Upon investigation it was discovered that the bus section control unit had failed in a state where all outputs were high. This failure mode is not acceptable since it can cause customer outages.A  There are several control units installed at the utility's substations, resulting in an expedited need to get to the root cause of the failure and have it corrected. After a second board was removed, a chip similar in appearance to IC2 and named IC1 was found to have a similar brown spot on it.A  This chip controls general logic (Figure 2b). Latch-up studies by Altera's Reliability Department show that this product meets or exceeds JEDEC standard 78. Specific Findings From The Investigation Microscopic failure analysis of damaged Altera components (designated IC1 & IC2), taken from the control unit, showed carbonized deposit and blistering associated with Vcc and Vss power supply pins on both devices. 1) Control Unit Power Supply Modification - In the course of an RCA following control unit failure at the substation in the fall of 2009, the power supply unit was identified as the most likely cause of failure of the two EPLD chips (IC2, IC1:).
Later verification confirmed that transient behavior of the power supply in a working control unit systems did not differ from that of the power supply loaded with the normal resistive load described. To provide a reference for later comparison, Figure 7 shows typical +5V and +12V output response when a non-modified power supply is subjected to relatively fast power cycling on its input. A simple power supply modification (removal of Zener diode D13, see Figure 4) was determined to significantly reduce the magnitude of the transients appearing on 5V output when input power is interrupted and restored in rapid succession, similar to transfer switch operation. As can be seen from the schematic diagram, U2 pin1 is inherently protected from being driven to higher voltages.


As shown, with the removal of Zener diode D13 there is significant improvement in the +5V output transient performance, with no evidence of detrimental effect on other power supply regulation feedback characteristics. 2) Control Card Modification - Independent of power supply modifications, control unit Control Card line termination enhancements for IC2 EPLD signals have been added. Sidebars: In this case study a lightly loaded power supply combined with a high speed transfer switch resulted in over-voltages on key control processors. The importance of mains analysis is becoming more significant due to the increase in consumers with non-unity power-factor loads in industrial complexes and the affect this has on the supply. Even modern installations are not immune from supply fluctuation - almost any electrical equipment with a power supply can cause problems on the electrical supply network.
When they tried to interrogate the unit, it failed to communicate from the front or the rear ports.
Control unit grounding requirements: The grounding method described in control unit user's manual is very clear about the function of each grounding terminal on the back panel Proper grounding practices must be followed.
Carbonized mold compound were found near Vcc pins and Vss pins while blisters were observed near Vss pins for both units Figure 5 and 6.
This should lead the customer to review their system setup for possible sources of electrical overstress.
The divider formed by optocoupler transistor, R16 and R28 would not produce levels above 5V, and the remaining inputs to input amplifiers do not have any external Zener limiters either.


The excessive overshoot is minimized when rapid input power cycling is applied to the control unit power supply. Affected signals are: Individual Control point lines (C1 to C16) Control Master Trip, Control Master Close lines (CMT, CMC) Control enable line (C0) Each of these lines would be provided with 10k pull-down resistor to GND as shown in Figure 9 so that the inputs to the corresponding analog switches on the control card, normally driven by IC2 outputs, would not be left floating if IC2 were damaged and the control unit was energized. Sudden and unexpected deviations from permitted operating conditions in the electrical network of distributors can lead to the impairment of supply toother consumers.
At this point, the remaining boards were removed from the control unit, cross-referenced with a parts list, and inspected for failures.A  No other visible failures were noticed on the remaining boards. Transient overvoltage observed on 5V power output used for IC2, IC1 chips was quite significant and, in certain situations, exceeded 7.5V for short periods of time (single milliseconds). Fast input power cycling and power restoration changes the re-start conditions since the internal voltages would not completely decay to zero levels. As was observed during experiments, the state of the decaying 5V output at the instance when input power gets restored is the most critical factor affecting how hard the negative feedback line is driven. Not allowing the line to reach levels above 3.6V renders the feedback ineffective for short period of time and produces significant overshoot on the 5V output.



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