Transmission systems: EMS, FACTS and HVDC

Energy Management Systems (EMS)

The concept of an energy management system is not new in a power system, but it was commonly believed to be a computer-aided tools used by operators of electric utility grids to monitor, control, and optimize the performance of geographically dispersed generation and transmission assets in real-time. EMS is controlled by a SCADA (supervisory control and data acquisition) system and a certain number of advanced applications including forecasting and optimization. The increased use of renewable energy and distributed resources, along with the need to increase energy efficiency and energy savings, are the main drivers for the implementation and utilization of energy management systems.

Fig. (d)

A typical EMS system configuration is shown in Fig.(d). System status and measurement information are collected by the Remote Terminal Units (RTUs) such as frequency, actual generation, tie-line load flows, and plant units’ controller status to provide system changes and sent to the Control Centre  through the communication infrastructure. The front-end server in the EMS is responsible for communicating with the RTUs and IEDs. Different EMS Applications reside in different servers and are linked together by the Local Area Network (LAN).

EMS Applications include Unit Commitment, Automatic Generation Control (AGC), and security assessment and control. However, an EMS also includes Applications similar to those of a DMS and most of the tools used in a DMS such as: topological analysis, load forecasting, power flow analysis, and state estimation.

Energy Management System (EMS) is designed to reduce energy consumption, Improved operational security, improve the utilization of the system, increase services reliability, predict electrical system performance, and as well as optimize energy usage to reduce fuel cost and O&M cost. There are many objectives of an energy management software, including an application to maintain the frequency of a Power Distribution System and to keeping tie-line power close to the scheduled values. In EMS, scheduled values will be maintained by adjusting the MW outputs of the AGC generators so as to accommodate fluctuating load demands. The  operation of the generation units  can be optimize by calculating the required parameters with energy management action.

The total load in the power system varies throughout a day and its value also changes with the
day of the week and season. Hence, it is not economical to run all the units available all the time.
Thus, the purpose of Unit Commitment within a traditional power system is to determine in advance, the start and the shut down sequence of the available generators such that the load demand is met and the cost of generation is minimum.

Similarly, in a power system, AGC carries out load frequency control and economic dispatch. Load frequency control has to achieve three primary objectives to maintain: (1) system
frequency; (2) power interchanges with neighbouring control areas; and (3) power allocation
between generators at the economic optimum. AGC also performs functions such as reserve
management (maintaining enough reserve in the system) and monitoring/recording of system
performance. The main aim of economic dispatch problem is to minimize the total cost of generating real power at different plants in the system while maintaining the real power balance in the system.

The load flow analysis  involves the steady state solution of the power system network to determine power flows and bus voltages of a transmission network for specified generation and loading
conditions. These calculations are required for the study of steady state and dynamic performance
of the system. The system is assumed to be balanced and hence, single phase representation is used. These studies are important in planning and designing future expansion of power system and also in determining the best operation of the existing systems.

Security assessment and control may be understood using the widely used framework
shown in figure (e). This Application exercises control to keep the power system in a normal state. This framework considers the power system as being operated under two types of constraint: load constraints (load demand must be met, known as equality constraint), and operating constraints (maximum and minimum operating limits together with stability limits should be respected, known as inequality constraints). In the normal state, both these constraints are satisfied. The security assessment and control Application includes; security monitoring, security analysis, preventive control, emergency control, fault diagnosis and restorative control.

Fig. (e)

Power system state estimation is a process whereby telemetered data from network measuring points to a central computer, can be formed into a set of reliable data for control and recording purposes. It allow the calculation of margins to operating limits, health of equipments and required operator actions with high confidence despite of measurements that are corrupted by noise or missing of data.

Flexible alternating current transmission system (FACTS)

FACTS is defined as "A high voltage power electronic based system that provide control of one or more AC transmission system parameters to enhance control functionality, power transfer capability, system stability very efficiently and prevent cascade disturbances."

The FACTS technology is a collection of controllers, which can be applied individually or in coordination with others to control one or more of the interrelated system parameters, such as series impedance, shunt impedance, current, voltage, and damping of oscillations.

