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Learn how DC grids help Grid Operators

Today power systems are dominated by Alternating Current (AC). This is a fact. However, step by step we notice a slow but persistent penetration of Direct Current (DC) solutions which later result in adoption of large DC systems. The fast development of power electronics technologies allowed DC converters to operate at wide voltage level range, including transmission, distribution, and consumption, which brings us advantages, such as higher energy harvest from PV panels, lower CO2 emissions rates, higher reliability and controllability etc. In case of storage, it allows to use batteries with different chemistry and cells configurations. And due to the innumerous advantages of DC distribution systems such as reduced losses and easy integration of RES (renewable energy sources), energy storage and other DC native loads, nowadays they are gaining more and more attention.

In the previous posts we already addressed the strengths of the DC grids, but today we would like to focus on a specific stakeholder – Distribution System Operator (DSO). Let’s see how DC microgrids could facilitate the life of DSO and make the overall operation of power system more efficient, reliable, controllable and safer.

Growing Responsibilities of DSO

The work of DSOs is not easy. They are responsible for planning, maintaining and management of energy networks. Thanks to their proper work you have a privilege of reading this article using your electronic device whenever you want, regardless of factors such as extreme weather conditions etc. With the increasing penetration of Distributed Energy Sources (DERs) into our energy system, the DSO’s responsibilities multiply. So, in addition to its conventional work described above, nowadays a DSO also needs to perform other important tasks. Some of them are listed here:

· Peak load management through DERs

· Network congestion management

· Procure voltage support etc.

One of the possible solutions that DSO could adopt to make the operation of power grid easier is a development of microgrids. So, let’s learn more about its general benefits and let’s analyze in more detail the advantages that DC microgrids offer to the DSO.

Microgrids Benefits

The drivers that stimulate the microgrid development are multiple:

· Energy Security

Severe weather. The extremely weather conditions related disruptions may become more frequent and more severe due to the climate change. Therefore, microgrids can provide power to important facilities and communities using their DERs when the main grid goes down.

Cascading outages. Since electrical grids are run near critical capacity, even a small problem in a part of the system may affect an entire electrical grid.

Cyber and physical attacks. In a big extent, energy grid today relies on advanced information and communications technologies, making it vulnerable to cyberattack. Microgrids, through their decentralized architecture, are less vulnerable to attacks on individual pieces of key generation or transmission infrastructure.

So, the above-mentioned statements prove that microgrids development is something that we need globally adopt to make our energy system safer and more reliable. There are two types of microgrids that can be addresses: based on AC and DC technology. Of course, since we live in 21st century, we are going to talk about DC microgrids, which are more efficient when compared to AC counterparts, but also suitable for modern technologies we are using today.

DC microgrids and its benefits for DSO

Clean Energy integration. Due to the intermittent and uncontrollable nature of PV and wind energy, overgeneration and voltage control related issues occur more frequently. Microgrids are designed in a way to handle variable generation by using storage technology to locally balance energy production and loads. According to EU Fit to 55 plan, the number of RES should increase exponentially. The DERs cost construction for DC grid is lower due to cheaper DC-DC equipment comparing to DC-AC. Also, DC-DC converters are more reliable (due to lacking of rectifier), which benefits in decreased unscheduled maintenance cost.

Electric Vehicles (EVs) integration. The EU Road2Zero plan considers increase the number of public EV chargers from 0.4M in 2020 to 1.4M in 2025 and 3.4M in 2030. The EV charging demand is stressful for the existing AC grid, thus energy storages and solar panels already integrating into the charging stations utilizing DC interconnection. However, EV charger operator might sell back the energy from the storage to the grid. Considering the better efficiency of DC-DC conversion, the profit from trading is higher.

LED street lighting is an energy saving standard. The critical issue in LED light is “flickering” caused by AC ripple. To avoid this in AC grid, large storage capacitances are used. Despite that, due to outdoor temperature variation it subjects to fast aging, which results “flickering” appears ageing. The DC grid utilization completely avoid this problem and benefits in decreased LED power supply cost.

DC microgrids as Ancillary Services provider

And now let’s move to more technical talks. DC microgrids can provide a vast number of possible functions that improve the operation of energy grid. They can be used to strengthen weak points in the power system, to control power flow in the AC network and provide increased controllability and flexibility to grid operator. These functions are called Ancillary Services. Let’s look closer at each of them:

1. Artificial inertia: Weak AC systems may suffer from frequency variations. This results from the low ratio of rotating mass (inertia) related to synchronous machines. The DC microgrids can provide Ancillary Services by giving additional inertia to increase the local grid stability.

2. Frequency stability: Frequency deviation in an AC grid results from imbalances between generated and consumed power. DC microgrids, which have zero frequency, can mitigate the frequency deviation through their converters, thus restoring the frequency stability.

3. DC microgrids as a firewall in the AC grid: A DC system between AC grids can act as a “firewall” preventing disturbances spreading from one AC grid to another. When a power imbalance occurs on one part of the AC grid, the DC microgrid can alleviate the imbalance and prevent the disturbance from spreading to the rest of the network.

4. Power oscillation dumping: Electromechanical oscillations of the rotors in the synchronous machines may put pressure on the main AC grid. These oscillations indicate an operating working point close to the stability limit that wears down the governor systems of the turbines. To reduce these oscillations and maintain a safe power transfer, a control signal can be applied to the DC system, which is a valuable Ancillary Service.

5. Black start: Due to the interconnected power sources and storage means in a DC microgrid, the restoration process (black start) of an AC system can be fulfilled after a system power loss or blackout.

6. Maintaining synchronization: A DC microgrids may provide Ancillary Services to the AC system by maintaining synchronization. DC systems can support the power flow in the AC grid, reducing the risk of falling out of step and losing synchronization.

According to the EU Climate Target Plan, 38-40% of all produced energy must have a renewable origin. From one side it means that with this target we manage to alleviate climate change, what is excellent! From other side, considering our current energy grid structure that is dominated by AC systems, it will be very challenging for DSO to guarantee a reliable and safe system operation. From the previous paragraphs we learned that DC microgrid could be a perfect ally for a DSO to cope with increasing pressure, since because of smaller losses in DC grids, we can simply use less RES to reach the same production levels.

In addition, considering the latest news regarding potential power outages during upcoming winter in EU due to (gas) power (and heat) generation insufficiency, we claim that with local DC microgrids equipped with local RES and battery storage (including V2G) it would give more support for the full grid not to fall apart.

If we want to live in a carbon-neutral society, we must think in a new way, stop depending on reliable, but such obsolete and inefficient technologies (AC grids) and move on with innovation. At UBIK we believe that DC microgrids make part of this innovation that will bring all of us to the sustainable, safe & green tomorrow.

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