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March 2020


By Brad Micallef, President and Managing Director & President

Utility witness testing is a critical moment for any utility-scale PV solar Facility. Only once witness testing is passed to the satisfaction of the Utility, may the Facility operate commercial to the fullest contractual capacity.

Witness testing is most often comprised of four (4) electrical fault tests, each is to demonstrate the ability for the PV solar Facility to anti-island. That is to say, the Facility’s inverters (generating units) need to cease operation within a prescribed time lime (typically less than 2 seconds) once an abnormal grid condition is present.

Each of the tests creates an abnormal grid condition, when the generating units should cease operation, then the condition is restored and the generating units should wait 300s (5mins) before resuming generation.

If the generating units fail to cease generation, or fail to start on time following restoration of a fault condition, the test is considered to be a failure, and the Facility cannot achieve commercial operation.

Testing typically includes:

  • 1. Three-Phase Drop Test - All three Utility phases are OPEN’d simultaneously.
  • 2. Single-Phase Drop Test - Each utility phase is OPEN’d {A, B, and C}

The first (three-phase drop) test is typically passed easily, as the generating units loose grid voltage, frequency, and phase rotation.

The second set of (A, B, and C single-phase drop) tests are more difficult to pass.

When a single MV electrical phase is OPEN’s the phase voltage does not drop to zero, neither does the frequency, nor the phase rotation. This phenomenon, called regenerative voltage, is due to the electromagnetic windings inside the step-up voltage transformers present on site. With only two (2) electrical phases connected, the third OPEN phase presents voltage and frequency based on the electromagnetic influence of the remaining two (2) energized windings.

On the secondary (low voltage) side of the transformer (where the inverters) are electrically connected, detecting this phase loss is complicated further but the stabilizing effect the transformer magnetization has when stepping-down the voltage from the MV to LV windings as well.

The ability to pass the MV single-phase loss is completely dependent on the inverter manufacturer’s firmware and settings. Under real-world conditions, results may vary.

Most Developers and EPCs understand the easiest way to ensure testing success is to use a more reliable and predictable fault detection means that will create a three-phase OPEN fault condition to ensure the generating units cease operation as expected. In other words, a recloser.

What is commonly referred to as a “Recloser” is actually two products paired together. The first is a MV rated three-phase vacuum fault interrupter. A large breaker. The second is a protection relay, a special computer that watches all three-phase voltages and currents with special programming to identify fault events and then OPEN the circuit (all three-phases) when a fault occurs.

Note, the term “recloser” is still missing. The act of “reclosing” is a behavior of the protective relay.

During a storm, a branch swinging in the rain and wind may temporarily create a fault on overhead MV power lines that causes a protective relay to trip a MV breaker OPEN. The same protective relay can be programmed to wait a period of time (maybe 1 min) after a fault and then attempt to re-CLOSE the circuit to see if the fault remains. If there is no fault, the circuit has restored itself. If the fault remains, the protective relay can OPEN the circuit again.

This process of OPEN’ing and then CLOSE’ing repeated times, with varying delays is referred to as “reclosing” and it is an essential part of how overhead MV distribution infrastructure recovers from temporary faults automatically.

Taking that knowledge back to the PV solar Facility and witness testing, the same protective relay and breaker arrangement can also be used to OPEN when there is a fault, and then automatically CLOSE when that fault is resolved.

Adding an overhead pole-mounted recloser is both expensive, frought with long lead-times, ad requires custom programming as well.

The myPV ClearSky Interconnection includes a three-phase breaker and protective relay built-in. With short lead-times and competitive costs, adding recloser functionality no longer needs to be a cost driven decision, it can be a strategic alternative. In addition to hardware, the ClearSky’s protective relay is pre-programmed with witness testing in mind. If your Facility’s inverters cannot successful pass the witness test, enabling the additional logic already in the protective relay is literally a button press away.

Why put your next project at risk?  Specify a ClearSky Interconnection, and know that your witness test is a test you can pass every time.

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