Sarina Adhikari

Real and reactive power control of a three-phase single-stage PV system and PV voltage stability

Grid-connected photovoltaic (PV) systems with power electronic interfaces can provide both real and reactive power to meet power system needs with appropriate control algorithms. This paper presents the control algorithm design for a three-phase single-stage grid-connected PV inverter to achieve either maximum power point tracking (MPPT) or a certain amount of real power injection, as well as the voltage/var control. The switching between MPPT control mode and a certain amount of real power control mode is automatic and seamless. Without the DC-to-DC booster stage, PV DC voltage stability is an important issue in the control design especially when the PV inverter is operating at maximum power point (MPP) with voltage/var control. The PV DC voltage collapse phenomenon and its reason are discussed. The method based on dynamic correction of the PV inverter output is proposed to ensure PV DC voltage stability. Simulation results of the single-stage PV system during system disturbances and fast solar irradiation changes confirm that the proposed control algorithm for single-stage PV inverters can provide appropriate real and reactive power services and ensure PV DC voltage stability during dynamic system operation and atmospheric conditions.

Properly understanding the impacts of distributed resources on distribution systems

The subject paper discusses important impacts of distributed resources on distribution networks and feeders. These include capacity, line losses, voltage regulation, and central system support (such as volt/var control via central generators and substation) as the number, placement and penetration levels of distributed resources are varied. Typically, the impacts of distributed resources on the distribution system are studied by using steady-state rather than dynamic analysis tools. However, the response time and transient impacts of both system equipment (such as substation/feeder capacitors) and distributed resources needs to be taken into account and only dynamic analysis will provide the full impact results. ORNL is completing a study of distributed resources interconnected to a large distribution system considering the above variables. A report of the study and its results will be condensed into a paper for this panel session. The impact of distributed resources will vary as the penetration level reaches the capacity of the distribution feeder/system. The question is how high of a penetration of distributed resource can be accommodated on the distribution feeder/system without any major changes to system operation, system design and protection. The impacts will most surely vary depending upon load composition, distribution and level. Also, it is expected that various placement of distributed resources will impact the distribution system differently.

P-Q and P-V Control of Photovoltaic Generators in Distribution Systems

In this paper, simultaneous control of active power and volt/var is explored with photovoltaic (PV) generators in distribution systems. The PV active power output can be controlled in the load-following mode when sufficient solar power is available to supply a local load, or the maximum power point tracking (MPPT) mode when a local load is large or injection to the system is allowed. Two selected control approaches, P-Q control in the load-following mode and P-V control in the MPPT mode, are investigated in this paper. The P-Q control is implemented with a relatively simple approach, while the P-V control demands an extra MPPT logic, which is solved based on a power balance between the dc and ac sides in a two-stage PV configuration. The control algorithms are tested with the IEEE 13-bus distribution feeder with various system conditions like the presence of multiple PV generators, imbalance, harmonics, and faults. The MATLAB and SimPowerSystems simulation results clearly demonstrate the capability of the proposed control in maintaining the P/V bus as either a P-Q or P-V bus depending on different applications.

Improving an Unjustified Common Practice in Ex Post LMP Calculation

A common practice for the Ex Post LMP calculation at a number of U.S. ISOs uses a small constant range, typically from -2.0 to +0.1 MW, as each generators lower and upper bounds. This paper shows that this is an unjustified practice as the marginal units and LMP may change if the bounds are changed. A simple yet effective improvement is proposed.

Hierarchical Utilization Control for Real-Time and Resilient Power Grid

Blackouts in our daily life can be disastrous with enormous economic loss. Blackouts usually occur when appropriate corrective actions are not effectively taken for an initial contingency, resulting in a cascade failure. Therefore, it is critical to complete those tasks that are running power grid computing algorithms in the Energy Management System (EMS) in a timely manner to avoid blackouts. This problem can be formulated as guaranteeing end-to-end deadlines in a Distributed Real-time Embedded (DRE) system. However, existing work in power grid computing runs those tasks in an open-loop manner, which leads to poor guarantees on timeliness thus a high probability of blackouts. Furthermore, existing feedback scheduling algorithms in DRE systems cannot be directly adopted to handle with significantly different timescales of power grid computing tasks. In this paper, we propose a hierarchical control solution to guarantee the deadlines of those tasks in EMS by grouping them based on their characteristics. Our solution is based on well-established control theory for guaranteed control accuracy and system stability. Simulation results based on a realistic workload configuration demonstrate that our solution can guarantee timeliness for power grid computing and hence help to avoid blackouts.

