The Current Transformer Unit - Parallel Switch. Paralleling Different Size Generators

A switch is installed on the secondary side of the current transformer to short-circuit both the CT and the burden resistor, preventing any signal from reaching the generator. When set to the “Unit” position, the switch allows the generator to operate independently of the parallel generating system, eliminating the effects of the droop circuit.

When a generator operates independently in a parallel droop system without the short-circuit switch set to the "Unit" position, an unwanted droop occurs in the generator's output voltage. The same effect happens in a crosscurrent system, though to a lesser extent, because other burden resistors in series function as a voltage divider, proportionally reducing the voltage.

In a crosscurrent loop, when one generator is removed from parallel operation to run independently, the unit-parallel switch plays a crucial role in maintaining the stability of the remaining paralleled generators. If the non-paralleled generator is disconnected from the line without shorting its current transformer and burden resistor, the parallel system voltage will fluctuate. Since the non-paralleled generator operates at a different speed and frequency than the paralleled generators, the current through its burden resistor will have a continuously varying phase angle relative to the current through the burden resistors of the paralleled generators.

This varying phase angle causes the regulator of the non-paralleled generator to alternately increase and decrease its excitation, resulting in small periodic fluctuations in the generator’s output voltage. The frequency of these voltage fluctuations corresponds to the difference in frequency between the non-paralleled generator and the paralleled generators.

Drawing 32:  Unit - Parallel Switch

When adding an external unit-parallel switch to a paralleling circuit, careful consideration must be given to the distance between the switch and the current transformer’s secondary terminals (Drawing 32). If the connection requires a significant length of wiring, the resistance of the wire may become high enough to allow some current to flow through the burden resistor.

Even a small current passing through the burden resistor will generate a corresponding small voltage, which can cause a slight droop in system voltage if the system is operating with reactive droop compensation. If reactive differential compensation is used, a similar effect will occur, as described in the previous section on the crosscurrent loop.

To minimize these effects, the wiring between the current transformer and the shorting switch should be kept as short as possible.

When using a reactive differential compensation circuit, it is recommended to integrate the unit-parallel switch as an auxiliary contact on the main generator breaker. The auxiliary contact should be closed, shorting the current transformer (CT) secondary, whenever the main generator breaker is open.

The moment the main generator breaker closes, connecting the generator in parallel, the auxiliary contact should open, removing the short from the CT secondary and allowing it to transmit the reactive load signal. This setup prevents voltage droop from occurring and eliminates the fluctuating voltage of the oncoming generator, as described earlier.

Drawing 33: Generator Phase Sequence (Current Transformer Polarity Relationship)

Current Transformer Placement on Unmarked Phases

As previously mentioned, the current transformer (CT) can be connected to any generator phase, provided the system voltage is taken from the appropriate line voltage to maintain a 90-degree electrical phase difference with the burden resistor voltage.

If the phase designations (A, B, and C) cannot be determined, the paralleling CT should be installed on the generator phase that does not supply the sensing voltage to the regulator. In cases where the voltage regulator is conDrawingd for three-phase sensing, the CT must be placed on the phase where the voltage regulator senses the B phase (Drawing 33).

The polarity connection of the CT is crucial in ensuring proper generator paralleling when phase rotation is unknown. An incorrectly connected CT will cause the generator voltage to rise under a lagging inductive load instead of exhibiting the expected voltage droop.

Crosscurrent Compensation and Current Transformer Selection

When using crosscurrent compensation to parallel generators, it is recommended to keep the crosscurrent connecting loop open at one point until proper paralleling is achieved using parallel droop compensation.

Current Transformer Selection

Generators with different kVA ratings and line voltages have varying line currents. When paralleling generators of different kVA ratings, the current transformers (CTs) must have appropriate ratios to ensure that their secondary current output matches that of the other CTs in the system. Typically, a CT has a nominal secondary current of 5 amperes with a maximum burden of 25VA. To achieve a secondary current of 5A, the primary current must match the CT’s primary rating. Ensuring proper CT ratio matching with the generator’s line current is a critical detail that must not be overlooked. (Refer to Table 1 for typical CT design specifications).

If the generator’s line current falls between the primary ratings of two available CTs, the CT with the higher primary rating should be selected. This ensures that the CT remains within its maximum limits.

