Engineering Better Transmission With STATCOM

Static Synchronous Compensators (STATCOM) used in HVDC systems to provide reactive power compensation are well-known to engineers. Here, EEPower Editor-in-Chief Barbara Vergetis Lundin talks with Eaton Utilities’ Electronics Division Senior Field Application Engineer Akos Labady to learn more about the technology and how engineers can use it to successfully engineer future power plants.

 

Basic STATCOM configuration and main components. Image used courtesy of GE
 

EEPower: Please provide a general overview of STATCOMs and how power engineers might work with them.

Akos Labady: STATCOMs, from a hardware point of view, are a series of connected inverters precisely controlled by an operating software that compensates for voltage and frequency disturbances on the electric grid. 

Disturbances happen every time the electric production on a given network is not matched with the load, meaning STATCOMs (or SVI) continuously work to keep the grid voltage and frequency (50 or 60 Hz +/1 Hz) stable. At the same time, the power plants, solar fields, wind farms, and hydro plants try to adjust to the actual load requirement on the consumer side. 

The electric energy production units have a certain level of flexibility to adjust to the load, but normally it takes seconds to get readjusted, and the delayed periods are supposed to be managed by passive components like synchronous condensers, shunts or capacitive reactors, or a smart solution like the STATCOM. 

STATCOMs are effectively and efficiently managing reactive powers in the scale of hundreds or MW power level already. 

 

EEPower: How are STATCOMs evolving? What are some of the major technical innovations over the last five years?

Labady: Currently, STATCOMs are the most effective way to manage reactive power fluctuations. They are still evolving in terms of efficiency or cost but started to spread and have demonstrated performance. There are different ways to connect the H-Bridge or Half-Bridge converters, and every project may need a differently-tuned setup for optimal performance and cost. 

The need for STATCOMs is increasing, especially in Europe, which would like to move away from fossil energy sources and fossil-fueled power plants. 

These traditional power plants represent a great level of grid inertia resulting from their massive mechanical generators/turbines naturally providing spinning reserves. Removing such high-inertia devices from the grid results in more unstable grids which have trouble responding to disturbances or load fluctuations. Adding more PV or wind energy sources helps, but those have very low inertia since they all are connected through an inverter to the grid with a power limit that can’t be exceeded. This means that removing 1 MW of fossil power will require several MWs of PV and/or wind to have the same inertia in return. 

STATCOMs can help to add electric inertia to the system so more and more fossil power can be replaced by renewable alternatives. 

 

EEPower: What innovations/advancements can we expect to see in the future?

Labady: There are developments toward adding active power balancing features to the current STATCOMs, calling them Active or Enhanced STATCOMs. Active power is an efficient tool to manage the grid frequency during power perturbation. In reality, it means injecting hundreds of MWs of power for some seconds either on the power plant side or the TSO (Transmission System Operator). 

There are two major types of frequency regulation:

1. FFR–fast frequency response is used mainly at power plants, particularly hydro plants. These events have a well-defined sequence that may require five to 30 seconds of active power injection or absorption.
2. SIR–synthetic inertia response normally occurs at the TSO at higher power but requires a few seconds of active power injection or absorption.

 

EEPower: What are the challenges facing STATCOMs?

Labady: STATCOMs are generally a large investment for TSOs or power plants. Enhanced/active power STATCOMs are even steeper as there is an energy storage system that has to be high-performance, reliable, and safe in a medium voltage environment. 

Currently, there are no enhanced STATCOMs installed, and further challenges may be identified during the first site installations.

 

EEPower: How can those challenges be addressed?

Labady: Careful design considerations and precise sizing are the keys to addressing the high cost and reliability challenges. Identifying a scaleable and reliable energy storage system is the key to success. 

 

EEPower: Why are supercaps important in STATCOM systems? What advances have been made with this technology? What is on the horizon?

Labady: In general, supercaps are the most cost-efficient energy storage option for charge/discharge times of 0.1-15 seconds. In this way, supercaps are the major focus for developing enhanced STATCOMs both for FRR and SIR. 

Supercaps have the following advantages over batteries or other energy storage systems:

• Scaleable modular solution: Configurations are flexible and can precisely be tuned for the energy storage job.
• One million-plus cycle life: With STATCOMs, it’s difficult to predict the total number of charges or discharges over the lifetime, and it may vary year over year.
• Operating temperature range -40 C/+65 C: Flexible in terms of installation location.
• 80%+ roundtrip efficiency by the end of life and heaviest charge/discharge powers. Normally, the efficiency is expected to be 95%+.
• Flexible (electrostatic) charge voltage/mechanism: The actual nominal voltage can be set to have the supercaps have enough reserve to deliver the required power while, at the same charge level, absorbing the same amount of energy in seconds without damaging the caps. 
• High availability: Charge/discharge sequences are repeatable in a matter of seconds apart without damage/overheating.

 

EEPower: What challenges do supercaps pose to engineers, and how can engineers address those challenges?

Labady: The following are some challenges of supercaps, along with solutions:

1. Safety: Current supercap solutions are developed for LV (low voltage) system integration. This means the current solutions are difficult to integrate into higher voltage (1500 V+) inverters and floating voltage power modules. In MV (medium voltage) environments, different isolation and safety rules apply to LV and specifically developed supercapacitor modules needed to integrate in STATCOMs safely. The control level circuits, communication interfaces, and mechanical housing need to be floating and heavily insulated. Eaton’s supercap engineering team is currently working to provide an optimized, easy-to-integrate supercapacitor solution.
2. Performance: Since the power events in STATCOMs last for seconds, the charge/discharge currents may reach over a thousand amperes per supercap cell, which causes unwanted voltage drops on the DClink and limits the converters/inverters' working range. The Equivalent Series Resistance (ESR) is the key factor for the supercap modules for this reason. Eaton’s engineering team strives to deliver the industry's lowest ESR supercap modules featuring advanced terminal designs and laser-welded internal structures. 
3. Lifetime: Supercaps are required to operate for a minimum of 10 years, preferably 20 years, in an enhanced STATCOM application, which requires a reliable and effective cell management system and low-degradation cell design, specifically from an ESR point of view. Eaton’s supercapacitor modules are developed to have low ESR degradation topped by a state-of-the-art cell management & monitoring system, which is ready for the 20-year lifetime challenge.