Enhancing grid stability in a renewable world with the unique UPFC PLUS technology

Overcoming today’s grid challenges requires technology that can easily integrate into existing infrastructure whilst providing grid stability. German Kuhn, FACTS Product Manager at Siemens Energy discusses the advantages of the new UPFC PLUS system for effective dynamic load flow management. 

By Stephen Ballard

What are the trends and challenges faced by the electricity transmission grid that make the unified power flow controller (UPFC) a vital tool?

There is increased demand and rapid growth in renewable generation in many parts of the world and the existing network infrastructure is struggling to cope with these pressures. The requirements that decentralized energy systems with unregulated load flows and the liberalized electricity markets are increasingly making is causing concern for grid operators.

This pressure on the grid is overloading existing transmission lines and that can cause thermal overloads. Then there is also an increasing number of cases where the frequency and voltage approach or even exceed the allowable limits. We believe UPFC PLUS will increase the efficiency of the system and thus, increase the utilization of the grid.

How do increasing levels of renewable energy generation affect the grid’s performance?

Renewables enter the grid by way of converter-based technology, whereas the conventional power plants use rotating equipment such as generators. Generators provide system services such as inertia that renewables may not fulfil. Also, renewables do not deliver a constant or predictable power level. When the wind is blowing or the sun is shining, they produce a lot of power, but at night and when the wind is low there is less power. These load flow changes cause significant problems for the transmission grid.

How does the lack of reactive power affect grid stability?

Reactive power is mainly needed to stabilize the system voltage. If the system is not loaded in a low load condition, the voltage goes up and vice versa the voltage drops down in case of high load condition. The power quality is defined by a very narrow band of voltage that must be maintained to ensure consistent power quality. If you get out of this band, consumers and industrial plants may have damages. This can cause economic losses as well as risks of a blackout in the energy system.

The obvious solution would be to build more power lines. Why is that a viable option?

Getting the approval to build a new power line is simply difficult in Germany, mainly because of lack of space and public acceptance. The approval process for new AC power lines can take several decades, so that is a process that transmission system operators (TSOs) want to avoid if possible.

Can you explain to me how UPFC PLUS helps to alleviate this congestion?

UPFC cannot increase the capacity, but it can balance the load flow, that is the main feature. It can help to alleviate the flow on loaded lines and push the power to lower loaded lines. It balances the flow by changing the line impedances, which influences the load flow, so that more power can flow through the system.

There are several other load flow control technologies that can be used for stationary load flow and there are plenty of these units planned in Germany. UPFC adds the vital element of dynamic load flow control.

What is redispatch, why should it be avoided and how can UPFC PLUS help in this?

Redispatch is required when a line section is overloaded because of a higher renewable power feed. In this situation the TSO is forced to reduce the power input from renewables, to prevent overloading, meaning that he cannot fully utilize the available renewable energy. However, the power is still needed so the TSO must increase the power generated by, for example, gas turbines. So, there are now two aspects, reducing the renewable power and ramping up the gas turbine. Gas turbines in conventional power plants are designed for constant power generation so for fast ramp-up they will run inefficiently. They are ramped up to support the system and after the redispatch is cleared you must return to the renewable power park. This procedure is called redispatch and causes financial losses to the public economy TSO and so TSOs they want to prevent this.

What are the advantages that UPFC PLUS has over other FACTS technologies?

There are several FACTS technologies that can be used for different purposes.

UPFC PLUS is a series compensation device that is designed to conduct load flow transfers and dynamic load flow control. To date there are no other FACTS devices that provide the same performance as the UPFC for this particular purpose.

In stationary load flows you can set up for constant power values and the system will transmit the required power. In case of disturbances, such as when you lose a line due to a lightning strike, or some other fault scenario, this will create instability in the system, but the power is still needed at the end of the line. To achieve this the power must be routed immediately through the network by another route. This disturbance can also cause oscillations in voltage as well as frequency. The static devices are not able to control voltage and frequency, but dynamic elements such as the UPFC PLUS can do that through dynamic controllers. 

What is the next step in the UPFC PLUS and FACTS evolution?

I believe that we can convince one of the TSOs worldwide, maybe one in Germany, to establish a pilot project.

We are prepared and ready to provide an offer. But alongside that we are working on our technology roadmap to add additional stability functions, increase the rating if needed, or even offer smaller sized applications. This direction will be decided as the use ramps up as we need the experience from our customers and from the first pilot project to define what direction we want to develop the product moving forwards.

October 30, 2020

Stephen Ballard is a freelance technical author.

Combined picture credits: Siemens Energy