Effects of Incorrect CT Selection

Using a CT with a lower primary rating than required:

• Higher line current forces a secondary current exceeding the nominal 5A.
• Excessive secondary current increases the burden on the paralleling circuit beyond the CT’s design limits.
• CT saturation occurs, altering inductance values and shifting the phase angle of the signal sent to the voltage regulator, leading to improper regulator response.

Using a CT with a higher primary rating than required:

• Insufficient secondary current is supplied to the paralleling circuit.
• Paralleling operation becomes weak or uncontrolled.
• In some cases, a higher CT ratio can be used by doubling the primary turns of the generator line to bring the secondary current closer to the nominal 5A.

For optimal selection, choose a CT with a ratio that provides a secondary current as close as possible to 5A. However, satisfactory paralleling can still be achieved with a minimum of 3A at full load rated power factor.

Current Transformer Requirements For Droop Compensation:

Instrument Accuracy:

• Must provide accurate magnitude and phase (phase may be shared).

Ratio Selection:

• Minimum of 3A secondary current.
• Maximum of 5A secondary current.

For Crosscurrent Compensation:

Current Transformer Ratios:

• Must be equal for generators with the same ratings.
• Must be proportional for generators with different ratings.

Burden Resistors:

• Must be the same for all units.

Grounding:

• No grounding in individual CT secondaries.

CT Burden Requirements:

Basler AVRs have burden values between 0.2 and 1 ohm, with nominal currents of 5A or 1A.

CT Capacity:

• Must be sufficient to drive all connected loads, including lead wire resistance.

Breaker Auxiliary Contact and Wiring:

• Must have low resistance to effectively short the CT when required.

Paralleling Different Size Generators

When paralleling generators of different kVA ratings, each generator will have a different line current. To ensure compatibility, current transformers (CTs) with different ratios must be used to step down these varying line currents to a standard nominal secondary current.

Droop Operation

• The droop adjustment ensures that each generator experiences the same percentage of voltage droop at its rated load.
• This allows generators to be paralleled while each unit carries its proportional share of the reactive load.

Reactive Differential Compensation

• For generators using reactive differential compensation, equal secondary currents are required to cancel opposing currents and eliminate system voltage droop.
• Unequal secondary currents from different CTs create imbalances in the crosscurrent loop, leading to circulating currents.
• To reduce imbalance:

1. Set all burden resistors to maximum resistance.
2. Adjust the burden resistor of the CT with the smallest secondary current by slightly decreasing its resistance to allow more current flow.
3. Only make small adjustments to prevent excessive sensitivity loss while still reducing imbalance.

If the secondary current variation among generators is only a few tenths of an ampere, the resulting imbalance in the crosscurrent loop or droop circuit will be negligible.

Paralleling Generators – Case Studies

To illustrate the considerations and required paralleling equipment, two examples of three-generator systems are presented:

1. Drawing 34: Three generators connected with crosscurrent compensation.
2. Drawing 35: Three generators connected with parallel droop compensation.

Both systems consist of:

• Two 625 kVA generators (500 kW each).
• One 125 kVA generator (100 kW).

Each configuration demonstrates how different paralleling methods impact load sharing and system stability.

Drawing 34: Cross-Current Compensation Connections

Drawing 35:  Parallel Droop Compensation Connections

Each generator operates with a line-to-line voltage of 480 volts, allowing them to connect to a common bus. Based on the provided specifications, we calculate the current for each generator using the apparent power (KVA) formula:

KVA = 3 I_LV_L

For the 500 KVA generators, this results in a current of 752 amperes, while the 125 KVA generator has a current of 150 amperes. 

The voltage regulators selected for the generators, which utilize parallel droop compensation, demonstrate that voltage regulators do not have to be identical to operate effectively in parallel. Both 625 KVA generators are equipped with Basler SSR regulators, and the 125 KVA generator uses a generic voltage regulator.

Basler voltage regulators and paralleling modules are designed with a built-in isolation transformer to handle cross-current compensation. This isolation transformer is crucial to avoid interference from cross-current connections between the generators affecting the regulators’ sensing circuits.

To incorporate paralleling capabilities into a Basler regulator, you can refer to Drawing 35 for guidance. The SSR regulators used with the 625 KVA generators come with the option for paralleling provisions. In contrast, the generic regulator lacks built-in paralleling capabilities, so we will need to utilize a Basler APM 300 paralleling module.

