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Thursday, May 2, 2019

Explaining The Short Intermittent Power Outages GPA Customers Are Experiencing


The short outages you may experience in your area may be due to the loss of a generator or solar photovoltaic (PV) farm; resulting in the island wide power system automatically dumping your circuit in order to prevent an island wide black out. This is an ongoing problem which is explained herein.  GPA is constructing 40 megawatts of energy storage batteries to be completed in 2019, to mitigate these types of outages. 

The Guam Power Authority appreciates your understanding and patience as GPA works to resolve this issue.

Guam’s island wide power system operates as an isolated grid, making it more vulnerable to issues that may result in a total system blackout. One issue includes the sudden loss of generation. This may arise from a generator going offline or a large reduction in power output from solar photovoltaic (PV) systems, due to passing clouds or inverter trips.  GPA must supply the exact power as required by customers for stable grid operation. If GPA online generators cannot ramp up their output fast enough to makeup this balance, the system automatically drops load to bring the demand and generation back into balance. The time to effectively act to prevent a blackout for the Guam grid is much shorter than it takes to bring new generation online, so GPA must rely on fast-acting automatic system protection.

GPA uses an “under frequency load shedding” (UFLS) system protection scheme to protect the grid from a sudden loss of generation. UFLS helps stabilize the power system preventing a black out, as it allows the system to recover.  The UFLS uses several levels or stages of protection relays that would load shed distribution feeders. The relay levels are set to load shed at different frequency set-points and at different time delays after sensing decreases in frequency. GPA uses several stages of load shedding to match the generation lost to the system.

GPA groups distribution feeders in 20 to 25 megawatt blocks and assigns them to one of five stages of UFLS.  Once customers on the first stage experience 10 events of load shedding, these feeders are rotated by GPA engineers and relay teams, so that the customers in the UFLS scheme have some relief from the outages experienced. Stage 1 activates first. If more than 20 to 25 megawatts are needed to be load shed to make up the generation shortfall, then the next stage(s) may also then be activated. Thus, higher stages activate less frequently than the stage below it. UFLS outages on these circuits are usually less than 5 minutes. Some feeders, such as those serving hospitals and other critical infrastructure or services, are not placed into the UFLS scheme.

Photovoltaic (PV) solar energy fluctuates with the weather and can ramp up or ramp down quickly without warning. Passing clouds, on an otherwise sunny day, can adversely affect the output of solar PV systems onto the grid. GPA’s current generators cannot react fast enough to make up for large and fast drops in solar production.

GPA’s new base load power plant will use the latest “flexible” generation technology and will react more quickly than current generators. The new generation will quickly and easily smooth out the intermittent power output of solar PV systems. GPA’s intention is for the new power plant, along with energy storage batteries, to almost completely alleviate load shedding events caused by drops in total solar production and the sudden loss of generation.
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