Impacts of varying penetration of distributed resources with & without volt/var control: Case study of varying load types

This paper provides a follow-up to an earlier one on impacts of distributed energy resources (DR) on distribution feeders. As DR penetration level on the feeder increases, there can be impacts to distribution system/feeder capacity, line losses, and voltage regulation. These can vary as the penetration level reaches the capacity of the distribution feeder/system or loading. The question is how high of a DR level can be accommodated without any major changes to system operation, system design and protection. Our objective for this work was to address the question of how the DR impacts vary in regards to both DR voltage regulation capability and load mix. A dynamic analysis was used to focus on the impacts of DR with and without volt/var control with different load composition on the distribution feeder. The study considered an example 10MVA distribution feeder in which two inverter-based DRs were used to provide voltage regulation. The results due to DR without voltage regulation capability are compared with DR capable of providing local (at its bus) voltage regulation. The analysis was repeated for four different feeder load compositions consisting of (1) constant power, (2) constant impedance, (3) constant current and (4) ZIP (equal combination of the previous three).

Utility-Side Voltage and PQ Control with Inverter-Based Photovoltaic Systems

Abstract Distributed energy resources (DER) are relatively small-scale generators or energy storage units that are located in close proximity to load centers. The DERs that are integrated to the grid with the power electronic converter interfaces are capable of providing nonactive power in addition to active power. Hence, they are capable of regulating the voltages of weak electrical buses in distribution systems. This paper discusses voltage control capability of photovoltaic (PV) systems as compared to the traditional capacitor banks. The simulation results prove the effectiveness of dynamic voltage control capability of inverter-based PVs. With proper control algorithms, active and nonactive power supplied from DERs (e.g., solar PVs or micro-turbines) can be controlled independently. This paper also presents the scenario of controlling active and nonactive power supplied from a PV array to track and supply the local load.

Aggregation of multiple induction motors using MATLAB-based software package

This paper presents an aggregate model of multiple induction motors using a single equivalent circuit model, which simplifies the computation while simulating dynamic behavior of large power systems. The simulations and analysis are carried out using MATLAB-based software package. The performance of the aggregation model of multiple motors is verified by comparing the results obtained from the sum of individual induction motors and the aggregation model. The results obtained are satisfactory and reasonable.

Air conditioning stall phenomenon - Testing, model development, and simulation

Electric distribution systems are experiencing power quality issues of extended reduced voltage due to fault-induced delayed voltage recovery (FIDVR). FIDVR occurs in part because modern air conditioner (A/C) and heat pump compressor motors are much more susceptible to stalling during a voltage sag or dip such as a sub-transmission fault. They are more susceptible than older A/C compressor motors due to the low inertia of these newer and more energy efficient motors. There is a concern that these local reduced voltage events on the distribution system will become more frequent and prevalent and will combine over larger areas and challenge transmission system voltage and ultimately power grid reliability. The Distributed Energy Communications and Controls (DECC) Laboratory at Oak Ridge National Laboratory (ORNL) has been employed to (1) test, (2) characterize and (3) model the A/C stall phenomenon.

Constructive back-feed algorithm for online power restoration in distribution systems

The paper proposes a simple yet effective back-feed restoration algorithm to reinstate supply to the loads following a distribution system fault. The algorithm tries to balance the load served by the individual substations in the post fault restoration strategy. The proposed algorithm is essentially a constructive network tracing algorithm which comes up with final network restoration strategy by determining the switching sequences of the feeder switches like circuit breakers, sectionalizers, etc. Simple example systems are considered to demonstrate the algorithm. The algorithm is considered to be applicable to the Smart Grids in which reliability of the supply is a major concern.