Performance of synchronous motors loss-of-synchronism protection
Synchronous motors are rarely designed to operate asynchronously for extended periods of time. When a synchronous motor experiences a mechanical load exceeding its pull out torque, it enters asynchronous operation, under which the interaction between the stator and rotor magnetic fields causes damaging mechanical stress and vibrations. Also, induced currents in the rotor cause extra heating that quickly damages the rotor windings. Asynchronous operation must be detected and dealt with by allowing time for resynchronization or by tripping the motor to avoid thermal overload and damage. Two protection elements are often used to detect asynchronous motor operation: the power factor element and the out-of-step element. This paper compares the performance of the two elements to protect a 20 MW synchronous motor subjected to pull out conditions.
Leveraging Digital Relays for Protection of Pumped Storage Hydro
Currently the U.S. has about 20 GW of pumped storage hydro. An additional 30 GW of new capacity has been proposed for support of renewable sources. This paper describes the challenges associated with protecting these units and how digital protective relays provide simpler and more effective protection. Switching from generator to pump mode impacts elements that use sequence components. A second impact occurs if the differential zone includes a reversing switch. A third item is a bad status signal from the reversing switch. The paper describes how a digital relay can address these issues. Protecting units that start as induction motors requires protection for the damper winding, which was typically provided by an overcurrent relay or even a simple timer. An alternate method uses a motor thermal model. The paper discusses the merits of this solution and offers some setting guidelines. In the past, units that started while connected to a variable-frequency source used overcurrent and overvoltage relays with wide frequency responses to supplement the differential and neutral overvoltage elements. If a digital relay with tracking frequency is used, these supplemental functions are no longer required. In early electromechanical schemes, out-of-step protection took the form of an inverse-time overcurrent relay. This paper discusses the advantages of power and impedance-based elements present in digital relays that can provide a superior method for protecting against out-of-step conditions. Regarding the generator step-up transformer (GSU), if it is downstream from the reversing switch, then the transformer effectively changes its vector group. A second issue can occur if the tap from the GSU is used to implement lowvoltage starting, which effectively changes the turns ratio of the GSU. The paper discusses how both of these issues can be addressed in a digital relay. When shutting down a hydro machine, it must be quickly brought to a standstill to avoid bearing damage. Dynamic braking is an effective way to stop these machines but introduces challenges for protection. I. OVERVIEW OF PUMPED STORAGE HYDRO During the last two decades, renewable energy projects have gained momentum, and at present, a large installed base of wind and solar energy sources exists worldwide. These sources are intermittent, and maintaining grid stability requires bulk energy storage. Pumped storage hydro (PSH) units are hydro units that can operate in generating or pumping mode, moving water either from the upper reservoir to the lower reservoir or vice versa, as shown in Fig. 1. The current round-trip energy efficiency of PSH exceeds 80 percent [1], and primarily for this reason, PSH units have historically been the only practical method to store and manage large amounts of grid energy. Therefore, they play a vital role in the operation of power systems with a large ingression of renewable energy sources.
Parboiled rice effluent: A wastewater niche for microalgae and cyanobacteria with growth coupled to comprehensive remediation and phosphorus biofertilization
Abstract The potential of microalgae and cyanobacteria for bioremediation of wastewater by nutrient uptake combined with simultaneous biomass production is a well recognized perception of today's world. The present study illustrates the treatment of a highly polluted wastewater generated during parboiling of paddy in rice mill industries, widely operational in developing countries where rice is the staple food crop, with the help of microalgal and cyanobacterial isolates capable of growing at high rates in parboiled rice mill effluent (RME). This endeavor leads to comprehensive bioremediation of the said effluent and subsequent use of the harvested biomass as slow release phosphorus biofertilizers and the treated effluent for crop irrigation. The RME-acclimatized algal consortium demonstrated highest growth in terms of fresh weight and greatest remediation efficiency at the end of 36 days' treatment of RME, with 93.9% phosphorus and 100% ammonia-nitrogen removal, 98.7%, 91.6% and 93.5% reduction in biological oxygen demand, chemical oxygen demand and total dissolved solid, respectively, and an increment of 186 ± 0.3 mg L − 1 dissolved oxygen, bringing down the pollutants level well below the discharge limits suggested by Central Pollution Control Board, India. Additionally, microalgae in the consortium aggregated in clumps spontaneously in presence of the filaments of Phormidium sp. facilitating easy harvest. The RME-acclimatized algal consortium demonstrated highest accumulation of polyphosphate (poly-P) (0.76 ± 0.01% of dry weight) as well as highest release of phosphorus in non-sterile soil emphasizing the essential role of soil phosphorus solubilizing organisms to leach soluble phosphorus from the insoluble poly-P present in the biomass. The rice seedlings watered with treated RME also showed improved growth effect on shoot height and leaf width. The study results establish the suitability of RME as an excellent growth media for microalgae and cyanobacteria.