Dynamic Security Analysis of Shipboard Power Systems with Energy Storage for Pulsed Load Effect Mitigation

Improving the dynamic security and stability of a shipboard power system was studied by distributing energy storage elements. The notional ship power system under study has an AC network consisting of four alternators (two main and two auxiliary turbine generators), a DC microgrid involving distributed renewable energy generation and hybrid battery/super capacitor (SC) energy storage system. In addition to normal ship loads, we are considering the operation of a pulsed load. The security of the system was investigated given several contingency events. Such events include the loss of one of the main generators when the battery storage is conventionally connected at a single location on a DC bus. We have also studied the effect of distributing the battery storage over different DC zones in the system. Various case studies and events were studied. It was found that best system performance can be achieved when the battery storage is divided among the various DC zones of the DC network Power System Security Security analysis and assessment are very essential for the operation of any power system architecture. In this paper, the dynamic security was studied on a notional shipboard hybrid AC/DC power system example; especially during pulsed load conditions. Shipboard Integrated Power Systems (IPSs) have some challenging and unique features from a power system operation point of view [1]-[3]. Firstly, the IPS is an isolated power system, not supported with a relatively larger grid. Therefore, the system encounters faster dynamics due to the finite generation inertia and is more susceptible to disturbances. Secondly, transients on the load side may cause considerable fluctuations in the system since load/generation ratio is close to 1. Moreover, the system is physically small; hence the connecting cables have negligible impedance, which may cause instability issues among the different components of the system. Finally, the IPS is more susceptible to severe physical damages, especially during battle conditions, which must be taken into account while operating the system and for security analysis. With the presence of pulsed loads in such systems, dynamic security analysis must be performed during various contingencies while utilizing storage distribution in the operating architecture. The main security constraints considered are the voltage limits on the AC and DC buses in steady state and transient conditions during pulsed load conditions. In addition, the AC voltage frequency was allowed to oscillate only within acceptable limit. Finally, none of the system components, such as cables and transformers, should be overloaded. Table 1 shows the security constraints considered in this study. Table 1 Security constraints in the Study Parameter Secure Range AC Voltage Amplitude 0.9 < |Vac| < 1.1 AC Voltage Frequency 59.5 < f < 60.5 Loading of System Components Loading < 100% DC Voltage Level 0.9 < Vdc < 1.1 Shipboard Power System Architecture In this research, the IPS configuration is a zonal DC distribution system. The DC side consists of four different buses. Figure 1 shows a single line diagram of the IPS studied. The example system is notional shipboard power system with scaled down ratings for reduced scale test-bed verification. This system includes two 13.8 kW main generators (MTG) and two 10.4 kW auxiliary generators (ATG) connected in a ring bus configuration. The bulk of the load consists of two 50 kW propulsion motors, modeled as permanent magnet machines supplied by PWM drives, with hydrodynamic propeller models as the mechanical load. Each rectifier supplies one of two 0.318 kV DC busses. Furthermore, a DC generation system of 10 kW rated capacity, lithium-ion batteries with 3000 Ah rated capacity and super capacitors with 200 F are included in the DC microgrid. Figure 1. A single line diagram of the hybrid AC/DC system studied in this chapter. A PWM controlled DC-DC converter is used as an interface between the PV system and the DC bus. Moreover, a vector decoupling PWM controlled ACDC/DC-AC bidirectional converter was used for connectivity between the AC and DC sides. In the steady state case, the system voltages and loadings are within the normal limits. For transient simulations, we considered a pulse train of four pulses with a rate of 0.2 Hz, a duty ratio of 10% and amplitude of 20 kW. Each of the four DC zones may include, a pulsed load, battery, SC, a hotel load or a combination of these elements based on the event studied. As shown in figure 2, with no contingencies and applying the algorithm developed in [1], the super capacitors quickly respond at the beginning of the pulsed load due to its power density. However, its power drops and the battery takes over due to its higher energy density. The main and auxiliary generators encounter some swinging and oscillation. Super capacitor and battery, are capable of riding the system through the disturbance caused by the pulsed load. The system frequency oscillations are within the ±0.5 Hz limit, and we have no overloading at any generator according to figure 2 c. Transient values of voltages are within limits according to figure 2 d and in addition the steady state values of voltages are within limits. The generators rotor angle and MTG1’s power delta-curve were shown in 2 e and 2 f.