The ability of the transmission system to transmit power becomes impaired by one or more of the  following limitations:

• Thermal limit: For overhead line, thermal capability is a function of ambient temperature, wind conditions, conditions of conductor, and ground clearance. One should ensure that over-currents are within limits.
• Dielectric limit:  From insulation point of view, many lines are designed very conservatively. It is often possible to increase normal operating voltage by 10% or even higher. One should ensure that over-voltages are within limits.

• Stability limit: The stability issues that limit the transmission capability include:  transient stability, dynamic stability, steady-state stability, frequency collapse, Voltage collapse, and sub-synchronous resonance.

The FACTS technology can certainly be used to overcome any of the loading capability limits.

The main objectives of FACTS controllers are the following:

                1. Regulation of power flows in prescribed transmission routes.

                2. Secure loading of transmission lines nearer to their thermal limits.

                3. Prevention of cascading outages by contributing to emergency control.

                4. Damping of oscillations that can threaten security or limit the usable line capacity.

The implementation of the above objectives requires the development of high power  compensators and controllers. The technology needed for this is high power electronics with real-time operating control. The realization of such an overall system optimization control can be considered as an additional objective of FACTS controllers.

Types of FACTS controllers

Series controllers.

The series controller could be a variable impedance, such as capacitor, reactor, or a power electronics based variable source of main frequency, sub-synchronous and  harmonic frequencies (or a combination) to serve the desired load. In principle, all series controllers inject voltage in series with the line. As long as the voltage is in phase quadrature with the line current, the series controller only supplies or consumes variable reactive power. Any other phase relationship will involve handling of real power as well. Series controllers include SSSC, IPFC, TCSC, TSSC, TCSR, and TSSR.

1. Static synchronous series compensator (SSSC)
2. Static synchronous series compensator (SSSC) with storage
3. Thyristor-controlled series capacitor (TCSC); Thyristor-switched series capacitor (TSSC)
4. Thyristor-controlled series reactor (TCSR); Thyristor-switched series reactor (TSSR)

 

Shunt controllers.

As in the case of series controllers, the shunt controllers may be variable susceptance, variable source, or a combination of these. In principle, all shunt controllers inject current into the system at the point of connection. Even a variable shunt impedance connected to the line voltage causes a variable current flow and hence represents injection of current into the line. As long as the injected current is in phase quadrature with the line voltage, the shunt controller only supplies or consumes reactive power. Any other phase relationship will involve handling of real power as well. Shunt controllers include STATCOM, TCR, TSR, TSC, and TCBR.

 

1. Static synchronous compensator (STATCOM) based on VSC & ISC.
2. STATCOM with storage
3. Static VAR compensator (SVC). Most common SVCs are:

• Thyristor-controlled reactor (TCR)
• Thyristor-switched reactor (TSR)
• Thyristor-switched capacitor (TSC)
• Mechanically-switched capacitor (MSC)

 

Combined series-series controllers.

This could be a combination of separate series controllers, which are controlled in a coordinated manner, in a multiline transmission system or it could be a unified controller in which series controllers provide independent series reactive compensation for each line but also transfer real power among the lines via the power link.

The real power transfer capability of the unified series-series controller, referred to as Interline power flow controller (IPFC), makes it possible to balance both real and reactive power flow in the lines and thereby maximize the utilization of the transmission system. The term “unified” here means that the dc terminals of all controller converters are all connected together for real power transfer.

Combined series-shunt controllers.

This could be a combination of separate shunt and series controllers, which are controlled in a coordinated manner, or a UPFC with series and shunt elements. In principle, combined shunt and series controllers inject current into the system with the shunt part of the controller and voltage in series in the line with the series part of the controller. However, when the shunt and series controllers are unified, there can be a real power exchange between the series and shunt controllers via the proper link. Combined series-shunt controllers include UPFC, TCPST, and TCPAR.

