Smart Grid and Power Quality Management

By Mehrdad Boloorchi

Frequent occurrence of power quality events are the results of deficiencies in the power delivery system. Prudently incorporating and integrating of Smart Grid technology, which encompass varieties of monitoring, communicating, and digital computing technologies, into the power delivery system can provide a clean power to the end-user equipment with minimal disturbances.

To understand the effect of smart grid on power quality improvement first we review the power quality problems.

Power quality events could be categorized as follows:

• Voltage variation; which occur when the voltage is temporarily under or over a specified limit, called voltage sag or voltage swell, or if a complete loss of voltage occurs. Voltage sag or voltage dip is the most disturbing power quality (PQ) issue in power delivery system. Many of the critical and sensitive loads such as radio frequency generators in semiconductor manufacturing industries or flow rate dependent pumps in process industries shall not be affected during voltage sag.
• Frequency variation; which occurs when the system frequency is temporarily over or under a specified frequency limit. Generally, the impact of frequency variations in a large power delivery system on different types of equipment is almost insignificant, However, the frequency variations within an islanded Nano-grid could be larger than in a large power delivery system and therefore a large increase in volt per Hertz (V/Hz) ratio could cause saturation of the induction motors and their overheating.
• Voltage unbalance variations; which occurs if the voltage unbalances of three (3) phase power system is outside of a predetermined value range. The negative sequence currents flow along with positive sequence current results in increasing of motors losses or decreasing their efficiency, motors torque fluctuation, and motors failure. It also causes the increasing of cables losses and poor operation of UPS, inverters, and VFDs.
• Current Unbalance variations; which occurs if the current unbalances in three (3) phase power system is outside of a predetermined value range. Current unbalance occurs due to the load imperfection and results in voltage unbalance.
• Waveform distortion; which is a steady state deviation from an ideal sinusoidal wave of power frequency and includes DC offset, harmonics, interharmonics, notching and noise, o DC offset is the presence of a DC voltage or current in an AC power system and can cause transformer saturation, additional stressing of insulation, and other adverse effects.
  • Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the fundamental power frequency. Wave from distortion due to the harmonics causes problems such as communication interference, heating, and solid-state device malfunction.
  • Interharmonics can appear as discrete frequencies or as a wide-band spectrum. The main sources of interharmonics waveform distortion are static frequency converters, cyclo-converters, induction motors, and arcing devices. Power-line carrier signals can also be considered as interharmonics. The effects of interharmonics are not well known but have been shown to affect power line carrier signaling and induce visual flicker in display devices such as CRTs.
  • Notching is a periodic voltage disturbance caused by the normal operation of power electronics devices when current is commutated from one phase to another. During this period, there is a momentary short circuit between two phases. It can be a significant problem on weak power delivery systems, where it can produce noise currents causing control system misoperation.
  • Noise is unwanted electrical signals with broadband spectral content lower than 200 kHz superimposed upon the power system voltage or current in phase conductors or found on neutral conductors or signal lines. Noise in power systems can be caused by power electronic devices, control circuits, arcing equipment, loads with solid-state rectifiers, and switching power supplies. The frequency range and magnitude level of noise depend on the source, which produces the noise and the system characteristics. A typical magnitude of noise is less than 1% of the voltage magnitude. Noise disturbs electronic devices such as microcomputer and programmable controllers.
• Power delivery system transients; includes both impulsive and oscillatory variation of system voltages and currents. Impulsive transients are sub-cycle high frequency and high voltage/high current changes outside of a predefined range and oscillatory transients include sub-cycle low frequency (≤5kHz), medium frequency (5-500 kHz), and high-medium frequency (0.5-5MHz), and under/over voltage change outside a predefined range of voltage. The oscillatory transient can lead to transient over voltages, causing line insulators damage, tripping, component failure, hardware reboot necessity, and software glistens.

Smart Grid technologies and systems address the power quality needs for all kinds of end-user equipment of consumers.

Volt/Var management (VVM) system, which, typically operate to regulate distribution system voltage profiles within allowable limits and minimize reactive power flow, can provide significant benefits in the areas of conservation, efficiency, and peak reduction by optimizing the voltage levels for consumers along feeder lines. VVM also improves phase balancing and in conjunction with capacitor banks can flatten the voltage profile, facilitating the voltage regulation.

The VVM software is the key component of the VVM solution. The major function of VVM Software is to calculate optimal settings for voltage and reactive power control devices, including load tap-changer (LTC) controllers, regulators and capacitor banks.

VVM Software uses the advanced metering infrastructure (AMI) data to ensure that optimal set-points are calculated without violating any constraints.

A geographic information system (GIS) interface enables network model updates with the latest configuration data.

A SCADA system primarily monitor and control power delivery system, including substations, equipment.

Distribution automation (DA) system proactively solve problems before they affect customers or limit their effects if service interruptions do result.

The essential functions of DA are:

• Monitoring and control of distribution system devices that are outside of the substations battery limits and includes devices such as switches, reclosers, capacitors, regulators, sensors, meters, and fault current indicators (FCIs).
• Fault location, isolation, and restoration (FLIR) which provides the capability to locate and isolate a fault, and restore power to the entire upstream section, as well as, downstream section of the isolated faulty section.
• Real-time power flow which provides the capability of running power flow analysis using real-time telemetered data. This function enables calculation of system parameters at each system node in real-time.
• Auto transfer function is the capability of transferring one supply source to an alternative source when the main power supply source is lost. This function requires real-time monitoring and control of the system to make safe switching decisions that will be provided by the DA system.

The DA software is the key component of the DA system, providing the intelligence for find the fault location, isolate the fault and restore power to the healthy sections of a feeder of distribution system.

Advanced metering infrastructure (AMI) is an integrated system of smart meters, communications networks, and data management systems that enables two-way communication between utilities and customers.

AMI provides the ability to automatically and remotely measure electricity use, connect and disconnect service, detect tampering, identify and isolate outages, and monitor voltage, frequency and power factor.

The smart meters data can be identified easily as each meter has a unique identifier, the ability to time stamp data, power consumption, voltage sags, interruptions, harmonics and other power quality parameters. The collected data can be used to identify the power quality problems in their network.

Integrating the properly managed data pertaining the power quality into a system which collects, store and process all the data forms power quality analyzer (PQA). This PQA provides the capability to investigate any particular type of disturbances in power system as well.

While the smart grid technologies and systems as described above can monitor and analyze almost all kind of the power quality problems, however, voltage sags are still uncontrollable and unavoidable. To provide a voltage sag immunity, it is required to use the PQA and smart grid systems in conjunction with voltage sag mitigating solutions such as true on-line UPS, DC scheme equipped with battery and charger, devices with good voltage dip ride-through capability, and true-off delay timer. The noise problem can also be mitigated by using filters, isolation transformers, and some line conditioners.