Synchronization and Reactive Power Control in Power System

Power System Stability

In the world of power systems, synchronization and reactive power control are crucial to maintaining stability, efficiency, and reliability. Synchronizing various power sources, such as generators and grids, ensures they operate in harmony to meet the demand and support the system’s overall health.

Central to this process is the synchro check scheme, a protective mechanism that prevents the closing of circuit breakers under out-of-sync conditions, minimizing potential damage to equipment and ensuring the safe integration of power sources.

This article begins by exploring the synchro check relay (ANSI Device 25) and its role in verifying essential parameters—frequency, voltage, and phase angle—before allowing connection. We will examine why synchronization is critical, delve into the core functionality of synchro check relays, and discuss key considerations and potential impacts of out-of-sync closing.

Additionally, we’ll touch on practical applications, including the permissive features for manual and automatic closing.

Topics such as the Ferranti Effect, voltage correction using reactive devices, and real-world voltage stabilization through tools like ETAP will be covered in depth.

Finally, this article will address phase rotation—a critical factor in system reliability. With insights from commissioning experience at a 110kV substation, we will highlight the importance of correct phase sequence, examine the implications of phase rotation variations (such as ABC and ACB sequences), and discuss methods to ensure accurate phase alignment in diverse power systems.

Through a combination of theoretical insights, practical considerations, and case studies, this article provides a comprehensive guide to synchronization, reactive power management, and phase sequence verification for power system professionals.

Table of Contents:

1. Synchro Check Scheme:
   1. Why Synchronizing is Important
   2. How the Synchro check Relay Works (ANSI Device 25)
   3. Basic Synchronism Check Functionality
2. Key Considerations for Synchro check Relays:
   1. Impact of Out-of-Sync Closing
   2. Permissive for Manual or Automatic Closing
3. Reactive Power Flow and Voltage Matching in Paralleling Sources:
   1. Impact of Reactive Power Flow
4. The Role of Reactive Power in Voltage Control: Insights from EHV Network Operations
   1. Understanding Reactive Power and Its Use in Voltage Control
      1. Reactive Power’s Influence on Voltage: The Ferranti Effect:
      2. Voltage Correction Using Reactive Power Devices: A Case Study
      3. Reactive Power as a Tool for Voltage Control
5. ETAP Case Study: Voltage Stabilization Using Shunt Capacitors
6. Phase Rotation in Power Systems:
   1. Ensuring Correct Phase Rotation
   2. The Impact of Incorrect Phase Rotation
   3. Understanding Phase Rotation through Waveforms
7. Ensuring System Reliability: My Commissioning Experience with Phase Sequence Verification at a 110kV Substation in Saudi Arabia
8. Phase Sequence Variations in Power Systems: ABC vs. ACB
9. Countries where ACB or reversed phase sequence may be observed
10. BONUS (PDF) 🔗 Download Guide to Implementation of Out-of-Step Protection Schemes for Generators

1. Synchro check Scheme

In modern power systems, maintaining synchronization between different power sources or between a power source and the grid is vital for the stability and reliability of the system.

This process is managed by synchro check relays, also referred to as synchronism-check relays, which play a critical role in ensuring that key electrical parameters such as voltage magnitude, frequency, phase angle, and phase rotation are aligned before connecting two sources or closing a breaker.

Figure 1 – Automatic synchronizing

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1.1 Why Synchronizing is Important?

Whenever two different electrical sources or a source and the grid need to be interconnected, precise synchronization is required. Imagine two carts on a road; to connect them, they must be moving at the same speed and in the same direction. If their speed or direction differs, the carts could collide or become damaged.

Similarly, in electrical systems, if the voltage, frequency, or phase of two sources is not properly synchronized, the result can be catastrophic, leading to high inrush currents, system instability, and potential damage to equipment.

When two sections of a power system, such as two substations or a power plant and the grid, need to be connected via a circuit breaker, differences in voltage magnitude, phase, and frequency between the two sides can lead to a large current flow.

To avoid such issues, synchronization must be carefully monitored and controlled.

Figure 2 – Synchroscope installed on a panel

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1.2 How the Synchro check Relay Works (ANSI Device 25)

The synchro check relay (designated by ANSI device number 25) helps achieve safe synchronization. The relay monitors the voltage on both sides of the breaker, ensuring the following conditions are met before the breaker closes:

• Voltage magnitude on both sides must be within acceptable limits.
• Phase angle should not exceed a specified difference.
• Frequency of both sources must be synchronized.
• Phase rotation must match between the two sides.

If any of these parameters are out of tolerance, the relay will block the breaker from closing, preventing an out-of-sync connection. This blocking action helps avoid potential system instability, equipment damage, and safety hazards.

