Safe and reliable alternative energy

Consider what happened when a solar panel installed in a San Diego house caused a fire a couple years ago. According to newspaper reports, the problem began with smoke coming out of the inverter box. The homeowner flipped the cutoff switch to disconnect the power but while she was on the phone with an electrician, the converter box caught fire. The fire continued to burn even after a fire extinguisher was emptied on it. The San Diego Fire-Rescue Dept. eventually arrived, but all firefighters could do was keep the fire down to a smolder. Finally an electrician was able to cut the wires from the panels, extinguishing the source of energy, and the fire went out.

Home alternative energy systems based on solar and wind power are becoming popular, but safety is becoming more of an issue. As incidents like the one in San Diego show, system designers need to build in protections against short circuits and other failures. The power inverter and its control system are the place to focus.

A home alternative energy system typically has several major components that must work together. The defining component, naturally, is the power source. When solar panels are the power source, they are often equipped with a maximum power point (MPP) controller to boost efficiency. Residential wind generators, although less common, are also used both alone and to supplement solar panels as primary power sources.

Many alternative energy installations include battery banks to buffer the variable output of solar and wind generators as well as to store surplus energy for later use. These systems usually also include a battery charge controller as a core system element. Besides directing current flow into or out of the battery, the charge controller continually monitors both user power demand and the power generated.

Each of the power sources incorporates a dc-dc converter and feeds a common high-voltage dc bus known as the dc link. This dc link is what feeds power to the inverter that produces the system ac output. The dc link typically runs in the 600 to 1,200 V range to maximize the inverter efficiency at converting dc power to ac. Inverters for home power systems can range in capacity from 1 to 30 kW and operate by rapidly connecting the dc link voltage to the ac power mains. High-voltage insulated-gate bipolar transistors (IGBTs) typically serve as the switches, operating in pairs to provide both positive and negative output voltages.

The switching inside the inverter, typically at 50 kHz, provides an ON interval just long enough to charge the load capacitance on each main line to the instantaneous voltage that will properly mimic a one or three-phase, 60 Hz ac supply. Synthesis of the complex timing needed to realize this mimicry under all load conditions is the job of the system controller. The controller also monitors the power that drives the battery charge controller and manages the user interface the system offers. In addition, it watches for line faults and directs the system to give safe response if they happen.

The fact that the controller has a central role in both system operation and the user interface makes it important that the controller be isolated from the high voltages in the power pathways. The isolation prevents the possibility of the high-dc-link voltage arcing into the control lines, which could damage the controller electronics and possibly injure the user. All sensor lines into and signal lines out of the controller need such isolation, especially those driving the inverter IGBT power switches.

The inverter's central role in the power circuits means it must be highly reliable if it is to provide the 15-to-20 year life that consumers demand of alternative energy systems to justify their cost. The inverter should also be relatively immune to accidental damage because, at a cost of $2,000 to $4,000, it takes nearly 10% of the initial system investment to replace it.

To provide that reliability and protection, the inverter design must include circuitry to mitigate several known failure mechanisms in the IGBT switches. These failure mechanisms include switching transients that can arise during normal operation; low gate voltages that can happen when batteries are depleted; and short-circuits on the mains caused by faults or user error. Ideally, the IGBT drive electronics would provide both this fault protection and the controller isolation the system needs.

Isolation choices

Designers can use several different technologies to build electrical isolation into a home energy system. Some magnetic isolation devices, for instance, couple signals across a thin insulating barrier through magnetic induction. This approach may not be fail-safe, however. In a high-voltage electric fault situation, breakdown of this thin insulation barrier may cause a short circuit that can potentially be hazardous to someone touching the controls. Even in normal operation, magnetic isolation has a drawback: It is susceptible to electromagnetic interference (EMI) that could confound controller operation.

Capacitive coupling is another means of providing isolation. Here the insulation barrier is also a thin dielectric within the capacitor. Like magnetic coupling, capacitor-based isolation is susceptible to EMI, readily passing high-frequency noise to the controller.

Optical isolation is a third alternative and offers several advantages. Magnetic and capacitive coupling necessitate keeping high and low-voltage lines in close proximity. In contrast, optical isolation can be configured to keep low-voltage and high-voltage lines far away from each other. This significant distance virtually guarantees that no shorts can occur.

