Surge Transients Research Articles

Compressor Surge Identification in Innovative Heat-Pump Systems Equipped by Vaned Diffuser Centrifugal Compressor

Abstract In the present energy scenario, heat-pumps play an important role to improve energy efficiency in reversible cooling systems. In case of industrial size plants, centrifugal compressors are preferred with respect to other solutions; nonetheless, they work in variable operation and therefore they must withstand off-design conditions. Carrier provided the University of Genoa with a small size chiller rig equipped with an innovative high speed centrifugal compressor driven by a variable speed motor to study unstable operation of refrigerant closed loop systems. To this aim, vibro-acoustic signature analysis of such closed loop rig is performed mainly focusing on compressor mechanical response. This experimental characterization is firstly conducted in system stable operation. Afterwards, surge transients are obtained by acting on plant feeding lines valves; meanwhile, experimental signals are acquired at relevant plant locations to investigate system response just before instability onset. Indeed, system dynamics is affected by interposed volumes, and therefore the effect of heat exchangers in system response may be relevant. Therefore, system dynamics is analysed both in sub-synchronous frequency range and in high frequency region to assess how its vibro-acoustic response varies when moving from stable conditions towards surge. By doing so, suitable surge precursors can be defined by relying only on vibro-acoustic signals both in low and high frequency ranges to perform early surge detection in such chiller systems. The proposed approach main advantage is to exploit non-intrusive probes, which allow to define diagnostic indicators without interacting directly with the working fluid.

Pressurizer system dynamic model for transient control in PWR

Abstract The pressurizer system of pressurized water reactor (PWR) maintains the reactor coolant system pressure during steady-state operation and limits pressure changes during transients. The in/out surge transients will cause pressure variations and they are controlled by either the spray system or the heater system. The spray system is actuated when the pressure exceeds a preset value. The heater system is initiated when the pressure falls below a preset value. The fundamental understanding and a reliable modeling of the pressurizer system behavior under steady state and transient conditions are needed to simulate overall nuclear power plant behavior. In the present study, an algorithm using Python 3.7 is developed to represent the dynamic behavior of the pressurizer system under steady-state and during in/out surge transients. Moreover, RELAP5 code is used to simulate the pressurizer system during the prescribed transients. The analysis and assessment results demonstrate satisfactory control performance during the in/out surge transients that guarantee the safety of PWR during operation. Also, the comparison between Python algorithm and RELAP5 model illustrates the capability and effectiveness of the Python algorithm for dynamic simulation and control.

A Computational Method of Rotating Stall and Surge Transients in Axial Compressor

The onset of rotating stall and surge in compressors limits the operating range of aero-engines. Accurately predicting the key features during these events is critical in the engine design process. In this paper, a three-dimensional computational model for transient simulation of multi-stage axial compressors during stall is proposed. The kinetic equations describing the dynamic process of the compression system are constructed, with a 3D through-flow model for the compression part and a 1D gas collector model for the outlet part. The calculation of the source term is performed using the developed body-force model, which realizes the correlation between the deviation angle and the loss coefficient with the inlet parameters in various flow regions. Validated on a single-stage compressor and a single-rotor fan, the results show that the method is capable of capturing the stall and surge features correctly and that the three-dimensional structure of the stall cell can be captured. In addition, this model could be used for the analysis of the surge load, which is significant for the structural integrity of the compressor.

A Novel Energy Absorption-Based Protection Scheme for Surge Transients in Large-Scale Wind Farm System

Onshore wind farms are usually connected to the power grid by 35 kV transmission lines. Due to the high altitude of the wind turbines and their associated long transmission corridors, lightning activities are more frequent for onshore wind farms compared to the power plant in plain areas. In addition, the insulation levels of 10–35 kV distribution lines are generally low. All these facts make proper lightning strike protection a technical challenge for onshore wind farms. Taking a typical onshore wind farm as the testing object, the structure of the 35 kV distribution line connected to this wind farm is modeled by EMTP software in this paper. The leakage current and absorbed energy of the arrester at the head tower when lightning currents of different amplitudes inject into the overhead line are studied. By calculating the energy accumulated in the arrester within 100 µs, the causes of the lightning trip and damage to the arrester of the distribution line, i.e., the energy accumulation mechanism, are clarified. Eventually, the lightning protection updating scheme for this distribution line is proposed: a combination of refitting the porcelain cross arm at the head and end of the tower and filling the grounding body of the tower with a drag-reduction agent, which shows that the lightning resistance level of the distribution can be improved by 50 times in maximum.

