Voltage Rise Considerations for Utility-Interactive PV Systems
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Voltage rise is a commonly misunderstood concept among PV installers, especially those who have not worked on electrical distribution systems with parallel power supplies, such as a utility-interactive PV system. When a grid-connected inverter produces ac current, the impedance from the grid and inverter output-circuit conductors causes an increase in voltage at the inverter relative to the utility voltage. This phenomenon is commonly referred to as voltage rise.
Voltage rise is essentially a negative voltage drop on the circuit between the inverter and the point of common coupling (POCC) that causes the voltage to increase at the inverter ac bus. Greg Smith, technical training specialist at SMA America, points out: “It isn’t that the inverter must increase the voltage to push current into the grid, but rather the voltage rises because the inverter pushes current into the grid.” He explains, “Ohm’s Law that V = I x R applies when current is pushed against the impedance of the grid.”
Whether you think of this change in voltage as voltage rise at the inverter ac bus or voltage drop between the inverter ac bus and the POCC, the net effect on system performance is negative and must be considered during system design and specification. For example, the percentage of voltage lost due to voltage drop between the inverter and POCC is proportional to the percentage of power, energy and revenue lost, as illustrated by the following relationships: power = voltage x current; energy = power x time; energy = $.
Other impacts are not as easy to quantify and predict. Voltage rise at the ac bus can also cause the inverter to disconnect from the grid if the voltage exceeds its upper ac operating voltage limit. Once the inverter trips and ceases to export current, it must monitor the utility source for at least 5 minutes before attempting to reconnect. Therefore, voltage rise is a potential cause of nuisance tripping and may result in unnecessary system losses or avoidable service calls.
Understanding the interaction between inverter and utility operating-voltage ranges is key to understanding how, when and why nuisance tripping occurs due to voltage rise.
Inverter operating voltage. The UL 1741 safety standard, which is based on IEEE 1547 requirements, defines ac operating voltage range for a utility-interactive inverter as 12% to 10% of the nominal grid voltage. Table 1 shows the voltage trip point according to inverter capacity and service voltage. Per this standard, an inverter must disconnect from the grid when the voltage at the inverter ac bus is outside this range.
For example, consider the typical residential service in the US, which has a nominal voltage of 240 V. Per UL 1741, the minimum inverter ac operating voltage on this service is 12% less than nominal, or 211.2 V (240 V x 0.88), and the maximum inverter ac operating voltage is 10% above nominal, or 264 V (240 x 1.1).
While 211 V to 264 V is the maximum inverter operating range for a 240 V interconnection, the effective range may be somewhat narrower. Some manufacturers intentionally set the upper limit of their inverter operating- voltage range 9% above nominal to ensure that their inverters do not fail UL compliance tests. Therefore, to be conservative, designers may want to assume that the effective upper voltage limit for a 240 V service is actually 109% of nominal, or 261.6 V (240 V x 1.09).
Service voltage range. While the inverter operating-voltage range is set by the manufacturer in relation to the nominal service voltage, the actual grid voltage maintained by the utility fluctuates. The American National Standards Institute (ANSI) defines the voltage tolerance for electric power systems in ANSI C84.1. There are two voltage ranges defined in this standard: Range A is the optimal service voltage range, and Range B is the acceptable service voltage range. According to ANSI C84.1, the maximum Range A service voltage is 105% of nominal, and the maximum Range B service voltage is 105.8% of nominal.
Compound effects. When the utility service voltage is at the high end of its operating range, the likelihood of inverter nuisance tripping due to voltage rise increases. Again, consider a 240 V nominal residential interconnection. If the utility is operating at the high end of ANSI Range A, then the service voltage that the inverter is interconnecting to could be as high as 252 V (240 V x 1.05). Since occasional “voltage excursions” are allowed into Range B, the utility would consider a service voltage of 254 V (240 V x 1.058) to be acceptable. Under these circumstances, 3% or 4% of voltage rise at the ac input bus could cause the inverter to disconnect from the utility grid.