TEV DUMP SWITCH FAILURES OF 2/10/09 Dan Wolff/EE Support

Quench Protection in TEV

TEV Dump/PS Arrangement * Total of 12 Dumps located around the ring in the “2” and “3” buildings * Dumps and power supplies are controlled by both the QPMs directly and the FBP/Dump loops. * The FBP/Dump loops provide a backup system for the QPMs to initiate a dump. They also provide the means for the power supplies to initiate ring-wide trips. * TEVATRON is one large electrical circuit with all the magnets, power supplies and dumps in series.

Dump/PS Block Diagram * Original dump consists of a DC breaker, a series SCR and a commutating circuit. Normally, the commutating circuit operates abruptly and the series SCR stops conducting before the DC breaker opens. * The DC breakers have 2 means of opening: a fast trip operates in less than ms and the slow opening of the main coil occurs in a few tenths of seconds. * Dump capacitor limits the re-applied dv/dt on the switch devices * New dump consists of 2 solid state switches in series: Insulated Gate Bipolar Transistor (IGBT) and Integrated Gate Commutated Thyristor (IGCT). * The switches alternate as primary and secondary elements. The primary turns off first and the secondary turns off about 6 ms later. During this time circuitry checks that the primary element worked properly.

Dump/PS Filter Cabinet

5000 Amp DC Breaker

Series/Shunt Module

IGBT Module

Dump/Fast Bypass Loop Loop controller located in MCR

Sequence of Events

Short Explanation of Events Dump/FBP and Permit Loops trip due to power failure. Dumps “Fire” (Dump switches turn off) after 60 ms delay. 10 ms later most Dump switches turn back on (noise on the dump loop or loop momentarily came up again)!!!!! Two original dumps with fast DC Breakers do not turn back on. Once open the breakers would take many line cycles to re- close. IGBTs and some IGCTs in new solid-state (SS) dumps and some SCRs in original dumps are damaged by energy in the 4.8mF cap bank (charged to 1200 volts)!!!!! The QPMs respond to the dump activations by disabling their stay-alive signals to the dump controllers. 70 ms later dump switches are locked (latched) off.

Short Explanation of Events (cont.) F3 (SS) dump re-opens in response to QPM request but B3 and C2 SS switches are compromised and continue to conduct current but with a significant voltage drop (failure mode of IGBTs). The original dump SCRs cannot be turned off again because the 1900 uF commutating cap bank has been discharged by the first turn-off. However, the slower DC Breakers begin to open at various times between.2 and.6 seconds. At this time all dump resistors are in the circuit accept for B3 and C2. After 4 or 5 seconds the dump switches in B3 and C2 burn open. Thus by 5 seconds into the trip all dump resistors are in the circuit.

Voltage to ground on each side of C2 Dump

Damaged Dump

Damaged IGCT Module

Damaged IGBT Module

Conclusions If a dump is initiated by a Dump/FBP loop trip, there is a ms window within which the dumps could re-close if the loop experiences significant noise or comes back up. The original dump system is somewhat immune to this occurrence through the action of the DC Breaker. However, there are 7 DC Breakers in the ring whose fast trip response was a little too slow in this case. In these dumps the SCRs can sustain damage. The re-closure of the dumps in the new solid state system is catastrophic causing failures in both the IGBTs and IGCTs. Relatively simple changes in the controls and power circuit of the dumps can mitigate the problem (diode in series with 4.8 mF cap and a re-closure lock out in the dump control system). The re-closure lock out has been implemented as of 2/14/09. For added protection the diodes will be installed during shut down opportunities. The TEV magnets that quenched were not directly at risk. The additional voltage to ground experienced was limited to a few locations and no more than 25% higher than the normal hipot voltage. The dumps all operated in sufficient time to protect the safety leads.

Voltage to Ground +.68s

Voltage to Ground +4.9s

Voltage to Ground – oversampling Buffers

Voltage to Ground – oversampling buffers