When the physicians of the power system (planners and operators) treat for resource inadequacy, congestion, instability and all the modern-day maladies of competitive power markets, their regimen may come with an increasingly common side effect – harmonics. The utilization of static var compensators (SVC), induction generators, source converters, underground and submarine cables, direct current converters, to name a few, to provide solutions to power system problems can lead to increasing harmonic penetration in the power system. Harmonic generating equipment coupled with system resonance conditions effects are cumulative and can be detrimental to system operations if not mitigated.
Sources of Increasing Harmonic Penetration
The following are the usual suspects for escalating harmonic penetration in transmission systems:
Reactive Power Compensation. To address local voltage problems, utilities use capacitor banks. Capacitors in transmission systems are usually switched in blocks that may result in harmonic resonance. The switching events can be cyclical over the course of daily operation or in response to various system contingencies.
Wind Turbine Generator Speed Controls. Variable speed controls of wind turbine generators employ power converters that produce harmonics at the point of common coupling (PCC).
Wind Plant Power Factor Compensation and Low Voltage Ride Through Capability. Wind turbines use induction generators that require some form of power factor compensation to maintain unity power factor at the low voltage PCC. The compensation may take the form of capacitor banks. Wind turbine generators are further required to have ride-through capability for low voltage conditions associated with system faults and disturbances similar to synchronous machines. Dynamic compensation to attain acceptable voltage response during system disturbance may be specified. The dynamic compensation can take the form of Static Var Compensator (SVC), Static Compensator (STATCOM) and other forms of static var device made up of high power electronics. Such devices produce harmonics as a consequence of their operation.
Flexible Alternating Current Transmission System (FACTS) devices. These devices are mostly considered for flexible control of the transmission systems. Power flow control and reactive power control are usually the application of FACTS. Sometimes, these devices are useful for postponing transmission line investments or replacing retired generation. Utilizing power electronic components, these devices, even equipped with filters, can result in harmonic resonance in combination with the system impedance.
High Voltage Direct Current (HVDC) Installations. Though provided with harmonic filters to smooth the AC output, HVDC can resonate with the system at various operating conditions thus increasing harmonic content of transmission bus voltages and line currents.
Underground and Submarine Cable Installations. To solve the problems of right-of-way and environmental constraints, utilities may consider underground or submarine transmission cables which have high charging characteristics that can be comparable to reactive power compensation.
The transmission system is a continuously changing large scale system that is exposed to different generation dispatches, load levels and line status conditions at various times during the day, month or year. For one, the system is exposed to multiple generation dispatches influencing interface flows and line loadings. Each change in generation dispatch results in a different short circuit profile. The consequent variation in apparent impedances interacts in unique ways with harmonic sources in the transmission system. This is further impacted by changes in the system configuration such as may be due to outages, operational procedures and maintenance. In addition, changes in load level and composition present diverse modes for the damping of harmonics.
The changes in harmonic supply and damping may result in more or less harmonics penetrating the grid at various times. Furthermore, the risk of reaching resonant conditions is a critical concern for operators and consumers alike.
The level of harmonic penetration is determined by harmonic measurements (hardware) and harmonic simulations (software). Measurements account for whether or not recommended standard harmonic limits are met over a certain period of monitoring.
Evaluation of harmonic measurements, as a minimum, are made against existing recommended or regulatory harmonic limits utilizing industry accepted indices. The monitoring period needs to capture all the various operating scenarios for proper analysis of the increased harmonic content window. Harmonic simulations accounting for proper modeling and harmonic power flow utilize electro-magnetic transient software. Power flow transmission models can be easily converted to such platform nowadays. Another option is programming typical steady-state power flow software to compute for frequency response. Such software solutions compute a so-called “frequency scan”. Frequency scan analysis is initially conducted to achieve perspective on the system response to additions of components to the system and/or to different system operating scenarios. In figure 1, a frequency scan of one transmission system bus with respect to several operating conditions results in resonance to 3rd (black) and 5th (green) harmonic frequencies. To further dig deeper into the resonant situation, harmonic indices such as total harmonic distortion and individual harmonic distortion should be conducted. If harmonic distortion magnitudes surpass accepted industry limits, various harmonic mitigation solutions are investigated.
Figure 1: Harmonic Frequency Scan at a Transmission System Bus at various contingencies.
High power electronic devices used for power system control generates harmonics that when coupled with harmonic resonance can produce unacceptable harmonic quantities that may impact equipment loading, nuisance protection tripping and unnecessary added heating in system components.
Knowing the potential and actual levels of harmonic penetration gives transmission operators and planners the opportunity to apply mitigation measures. Typical planning mitigation may involve the installation of harmonic filters at strategic locations.
Other solutions may include re-tuning of controls of power electronic converters, reconfiguration of existing harmonic filters and investigation of cable parameter sensitivity to harmonic resonance. The latter may result in choosing a cable with different characteristics, for instance.
Harmonic penetration in the transmission system increases as the system is more heavily utilized and transmission and generation additions result in conditions of resonance with existing components at certain system operating states. The level of harmonic dispersion is determined by conducting harmonic measurements and simulations. Knowing the potential levels of harmonic penetration allows for opportunities to mitigate through operating procedures or planning solutions such as implementing filters.
D. Mueller, “Case Studies of Harmonic Problems, Analysis and Solutions on Transmission Systems”, 9th International Conference on Electrical Power Quality and Utilization, Barcelona, Spain, October 9-11, 2007.