به هنگام موازی نمودن دو یا چند ترانسفورماتور که هر یک مجهز به سیستم کنترل ولتاژ اتوماتیک و OLTC هستند، ارتباط سیستمهای کنترل مجزا جهت به حداقل رساندن جریانهای راکتیو گردشی بین ترانسها ضروری است. نحوه ارتباط با چندین مد مختلف شناسایی می شود که مد ( Master/follower (master/slaveیکی از آنها است. در این مد کنترلی سیستم کنترل یکی از ترانسها مستر شده و از طریق کنتاکتها و رله های کمکی پیش بینی شده در مدار کنترل تپ چنجر دیگر ترانسها وضعیت فیزیکی سیستم کنترل مستر را تقلید و دنبال می کنند. این مد کنترلی به دلیل پیچیدگی و وابستگی به کارکرد تعداد زیادی المان الکترومکانیکی امروزه کمتر مورد استفاده قرار می گیرد.
Where two or more transformers with automatically controlled on-load tap changers are operating in parallel, it is normally necessary to keep them either on the same tapping position or a maximum of one tap step apart. If transformers are operated in parallel on different tappings circulating currents will be set up and in general one step is the most that can be tolerated.
Numerous procedures have been identified over the years to accomplish LTC transformer paralleling using electronic control. These are listed with some limited description along with alternative names sometimes used:
1. Negative reactance (reverse reactance): Seldom used today except in some network applications, this is one of the oldest procedures accomplished by other than mechanical means. This means of paralleling is the reason LTC controls are required by the standard to provide negative X capability on the line-drop compensation.
Advantages: Simplicity of installation. The system requires no apparatus other than the basic control, with LDC X set as a negative value. There is no control interconnection wiring, so transformers can be distant from each other.
Disadvantages: Operation is with a usually high –X LDC set point, meaning that the bus voltage will be lowered as the load increases. This can be sufficiently compensated in particular cases with the additional use of +R LDC settings.
2. Cross-connected current transformers: Unknown in practice today, the system operates on precepts similar to the negative reactance method. The LDC circuits of two controls are fed from the line CT of the opposite transformer.
Advantages: System requires no apparatus other than the basic control, but it does require CT circuits to pass between the transformers.
Disadvantages: System may need to be operated with a value of +X LDC much higher than that desired for LDC purposes, thereby boosting the voltage too much. The system can be used on two transformers, only.
3. Circulating current (current balance): The most common method in use in the U.S. today; about
90% of new installations use this procedure. It has been implemented with technical variations by several sources.
Advantages: Generally reliable operation for any reasonable number of paralleled transformers.
Uses the same CT as that provided for LDC, but it operates independently of the line-drop compensation.
Disadvantages: Control circuits can be confusing, and they must be accurate as to instrument transformer polarities, etc. Proper operation is predicated on the system being such that any significant difference in CT currents must be due only to circulating current. Matched transformers will at times operate unbalanced, i.e., at differing tap steps under normal conditions.
4. Master/follower (master/slave, electrical interlock, lock-in-step): Used by most of the 10% of new installations in the U.S. that do not use circulating current. It is used much more commonly elsewhere in the world than in the U.S.
Advantages: Matched transformers will always be balanced, resulting in minimal system losses.
Disadvantages: As usually implemented, involves numerous auxiliary relays that may fail, locking out the system.
5. Reactive current balance (delta var): Generally used only when special system circumstances require it. Operates so as to balance the reactive current in the transformers.
Advantages: Can be made to parallel transformers in many more-complex systems where other methods do not work.
Disadvantages: May be more expensive than more common means.
The two most common paralleling procedures are master/follower and circulating-current minimization.
The negative-reactance method also deserves summary discussion.
With Master/follower control system one of the units is selected as the master and the remaining units operate as followers. Built-in contacts in the on load tap changer mechanisms are connected so that once a tap change has been completed on the master unit each follower is initiated in turn from the interconnected auxiliary contacts to carry out the tap change in the same direction as that carried out by the master. A simplified schematic diagram of the master and follower circuit is shown in Figure below. The disadvantage of master/follower schemes is their complexity, so that nowadays they are very seldom used.
With simultaneous operation, all tap changers of a group are arranged to start their operation at the same time. This is a simpler arrangement although it is still necessary to provide lock-out arrangements to take care of any individual failure.
In microprocessor base AVR the Parallel Control Unit (PCU) is used for controlling two Transformers in parallel. The transformers have to be identical i.e. with the same number of taps.
'Raise' & 'Lower' NO contacts from AVR1 & AVR2 are connected to the PCU, & the outputs from the PCU are connected to the OLTC1 & OLTC2 respectively.
Switches on the front panel of PCU allows for selecting AVR1 or AVR2 as master & the other as the follower, or both AVRs as independent (masters).
Tap Position of both the Transformers are displayed on the PCU. In Master / Follower Mode if the two tap numbers are not equal, pulses are automatically given to the Follower to make it equal to the Master. When the two taps are equal and when the master AVR gives either Raise / Lower Pulse to the transformer, the tap numbers are compared after the set delay & if there is a difference in the tap numbers, the Raise / Lower pulse are given to the follower.
In case the tap position of the two transformers in parallel is unequal for several minutes, Out of step indication is given & both OLTCs are blocked.
External A/M Switch (NO contacts) is sensed for determining Auto / Manual mode of operation. When manual mode is selected, the PCU is blocked i.e. no output pulses are given to the OLTCs. The PCU is also blocked when both the AVRs are set in follower mode. External Bus Coupler ( NO ) contacts are sensed. If the input contacts are closed, the Bus Coupler output closes with delay, and is indicated by Parallel Mode Indication on the Front Panel.