What You Need to Know About Solar Design

by Bernard Richard, Claude Gudel and Stephane Rollier, LEM

The Solar Energy Industries Association (SEIA) reports that solar energy is the fastest-growing energy technology in the U.S. SEIA said the U.S. solar industry was worth $6 billion in 2010, up 67 percent from 2009. In addition, solar energy capacity installations reached 956 MW in 2010, more than double the installations in 2009. They are projected to grow significantly in 2012.

Germany is the world leader in solar installations. It installed 3.8 GW in 2009, amounting to more than half of the estimated 7.2 GW added worldwide that year. Germany’s total photovoltaic (PV) capacity in 2009 was 9.8 GW, and another 3.8 GW was added in the first six months of 2010. Germany’s pioneering position on PV is being repeated in many countries worldwide.

As with any manufactured product, economies of scale apply to solar equipment and as production volumes increase, PV system prices decrease, raising investors’ interest.

With renewable energy sources continuing to see increased deployment worldwide and solar photovoltaic technology at the forefront, understanding inverter design options has never been more important.

Inverter Types

Solar energy facilities are connected to the grid through inverters. A facility can connect through the inverter with or without a transformer and with or without a battery. A connection that is made without galvanic isolation is a trasnsformerless connection.

Different inverters exist for different purposes. They vary by size, efficiency, weight, range and galvanic insulation. Selecting the most efficient design for specific renewable energy needs can be difficult. Following are the advantages and disadvantages of various solar inverter designs.

Option 1: Low-frequency transformer inverter design, illustrated in Figure 1 (Page 90).

Advantages:

  • Galvanic insulation
  • No direct current (DC) current injection in the grid
  • Cost-effective
  • Reliable

Disadvantages:

  • Lower-efficiency
  • Large
  • Heavy

Option 2: High-frequency transformer inverter design, illustrated in Figure 2 (Page 90).

Advantages:

  • Galvanic insulation
  • Small
  • Light

Disadvantages:

  • Lower-efficiency
  • DC current injection in the grid
  • Many components, which diminish reliability

Option 3: Transformerless (without DC chopper) inverter design, illustrated in Figure 3 (Page 92).

Advantages:

  • Small
  • Light
  • High-efficiency

Disadvantages:

  • No galvanic insulation
  • DC current injection in the grid
  • Leakage current

Option 4: Transformerless (with DC chopper) inverter design, illustrated in Figure 4 (Page 93).

Advantages:

  • Small
  • Light
  • High-efficiency
  • Wide voltage input range

Disadvantages:

  • No galvanic insulation
  • DC current injection in the grid
  • Leakage currents

MPPT Control, Inverter Control and Protection

Instantaneous DC current and PV panel voltage output must be measured in all options to establish the maximum power point tracking (MPPT), the point at which the maximum output power can be extracted from the solar panel. In addition, current measurement is needed as an input to the inverter’s control loop, and to ensure protection against short circuit or overload. Open-loop and closed-loop Hall effect technologies are used for the current and voltage transducers.

DC Current Injection Measurement

Transformerless designs (options 3 and 4) have numerous advantages, making them more successful in the market. They have one main disadvantage, however. The DC current an inverter can inject into the grid is limited to 10 milliamps (mA) to 1 amp (A), according to different standards in different countries. (Relevant standards include IEC 61727, IEEE 1547, UL 1741 and VDE 0126-1.) This requires transducers with high accuracy (better than 1 percent) and low offset and gain drifts, making transformerless technology ideal for closed-loop fluxgate transducers.

This disadvantage is not limited to transformerless designs. High-frequency transformer inverter designs (option 2) also are subject to these limits.

Leakage Current Measurement

Transformerless inverters (options 3 and 4) can experience leakage currents.

Potential hazards could come from a leakage capacitance between the solar panel and roof, offering a path for the leakage current. A leakage path from the AC line back to the panel can raise the panel to line voltage, creating an electric shock risk. Leakage currents also can be responsible for electromagnetic interferences, grid current distortion and system losses.

