Flexible AC Transmission Systems

The addition of renewables-based power generation to the energy mix, phase-out of thermal power plants, new HVDC systems, and the extension of power supply systems to remote areas influence the stability of transmission networks. 

Synchronous condensers offer grid stability e.g. for transmission systems with a high amount of renewable infeed by providing short-circuit power, inertia and reactive power compensation.

Power producers and system operators are faced with increasing demands for bulk power transmission, low-cost power delivery, and high system reliability. At the same time, bottlenecks cause limitations in power transmission. For highly dynamic load compensation in the industry a very fast compensation system is called for. 

The innovative SVC PLUS® for Reactive Power Compensation is a cost efficient, space saving, flexible solution to increase dynamic stability and power quality of the grid, based on multilevel Voltage-Source Converter (VSC) technology.

As the infeed of power from renewable sources continuously replaces conventional synchronous power generation, the grid frequency is getting more sensitive due to the reduced amount of rotating machines. Now grid operators are faced with the challenge to provide sufficient system inertia of synchronous generators with high rotating masses to stabilize the grid.

An SVC PLUS® Frequency Stabilizer can solve this challenge as it is able to emulating system inertia by boosting high active power into the grid when needed. In addition it offers voltage support by means of reactive power compensation.

Mechanically switched devices (MSC/MSCDN) are the most economical reactive power compensation devices. MSCs are a robust solution for voltage control and network stabilization under steady state conditions. The MSCDN additionally offers a damping network to avoid resonance conditions in the electrical system. 

And in industrial plants with rising amounts of converter fed drives and other nonlinear consumers the need for efficient reduction of harmonic voltages along with power factor improvement becomes increasingly important – tuned filter circuits are a remedy.

The fixed series capacitor (FSC) is part of the series compensation portfolio and makes use of capacitors. They provide an increase in transmission system stability and capacity for power transmission.

Decentralized energy systems and the associated uncontrolled power flows pose new challenges for the existing AC grids. Thermal overloads in the lines and a growing number of cases in which frequency and voltage come critically close to acceptable range limits, or even exceed them, threaten grid stability and the transmission infrastructure. The Unified Power Flow Controller (UPFC PLUS) controls power flow in just milliseconds thus stabilizes the AC grid even in critical situations (Grid Code N-1). UPFC PLUS lets you get the most from your existing grid capacity while maintaining maximum protection, reducing the risk of power failures, and minimizing redispatch costs.

Siemens Energy's MVDC PLUS® medium-voltage DC solution is a powerful system for managing future distribution grids and regional transmission networks. The solution provides power flow control, long distance transmission, increased feed-in, transmission autonomy and grid connection.

SVC (Static Var Compensators) and SVC PLUS® (STATCOM) provide reactive power, enhance reliability by assisting fault recovery, reduce black-out risk and regulate the power factor. MSCDN (Mechanical Switched Capacitor with Damping Network) is a simple solution for voltage control and network stabilization on a steady state basis.

In three-phase AC grids problems are often caused by temporarily increased load on one phase for example for trains use the transmission system while passing by. Fast and efficient load balancing for each phase is also required in historically developed infrastructures in challenging environments, e.g. for metro stations in megacities.
Therefore, utilities require enhanced solutions that combine the advantages of SVC solutions with less space requirements as well as lower noise requirements.
Siemens Energy' SVC PLUS® with its flexible, modular concept could be the perfect match. The electric characteristics of the SVC PLUS® differ from a classic SVC. Its current control leads to a superior undervoltage performance, which means that the SVC PLUS® can support the network longer and at lower voltages.

The share of renewables in the energy mix increases and conventional power plants are switched off in order to lower CO₂ emissions. These lead to the decrease of short circuit power and inertia in the grids. Converter-based renewable power generation does not contribute short-circuit power to the transmission network and needs a certain level of it to operate stable.

SVC PLUS® Frequency Stabilizer stores energy in super capacitors and releases the energy whenever the grid needs it. It therefore avoids frequency drops and enables further renewable power integration to the network. It is a pure power electronic system based on proven SVC PLUS® technology.

Synchronous condenser solutions support transmission systems with short-circuit power, reactive power and inertia to stabilize the grids and to prevent blackouts. They feature high efficiency, low noise emissions and low installation and commissioning costs. Inertia can be extended by the use of a fly wheel.