Lesson – Synchronization of Machine with Grid

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1.3 Basic Synchronism Check Functionality

In a typical modern digital relay setup, the synchronism check function operates by comparing the three-phase voltage from one side of the breaker (running source) with the single-phase voltage from the incoming side (incoming source). This setup allows the relay to monitor the critical parameters and determine if the two sources are in sync.

Usually, phase-A is selected for synchronization, but modern relays offer flexibility, allowing users to choose any of the three phases (A, B, or C) or configure custom phase angle selections.

Relays typically have four voltage inputs:

• Three inputs for each of the three phases on the running side.
• One input for the sync voltage (typically phase-line or phase-neutral from the incoming source).

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2. Key Considerations for Synchro check Relays

There are cases where synchronization may not be possible due to the nature of the system configuration. Common scenarios where synchronization cannot be achieved include:

Scenario #1: Voltage Transformer (PT) Connections

• If the running voltage is connected via a wye-connected PT and the sync input is derived from a phase-phase connection, synchronization is not possible.
• Similarly, if the relay is connected to a delta PT while the sync input is phase-neutral, the two voltages will be incompatible.

Scenario #2: Transformer Configurations:

• When the sync input is from the secondary side of a delta-wye transformer, the phase shift introduced by the transformer will prevent synchronization with the primary side voltage.

Watch Video – Synchronism Check Elements in Protective Relays

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2.1 Impact of Out-of-Sync Closing

Closing a breaker when the voltages on both sides are not synchronized can have significant negative consequences. The most immediate effect is the flow of large equalization currents, which can severely damage the breaker, transformers, or other equipment.

In extreme situations, out-of-sync closing can cause voltage dips, system oscillations, and even blackouts. Therefore, ensuring proper synchronization before closing a breaker is not just a technical requirement, but a critical safety measure.

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2.2 Permissive for Manual or Automatic Closing

The synchro check relay provides a permissive signal for breaker closing. This signal can be used for both manual and automatic source paralleling, depending on the operational needs of the power system. In manual mode, operators can manually close the breaker once the relay indicates that conditions are within acceptable limits.

In automatic mode, the breaker will automatically close once synchronization is achieved.

The flexibility of modern digital relays further enhances their functionality, allowing customized synchronization settings for complex power system configurations.

Figure 3 – Single line Diagram of Basic Synch Check Scheme

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3. Reactive Power Flow and Voltage Matching in Paralleling Sources

When two electrical sources are to be paralleled, it is critical that they have matching voltages to ensure stable and efficient operation. If the voltages between the sources are unequal, it results in the flow of reactive power (Vars) between them, which can lead to undesirable effects on the system.

Although minor voltage variations in the range of 1-5% are generally acceptable, significant differences in voltage magnitudes can cause excessive reactive power flow, leading to inefficiencies and potential damage.

The flow of reactive power between two interconnected sources occurs because of the voltage difference, even if the phase angle remains the same. The equation governing the flow of reactive power (Q) between two sources, assuming zero phase angle difference, is expressed as:

Where:

• Q = represents the reactive power (Vars).
• E_a = Voltage of source 1
• V_a = Voltage of source 2
• X_s = Impedance between the sources.

From this equation, it is clear that the larger the voltage difference between the sources, the greater the reactive power flow.

Related Course – MATLAB/Simulink Course: Power System Simulations (Load Flow, Short Circuit & Stability)

MATLAB/Simulink Course: Power System Simulations (Load Flow, Short Circuit & Stability)

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3.1 Impact of Reactive Power Flow

Even though the two sources may have the same frequency and phase, the mismatch in their voltage magnitudes will still lead to the exchange of reactive power. This is illustrated by the waveform where the two signals have identical frequencies and phases, but their voltage levels differ.

Reactive power flow, especially when excessive, causes unnecessary heating in the components that interconnect the sources, such as transformers, switchgear, and cables. This heating can lead to increased losses, reduced efficiency, and accelerated aging of the equipment.

When paralleling sources, it is essential to ensure voltage matching to prevent the flow of excessive reactive power, thereby protecting the integrity of the electrical system and improving overall efficiency.

Figure 4 – Voltage waveform of two source with same frequence and phase angle but different voltage levels

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4. The Role of Reactive Power in Voltage Control: Insights from EHV Network Operations

4.1 Understanding Reactive Power and Its Use in Voltage Control

In the previous section, we discussed the importance of maintaining voltage within acceptable limits when synchronizing two sources. One key reason for this is that any difference in voltage between the sources causes the transfer of reactive power between them.

This reactive power flow can result in excessive heating, system instability, and inefficiency.

From my experience in network operations at 500kV voltage levels, particularly during the energization of Pakistan’s first HVDC project, I gained firsthand insights into how reactive power affects voltage levels in a transmission network and the methods used to control it.