HVDC

A high-voltage direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current (AC) systems. For long distance transmission, HVDC systems may be less expensive and suffer lower electrical losses. For underwater power cables, HVDC avoids the heavy currents required to charge and discharge the cable capacitance each cycle. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may still be warranted, due to other benefits of direct current links.

HVDC allows power transmission between unsynchronized AC transmission systems. Since the power flow through an HVDC link can be controlled independently of the phase angle between source and load, it can stabilize a network against disturbances due to rapid changes in power. HVDC also allows transfer of power between grid systems running at different frequencies, such as 50 Hz and 60 Hz. This improves the stability and economy of each grid, by allowing exchange of power between incompatible networks.

 

 

Advantages of HVDC

(a) More power can be transmitted per conductor per circuit.

The capabilities of power transmission of an AC link and a DC link are different. For the same insulation, the direct voltage V_d is equal to the peak value (√2 x rms value) of the alternating voltage V_p.

For the same conductor size, the same current can transmitted with both DC and AC if skin effect is not considered i.e., I_a = I_d. Thus the corresponding power transmission using 2 conductors with DC and AC are as follows.

                d c power per conductor               P_d = V_dI_d

                a c power per conductor               P_a = V_aI_a cosφ

Now the ratio of these two power,         

Thus it is clear from above ratio that the more power transmission with DC over AC

In general, we are interested in transmitting a given quantity of power at a given insulation level, at a given efficiency of transmission. Thus for the same power transmitted P, same losses P_L and same peak voltage V_m, we can determine the reduction of conductor cross-section A_a over A_d

Let R_d and R_a be the corresponding values of conductor resistance for DC and AC respectively, neglecting skin resistance.

 

It is seen that only one-half the amount of copper is required for the same power transmission at unity power factor, and less than one-third is required at the power factor of 0.8 lag.

(b) Use of Ground Return Possible

In the case of HVDC transmission, ground return (especially submarine crossing) may be used, as in the case of a monopolar DC link. Also the single circuit bipolar DC link is more reliable, than the corresponding AC link, as in the event of a fault on one conductor, the other conductor can continue to operate at reduced power with ground return. For the same length of transmission, the impedance of the ground path is much less for DC than for the corresponding AC because DC spreads over a much larger width and depth. In fact, in the case of DC the ground path resistance is almost entirely dependent on the earth electrode resistance at the two ends of the line, rather than on the line length. However it must be borne in mind that ground return has the following disadvantages. The ground currents cause electrolytic corrosion of buried metals, interfere with the operation of signalling and ships' compasses, and can cause dangerous step and touch potentials.

(c) Smaller Tower Size

The DC insulation level for the same power transmission is likely to be lower than the corresponding AC level. Also the DC line will only need two conductors whereas three conductors (if not six to obtain the same reliability) are required for AC Thus both electrical and mechanical considerations dictate a smaller tower.

(d) Higher Capacity available for cables

In contrast to the overhead line, in the cable breakdown occurs by puncture and not by external flashover. Mainly due to the absence of ionic motion, the working stress of the DC cable insulation may be 3 to 4 times higher than under AC Also, the absence of continuous charging current in a DC cable permits higher active power transfer, especially over long lengths.

(e) No skin effect

Under AC conditions, the current is not uniformly distributed over the cross section of the conductor. The current density is higher in the outer region (skin effect) and result in under utilisation of the conductor cross section. Skin effect under conditions of smooth DC is completely absent and hence there is a uniform current in the conductor, and the conductor metal is better utilised.

(f) Less corona and radio interference

Since corona loss increases with frequency (in fact it is known to be proportional to [f+25]), for a given conductor diameter and applied voltage, there is much lower corona loss and hence more importantly less radio interference with DC. Due to this bundle conductors become unnecessary and hence give a substantial saving in line costs. [Tests have also shown that bundle conductors would anyway not offer a significant advantage for DC as the lower reactance effect so beneficial for AC is not applicable for DC]

 

(g) No Stability Problem

The DC link is an asynchronous link and hence any AC supplied through converters or DC generation do not have to be synchronised with the link. Hence the length of DC link is not governed by stability. In AC links the phase angle between sending end and receiving end should not exceed 30^o  at full-load for transient stability (maximum theoretical steady state limit is 90^o).

