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3.3 Basics for the Design of a Distributed Generation Power Conversion System

Special issues and concerns must be addressed when dealing with medium and high power systems. These issues arise in instances where a design is being done for the support of power delivery to the utility and when incorporating the concepts of both DG and intentional islanding.   The following sections detail the reasoning behind the selection of using a VSI, common standards and regulations used for DG interconnections, and descriptions of challenges faced while implementing medium and high power systems.

3.3.1 Focus on VSI of the PCS

The inverter portion of the PCS was selected as a VSI as previous described.   This type of inverter was selected not only because of the readily available power electronics building block (PEBB) based inverter system, but also because of the type of control systems to be implemented. PEBBs have the ability to standardize the converter system. The PEBBs developed and leveraged for this study were insulated gate bipolar transitors (IGBTs) with anti-parallel diodes (APD) based switches. VSI typically consist of a transitor device (like the IGBT) with an anti-parallel diode (APD) to allow for the bi-directional current flow. This makes the selection of VSI a natural choice after the PEBBs were integrated into the design.

As mentioned above, power will be regulated through the PCS via the control. With the VSI's inherent bi-directional current flow capabilities and the utility's fixed voltage, the PCS can now simply control the power flow by means of current regulation [1, 5, 9, 17, 18].   The control system must allow for both the grid-connection and islanding modes of operation. Direct regulation, current for grid-connected mode and voltage for islanding mode, is preferable to the alternatives. It allows for the system to achieve zero steady state error in output. The VSI's bidirectional current flow fits the demands of DG technologies in an intentional islanding framework rather nicely.

3.3.2 Standards and Common Practices for Grid Interconnections

The following standards are considered guidelines during the design process of the PCS and control thereof:

•  ANSI/IEEE C84.1 - 1995, [19]

•  IEEE 519 - 1992, [20]

•  IEEE 929 - 2000, [21]

•  IEEE 1547 - 2003

•  UL 1741, [22]

These standards provide guidelines and specifications for the interconnection and control of DERs to the utility grid. The following are brief summaries of each standard:

ANSI/IEEE C84.1 - 1995 standard deals with common line voltages at different distributions levels (ie: residential power is single phase and an RMS voltage of 120 V, where as some commercial sites have 3 phases with an RMS voltage of 240 V).

IEEE 519 - 1992 are recommended practices and requirements for the harmonic control of electrical power systems.   It sets maximum total harmonic distortion (THD) limits on voltages and currents that a power system is allowed. Therefore, the PCS cannot inject harmonics into the grid that cause the system to go above these limits set forth by the standard and the PCS should filter these harmonics [23].

IEEE 929 - 2000 are recommended practices for the utility interface of photovoltaic (PV) systems.   Though written for PV inverters, the guidelines and specifications can be adapted to be used for an inverter connecting a DER to the utility.

IEEE 1547 - 2003 is the standard for the interconnection of distributed resources to the utility grid.   This standard outlines requirements and specifications that the conversion systems of the DER have to meet to be allowed to connect to the utility.   This standard does not deal with the concepts and issues of intentional islanding, and currently dictates that the DER shall disconnect from the distribution system when islanding events occur.   As noted above the standard does leave open a section for consideration of intentional islanding in future revisions of the standard.   An analysis of 1547 raising questions to issues proposed by it can be found in [24].

UL 1741 is the Underwriters Laboratories' testing standards for equipment as they relate to IEEE 1547.

3.3.3 Challenges for Medium & High Power Inverters

Switching Frequency & Line Conditions -

The selected PCS is a pulsed-width modulation (PWM) based system. This makes it advantageous to push the switching frequency as high as possible.   Higher switching frequencies translate to reduction in passive component (inductors and capacitors) sizes. However, with the present semiconductor switching devices available for medium and high power systems (GTOs, ETOs, IGBTs, etc...), device limitations require that the system switching frequencies be on the order of kilohertz to the tens of kilohertz range.   This, along with a utility line frequency of 60 Hz and low resonant frequency of the output line filter, brings about special considerations to the design of control systems [10].

Due the fact that the resonant frequency of the VSI output filter tends to be a few decades below the switching frequency (in order to allow filtering of the negative affects of the PWM switching) and the frequency generally needs to be one to two decades above the line-frequency to allow the system's natural dynamics to behave properly. This means that the control loop must be designed within a narrow bandwidth. In addition to filter effects, the loading conditions of the converter also affect the control design.

Digital Delay -

It has become common in converter systems for the control to be digitally implemented through digital signal processors (DSPs); especially in 3 phase, medium to high power systems that require complex calculations for coordinate transformations from stationary reference frames to that of rotating reference frames for use in the control.   Also, analog circuitry makes the implementation of these transformations virtually impossible. Further, limiting of the states (controlled variables), controller anti wind-up, and other protection protocols are easily transported digitally to places where they are more complex to build in an analog control system.  

Synchronization to the Grid -

Another fundamental aspect to consider in the design of DER to grid connected PCS is that of synchronization to the utility system. The control needs measurements of the frequency and line-angle of the utility to properly ensure that it can regulate the real and reactive power flow through the PCS during the periods of time which the PCS is interconnected to the grid. These measurements are obtained through the implementation of phase-lock loops (PLL), which will use voltages of the PCS and grid to track the frequency and angles. Without these measurements to synchronize the PCS to the utility, the power flow to/from the PCS will be incorrect, and protection and safety issues arise [25].

The control system also needs these measurements for use in the coordinate transformation calculations of the rotating reference frame .

Detection & Re-closure of/to the Grid -

One of the key features of operating a DER interconnected to the grid, and running it in both islanding and grid-connected modes, is that the system has to have the capability to autonomously detect when disturbances on the grid occur (over/under voltages and/or frequencies, line faults, faults to ground, etc...).   This ability to determine if it needs to disconnect from the utility is to protect itself and the surrounding Area EPS from the grid disturbances [1, 4].

The advantages of the PCS being able to autonomously detect potentially hazardous disturbances on the grid not only aids in the creation and operation of fault protection schemes, but also helps the control switch modes of operation from a "grid-connected" mode to an "islanding" mode.   Common practices and proposed new detection and re-closure schemes are not within the scope of this module.

This module serves to present the rationale behind choosing the VSI as the common feature for DG technologies. Both the common standards and regulations for DG technologies and the challenges to medium and high power inverter design inform this choice.

 

 

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