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معمای سیستم DC شماره 13 - رله ارت فالت دی سی
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نویسنده : مهدی
 
#1
ساعت: 11:24 - تاریخ: 2 تیر 1393
استراتژی آشکار سازی ارت فالت در سیستمهای DC به زمین شدن یا نشدن و همچنین نحوه زمین نمودن باتری در سیستم بستگی دارد. روشهای متنوعی نظیر تقسیم پتانسیل در سیستمهای DC کمکی رایج است که در زیر بصورت خلاصه به آنها پرداخته شده است.
در سیستم تحریک ژنراتورها نیز روشهای متنوعی وجود دارد که در زیر به برخی پرداخته شده است.
در سیستمهای پیشرفته تر حفاظت تحریک ژنراتور از روش تزریق جریان 20 هرتز به سیم پیچی تحریک استفاده می شود که می توانید برای اطلاع بیشتر، به معماهای حفاظت همین سایت مراجعه فرمائید.

Bridge Circuit
The traditional DC ground fault detection circuit is comprised of a voltage divider network, represented as a resistor network R1 and R2, where the common connection is grounded electrically at the mid-point of the battery. The common connection references the battery to ground while still providing a measure of isolation from ground. Under normal operation, R1 and R2 each support a voltage equal to half the total battery voltage. An inadvertent ground on the + or – DC bus shorts out the respective R and the DC bus drops to a ground potential. The bridge circuit is widely used with the Isolated Input UPS configuration. Graphical representation of the bridge circuit is presented in Figures 1 and 2.

DC Current Sensing Summation Circuit
DC current sensing summation (residual current) measures both the positive and the negative bus currents in a Hall-element current sensor. DC current in the positive leg and DC Current in the negative leg are summed. The normal condition is that the instantaneous sum of the positive and negative current flow will equal zero. The circuit alarm on a current threshold setting that is greater than zero. The assumption is that if the current sum is not zero it must be outside of the normal condition path, i.e. a ground fault on the battery. The DC current sensing summation circuit is used with the non-isolated UPS configuration. Graphical representation of the DC current summation circuit is presented in Figure 3.

Hybrid and Proprietary Circuits
There reportedly are variations on the circuit designs described above as well as alternative detection circuit configurations.
In addition to the DC current summation circuit, it is reported that the AC ground fault detection provided with module input circuit breakers offers an alternative means of detecting a battery ground fault on non-isolated input systems. The limitations to this method are discussed later in this report.
A proprietary detection circuit design described by one of the UPS manufacturers applies an AC potential across the battery.
A ground fault in the battery will result in an AC current which is then detected and an alarm is activated.
Detection circuit sensitivity levels are somewhat ambiguous given specific methodologies. The following presents a description of the relative sensitivity for each of the detection circuits provided.
Resistance or Impedance bridges are common for ungrounded battery systems and specifically Isolated Input UPS module.
These detection circuits are reportedly the most sensitive and most prone to nuisance alarms. Consequently, the UPS manufacturer may set alarm trigger levels to limit nuisance alarms. Additionally, the location of the ground fault and the resistance of the fault path can affect the detection and alarm capability of the circuit.
DC Current Summation circuits using a Hall Effect CT enclosing the positive and negative circuit conductors, and not the equipment grounding conductors. This methodology is applicable to grounded battery systems, specifically non-isolated UPS modules. Any leakage/fault current to ground will appear as an output from the CT. On the smaller UPS modules with very low battery load levels these can be effective and reliably detect very low leakage/fault current levels, i.e., less than 100ma.
In larger UPS systems, as the battery system power levels go up, so do the detection current levels. This is due to the size of the Hall Effect CT's required and their corresponding error tolerance. A 1200A battery ground fault detection system with a properly rated Hall Effect CT may not be able to reliably detect less than 20A of battery ground fault current.
AC signal injection is used for an ungrounded battery system, and uses an AC signal source to impose an AC voltage to ground on the ungrounded battery string. This AC voltage between the battery and ground has no detrimental effects to the battery system. Consequently, a battery ground fault will create a corresponding AC fault current which can be detected using a conventional AC current CT. The fact that an AC current signal is used for detection purposes makes this an attractive option for larger battery systems, i.e. an AC current CT is used for detection and the detection level is independent of the DC battery power level. The success of such a detection method has not been verified to date.
AC ground fault detection is reportedly used by some UPS manufacturers as an alternate means of detecting a battery ground fault condition primarily on the non-isolated input UPS. This methodology relies on the ground fault protection provided with a UPS module input circuit breaker. The application relies on the fact that in a battery fault to ground there is an AC characteristic to the fault current reflective of the rectifier switching activity. The sensitivity of an AC ground fault circuit to detect the levels of DC fault current discussed above is questionable. The primary function of the AC ground fault circuit is to detect questionable levels of AC ground fault current while avoiding unwanted nuisance tripping if set too low. Ground fault protection is not a standard offering with all UPS modules and may not be an option.



دو روش آشکار سازی ارت فالت در سیستم تحریک ژنراتورها نیز در زیر آمده است.

 
نویسنده : حمید - از: فیروزآباد فارس
 
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