Balancing safety and cost-effectiveness in solar power inverter installations
In response to rising fossil fuel costs and environmental concerns, installation rates of solar photovoltaic panels, are rising rapidly. A further factor encouraging PV deployment has been incentives in the form of favorable feed-in tariffs to national grids; almost 99% (Reference 1) of the energy produced by solar installations is “grid-connected” via an inverter. All PV installations demand accurate measurement of generated current, both for efficient control of the inverter, and protection purposes.
Grid-connected inverters may, or may not include a transformer to provide galvanic isolation: transformer-less configurations incur a higher risk of leakage to earth. Four main inverter designs are commonly encountered. Two designs use a transformer (at low or high frequency) and two designs are transformer-less; with or without a DC chopper or step-up converter. For cost but also size, efficiency, weight reasons, transformers are less favoured for new designs.
MPPT control, inverter control and protection
For each different topology, instantaneous DC current and voltage output of the PV panel must be measured, to establish the MPP (Maximum Power Point) at which the maximum output power can be extracted from the solar panel. Current measurement is also needed as an input to the control loop of the inverter, 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
In transformer-less designs and in high-frequency transformer configurations, the DC current that an inverter is permitted to inject into the grid must be limited to a maximum value of between 10 mA and 1 A, according to different standards that apply in different countries (relevant standards include IEC 61727, IEEE 1547, UL 1741, and VDE 0126-1). This requires transducers with very high accuracy (better than 1%) and very low offset and gain drifts; an ideal technology is the closed loop Fluxgate transducer (Fig 1).
Figure 1: CKSR current transducers have the ideal characteristics for control of DC current injected into the power grid by solar inverters.
Leakage current measurement
Transformer-less inverters without galvanic isolation have a potential for leakage currents to occur. (Fig 2) Some of the hazards include:
- capacitance between solar panel and roof may offer an AC leakage current path
- any leakage path from the AC line back to the panel can raise the panel itself to line voltage, leading to a risk of electric shock
- leakage currents can be responsible for electromagnetic interferences, grid current distortion and losses in the system.
The ideal measurement of leakage current for safety purposes should be non-contact and non-intrusive. AC, 50/60 Hz leakage currents will be small, typically in the range up to 300 mA, and can be measured as the residual component from a differential measurement of currents in several conductors. The measurement must detect a sudden increase of 30 mA in leakage current, which may indicate a person touching a panel. Requirements again include accuracy and, especially, low offset and gain drifts, to ensure resolution of these small measured currents; the ability to accommodate several conductors, to cater for single or three phase system within the transducer aperture is a major advantage.
Earth fault current measurement
Safety monitoring must detect an earth fault current arising from an insulation defect in transformer-less designs; this current could be AC or DC, depending where the fault occurs; and depending on whether the PV panel is grounded or not. (see Fig.3) Similar requirements to those for the leakage current measurement apply. Accuracy, while still important, is less of a consideration in this case as short-circuit currents are higher than the leakage currents. All of these residual current measurement needs in PV transformer-less inverters designs are a fundamental safety requirement; and they must conform to all relevant standards.
Figure 3: Potential paths for earth fault currents in a transformer-less inverter design (without DC chopper)
Closed-loop fluxgate technology
Closed Loop Fluxgate technology offers the necessary accuracy, reliability and isolation when measuring small currents; LEM applied it to create “CTSR” current transducers (Fig 4). Closed Loop current transducers measure current over wide frequency ranges, including DC.
In higher frequency ranges, these transducers function in the same way as (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 and a voltage is applied to the secondary circuit that, overall, 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 developed per unit of current linkage, or “Open Loop sensitivity”. The Fluxgate also exhibits low offset drift. The magnetic head of the CTSR has been 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 A, when primary currents are several tens of Amps in each wire).
The CTSR additionally contains a self-test function and a demagnetization function, that removes any magnetization offset; and, it is suitable for use on both single-phase and multi-phase grids. Within the device a custom IC carries out signal processing; circuit elements form an oscillator coupled to the Fluxgate that drives it into saturation each half cycle, at a frequency of several hundred kHz. A DC magnetic flux present in the fluxgate core has the effect of altering the duty cycle of the driving voltage (Fig 5) and this change indicates the value of that residual flux.
Figure 5: A high frequency signal drives the Fluxgate into saturation and the presence of a residual flux appears as an unbalanced duty cycle
The signal processing stages comprise duty cycle demodulation, frequency response compensation, an integrator and a bridge amplifier that provides 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 difference amplifier is used which is also part of the IC.
The magnetic core comprises a pair of magnetic shells (Fig 6) containing the fluxgate a construction that protects the fluxgate against any parasitic magnetic field. Closed Loop Fluxgate technology has achieved accurate measurement of very small residual DC or AC currents with very low offset and gain drifts over the wide operating temperature range from –40 to +105°C. The devices are PCB mountable, lightweight (28g) components, with a 20.1 mm diameter aperture for multiple conductors. The residual-current capability measures the sum of all of the instantaneous currents flowing through the aperture, in single- or three-phase configurations, and tolerating an overload pulse of 3300 A for 100 µsec, with a rise time of 500 A/µsec. Conductors may be carrying primary currents of up to 30 A/wire, AC or DC.
Standards compliance
The transducers meet the demands of the latest relevant standards such as VDE 0126-1-1, UL 1741, DK 5940, and IEC 61010-1, in parameters such as creepage and clearance distances (11 mm) and CTI (comparative tracking index) of 600 V. Power supply requirement is +5 VDC; an additional pin provides access to the internal reference voltage (2.5 V) which can be used as the reference voltage of an A/D converter; this extra pin can also accept a reference provided by an external Digital Signal Processor or A/D converter, to cancel reference temperature drift.
The transducer design allows for a variant with four primary integrated current conductors in for PCB-mounting (three phase currents plus test or neutral). Higher current variants measuring up to 3 ARMS are also possible.
Conclusion
With efficient conversion electronics contributing to ensuring maximum profitability and returns from feeding power into national grids, solar energy is a competitive energy source that will see greater deployment worldwide. Advanced transducer technology provides one key element that will underpin the quality, safety, reliability and efficiency of that new generating capacity.
About the authors
Bernard Richard (bri@lem.com) Business Development Manager, Renewable Energy and Power Supply; Claude Gudel Research & Development Senior Engineer; and Stéphane Rollier Product and MarCom Manager.
Reference 1; IMS PV Inverter Database – Premium Edition – Latest Output – 7th April 2011.
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