Ruifeng Yan

Investigation of Voltage Stability for Residential Customers Due to High Photovoltaic Penetrations

Several studies on voltage stability analysis of electric systems with high photovoltaic (PV) penetration have been conducted at a power-transmission level, but very few have focused on small-area networks of low voltage. As a distribution system has its special characteristics-high R/X ratio, long tap switching delay, small PV units, and so on-PV integration impacts also need to be investigated thoroughly at a distribution level. In this paper, the IEEE 13 bus system has been modified and extended to explore network stability impacts of variable PV generation, and the results show that a voltage stability issue with PV integration does exist in distribution networks. Simulation comparisons demonstrate that distribution networks are traditionally designed for heavily loaded situations exclusive of PVs, but they can still operate under low PV penetration levels without cloud-induced voltage-stability problems. It is also demonstrated that voltage instability can effectively be solved by PV inverter reactive power support if this scheme is allowed by the standards in the near future.

Voltage Variation Sensitivity Analysis for Unbalanced Distribution Networks Due to Photovoltaic Power Fluctuations

In a geographically small distribution area, fast moving clouds may cover the whole area within a short period causing photovoltaic (PV) power to drop. When a feeder loses PV power support, bus voltages will decrease. In an unbalanced network, asymmetrical spacing and non-transposition of line configurations can result in different voltage drops for each phase. This may potentially cause some voltage problems after a decline in PV generation, such as an extremely low voltage magnitude of a certain phase and an unacceptable voltage imbalance level at a remote bus. This paper proposes a method of analyzing voltage variation sensitivity due to PV power fluctuations in an unbalanced network (unbalanced line configuration and phase loading levels). Based on this method, a network reconfiguration solution is developed to solve the voltage problems. This solution utilizes unbalanced line characteristics and realizes the potential of the network, so no extra compensation devices are needed for network support.

Investigation of Voltage Imbalance Due to Distribution Network Unbalanced Line Configurations and Load Levels

In distribution power systems line transposition is not a common practice and phase loading levels are always changing. Therefore, perfect balance is never achieved at a distribution level. Non-transposed lines can cause unequal voltage drops between phases and this may be further exacerbated by uneven loads in each phase. Consequently, the voltage magnitude of a certain phase may suffer a severe decrease, breaching the lower voltage limit or unbalanced three-phase voltages at a remote bus may violate the percentage voltage imbalance accepted in the standards. This paper examines the mechanism of causing uneven voltage drops and investigates the impacts of unbalanced line configurations with different phase loading levels on network voltage imbalance. Methodologies are proposed to analyze voltage drops affected by line and load imbalance. Based on these methods, general guidelines are recommended to mitigate the voltage problems, which are useful for network analysis, planning and reconfiguration.

Regulation of distribution network voltage using dispersed battery storage systems: A case study of a rural network

The drive for power networks throughout the world to utilize clean, green renewable generation poses a number of issues for distribution and transmission companies. Two of the most popular forms of renewable generation, wind and photovoltaic (PV) face large fluctuations in their generation profile. This variation means that peak generation and peak customer demand are often greatly divergent. Battery energy storage can help to buffer wind and PV generation, capturing a portion of the energy produced during light load and exporting it back onto the network as required. Moreover, energy storage can be utilized to load shift and regulate network voltage. This paper investigates the use of battery storage in regulating network voltage, in particular, the different strategies which may be employed in controlling the storage unit. Simulations are coupled with real data, taken from a rural single wire earth return (SWER) network, showing that battery storage is capable of boosting the network voltage.

A Time-Dependent Approach to Evaluate Capacity Value of Wind and Solar PV Generation

Contribution of renewable energies in power systems is increasing due to continuous growth of wind and solar generators. Because of intermittency and uncertainty of these resources, conventional reliability evaluation methods are not applicable and different techniques have been developed to model these generators. However, most of these methods are time-consuming or may not be able to keep time dependency and correlations between renewable resources and load. Therefore, this paper intends to improve the existing methods and proposes a fast and simple approach. In this approach, wind power, photovoltaic (PV) generation, and electricity demand have been modeled as time-dependent clusters, which not only can capture their time-dependent attributes, but also are able to keep the correlations between these data sets. To illustrate the effectiveness of this framework, the proposed methodology has been applied on two different case studies: 1)IEEE RTS system and 2)South Australia (SA) power network. The developed technique is validated by comparing results with sequential Monte Carlo technique.

