7.4 Procedures currently in place to mitigate issues associated with intermittent generation

Xcel Energy explains in [60] how effective their voltage regulation control has been for their Colorado State University (CSU) 5 MW PV System - 2 MW for Phase I and 3 MW for Phase II.

The characteristics of the feeder are:

  • 76% penetration during light loads
  • PV site is 10.4 km from substation
  • grounded wye configuration @ 13.2 kV
  • two sets of three single phase bi-directional regulators.

The voltage regulators are set upstream from the PV site set at a 1V Bandwidth, and are also used for line drop compensation. A quarter of a million operations were recorded due to the narrow bandwidth with no customer complaints received. The time-frame for this operation was not mentioned.

Phase II will add an additional 3 MW of PV. Plans to manage voltage include:

  • all capacitors on the feeder to be switched off to prevent over-voltage
  • if over-voltage still occurs, the six units of 500 kW PV inverters will be set to 100 - 150 kVar (600 - 900 kVar total) leading in order to reduce the voltage by absorbing reactive power.

Figure 74 shows the effectiveness of the voltage regulation during periods of reduced power output for Phase I of CSU. The voltage is seen to be well regulated despite drops in active power.

Figure 74 Voltage regulation at CSU (Phase I) [60]

PEPCO [58] trialled an Absorbing Power Factor (APF) solution combined with a Load Tap Changing (LTC) transformer and voltage regulator to mitigate voltage fluctuation issues for a 20 MW PV system shown in Figure 75. The results are shown in Table 12. The mitigation attempts result in little change in maximum voltage fluctuation at the PV site (4% reduction) and has no impact at the substation. It appears that the system is on dedicated feeders, so the PV would be the dominant source of the fluctuations.

Figure 75 PEPCO 20 MW system [58]

Table 12 Voltage fluctuations solutions PEPCO 20 MW PV

Solution Maximum Steady State Voltage at PV Site (V) Maximum Voltage Fluctuation at the PV Site (V) Maximum Voltage Fluctuation at the Substation Bus (V)
Without mitigation 134.4 7 0.8
0.98 absorbing power factor
103V LTC reference voltage
Voltage regulator 3.4 miles downstream from Substation set @ 118V

124.4

6.7

0.8

Table 13 shows the results for voltage mitigations attempts (and their associated costs) using various solutions for a 1.9 MW system. The impact of the APF solution is far greater (48%) for the smaller system than for this 20 MW system where only a 4% reduction is obtained.

Table 13 Voltage fl uctuation solutions PEPCO 1.9 MW PV [58]

Solution Maximum Steady State Voltage (V) Maximum Voltage Fluctuation at the PV site (V) Maximum Voltage Fluctuation at the Upstream Regulator (V) Cost ($)
Without Mitigation 125.3 2.3 1 0
Absorbing Power Factor Solution 124 1.2 0.2 2200
500kVA/1500kWh Battery 125 0.5 0.1 1115014
750kVA/3000kWh Battery 124.7 0 0 2189390
477 AAC Reconductor 124.9 1.3 1.1 26600

For the Absorbing Power Factor (APF) solution, a power factor of 0.97 is used. The APF solution is obviously the cheapest but its mitigation impact is limited. Comparing the 500kVA and 750kVA battery solutions in Table 13, the cost of the 750kVA battery is almost double that of the 500 kVA for an additional 20% reduction in maximum fluctuation at the PV site, and only 10% at the regulator. The question is to what extent fluctuations need to be reduced.

Spain has a requirement that any renewable generation installation greater than 10MW maintains an inductive power factor of between 0.98 and 0.99 to eliminate sudden changes in the voltage profile corresponding to the transitions in off-peak-intermediate-peak periods and to avoid system over-voltage problems. Requirements for wind turbines/facilities to comply with new voltage ride-through standards have been introduced to reduce generation tripping due to voltage dips [55]. The effectiveness of these strategies was not mentioned.

Portugal [56] also introduced a new code, through their Ministry of Economy and Innovation, for technical requirements for wind generation. This code will facilitate fault ride-through and reactive power requirements to manage voltage dips.

Through its Golden Sun project, China [57] is seeing technical issues which need to be addressed, such as fluctuation in voltage and frequency, power quality, protection and energy management. The Golden Sun project resulted in 73 MW of BIPV and BAPV installed in 2009.