Method of adaptive current protection against earth faults in 6–10 kV cable networks with an insulated neutral

Simple and reliable zero sequence overcurrent protection in distribution 6–10 kV cable networks with an insulated neutral is most widely used for protection against single phase earth faults. However, protection of this type in many cases does not provide the required sensitivity to internal (inside the protected zone) faults as it must be tuned to the response current from surge transients during external faults through an intermittent arc. It is possible to increase the sensitivity if adaptive current protection is applied. However, the known methods for its implementation are only effective for stable faults through transient resistance but do not provide high dynamic stability of operation in transient conditions in case of arc intermittent earth faults that are the most dangerous for the network. Therefore, an urgent problem to be solved now is improving the principles of adaptive current protection against earth faults. To compare the efficiency of the known and proposed principles of adaptive current protection implementation taking into account the complexity of transients during earth faults through an intermittent arc in 6–10 kV cable networks, we used Matlab simulation with the SimPowerSystem and Simulink extension packages. The research into the operation algorithms of adaptive current protection against earth faults was carried out on simulation models of 6–10 kV cable networks with an insulated neutral and with neutral grounding through a high-value resistor. The studies on the simulation models have shown that the known methods of implementation of adaptive current protection against earth faults based on the use of full zero sequence currents and voltage are ineffective during intermittent arc earth faults. The authors propose a method of adaptive current protection against earth faults in 6–10 kV cable networks with an insulated neutral and with neutral grounding through a high-value resistor that provides a significant increase in dynamic stability of transient operation with arc ground faults and allows using only zero sequence current and voltage components of the operating frequency of 50 Hz as the actuating quantities. The proposed method of implementing adaptive current protection against earth faults in 6–10 kV cable networks with an insulated neutral and with neutral grounding through a high-value resistor does not only increase the sensitivity of this protection type to earth faults through transient resistence and dynamic stability of operation in transient condiitons in case of arc intermittent earth faults but also broadens the range of its possible applications

Flow instability evolution in high pressure ratio centrifugal compressor with vaned diffuser

Compressor stability is an essential issue for developing single-stage high pressure ratio centrifugal compressors. In this paper, a comprehensive stability map of a centrifugal compressor stage with the peak pressure ratio of 6.2 has been illustrated. Fast response pressure transducers are mounted inside the casing or the back-plate wall to measure internal transient behaviors. From Fourier analysis of pressure signals, various instabilities across time and length scales are identified. At low speeds, the impeller inlet rotating instability develops in scale and evolves into the inducer stall, which eventually induces the mild surge and deep surge of the compression system. At middle speeds, the compressor successively experiences stable state, mild surge, rotating instability, and deep surge. The mild surge region coincides with the dip region of the compressor S-shape pressure rise curve, and deep surge occurs when the compressor pressure rise raises to the second peak. At high speeds, mild surge and deep surge abruptly occur without preceding stall or rotating instability. To explain the complex surge behavior, the mechanical analogy between the compression system and the mass-spring-damper system is applied. Both mild surge and deep surge transients are found to belong to the dynamic instability of the mass-spring-damper system, and the exact form of the surge state will be determined by eigenvalues of the system governing equation.