The latest innovation from Siemens Energy for load flow control is the high sophisticated Unified Power Flow Controller (UPFC PLUS) with an additional function for voltage control. It is based on proven converter technology from Siemens Energy just like the MVDC PLUS® (Medium-Voltage Direct Current) system which offers load flow control via DC transmission. The passive Fixed Series Capacitors (FSC) offer stability of long-distance transmissions by compensating the inductive reactance. The transmission of active power is mainly limited by the impedance of the transmission line, comprising the ohmic resistance plus the capacitive and inductive reactance. They help to better utilize existing overhead lines by increasing their transmission capacities and contribute to steady state and dynamic stability of the system.

Rapidly changing loads in industrial plants lead to undesirable voltage fluctuations caused by a varying load current. For quick changes this is referred to as flicker. Grid operators set stringent limits to the permitted flicker level imposed by industrial plants to avoid disturbances in the network. Flicker in lighting – critical for human wellbeing, it affects efficiency and performance of components in industrial plants, too.

SVC Classic and SVC PLUS® compensate and stabilize load-induced voltage fluctuations within a very narrow tolerance band and ensure compliance with grid requirements. With SVC PLUS®, flicker reduction factors above 4.5 (even into the 7 range) can be achieved.

For loads with fast varying reactive and active power, a dynamic reactive power compensator – the industrial SVC Classic with a thyristor-controlled reactor (TCR) and integrated filter circuits will be the right solution. Especially when arc furnaces are involved, the highly dynamic industrial SVC PLUS® will deliver best results.

High voltage stability and reduced flicker enable an increase in production and better utilization of cables, transformers, and drives. Steel mills especially benefit from compliance with utility requirements, as a penalty shutdown by the utility is a realistic threat. Due to the typically remote locations in the mining industry, voltage stabilization is important for reliable and efficient production.

Nonlinear loads from converters or arc furnaces are a source of unwanted generate harmonic voltage distortion at its grid connection point that could disturb other consumers connected to the network. They generate losses in all network elements, without contribution to driving a machine, melting steel or other tasks of industrial production.

According to the grid operators, harmonic voltage distortion should not exceed certain limits. To achieve that, harmonic currents should be “directed” to flow into harmonic filtering devices, or currents must be generated with a waveform neutralizing the harmonic component of the load current.

As a solution tuned filter circuits can be applied, carefully selected in accordance with network requirements and can be part of the SVC system. The most advanced approach however for low order harmonics (approx. up to 13th) harmonic is active filtering, a task performed by the very fast SVC PLUS® control concept practically as a byproduct and including inter-harmonics.

Grid compliance secures connection permit with the network and the high power quality means lower stress to plant equipment (drives, pumps, transformers etc.) and to avoid losses due to reduced harmonics.

Significant inductive loads like motors are present in all fields of industry. In the metallurgical industry arc furnaces constitute the largest inductive loads – operating at an extremely bad power factor. The reactive power required by such loads causes degrading of power factor (or cos φ). As also reactive power needs to be transported through the network, this leads to bad utilization of all transmission components such as cables, transformers, etc.

Different loads have different characteristic power factors. Some loads have a clearly defined and nearly constant power factors, for others it can vary rapidly and within a wide range. Extreme behavior comes with electric arc furnaces. Big drives for ore mill applications, rolling mill drives and pumps/compressors show fluctuating power factors due to variation in their operating point.

The main challenge is to measure the load behavior very fast to achieve according control of the compensation system: dynamic loads require dynamic compensation.

SVC und SVC PLUS® dynamically improve the power factor. If this variable inductive power is countered by a dynamically controlled capacitive power, the power factor can be stabilized at high level.

SVC and SVC PLUS® enable improved utilization of the complete electrical system – as with a good power factor only active power needs to be transported. Losses caused by reactive currents can be avoided. A good power factor will furthermore prevent penalties by the utility.

Unsymmetrical loads are not – or at least not completely – balanced at all times. Single-phase loads or arc furnaces can cause uneven phase currents in the three-phase supply system and uneven phase voltages.

Limit unwanted effects

Generators are three-phase sources of energy that is transported through the grid up to the consumers. The whole system works best when all three phases are evenly loaded. But certain loads are only single phase, and certain three-phase loads can become extremely unsymmetrical in special operating conditions, e.g. an electric arc furnace for melting steel can be anything between balanced up to only two-phase operation at worst.

In order to limit unwanted effects of such unbalanced conditions in the grid, utilities allow only a limited degree of unbalance, and the owner of such consumers has to take measures to evenly distribute the power consumption of his plant to the three phases at his connection to the utility grid.

Compensation systems capable of independent control for each phase, like SVC Classic and SVC PLUS®, allow distinct injection of capacitive or inductive power into each phase. In this way symmetric balanced loading of the feeding network can be achieved despite asymmetry in load.

Load balancing enables better utilization of the plant distribution system and compliance with utility requirements.