the transmission system is reduced, the current flowing from the transmission system to serve a given load is reduced along with the associated electrical losses, and the voltage is increased at the point of the capacitor. A portion of the capacitor banks are on (energized and in service) at all times (“fixed” banks). However, to account for the load variations and reactive power variations that occur throughout the day, some of the capacitor banks are equipped with switches (“switched banks”) that allow the utility to place these capacitor banks in or out of service as needs vary during the day. Capacitor banks are equipped with controllers that use local measurements to determine when to switch the capacitor bank off or on. For example, when the voltage measured at the location of the capacitor bank is low, the controller would close the switch to place the capacitor bank in service. When the voltage is high, the controller would open the switch to remove the capacitor bank from service. The conventional approach works best during peak load seasons, when low voltage conditions due to the voltage drop caused by peak current are most likely to result in capacitor banks being switched on when needed. However, during off peak loading periods, the voltage drop may not be sufficient to trigger capacitor bank switching. Therefore, while the voltage level is adequate during light loading seasons, the reactive power flow remains high causing excess losses.
Without such adjustments, voltage at the end of some feeders might sag to unacceptably low levels at peak load periods, while voltage close to the substation might rise to unacceptably high levels at minimum load.
Distribution Voltage Control
Volt and Var control Type 1 - Stand alone controllers
Traditionally, feeder voltage regulators and switched capacitor banks are operated as completely independent (stand-alone) devices, with no direct coordination between the individual controllers. This time-honored approach is effective for maintaining acceptable voltage and reactive power flow in the vicinity of the controllers, but typically does not produce optimal results for the entire feeder.
The conventional approach has generally been successful at accomplishing the primary objectives for this equipment. However, this approach has several key limitations:
- The system is not continuously monitored, so controller failures and malfunctions are not automatically detected. Line capacitors are particularly failure-prone. Without continuous monitoring, these devices may be switching on and off at the wrong time, or may be totally inoperative due to a blown fuse. This condition can go undetected until the problem deteriorates into a more serious and potentially unsafe problem.
- The system lacks flexibility to respond to changing conditions on the distribution feeders. Controller settings work well under normal circumstances. However, if the feeder is reconfigured for any reason (for example, while a faulted portion of the feeder is being repaired), the controller settings may not produce the desired results.
- The system cannot be used to respond to system emergencies. Occasionally, distribution utilities are called upon to place all switched capacitors in service as rapidly as possible to respond to power grid emergencies. Since the stand-alone controllers lack remote control capabilities, it is not possible to rapidly switch all capacitor banks on demand.
Smart Distribution Voltage Control
Smart Distribution voltage control provides significantly increased operating flexibility over conventional voltage control. While conventional voltage control is primarily intended to maintain acceptable voltage along the feeder, Smart distribution voltage control enables the user to achieve other operating objectives in addition to the primary function of maintaining acceptable voltage. The most common smart distribution voltage control function is Conservation Voltage Reduction (CVR). With CVR, the system intentionally lowers the voltage on the distribution feeder to the lowest acceptable voltage value to achieve valuable benefits to the electric utility and consumers, such as reduced demand and energy consumption. As long as the feeder voltage remains above the minimum acceptable value, there is no adverse impact on the customer.
Smart voltage control uses many of the same components as conventional voltage control, such as tap changing transformers and voltage regulating controllers. Smart voltage control also includes a main processor or other intelligent controller that executes additional control logic using available current and voltage measurements to achieve the specified objective function. The CVR, the main processor uses the available measurements to determine when it is possible to intentionally lower the feeder voltage (within low limit constraints) to achieve valuable benefits such as improved efficiency, reduced electrical demand, and lower energy consumption. The DA voltage control in its simplest form consist of a series of rules that specify what control actions (if any) should be performed for the real-time current and voltage measurements.
Smart VAR control provides a more effective way to maintain voltage level along the feeder and minimize electrical losses under all loading conditions. Like the conventional approach, Smart VAR control uses switched capacitors to control VAR flow and feeder voltage as feeder conditions vary during the day. However, rather than basing the control actions solely on local measurements, the VAR control function bases switching decisions on measurements taken at the substation end of the feeder, where total VAR flow to the feeder is readily observable. When DSCADA detects that VAR flow to the feeder is excessive, it uses remote control facilities to operate the switched capacitor banks as needed.
Advantages of smart VAR control over conventional methods include:
- Ability to determine that the system is operating correctly by observing the expected changes in reactive power flow following a switching request. If no change is observed immediately following a switching request, this indicates a possible malfunction that should be investigated.
- Ability to override the normal control during when necessary. For example, during a system emergency, the system operator can command all switched capacitor banks to switch on to provide relief to the bulk power grid.
Volt and Var control Type 2 - Integrated Volt and var Control (IVVC)
Feeder voltage and feeder reactive power flow are closely related and dependant variables. Control actions to change one of the variables can result in opposing control actions to change the other variable. For example, raising the voltage using the substation transformer LTC can produce a voltage rise that could cause capacitor bank controls to remove a capacitor bank form service, thus lowering the voltage. Similarly, placing a capacitor bank in service could cause the LTC to lower the voltage at the substation. While such conflicting control actions generally do not produce unacceptable electrical conditions on the feeder, they do produce conditions that are less efficient and not optimal. The coordinated control of voltage and reactive power is needed to determine and execute volt-VAR control actions that are truly optimal.
Integrated Volt VAR Control (IVVC) is an advanced SD function that determines the best set of control actions for all voltage regulating devices and VAR control devices to achieve a one or more specified operating objectives without violating any of the fundamental operating constraints (high/low voltage limits, load limits, etc.). IVVC operating objectives may include:
- Minimal electrical losses
- Minimal electrical demand
- Reduced energy consumption
- Weighted combination of the above
It is also possible to bias the recommended control actions to minimize the number of operations for specified load tap changers, regulators or capacitor banks that are nearing end of life or end of maintenance cycles.
IVVC uses an on-line power flow (OLPF) function and available real time measurements to compute the conditions that exist at any point on the feeder, total electrical losses, and other parameters that are not practical to monitor directly. The OLPF results are delivered to an “optimizing engine”, which is software designed to determine the correct set of control actions to achieve “optimal” conditions required by the electric utility. These control actions are then sent to the proper device controllers via SCADA.
Volt and Var control Type 3 - Integrated VVO
The Smart Distribution Volt-VAR Control and Optimization (VVC&O) application offers new capabilities that go well beyond those of conventional stand-alone controllers, providing significant benefits for electric distribution utilities.
In teh future the Integrated VVO will have to integrate all voltage information to include in the near real time simulation. These information are expected to come from Distribution Sensors, major distribution equipment controllers, AMI- Smart meters or any other device that can provide voltage information.
Volt and Var control Type 4 - Adaptative VVC
AVVC system “learns” from previous V-V control actions. It always aks “What happened last time system was in a given state and LTC tap position changed?” and records the event for future actions.