FACTS devices | sudaspace

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(Image of a variable Shunt reactor used in a transmission grid http://www.bpress.cn/list/di/pic/62/49762.jpg)

In this blog post I would like to talk about Flexible AC transmission Devices (FACTS). I have being studying how these devices are being used in controlling the load flow in electrical grids, therefore I thought to share what I have come across with you. In past few decades as the power consumption of population increased in exponential way power industry had to expand their ability to service people by constructing new power lines, transformers and power plants. At the same industry observed that there is reduction in rate of power consumption. This was due to increase in efficiency in power equipment, economic recessions. Because of this expanding and constructing new infrastructure was not a good solution economically. And also due to control schemes used in controlling grid and handle faults were not fast enough as they consisted of mechanically controlled circuit breakers. The long switching periods and discrete operation make them difficult to handle frequently changed loads smoothly and damp out the transient oscillations quickly. In order to compensate these drawbacks, large operational margins and redundancies in equipment used were maintained. These reasons caused electrical utility companies to have higher cost of ownership and longer payback periods for their investments. In order to overcome these problems, a utilization of assets was preferred. FACTS devices were introduced as a result of this.

The development of FACTS was a by-product of development in Power Electronics. Power electronics combines the advantages of semiconductor devices and the high power rating requirements in electrical power engineering. High speed switching, reliability and accuracy were some of the qualities FACTS obtained from using power semiconductor devices such as Thyristor. Due to advancements in Power Electronic technology and advanced control technology, FACTS has become the preferred choice to control Voltage, active and reactive power flow, transient and steady state stabilization that improves the operation and functionality of existing grid assets. FACTS devices increase the efficiency of in terms of both technical and economical ways by allowing operators to distribute generated power through less congested paths to consumer hence reducing overall generation capacity. This will reduce the operation costs in electrical utility.

Before the discussion about FACTS it’s better to have some knowledge in theory behind how power flow in a line being controlled and what are the parameters to be considered.

Simplified electrical system with two buses

The above diagram shows the schematic of a transmission line. Although in reality there is a resistance in the line that causes real power losses it can be neglected as this value is very small one compared to the reactance of the line (typically 10 times smaller). The next figure shows the vector diagram for the power line. In most cases the two buses have the same voltage magnitude (V₁=V₂) but different angles. This angle difference is used to control the power flow direction. There are two main equations used to model the power flow.

Equation 1- Active Power Flow and Equation 2- Reactive Power flow

These equations model the active and reactive power flow at bus 2. In here. These two equations shows the power flow will depend on the voltage magnitudes, the phase angles and the impedance of the transmission lines. By controlling those 3 parameters electrical utilities can control the power flow in a transmission line.

There are two methods used in power flow controlling,

Series compensation
Shunt compensation

Series compensation

According to equation1 the active power flow will depend upon V₁, V2, Sin δ and X_L. In series compensation power flow control is done by controlling X_L, the impedance in line. This is the overall line impedance not just the actual line impedance due to the reactance in line. A series connected capacitor or a reactor can inject a voltage causing this reactance to change hence changing the power flow in that particular line.

If a capacitor is added. This V_C is called quadrature voltage since it’s shifted by 90 degrees. This effect will add a voltage drop which will oppose the voltage drop in line. The power flow will be increased as a result of this

If a series reactor is added . This V_Lwill add a voltage drop which will cause power flow to be reduced.

Shunt compensation

Shunt compensation is used utility to regulate the voltage profiles of buses. Shunt connected reactors can reduce line over voltages by consuming reactive power and Shunt connected capacitors can inject reactive power to bus to increase the voltage. These methods help to maintain the voltage at a busbar closer to it’s rated value (1pu).

FACTS devices usually employees one or two of these methods to control the Voltage, phase angles or line reactance .

There are many FACTS techniques are being used at present but all can be categorized into 4 categories.

Series Controllers
Shunt Controllers
Combined series-series controllers
Combined series-shunt controllers

Series Control FACTS

Series connected FACTS could be a variable impedance such as a reactor or a capacitor or power electronics based variable source of main frequency. All series reactors inject a voltage in a series with the line. As long as the injected voltage is in phase quadrature with the line current, the series controller supply or consume reactive power. If the phase difference is different than this it will handle active power as well.

Shunt control FACTS

A shunt controller could be a variable impedance or a viable source or a combination of both. Shunt connected controllers inject current at the point of confection. Although it may seem like impedance devices are not capable of doing this they can create a variable current flow using varying impedance hence it in a way controls the current in line. The injected current is in phase quadrature with line voltage. Therefore it will only consume or deliver reactive power. Any other phase difference will cause it to handle real power as well.

Combined Series-Series FACTS

Combined Series-Series controllers can be used to control both active and reactive power in transmission lines. This method is often used in multiline transmission systems. These types of controllers can be operated in two methods.

Combination of separate series controllers which are operated in a coordinated manner in order to realise power flow control.
Unified controllers, several series controllers provide independent series reactive compensation for each line but also has the ability to transfer real power among transmission lines using power link. The real power transfer capability of unified series-series controller maximize the utilization of transmission systems by giving better control over both active and reactive power.

Combined Series-Shunt FACTS

Combined Series-Shunt can be used to control both active and reactive power in transmission lines. There are two types FACTS technologies to achieve this.

Combination of series and shunt which are controlled in a coordinated manner in order to realise power flow control.
Unified Power flow controller which contain both series and shunt components, in theory combined shunt and series controllers inject current into the system with the shunt part and voltage in series in the line with the series part. When the shunt and series parts are combined there can be a real power transfer between these two via power link.

Although both series and shunt FACTS have the same objective, Achieving better control over power flow in grid, these the effectiveness of these are different in different scenarios. Therefore it’s better to have a look into some of the advantages and disadvantages in both technologies.

Series controllers are more effective in controlling the current/power flow and damp oscillations than shunt controllers if both have same MVA ratings.
Shunt controllers are a good solution to control voltage at a point of connection and around that area. Therefore shunt controllers are often used to control the voltage level at substation bus as it serves the bus node independent of the individual lines connected to it.
One disadvantage of series controllers are that they have to be designed to handle dynamic and contingency overloading as they are connected in series with power lines.

Advantages of FACTS devices

Improving Power Transfer Capability.
Confining Power Flow to designated routes.
Transient and dynamic stability improvements.
Ability regulate voltage hence avoiding cascading power outages.
Damping of power system oscillations.
Ability use transmission lines close to their thermal limits.
Improving system reliability.

Drawbacks in FACTS

Very expensive to design and implement.

Device complexity in design and implementation. These devices are often rated in MVar power range and rated voltages are the voltages equals to the system voltage they are connected to. In power transmission this could be high as 132 kV and above.

Since the high power requirements, the power electronic devices used have to withstand high power and voltages causing high costs in construction.

All conventional FACTS devices used are lumped in nature. Generally these are designed to withstand the load growth in next 30 years. This approach causes poor return of investment to grid owner which is a major drawback.

FACTS devices are custom designed according to the requirements of the clients. This results in longer design and construction cycles. This will add additional costs to the devices.

Shut down or failure of one of these FACTS have a significant impact on grid as the number of devices used in a particular electricity grid are limited.

References