Wide Area Protection and Control Scheme

Authors: Jose V. Espinoza, AMM-Guatemala, Fernando Calero, Armando Guzman, Mangapathirao V. Mynam, and Eduardo Palma, Schweitzer Engineering Laboratories, Inc., USA

Figure 1 illustrates the Central American Electrical Interconnection System, or SIEPAC (Sistema de Interconexión Eléctrica de los Países de América Central). A 230 kV transmission network wheels power among the members of this power system. A 400 kV line from the Tapachula substation in Mexico to the Los Brillantes substation in Guatemala interconnects the two power systems. A power transformer steps down the voltage level to 230 kV, which is the SIEPAC transmission voltage level. The transmission system in Guatemala distributes the imported power to the local loads, but mostly wheels the power from Mexico to the SIEPAC system via the Ahuachapan substation in El Salvador.

The simplified diagram of the Guatemalan power system (Figure 2) shows the geographical location of the substations. The transmission lines from Moyuta to Ahuachapan and from Aguacapa to Ahuachapan connect Guatemala to El Salvador.

Synchronized Measurement in Guatemala
In 2008, the wholesale electric market administrator Administrador del Mercado Mayorista (AMM) identified the benefits of using synchrophasor technology to operate the power system in Guatemala.

AMM coordinates the operation of power plants and transmission lines in Guatemala and the international interconnections with Mexico and the rest of Central America to optimize operating costs in an open-market environment. AMM also regulates short-and long-term prices in transactions of the power market. However, the most important function of AMM is to guarantee the delivery of reliable electrical energy to the loads in Guatemala. The personnel at AMM are responsible for operating the system in the most efficient way, supervising the power system in real time, and managing electric energy transactions.

While AMM is not particularly responsible for the operation and maintenance of transmission lines, the main mandate for AMM is to secure the operation of the Guatemalan power system. Because of this challenge, AMM implements the emergency control systems required to meet this main purpose.

Based on the knowledge of the characteristics of SIEPAC interconnections and operating experiences with the Mexican power system, AMM conducted several power system stability and operational studies. Also, during the operation of the power system, occurrences of instability and blackouts provided information to propose a wide-area protection scheme (WAPS) to aid in guaranteeing secure operation of the power system. The implementation of the AMM wide-area supplementary control scheme (SCS) started in 2011.

Since June 2010, SIEPAC has been experiencing large power oscillations. Figure 4 shows the total active power exchange between Guatemala and El Salvador prior to a disconnection event on November 27, 2011. AMM coordinated with the transmission line operators and implemented control schemes in transmission line relays using the programmable logic capabilities of the relays. These schemes detected overloads in the interconnection lines and qualified their operation with time duration to open the corresponding interconnection.

One of the challenges of operating the interconnected system is complying with the operational security requirements of the interconnection with Mexico. In the past, the link with Mexico was not allowed to operate during certain periods of the day when the system is more prone to instability. This operation limitation was not in the best interest of AMM because not having the link with Mexico can compromise the security of the Guatemalan power system. AMM also has economic reasons for this interconnection. The event shown in Figure 4, as well as other occurrences that ended in blackouts, prompted AMM to find the proper tools to guarantee the stable operation of the power system. AMM realized the potential of time-synchronized phasor measurements (synchrophasors) to provide information and flexible tools to operate the power system in stable conditions.

Synchrophasor-Based Monitoring System: AMM now monitors the operation of the Guatemalan power system using supervisory control and data acquisition (SCADA) and synchrophasors. A traditional SCADA system is in operation supplying the data to the operators every 4 seconds.

AMM operators also have access to visualized, real time synchrophasor data, which are updated 30 times per second. Figure 3 illustrates a typical human-machine interface (HMI) used by the operators, showing power and voltage measurements in key nodes of the system. The faster rate allows the operator to view and recognize transients never seen in the SCADA system. Preexisting but invisible phenomena are now visible and actionable due to low latency and the simultaneous measurement of information (data coherence). This overcomes the infrequent and incoherent data collection associated with typical SCADA client-server communication. One example is the recovery of the power system after a transmission line fault. The operators can visualize the power oscillations, generation trips, and power flows in the interconnection lines.

AMM has found that synchrophasor data and the visualization tools greatly complement traditional SCADA systems in the control center.

Real Time Control Using Synchrophasor Data: Synchrophasor data can be used to take control actions. AMM identified this technology and implemented several schemes as part of its wide-area SCS based on synchronized measurements refreshed 30 times per second. This refresh rate allows for fast control applications with operating times of less than 100 milliseconds, which is not possible with the slower refresh rate (once every 4 seconds) of traditional SCADA.

The coherent nature of the synchrophasor measurements allows the implementation of both simple and sophisticated algorithms. A simple algorithm, for example, is the sum of two active power quantities measured from different nodes on the network. With traditional SCADA measurements that are not synchronized, arithmetic operations using noncoherent measurements are questionable. AMM also uses modal analysis techniques on the sum of two time-synchronized power measurements. The wide-area SCS sums the active power flowing in the two interconnection lines to El Salvador and applies modal analysis to the derived quantity. Angle differences are also available for control. AMM will evaluate the use of angle difference data when more phasor measurement and control units (PMCUs) are in service.

BeijingSifang June 2016