(h) Asynchronous interconnection possible

With AC links, interconnections between power systems must be synchronous. Thus different frequency systems cannot be interconnected. Such systems can be easily interconnected through HVDC links. For different frequency interconnections both convertors can be confined to the same station. In addition, different power authorities may need to maintain different tolerances on their supplies, even though nominally of the same frequency. This option is not available with AC With DC there is no such problem.

(i) Lower short circuit fault levels

When an AC transmission system is extended, the fault level of the whole system goes up, sometimes necessitating the expensive replacement of circuit breakers with those of higher fault levels. This problem can be overcome with HVDC as it does not contribute current to the AC short circuit beyond its rated current. In fact it is possible to operate a DC link in "parallel" with an AC link to limit the fault level on an expansion. In the event of a fault on the d.c line, after a momentary transient due to the discharge of the line capacitance, the current is limited by automatic grid control. Also the DC line does not draw excessive current from the AC system.

(j) Tie line power is easily controlled

In the case of an AC tie line, the power cannot be easily controlled between the two systems. With DC tie lines, the control is easily accomplished through grid control. In fact even the reversal of the power flow is just as easy.

Inherent problems associated with HVDC

(a) Expensive convertors

Expensive Convertor Stations are required at each end of a DC transmission link, whereas only transformer stations are required in an AC link.

(b) Reactive power requirement

Convertors require much reactive power, both in rectification as well as in inversion. At each convertor the reactive power consumed may be as much at 50% of the active power rating of the DC link. The reactive power requirement is partly supplied by the filter capacitance, and partly by synchronous or static capacitors that need to be installed for the purpose.

(c) Generation of harmonics

Convertors generate a lot of harmonics both on the DC side and on the AC side. Filters are used on the AC side to reduce the amount of harmonics transferred to the AC system. On the DC system, smoothing reactors are used. These components add to the cost of the convertor.

(d) Difficulty of circuit breaking

Due to the absence of a natural current zero with DC, circuit breaking is difficult. This is not a major problem in single HVDC link systems, as circuit breaking can be accomplished by a very rapid absorbing of the energy back into the AC system. (The blocking action of thyristors is faster than the operation of mechanical circuit breakers). However the lack of HVDC circuit breakers hampers multi-terminal operation.

(e) Difficulty of voltage transformation

Power is generally used at low voltage, but for reasons of efficiency must be transmitted at high voltage. The absence of the equivalent of DC transformers makes it necessary for voltage transformation to carried out on the AC side of the system and prevents a purely DC system being used.

(f) Difficulty of high power generation

Due to the problems of commutation with DC machines, voltage, speed and size are limited. Thus comparatively lower power can be generated with DC

(g) Absence of overload capacity

Convertors have very little overload capacity unlike transformers.

Classification of HVDC links:

DC links are classified into Monopolar links, Bipolar links, and Homopolar links.

Monopolar

In the case of the monopolar link there is only one conductor and the ground serves as the return path. In a monopole configuration, one of the terminals of the rectifier is connected to earth ground while other one, at a potential high above or below ground, is connected to a transmission line. The link normally operates at negative polarity as there is less corona loss and radio interference is reduced. A monopolar link is mainly used for submarine cable transmission.

 

Bipolar

In bipolar transmission a pair of conductors is used, each at a high potential with respect to ground, in opposite polarity. The junction between the two convertors may be grounded at one or both ends.  If both ends are grounded, each link could be independently operated when necessary.

Homopolar

The homopolar links have two or more conductors having the same polarity (usually negative) and always operate with ground path as return. When there is fault on one conductor, the entire converter is available for feeding the remaining conductor(s), which having some overloading capability, can carry more than normal power. In contrast, for a bipolar configuration reconnection of the whole converter to one pole of the line is more complicated and usually not feasible.