In the late 1980s, a large industrial customer was experiencing approximately 55 power outages per year. Anxious for a solution, the utility purchased and installed four automatic transfer switches (ATSs) and dozens of automatic reclosures in the mid 1990s. The ATSs and reclosures transferred approximately 8 sec after an outage. Combined with tree trimming and better maintenance, these improvements accounted for an 89% decrease in outages for the customer — an average of 6 per year. The next logical step was a faster ATS to further reduce outages and minimize transient disturbances. Recently, JEA engineers were called upon to install the first high-voltage, high-speed vacuum ATS, located at the customer's point of service.
The customer's operating conditions include 26.4kV service, 2.5MW demand (62A at maximum load), 0.9 load factor, and a minimum 24/5 occupancy (800kW on the weekends).
The high-speed ATS (see photo) switches the customer's load from a preferred 26kV source (Source 2) to an alternate 26kV source (Source 1). Source 1 is from a different substation, and both sources are served from the same 138kV transmission line. The distance from the ATS to Source 2 is 3.5 mi, the distance from the ATS to Source 1 is 4.0 mi, and the distance from Source 1 to Source 2 is 2.0 mi.
Initially, Source 1 was intended to be the primary source of power for the customer. But by the time construction was complete, Source 2 was chosen for two reasons — load balancing between the two substations, and the perception that Source 2 would be more reliable.
How Fast Is Fast Enough?
The purpose of the ATS is to transfer power from one source to an alternate source. From the customer's perspective, the transfer must be fast enough so it doesn't interrupt the plant's core processes. The effects of poor power quality on electrical equipment varies from component to component, but the following are critical factors: the nature, magnitude, and duration of the event; the sensitivity of the component to the event; the location within the customer's distribution system; and the age of the component.
The effects of poor power quality on plant processes are also customer-specific. This particular customer had superior tolerance to PQ events. Previous monitoring demonstrated that outages up to 250 ms (15 cycles) would not interrupt the customer's core processes — only the office computers would reset. However, waiting 250 ms allowed the motors to slow down enough for the restart inrush current to have a negative impact on the alternate circuit (Source 1). For that reason, and to test the ATS, JEA engineers chose to make the switch transfer as fast as possible — approximately 17 ms.
All of the settings inside the ATS that deal with transfer times are user-configurable from the control panel. Before determining the settings, the engineers contemplated these basic scenarios: a fault/sag on Source 2, a fault/sag on Source 1, and a fault/sag on the customer's side of the ATS.
In the event of a fault on Source 2, it was critical that the Source 2 part of the ATS opened before the Source 1 part of the ATS closed (also known as open transition). Otherwise, Source 1 could end up feeding the fault on Source 2. When the fault on Source 2 has been cleared (i.e., a fuse blows or circuit breaker opens), switching from Source 1 back to Source 2 can be delayed. This allows Source 2's distribution circuit to stabilize before picking up the customer load.
To minimize the disturbance, the transfer back to Source 2 is a closed transition transfer. This means Source 1 will be briefly synchronized and in parallel with Source 2. With the closed transition, approximately half of the current comes from each source. This significantly minimizes the impact of switching the load. A fault/sag on Source 1 inhibits the ATS from transferring until Source 1 has had a chance to stabilize. Otherwise, relatively minor sags on Source 2 could transfer the customer to a circuit having worse trouble. In addition, a fault on the customer's side of the ATS will inhibit the ATS from transferring.
Before installing the switch, JEA engineers performed low-voltage (120V) field testing. The measured transfer times were 13 ms. One of the settings (T3) was adjusted to slow it down to 18 ms for stable open-transition transfer.
The next step was to test the ATS at the customer's facility. The engineers used five power quality monitors at the following locations:
- voltage monitoring at the Source 1 substation
- voltage monitoring at the Source 2 substation
- voltage monitoring at the secondary of the customer's 26,400/480/277V transformer
- phase “C” voltage monitoring of Source 1, Source 2, and the load side of the ATS, inside the ATS NEMA 3R enclosure
- phases “A” and “B” voltage monitoring of Source 1 and Phases “A” and “B” voltage monitoring of Source 2, inside the ATS NEMA 3R enclosure
Engineers monitored the substations to see if any negative effects surfaced when the ATS transferred the load. With the ATS installed, but isolated from the customer using pad-mount isolation switches, the engineers simulated the loss of each source by opening and closing the proper isolation switches. Fig. 1 shows typical voltage waveforms during a 26kV no-load transfer. After verifying a transfer time of 17 ms, the engineers placed the ATS in service.
With the customer's permission, the engineers conducted the final test while supplying the customer with an 800kW load. Fig. 2 illustrates a typical voltage waveform during a 26kV, 800kW load transfer from Source 2 to Source 1. The ATS load-side voltages were distorted during the first cycle of the transfer. A combination of opening the isolation switch (i.e., removing Source 2) and the ATS produced the distortion. Notice how the voltage on the load side of the ATS remained strong during the rest of the transfer.
Finally, Fig. 3 shows the voltage waveforms during the transfer as measured on the secondary of the customer's 26,400/480/277V transformer 100 yd away. The customer's motors provided the voltage during the 17 ms it took for the ATS to finish transferring.
The high-speed ATS has now been monitored for a few months. On several occasions, the ATS saw a fault on the customer's side. One fault took the customer's 26kV fuse 60 ms to open and clear the fault. The fault's magnitude was enough to reduce the voltage, but it didn't blow the 100A fuses protecting the ATS. The ATS performed properly, allowing the customer's protective relaying to do its job. The ATS has transferred several times at high loads without impacting the customer. A typical transfer, as seen on the customer side of the ATS, is shown in Fig. 4.
High-voltage, high-speed vacuum ATSs offer utilities a new solution to many of today's power problems. It's important that the ATS be fast enough to keep programmable logic controllers (PLCs), adjustable-speed drives (ASDs), computers, and high-intensity discharge (HID) lights working without interruption. In the past, the only high-voltage solutions to automatically transfer power from a preferred to an alternate source were electromechanical switches or sub-cycle, solid-state transfer switches. The former were too slow and the latter were too expensive. The optimal solution was finding a device that combines reliable power sensing with high-speed automatic transfer switching.
Russell A Simmons is a senior engineer in customer technical services at JEA in Jacksonville, Fla. You may contact him at [email protected].
Fred J. Salem is a customer service technical adviser at JEA. You can reach him at [email protected].