UF –PNPI HV system status August 2008. Sergey Volkov Nikolai Bondar PNPI. HV system scope. Control computers 2 pc Host cards 6 pc Matsusada AU-5P60 type power supply 2 pc Master crates 2 pc RDB crate 8 pc Master boards 8 pc Remote boards 56 pc
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2016 individually controlled HV channels
HV system had the following main problem – very unstable behavior. (Host Card driver sent warnings “pre-trip” and “trip”, even at zero HV)
We found the following problems:
For proper work of the control line the time alignment of the control pulses should be within certain limits.
The system was designed for working with control line up to 100 meter of the twisted pairs cable with the time skew of 0.1ns/m. (Maximum time skew should be less than 10 ns.) Unfortunately our control line consists of few parts of different cable with total length about 100 m. As a result the total time distortion was very close to the critical value.
This problem was solved at CMS by changing the firmware at the Host card. Unfortunately the Host cards with new firmware doesn’t work in our control computers.
In our case we solved this problem by putting extra delay (15 ns) of the ADC Hold signal (CVADC in the control bus protocol).
The output voltage ripple is specified as 0.1% for primary power supply.
At 3 KV of output voltage this value may be up to 3 V. In our case we have 0.8 V of the ripple with 2 main frequencies - 30 KHz and 100 Hz.
30 KHz frequency is decreasing accuracy of the current measurement of the Master board in 2 times (accuracy about 5 uA).
This accuracy is not very good, but is not a crucial. It is possible to improve resolution by using simple LC filter.
(Output cable capacitance is about 5000 pF, it is equal 10 kOhm of AC resistance. Master board channel also has about 100 KOhm AC resistance.)
100 Hz frequency affects to the output voltage stability because of the feedback frequency of the Master board stabilization circuit very close to the 100 Hz. As a results the resolution of the current measurement may decrease up to trip value.
Unfortunately the 100Hz filter for 3-4 KV is a problematic thing.
So the CMS team decided to use saturated mode of the Master board
as a main working condition. In this case the voltage drop on the Master board channels about 100 +/- 5 V at maximum consuming current ( ~1.2mA).
To implement this mode of operation it is very important to have remote control of the primary power supply.
According experience of the CMS team we expect the floating the value of the voltage sensing resistor. So the recalibration of the voltage sensors is necessary during all time of operation. This procedure should be implemented in the system.
3. Infant mortality of some channel of the Master and Remote boards.
Unfortunately our board was under working conditions only few weeks.
So the failure of the channels still expected.
4. The recalibration of the voltage sensors is necessary during all time of operation. This procedure should be implemented in the system.