Charge regulators are electronic devices designed to protect batteries from overcharging. They are installed between the solar array termination boxes and batteries.
As described earlier, photovoltaic panels generate direct current, which can only be used by a limited number of devices. Most residential, commercial, and industrial devices and appliances are designed to work with alternating current. Inverters are devices that convert direct current to alternating current. Although inverters are usually
Figure 1.8 An inverter single line diagram. Courtesy of SatCon, Canada.
designed for specific application requirements, the basic conversion principles remain the same. Essentially, the inversion process consists of the following.
Wave formation process. Direct current, characterized by a continuous potential of positive and negative references (bias), is essentially chopped into equidistant segments, which are then processed through circuitry that alternately eliminates positive and negative portions of the chopped pattern, resulting in a waveform pattern called a square wave. Figure 1.8 shows an inverter single line diagram.
Waveshaping or filtration process. A square wave, when analyzed mathematically (by Fourier series analysis), consists of a combination of a very large number of sinusoidal (alternating) wave patterns called harmonics. Each wave harmonic has a distinct number of cycles (rise-and-fall pattern within a time period).
An electronic device referred to as a choke (magnetic coils) or filters discriminate passes through 60-cycle harmonics, which form the basis of sinusoidal current. Solid-state inverters use a highly efficient conversion technique known as envelope construction. Direct current is sliced into fine sections, which are then converted into a progressive rising (positive) and falling (negative) sinusoidal 60-cycle waveform pattern. This chopped sinusoidal wave is passed through a series of electronic filters that produce an output current, which has a smooth sinusoidal curvature.
Protective relaying systems. In general, most inverters used in photovoltaic applications are built from sensitive solid-state electronic devices that are very susceptible to external stray spikes, load short circuits, and overload voltage and currents. To protect the equipment from harm, inverters incorporate a number of electronic circuitry:
■ Synchronization relay
■ Undervoltage relay
■ Overcurrent relay
■ Ground trip or overcurrent relay
■ Overvoltage relay
■ Overfrequency relay
■ Underfrequency relay
Most inverters designed for photovoltaic applications are designed to allow simultaneous paralleling of multiple units. For instance, to support a 60-kW load, outputs of three 20-kW inverters may be connected in parallel. Depending on the power system requirements, inverters can produce single – or three-phase power at any required voltage or current capacity. Standard outputs available are single-phase 120 V ac and three-phase 120/208 and 277/480 V ac. In some instances step-up transformers are used to convert the output of 120/208 V ac inverters to higher voltages.
Input and output power distribution To protect inverters from stray spikes resulting from lightning or high-energy spikes, dc inputs from PV arrays are protected by fuses, housed at a junction box located in close proximity to the inverters. Additionally, inverter dc input ports are protected by various types of semiconductor devices that clip excessively high voltage spikes resulting from lightning activity.
To prevent damage resulting from voltage reversal, each positive (+) output lead within a PV cell is connected to a rectifier, a unidirectional (forward-biased) element. Alternating-current output power from inverters is connected to the loads by means of electronic or magnetic-type circuit breakers. These serve to protect the unit from external overcurrent and short circuits.
Grid-connected inverters In the preceding we described the general function of inverters. Here we will review their interconnection to the grid, which requires a thorough understanding of safety regulations that are mandated by various state agencies. Essentially the goal of design safety standards for inverters used in grid-connected systems, whether they be deployed in photovoltaic, wind turbine, fuel cell, or any other type of power cogeneration system, is to have one unified set of guidelines and standards for the entire country. Standard regulations for manufacturing inverters address issues concerning performance characteristics and grid connectivity practices and are recommended by a number of national test laboratories and regulatory agencies.
Underwriters Laboratories For product safety, the industry in the United States has worked with Underwriters Laboratories (UL) to develop UL1741, Standard for Static Inverter and Charge Controller for Use in Independent Power Systems, which has become the safety standard for inverters being used in the United States. Standard UL1741 covers many aspects of inverter design including enclosures, printed circuit board configuration, interconnectivity requirements such as the amount of direct current the inverters can inject into the grid, total harmonic distortion (THD) of the output current, inverter reaction to utility voltage spikes and variations, reset and recovery from abnormal conditions, and reaction to islanding conditions when the utility power is disconnected.
Islanding is a condition that occurs when the inverter continues to produce power during a utility outage. Under such conditions the power produced by a PV system becomes a safety hazard to utility workers who could be inadvertently exposed to hazardous electric currents; as such, inverters are required to include anti-islanding control circuitry to cut the power to the inverter and disconnect it from the grid network.
Anti-islanding also prevents the inverter output power from getting out of phase with the grid when the automatic safety interrupter reclosures reconnect the inverter to the grid, which could result in high voltage spikes that can cause damage to the conversion and utility equipment.
Institute of Electrical and Electronics Engineers The Institute of Electrical and Electronics Engineers (IEEE) provides suggestions for customers and utilities alike regarding the control of harmonic power and voltage flicker, which frequently occur on utility buses, in its IEEE 929 guideline (not a standard), Recommended Practice for Utility Interface of Photovoltaic (PV) Systems. Excessive harmonic power flow and power fluctuation from utility buses can damage customers’ equipment; therefore a number of states, including California, Delaware, New York, and Ohio, specifically require that inverters be designed to operate under abnormal utility power conditions.
Power limit conditions The maximum size of a photovoltaic power cogeneration system is subject to limitations imposed by various states. Essentially most utilities are concerned about large sources of private grid-connected power generation, since most distribution systems are designed for unidirectional power flow. The addition of a large power cogeneration system on the other hand creates bidirectional current flow conditions on the grid, which in some instances can diminish utility network reliability. However, it is well known that, in practice, small amounts of cogenerated power do not usually create a grid disturbance significant enough to be a cause for concern. To regulate the maximum size of a cogeneration system, a number of states have set various limits and caps for systems that generate in excess of 100 kW of power.
Utility side disconnects and isolation transformers In some states such as California, Delaware, Florida, New Hampshire, Ohio, and Virginia, utilities require that visible and accessible disconnect switches be installed outside for grid service isolation. It should be noted that several states such as California require that customers open the disconnect switches once every 4 years to check that the inverters are performing the required anti-islanding.
In other states such as New Mexico and New York, grid isolation transformers are required to reduce noise created by private customers that could be superimposed on the grid. This requirement is however not a regulation that is mandated by UL or the Federal Communication Commission (FCC).
PV Power cogeneration capacity In order to protect utility companies’ norm of operation, a number of states have imposed a cap on the maximum amount of power that can be generated by photovoltaic systems. For example, New Hampshire limits the maximum to 0.05 percent and Colorado to 1 percent of the monthly grid network peak demand.