The ideal leakage current measurement for safety purposes should be noncontact and nonintrusive. Alternating current (AC), 50/60 hertz (Hz) leakage currents will be small, typically about 300 mA, and can be measured as the residual component from differential current measurements in several conductors. The measurement must detect a sudden increase of 30 mA in leakage current, which might mean that a person is touching a panel. Requirements again include accuracy and, especially, low offset and gain drifts, to ensure resolution of these small measured currents.

Earth Fault Current Measurement

Safety monitoring must detect an earth fault current arising from an insulation defect in transformerless designs. This current could be AC or DC, depending on where the fault occurs and whether the PV panel is or isn’t grounded. The requirements are similar to those for leakage current measurement. Accuracy, while important, is less of a consideration for this case because short-circuit currents are higher than the leakage currents. All necessary residual current measurements in PV transformerless inverters are required for fundamental safety purposes and must conform to all relevant standards.

The Closed-Loop Fluxgate Technology Solution

Closed-loop fluxgate technology provides necessary accuracy, reliability and isolation when measuring small currents. LEM used it to create current transducers (CTSRs). Closed-loop CTSRs measure current over wide frequency ranges, including DC. In higher-frequency ranges, these transducers function like (passive) current transformers, but at DC, and in low-frequency ranges, the induced voltage in the secondary (measurement) winding is too low to drive enough secondary current to use the same principle. In this domain, the magnetic flux density in the transducer’s core is measured by a sensing element. A voltage is applied to the secondary circuit that maintains the flux density near zero, effectively creating a closed control loop.

CTSR Features

The CTSR transducer uses a fluxgate detector for feedback rather than the Hall device used in standard closed-loop transducers. This yields a higher voltage per unit of current linkage, or open-loop sensitivity. The fluxgate also exhibits low offset drift. The magnetic head of the CTSR was optimized to measure the residual current (the algebraic sum of the currents flowing in the wires that pass through the aperture of the transducer)—a maximum value under 1 amp, when primary currents in each wire are 10s of amps.

CTSR features include:

  • A self-test and demagnetization function that removes any magnetization offset and can be used on both single-phase and multiphase grids.
  • Signal processing carried out by a customer IC within the device.
  • Circuit elements of this IC that form an oscillator coupled to the fluxgate, driving it into saturation at each half cycle at a frequency of several hundred kHz.
  • DC magnetic flux presentation in the fluxgate core that alters the driving voltage’s duty cycle, indicating the residual flux’s value.
  • The IC’s signal processing stages—duty cycle demodulation, frequency response compensation, integrator and a bridge amplifier—that provide the secondary current. This output architecture can provide a higher (doubled) voltage to the secondary circuit. In this configuration, the load (or measurement) resistor is floating and a differential amplifier, which also is part of the IC, is used.
  • A magnetic core containing a pair of magnetic shells that houses the fluxgate. This protects the fluxgate against parasitic magnetic fields. Closed-loop fluxgate technology can accurately measure small residual DC or AC currents with low offset and gain drifts over operating temperatures that range from -40 C to 105 C.

Efficient conversion electronics contribute to maximum profitability by feeding power into grids, helping solar energy become a competitive energy source globally. Advanced transducer technology is a key element that will help ensure solar generation’s quality, safety, reliability and efficiency.

Bernard Richard is LEM’s business development manager. He joined LEM in 2001 as development manager and senior engineer in Japan before becoming application manager in Geneva in 2004. He’s held his current position since 2010.

Claude Gudel a senior engineer in LEM’s research and development department. He is head of LEM’s R&D electronics and magnetics know-how section and has been with LEM since 1985.

Stàƒ©phane Rollier is LEM’s marketing communication manager. He joined LEM in 1993 as a technical sales representative and became product manager in 2000. He has held his current position since 2010.

More PowerGrid International Issue Articles
View Power Generation Articles on PennEnergy.com
Previous articleELP Volume 90 Issue 1
Next articleMobile Combo Units

No posts to display