Modeling, Simulation, and Testing of Switching Surge Transients in Rapid Transit Vehicles DC Power Systems

During the operation of dc rapid transit systems, the rail passenger vehicles are subjected to surge transient events that can damage the on-board equipment and cause service interruption. During the testing phase, vehicle manufacturers must demonstrate the performance of the vehicles under the specified transients. However, system testing is generally not available during design. Engineering analysis must be performed to ensure that the design will meet the transient requirements, and that the overvoltage and overcurrent protective devices are coordinated on a system level. In response to these design challenges, detailed time-domain simulation models of a switching surge transient generator and the vehicle equipment have been developed and validated experimentally. These models are used to evaluate vehicle system parameters sensitivity, as well as to provide design guidelines for increased vehicle power system safety, reliability, and availability.

Analytical Study on Stall Stagnation Boundaries in Axial-Flow Compressor and Duct Systems

Stall stagnations in the system of axial-flow compressors and ducts occur in transition from deep surge conditions to decayed or converged stall conditions. The present study is concerned with the boundaries between the deep surges and the stagnation stalls on the basis of analytical results by a code on surge transients analysis and simulation. The fundamental acoustical-geometrical stagnation boundaries were made clear from examinations of the results on a variety of duct configurations coupled with a nine-stage compressor and a single stage fan. The boundary was found to be formed by three parts, i.e., B- and A-boundaries, and an intermediate zone. The B-boundary occurs for the suction-duct having a length of about a quarter of the wave-length of the first resonance in the case of very short and fat plenum-type delivery duct. On the other hand, the A-boundary occurs for the long and narrow duct-type delivery flow-path having a length about a fifth of the wavelength and relatively small sectional area in the case of short and narrow suction ducts. In addition to this, the reduced surge-cycle frequencies with respect to the duct lengths are observed to have respective limiting values at the stagnation boundaries. The reduced frequency for the B-boundary is related with a limiting value of the Greitzer's B parameter. The tendency and the characteristic features of the related flow behaviors in the neighborhood of the boundaries were also made clearer.

Monte Carlo analysis of wind farm surge arresters risk of failure due to lightning surges

This paper presents a Monte Carlo procedure intended for the assessment of the metal-oxide (MO) surge arresters risk of failure in onshore wind farms. It focuses on the energy withstand (absorption) capability of the MO surge arresters in relation to lightning surges and in terms of their risk of failure assessment. Presented methodology accounts for the fact that the lightning itself is stochastic in nature and that the MO surge arrester energy capability is a statistical quantity. The well-known backsurge phenomenon is employed as a means for studying the MO surge arresters energy stresses due to lightning surge transients (in onshore wind farms), where the associated transient (i.e. high-frequency) models of particular wind farm components feature prominently. Necessary numerical simulations are carried-out with the well-known EMTP-ATP software package. This procedure could be seen as beneficial in selection of the optimal MO surge arrester energy withstand capability for wind farm projects situated in areas marked with high keraunic levels and/or having high soil resistivity.

Converter-Switching Surges: Railroad Vehicle Power Converter Impact Studies

The application of new power electronics equipment to the rolling stocks of electrified railroad vehicles in recent years brought about the benefits of smooth acceleration and braking, higher efficient of propulsion systems, improved power quality, easy-and-simple maintenance of propulsion motor, and even lower procurement and service costs. Hence, they are generally regarded as a blessing and progress for the rail industry. The switching techniques and new power modules with fast response times are key components to make the above functions possible. If a railroad system is built with vehicles and traction power substations are designed for new equipment, then all power components should be designed to withstand the transient dynamic behavior, especially, the switching surge voltage and current. This is usually the responsibility of the general consultant or the turn-key contractor. Unfortunately, the situation in most electrified railroad systems is new, and renovated trains equipped with new converters and new IGBT power modules join service together with old trains and existing traction networks designed and commissioned decades ago. The impact of the switching surge voltage and current will become an issue of great concern, as illustrated by the two cases presented in this article. The problems presented in the two cases were eventually solved by some sort of soft-switching schemes as a remedy or correction solution. Yet, this problem should be addressed and examined in front of the commission phase to avoid impacts. In conclusion, clear specification of surge transients or permissible impacts need to be included in the procurement documents, and field tests are recommended to verify the compatibility of new power converters with the existing equipment in the entire system.

