Sampling on the high-voltage side and mitigation on the low-voltage side

The market's preference for the model of "sampling on the high-voltage side and mitigation on the low-voltage side" is not accidental. It is the optimal solution determined by a combination of factors including technical feasibility, economic efficiency, safety, and mitigation effectiveness. The logic behind this can be understood with a simple analogy:

A doctor performs a "blood test" (sampling) on the "main artery" (high-voltage side) and then conducts "targeted therapy" or "local medication" (mitigation) on the specific "organ" or "capillary network" (low-voltage load side).

I. Why Must Sampling Be Done on the High-Voltage Side?

Sampling on the high-voltage side (typically the 10kV or 35kV busbar) is essential to obtain the most comprehensive and accurate picture of the system's power quality status.

1. Global Perspective and the "Source of Truth":

   • The high-voltage busbar is the "heart" and "hub" of the entire distribution system. All power quality issues (harmonics, reactive power, etc.) generated by downstream low-voltage loads (non-linear loads) ultimately aggregate and are reflected back onto the high-voltage busbar.

   • Sampling at this point allows for the monitoring of the sum total of all power quality issues generated by all loads within the supply range of the substation. This provides the "global truth" of the system and is the sole basis for assessing compliance with utility requirements (e.g., the Chinese standard "Power Quality - Harmonics in Public Supply Network" GB/T 14549-93).

2. Data Representativeness and Stability:

   • The high-voltage side has a higher voltage level, greater short-circuit capacity, and lower system impedance, making the voltage waveform relatively more stable and stiff. The voltage waveform distortion is smaller,more accurately reflecting the true current distortion.

   • Sampling at a specific branch circuit on the low-voltage side only reveals the problems of that branch, failing to represent the state of the entire system—it's like "the blind men and the elephant."

3. Convenience and Standardization of Technical Implementation:

   • Voltage Transformers (VTs) and Current Transformers (CTs) on the high-voltage side are standard equipment. They are highly accurate, permanently installed, and provide ready-made, reliable measurement points for sampling. One only needs to install a power quality analyzer.

   • All assessments and settlements with the upstream grid are conducted at the high-voltage side, making data sampled here the most authoritative.

II. Why is Mitigation Preferred on the Low-Voltage Side?

Although the problem manifests on the high-voltage side, its root cause lies with the low-voltage loads. Mitigation, like treating a disease, requires addressing the root cause cost-effectively.

1. Economic Efficiency (The Most Critical Factor):

   • Equipment Cost: The cost of mitigation devices (e.g., APF, SVG) is directly related to the voltage level at which they operate and the compensation current they provide.

     • Voltage Level: The insulation等级 (rating), design complexity, and manufacturing cost of components for a 400V low-voltage APF/SVG versus a 10kV high-voltage APF/SVG are worlds apart. The cost of high-voltage equipment can be several timesor even ten times higher than that of low-voltage equipment.

     • Compensation Current: Mitigation capacity S = √3 * U * I. For the same compensation capacity (S), a higher voltage (U) requires a lower compensation current (I). This means high-voltage mitigation devices require lower current capacity, but the cost surge due to the higher voltage level far outweighs the cost savings from the reduced current.

   • Installation and Maintenance Costs: Low-voltage equipment is simpler to install, does not require stringent power outage plans or highly certified personnel for high-voltage work. Daily maintenance and troubleshooting are also safer and more convenient.

2. Mitigation Efficiency and Precision:

   • Principle of Locality: Harmonics and reactive power are best compensated locally, near where they are generated (by the loads), to prevent them from circulating throughout the system and causing unnecessary line losses and pollution. This is like sorting trash in the kitchen instead of waiting for it to pile up all over the house before cleaning.

   • Precision-Targeted Mitigation: On the low-voltage side, mitigation can be targeted at specific harmonic sources (e.g., a group of VFDs, an arc furnace, a charging station) for immediate and effective results. Mitigation on the high-voltage side is a "one-size-fits-all" approach that cannot specifically address issues from a particular load.

3. Safety and Reliability:

   • Mitigation on the low-voltage side avoids the risks associated with operating on the high-voltage system. Low-voltage APF/SVG technology is very mature and reliable. Even if a fault occurs, its impact is limited to the local low-voltage area and will not cause a total blackout of the entire high-voltage system.

4. Modularity and Scalability:

   • Enterprise loads are constantly growing. A strategy of "first mitigate the main issues, expand capacity as needed later" can be employed on the low-voltage side by simply adding modules. The difficulty and cost of expanding a high-voltage mitigation system after initial installation are extremely high.

III. Summary: The Logical Closed Loop of the Optimal Strategy

Thus, the market's choice forms a perfect logical closed loop:

1. [Identify the Global Nature of the Problem] → Sample on the high-voltage side to grasp the true state of the entire network's power quality and define the necessity and goals of mitigation.

2. [Analyze the Root Cause of the Problem] → Use data analysis to locate which low-voltage loads are the primary sources of the power quality issues.

3. [Solve the Problem Economically and Precisely] → On the low-voltage side, install more cost-effective, efficient, and safer mitigation devices (e.g., APF) targeted at these source loads.

4. [Verify the Effectiveness of Mitigation] → Use data from high-voltage side sampling again to verify that the power quality of the entire network meets standards after mitigation.

Exceptions: When is Mitigation on the High-Voltage Side Needed?

While "low-voltage mitigation" is the universal rule, exceptions exist where mitigation on the high-voltage side (installing SVG, SVC, or high-voltage APF) is necessary:

1. Extremely Dispersed Loads: For example, in a large park with hundreds of small workshops, each generating a small amount of harmonics. Installing an APF in every workshop is uneconomical; centralized mitigation on the high-voltage side is more appropriate.

2. Primarily Reactive Power Issues: If the main issue is a low power factor leading to penalty fees, and the reactive power demand is huge, installing a high-voltage SVG/SVC for centralized reactive power compensation might be the better option.

3. Extremely High Voltage Stability Requirements: For sites with instantaneous, huge impact loads like steel mills or rail transit, rapid reactive power compensation (SVG) directly on the high-voltage side is needed to stabilize grid voltage and prevent voltage sags.

4. Severe Space Constraints: If there is no space to install numerous low-voltage mitigation devices, the only choice may be to install one centralized high-voltage mitigation device in the high-voltage switchroom.

In conclusion, "High-voltage sampling, low-voltage mitigation" is a golden rule developed by the market through years of practice, striking the best balance between technology, cost, and effectiveness.