استاندارد مرتبط با موضوع IEEE 32 می باشد. ترانسهای زمین می باید تحمل منتجه مولفه صفر جریان دائمی گذرنده از سیستم همچنین جریان ارت فالت کوتاه مدت سیستم را دارا باشند. مقدار جریان اتصال کوتاه زمین خود فاکتور مهمی است که با توجه به الزامات مختلف بهره برداری، پایداری و حفاظتی سیستم انتخاب می شود. مدت تحمل این جریان اتصال کوتاه نیز بستگی به انتخاب طراح دارد که مقادیر مختلف 10 ثانیه تا 1 دقیقه را بخود اختصاص می دهد. به هر ترانس زمین باید همان قابلیت تحمل انتخاب شده برای NGR را دارا باشد. مثلاً در یک سیستم فشار متوسط با سیستم زمین LOW IMPEDANCE مقادیر نمونه 400 آمپر 10 ثانیه قابل ذکر است. مطابق استاندارد مذکور توان نامی دائمی یک ترانس زمین می باید حدود 3% توان کوتاه مدت 10 ثانیه ای آن باشد. در این ارتباط به متن و محاسبه نمونه زیر توجه فرمائید.
A grounding transformer provides a source for zero-sequence current.Grounding transformers used as the only ground source to a distribution circuit should be in service whenever the three-phase power source is in service. If the grounding transformer is lost, a line-to-ground causes high phase-to-neutral voltages on the un faulted phases, and load unbalances can also cause neutral shifts and over voltages.
A grounding transformer must handle the unbalanced load on the circuit as well as the duty during line-to-ground faults. If the circuit has minimal unbalance, then we can drastically reduce the rating of the transformer. It only has to be rated to carry short-duration (but high-magnitude) faults, normally a 10-sec or 1-min rating is used. We can also select the impedance of the grounding transformer to limit ground-fault currents.
Each leg of a grounding transformer carries one-third of the neutral current and has line-to-neutral voltage. Of the two types, wye-delta transformer and zig-zag transformer, the Zigzag grounding transformer is more commonly used. It is a three phase, dry type, air-cooled, auto transformer with no secondary winding.
Each phase has two identical windings, which are wound on different core legs to give the high impedance to normal phase currents. The windings are connected in a Wye configuration.
The neutral point is then connected either directly or through a neutral grounding resistor (NGR) to ground.
When a ground fault occurs, ground fault current flows through the fault, back through ground and the NGR to the Zigzag where the current is divided equally in each leg of the Zigzag. Since these three currents are all equal and in time phase with each other (zero sequence), and because of the special Zigzag winding connections, they see a very low impedance. This allows the ground fault current to flow back into the system.
It can be seen that the ground fault current is only limited by the resistance of the ground fault, the NGR, and the small reactance of the Zigzag.
The Zigzag transformer is continuously rated for a specific neutral current at rated phase-neutral voltage, without exceeding the temperature rise of the insulation class (class B up to 2400 volts, class H above 2400 volts).The saturation voltage level is normally 1.5 times the rated phase-to-phase voltage.
The current and time rating of the Zigzag, when used with an NGR, should be the same as the NGR.
The Zigzag should be connected to the system on the line side of the main breaker, as close as possible to the power transformer secondary terminals. When more than one power transformer is involved, one Zigzag is required for each. Care should be taken not to have more than one Zigzag connected to the same section of the system at the same time. They can also be connected to the main bus if the main bus will always be kept energized from an alternate source.
Each leg of a grounding transformer carries one-third of the neutral current and has line-to-neutral voltage. So in a grounded wye – delta transformer, the total power rating including all three phases is the neutral current times the line-to-ground voltage:
S = 3 x Vph x In/3 = Vph x In
A zig-zag transformer is more efficient than a grounded wye – delta transformer. In a zig-zag, each winding has less than the line-to-ground voltage, by a factor of 1.73 , so the bank may be rated lower:
S = (Vph x In)/1.73
The current and time rating of grounding transformer, when used with an NGR, should be the same as the NGR.
Also ANSI/IEEE Std. 32-1972 requires a continuous rating of 3% for a 10-sec rated unit.
So we can write:
S cont. = 0.03 S 10sec.
S cont. = 0.03 x (Vph x In)/1.73
FOR EXAMPLE :
In = 400 A
Vph = 20/1.73 KV
Duration of earth fault = 10 sec.
S cont. = 0.03 x (Vph x In)/1.73 = 80 KVA
EARTHING TRANSFORMER CONTINUOUS
RATED POWER SELECTION: