High AC voltage is a power quality issue that can be common in PV systems. ANSI C84.1 specifies the steady-state voltage tolerances for an electrical power system. The standard divides voltages into two ranges, Range A is the optimal voltage range and Range B is acceptable, but not optimal.

• Range A minimum voltage is 95% of nominal voltage
• Range A maximum voltage is 105% of nominal voltage
• Range B minimum voltage is 91.7% of nominal voltage
• Range B maximum voltage is 105.8% of nominal voltage

IEEE 1547, the Standard for Interconnecting Distributed Energy Resources (DER) with Electric Power Systems; and UL 1741, a product safety standard that defines the testing and certification requirements for DER systems that meet the IEEE 1547 requirements define the voltage ranges for PV inverters. This is range is 88% to 110% of nominal. Since this range is wider than what ANSI C84.1 specifies, this can lead to high AC voltages at the site.

Verifying these AC voltages are important because high voltages can cause core saturation and excessive heating in motors. This can create higher than normal inrush currents in motors that are found in air conditioners, washer, dryers and refrigerators that turn on and off multiple times a day. Premature lighting failure can be seen in both incandescent and fluorescent lighting. High voltage causes these lights to burn brighter, producing more heat and reducing their life cycle.

Unbalance voltage between phases is another problem utilities can face when solar is used. On a 3-phase system, if the solar array is not balanced between all the phases an unbalanced voltage condition can occur. The more single phase solar arrays connected to the grid, the worse the problem becomes. Most utilities limit this unbalance in their interconnection requirements.

Unbalanced voltages can become a serious problem in 3-phase motors. The resulting current unbalance in a motor can be 6 to 10 times higher than the voltage unbalance that creates it. This causes excessive heating and can burn out coils. A voltage unbalance of just 2% can equal a current unbalance of up to 20%. So it is important that a voltage negative sequence unbalance does not exceed 2% for 95% of the time. In order to combat this, utilities are having to change the tap settings on their transformers to compensate for these unbalanced conditions.

Transients are yet another issue that can arise with PV inverters. Solar reacts nearly instantaneously to changes in solar radiation and if the solar system does not have proper voltage conditioning, this can create high-speed transients. These high-speed transients can have adverse impacts on residential and commercial electronics. Modern electronics do have input filters that can filter out transients. These can fail when being exposed to repeated transients, leading to failures in these electronic devices, like flat screen televisions, microwave ovens and computer equipment.

Inverters convert DC current to AC current, this conversion can create harmonics. Harmonics are unwanted higher frequencies which superimpose on the fundamental waveform creating a distorted wave pattern. Harmonic currents are caused by non-linear loads connected to the distribution system. A load is said to be non-linear when the current it draws does not have the same waveform as the supply voltage. The flow of harmonic currents through system impedances in turn creates voltage harmonics, which distort the supply voltage.

Inverters tend to operate at relatively higher frequencies in order to maximize their efficiency. However, the higher the frequency the inverter functions at, the higher order harmonics it creates. It is not uncommon to see harmonic orders up above the 40th order.

Harmonics generate eddy currents in wires. These types of currents cause what is known as skin effect, which generates heat in wires. The higher the frequency of the harmonic, the greater the eddy currents and the more heating. High harmonic current can cause premature failures in motors and transformers as well as sensitive electronics.

Solar can affect the power factor that utility metering sees. Power factor (PF) is a measure of the phase difference between the voltage and current in an AC power system. In purely resistive loads (such as an incandescent lightbulb or electric kettle) the current is in phase with the voltage and there is 'unity' power factor. Capacitive and inductive loads (such as a capacitor banks or inductive motor respectively) will cause the current to 'lead' or 'lag' the voltage, resulting in a 'non-unity' power factor.

The supply of reactive power is very important in an AC power grid. The amount of reactive power produced by generators must closely match that which is being consumed. A leading power factor in the system (due to capacitive loads) causes the voltage to rise and a lagging power factor (due to inductive loads) will cause the voltage to fall. If reactive power is either under or over supplied, the voltage on the network may rise or fall to a point where generators must switch off to protect themselves thereby decreasing the generation and cause further problems.

On a site where the PF is metered by the utility and the site can be billed for poor PF it is important to maintain near unity PF. If the site is producing active power (kW) from the solar this won't be seen by the utility meter and will look like poor PF even though the reactive power (kVAR) used by the site remains the same. An example of this is the following; assume a site uses 100 kW and 10 kVAR of power, that is a PF of 0.9. Now if a 40kW PV system is installed reducing the 100 kW consumption to 60 kW, the 10 kVAR consumption stays the same, the PF drops to 0.86.