Estimation of maximum wind power penetration level to maintain an adequate frequency response in a power system

Wind energy has an immense potential to play a vital role with conventional energy sources. Therefore, integration of wind power is increasing in many power systems. Unlike synchronous generators, modern wind farms have lack of automatic frequency support capability following a disturbance. Thus, an addition of wind power does impact the frequency response of a power system. Inadequate frequency response may occur due to presence of a large wind generation. This paper proposes a methodology for estimating the maximum wind power penetration level to ensure an adequate frequency response in a power system. Frequency nadir and rate of change of frequency after an outage of the largest generator of a power system are taken into consideration. Impact of two different wind power integration strategies (direct replacement of a synchronous generator with a wind machine and 2/3 de-commitment, 1/3 re-dispatch approach) on the maximum wind power penetration level are investigated. This paper also attempts to identify a system parameter that could most significantly influence a penetration of wind power.

Frequency response and its enhancement using synchronous condensers in presence of high wind penetration

In recent years wind power integration has substantially increased in southern states of Australia. At present South Australia has the highest wind generation capacity of any region across the country. This large capacity penetration would likely displace existing synchronous generation fleet. These wind generators have neither enough inertia response nor governor support to control major frequency excursion. Under high wind power availability and cheaper import from neighboring region, South Australian grid could depend on few synchronous machines for frequency regulation. Under such a scenario, a big contingency may produce a severe frequency excursion in the network. Consequently, system may face a considerable amount of load shedding, which may degrade the standard of network service quality. To understand these issues, this paper investigates frequency response of a power system in presence of high wind penetration. The network under consideration loosely represents South Australian power system. Additional frequency control strategy, such as deployment of synchronous condensers to enhance network frequency response is also studied.

Large scale photovoltaic system and its impact on distribution network in transient cloud conditions

This paper presents simulation results from a real case study of a medium scale photovoltaic (PV) system integrated into an 11kV distribution network. The investigated system is a semi-rural part of an existing distribution network in Queensland, Australia with planned investments of more than 3MWp PV installation. The studied case is very interesting because of planned high penetration of PV power that exceeds the local peak load, even in high load conditions. In this paper, the authors are focusing on dynamic network conditions in the situation of a sudden drop in PV power production (shadow from clouds covering PV panels). It can result in a sudden drop of PV power generation and therefore significant voltage drops can occur. The investigated phenomena should be mitigated by control algorithms implemented within power system equipment such as On-Load Tap Changers (OLTC) and Series Voltage Regulators (SVR). The existence of significant time delays in control loops result in periods with grid voltage potentially without control.

Frequency response with significant wind power penetration: Case study of a realistic power system

Wind power penetration in power system is increasing rapidly in many countries due to its zero fuel cost and zero air pollution. Unlike the conventional synchronous generation, wind generation has different dynamic characteristics and hence influences the frequency response of a power system. This paper investigates the frequency response of a realistic power system with reasonable integration of wind power. The investigation is carried out on six different load scenarios, with three different wind power penetrations. The frequency sensitivity index and the rate of change of frequency (ROCOF) of the system are estimated. It is observed that the system may become more vulnerable in terms of frequency response as the penetration level of wind power increases. Based on different case studies, this paper also attempts to estimate the minimum synchronous generation requirement of the system after a synchronous generator trips to prevent possible under frequency load shedding (UFLS).

Investigation of the impacts of three-phase photovoltaic systems on three-phase unbalanced networks

More three-phase photovoltaic (PV) systems have been directly integrated to the electrical network at the distribution voltage level each year. Most of them are designed with balanced assumptions. In reality, under normal operating conditions distribution networks always have a certain degree of voltage imbalance, and these three-phase PV systems have already been implemented in such networks. However, their integration effects have yet to be fully identified and this should be thoroughly examined before the high PV penetration level causes serious problems in the near future. This paper studied the impacts of photovoltaic integration to an unbalanced distribution network. Using PSCAD and Matlab/Simulink softwares a three-phase photovoltaic model was built based on commonly applied balanced theories. This model was then implemented to the unbalanced IEEE 13 bus system. From simulation observation and mathematical analysis it has been determined that when voltage imbalance is within the allowable range defined by the standards the PV system achieves an approximately balanced current injection to each phase. As a result, it is suitable to implement a simplified current source model of the PV system for network analysis under such realistic operating conditions.