Parallel Operation of generators HYUNDAI (PMS DEIF) (video)

Automatic load sharing between parallel generators HYUNDAI (PMS DEIF) (video)

The SSR voltage regulators with these provisions do not include a built-in unit/parallel switch to bypass the current transformer and allow the generator to operate offline or independently from the others. Therefore, we will add an external unit/parallel switch to the SSR voltage regulators.

To further illustrate the process of paralleling different-sized generators in cross-current mode, Drawing 33 displays two Basler SSR regulators paired with either a regulator from another manufacturer or a Basler regulator using a different burden resistor value. For these generators to be interconnected in cross-current operation, it’s essential for the burden resistors to match. The Drawing indicates the addition of external resistors to the SSRs’ paralleling circuit to align the burdens with the APM 300. We calculated this value using the following method.

METHOD FOR DETERMINING BURDEN RESISTOR VALUE WHEN CONNECTING DISSIMILAR REGULATORS IN A CROSS-CURRENT COMPENSATION CONFIGURATION

1. Begin by identifying the VA rating and current signal of each regulator to calculate its equivalent resistance.

For example, for SSR Regulators: 10 VA, 5 Amps.

Other regulator:  25 VA, 5 Amps

2. Select the regulator with the lowest VA rating (voltage "V") and calculate the required series resistance to reduce the applied voltage from the higher VA rating to the voltage needed for the selected regulator. Refer to the diagram provided.

Drawing 36:   Voltage Regulator with External Matching Resistor

The required voltage is 2V, while the supplied voltage is 5V. To achieve the desired voltage, the series resistor must drop 3V. Given the previous examples:

• At 2V, the resistance is 0.4 ohms.
• At 5V, the resistance is 1.0 ohm.

Thus, the appropriate resistor value should be selected to ensure the voltage drop of 3V across it.

The wattage of the series resistor can be calculated from:

Since we apply a 50% derating to our resistors, the required wattage is 30 watts. The nearest standard wattage rating is 50 watts.

Drawing 37:  Parallel Generator

Troubleshooting Paralleling Issues

Abnormal Indications:

• Sudden increase in current immediately after paralleling
• Difficulty in adjusting voltage for null balance
• Load variations causing unbalance

If Immediate High Current Occurs:

1. Ensure paralleling is set to droop only.
2. Verify that kW load is properly shared.
3. Check sensing connections to the AVR.
4. Confirm the CT is in the correct phase.
5. Reverse the CT secondary polarity, if necessary.
6. Attempt to parallel again.
7. If all checks are correct, try closing the cross.

Verifying CT Polarity:

Place the CT on the correct phase.

• If there is no sensing connection or an E2 sensing connection, check the setup.

If the CT is on the correct phase, polarity outcomes:

• Correct polarity: The breaker closes normally.
• Reversed polarity: The breaker closes, causing an immediate high current.

If the CT is on the wrong phase, multiple issues may arise.

Finding the Problem:

• Check for small voltage across the droop input (a shorted CT may be present).
• Inspect the droop adjustment – is it at maximum or minimum?
• Verify polarity marks on PTs using a small battery and a voltmeter.
• Ensure proper PT and CT connections, confirming the CT is in the correct phase.

Drawing 38:  Polarity Check - Potential Transformer

Additional Testing

Basic Test:

• It is possible to parallel one machine without droop, but extreme caution is required.
• Verify whether the two machines can share the load if only one of the droop CTs is not shorted.
• If load sharing is possible, check the droop direction by increasing the AVR droop voltage adjustment.

Further Testing:

1. Connect an oscilloscope:

• Channel 1 → Terminals E1 and E3
• Channel 2 → Droop input, Terminals 1 and 2

2. Apply some resistive load current.

3. Observe the 90° phase shift between voltage and current.

Drawing 39:  AVR with Droop

Articles about Automatic Voltage Regulator:

1. Automatic Voltage Regulators. What is a generator AVR or Automatic Voltage Regulator?
2. Automatic Voltage Regulator and Parallel Operation of generators. Voltage droop
3. Automatic Voltage Regulator. Real power, Reactive power, Apparent power. KW, KVAR, KVA
4. Function of Voltage Regulator and Parallel Generator Operation
5. The Current Transformer Unit - Parallel Switch. Paralleling Different Size Generators
6. Checking and Troubleshooting a Reactive Compensation Circuit for the AVR in an Isolated AC Bus
7. ACB Trouble. The generator does not connect to the main busbars. Troubleshooting