Moreover, the LED/photodiode combination in an optocoupler is known to be immune from EMI because of its optical coupling path. Tests show optocouplers can withstand much higher electromagnetic fields than all other isolators currently available.

One example of optocouplers suitable for use in alternative energy inverters are those from Avago, which are approved and recognized by component-level safety standards. These standards include UL1577, CSA and IEC 60747-5-5. IEC 60747-5-5 is the official release of the International Safety Standard for optocouplers for reinforced insulation since 2007. Although this standard pertains to optical isolators only, other isolation technologies such as magnetic or capacitive have also obtained the certifications to the optocoupler safety standard. However, their recognition is limited to the obsolete IEC 60747-5-2 standard and to basic insulation only.

Basic insulation only provides minimal protection against electrical shock. It cannot be considered “fail-safe.” Therefore, devices offering only basic insulation should not be accessible to users. Reinforced insulation not only protects against electric shock, it is also a “failsafe” design that permits user accessibility to a device.

For high reliability in the inverter section of the home energy system, the IGBT transistor drivers must incorporate circuits to prevent or mitigate common failure modes that can damage the IGBT. Normally, the IGBT is operating in the transistor saturation region, which lets it conduct high currents with a low voltage drop (V_CE) across the transistor. To enter this state, the IGBT needs at least 12 V on its control gate.

There are several ways a home energy system can fail to meet that gate drive requirement. One is to experience an excessive load or a direct short on the ac mains connection to the inverter, which would result in abnormal current draw through the IGBTs. This excess current forces the IGBTs out of saturation, which causes the voltage drop across the transistor to rise. This voltage rise causes the transistor to begin heating, which can quickly lead to device failure.

The IGBT can also leave its saturation region if the gate control voltage drops below the minimum level. Conditions that cause this drop can include a decline in the logic supply voltage due to deep battery discharge, as might happen if there has been too little sunlight for a sustained period. As with excessive current draw conditions, the low gate voltage causes a low V_CE and heating that can lead to device failure.

There are also failure modes that arise from the inevitable parasitic inductance and capacitance in switching circuits. Parasitic inductance can cause voltage spikes to arise if the IGBT shuts its output down too quickly, and these spikes can be large enough to damage the device. The energy stored in a parasitic capacitance (called Miller Capacitance) at the IGBT's control gate can keep one switch of the pair closed too long, resulting in a short circuit across the dc link.

Fortunately, protective circuits integrated into the Avago ACPL-332 gate drive optocoupler can be designed into an inverter to prevent or mitigate all these failure modes as well as signal the system controller about the occurrence of a fault. An under-voltage lockout circuit, for instance, can clamp the gate voltage to ground (keeping the IGBT turned off) in the event the supply voltage is too low to properly drive the gate. Such a circuit should include hysteresis, however, to avoid oscillation when the supply voltage is at the lockout threshold.

Another useful protective circuit is a desaturation detector. This circuit monitors the V_CE voltage across the IGBT. If this voltage rises above about 7 V, the IGBT is out of saturation and in danger of damage from excessive heating. The detector circuit shuts down the IGBT gate driver during desaturation, eliminating the danger.

Careful circuit design and board layout can reduce the parasitic inductance in the inverter, hence reducing the magnitude of transient voltages when the IGBT switches off. But design practices alone cannot guarantee to eliminate the danger. Designing the IGBT driver for a soft turn-off, however, can ensure that transient voltages never become dangerous.

Parasitic Miller capacitance cannot be eliminated through board design, but a clamping circuit can eliminate its effect in keeping an IGBT turned on too long. An active Miller clamp can monitor the gate voltage when turning off the IGBT, stepping in to short out the parasitic capacitance when the gate voltage drops below a threshold, guaranteeing shut-down timing.

The protection against fault conditions that these circuits provide, as well as the protection against short-circuits that isolation provides, are both essential for the safety and reliability home energy systems demand. Optocouplers provide the kind of superior isolation needed to ensure user safety and prevent component damage despite the high voltages present in the dc link. Protective circuits in the IGBT gate drivers help extend inverter lifetime by preventing the most common failure modes. Together, these functions can help alternative energy systems provide the reliability that consumers demand.