Time to surge concept and surge control for acceleration performance

Surge is a dangerous instability that can occur in compressors. It is avoided using a valve that reduces the compressor pressure. The control of this valve is important for the compressor safety but it also has a direct influence on the acceleration performance. Compressor surge control is investigated by first studying the surge phenomenon in detail. Experimental data from a dynamic compressor flow test bench and surge cycles measured on an engine is used to tune and validate a model capable of describing surge. A concept named time to surge is introduced and a sensitivity analysis is performed to isolate the important characteristics that influence surge transients in an engine. It is pointed out that the controller clearly benefits from a feed-forward term due to the small time frames associated with the transition to surge. In the next step this knowledge is used in the design of a novel surge controller. This surge controller is then compared to two other controllers and it is shown that it avoids surge and improves the acceleration performance by delivering both higher engine torque and turbo shaft speed after a gear change.

Vestibulo-Ocular Reflex to Transient Surge Translation: Complex Geometric Response Ablated by Normal Aging

The linear vestibulo-ocular reflex (LVOR) to surge (fore-aft) translation has complex kinematics varying with target eccentricity and distance. To determine normal responses and aging changes, 9 younger [age, 28 +/- 2 (SE) yr] and 11 older subjects (age, 69 +/- 2 yr) underwent 0.5 g whole body surge transients while wearing binocular scleral search coils. Linear chair position and head acceleration were measured with a potentiometer and accelerometer. Subjects viewed centered and 10 degrees horizontally and vertically eccentric targets 50, 25, or 15 cm distant before unpredictable onset of randomly directed surge in darkness (LVOR) and light (V-LVOR). Response directions were kinematically appropriate to eccentricity in all subjects, but there were significantly more measurable LVOR and V-LVOR responses (63-79%) in younger than older subjects (38-44%, P < 0.01). Minimal LVOR latency averaged 48 +/- 4 ms for younger and significantly longer at 70 +/- 6 ms for older subjects. In the interval 200-300 ms after surge onset, horizontal LVOR gain (relative to ideal velocity) of younger subjects averaged over all target distances was 0.55 +/- 0.04 and was significantly reduced in older subjects to 0.33 +/- 0.04. Horizontal V-LVOR gain was 0.58 +/- 0.04 in younger and significantly lower at 0.35 +/- 0.06 in older subjects. Vertical gains did not differ significantly between groups. Target visibility had no effect in either group during the initial 200 ms. The LVOR and V-LVOR were augmented by saccades in younger more than older subjects. Aging thus decreases LVOR velocity gain, response rate, and saccade augmentation, but prolongs latency.

Maximum insulation stresses under transient voltages in the HV barrel-type winding of distribution and power transformers

Analytical as well as experimental results are presented for voltages developed in HV (high-voltage) windings of dry-tyre power and distribution transformers when subjected to surge transients. Although the existence of this phenomenon has been known for years, it has not always been taken into consideration in the calculation of insulation stresses. The possibility of accurate prediction of the developed voltages is demonstrated. >

Determination of compressor in-stall characteristics from engine surge transients

A technique for extracting the in-stall pumping characteristics for an axial flow compressor operating in an engine system environment is developed. The technique utilzes a Hybrid computer simulation of the compressor momentum equation in which actual transient data are used to provide all terms but the desired compressor characteristic. The compressor force characteristic as a function of corrected flow and speed results from the computation. The critical problem of data filtering is addressed. Results for a compressor operating in a turbofan engine are presented and comparison is made with the conventional compressor map. The relationship of the compressor surge charcteristic with its rotating stall characteristic is explored. Initial interpretation of the measured results is presented. Previously announced in STAR as N84-22566

Analog Computer Study of Switching Surge Transients for a 500-ky System

Discussion is presented of the results of an analog computer study which was conducted to determine the switching surge transients on the planned 500-kV Southern California Edison Company system and to find a means to limit them to a magnitude which would give economic design parameters for the system.