CH 9.1: Explain the functions and equipment of Electric System Control Centers C

Question

CH 9.1: Explain the functions and equipment of Electric System Control Centers CH 9.2: Describe how SCADA enables remote control of substation equipment CH 9.3: Explain the functions of the Energy Management Systems CH 9.4: Describe the EMS software tools used by system operators CH 9.5: Explain how synchro phasors and Wide Area Monitoring Systems help system reliability and security CH 9.6: Describe the types of telecommunications systems used by power companies CH 9.7: Discuss how advances in digital substation equipment and automation impacts efficiencies in system control, equipment wiring, and functionality capabilities.

Explanation / Answer

Answer 9.1
The electric system control center (ECC) has
traditionally been the decision-center for the
electric transmission and generation
interconnected system. The ECC provides the
functions necessary for monitoring and
coordinating the minute-by-minute physical and
economic operation of the power system. In the
continental U.S., there are only three
interconnected regions: Eastern, Western, and
Texas, but there are many balancing areas (also
known as control areas), with each balancing
area having its own ECC.

Maintaining integrity and economy of an interconnected power system requires significant
coordinated decision-making. So one of the
primary functions of the ECC is to monitor and
regulate the physical operation of the
interconnected grid.
Most areas today have a two-level hierarchy of
ECCs with the Independent System Operator
(ISO) performing the high-level decision-
making and the transmission owner ECC
performing the lower-level decision-making.
A high-level view of the ECC is illustrated in
Fig. 2 where we can identify the substation, the
remote terminal unit (RTU), a communication
link, and the ECC which contains the energy
management system (EMS). The EMS provides
the capability of converting the data received
from the substations to the types of screens

ECC Components
The system control function traditionally used in
electric utility operation consists of three main
integrated subsystems: the energy management
system (EMS), the supervisory control and data
acquisition (SCADA), and the communications
interconnecting the EMS and the SCADA
(which is often thought of as part of the SCADA
itself). Figure 3a [1] provides a block diagram
illustration of these three integrated subsystems
and Fig. 3b provides a closer view. The SCADA
and communications subsystems are indicated
in the dotted ovals at the top left hand corner of
the figure. The rest of the figure indicates the
EMS.

Answer 9.2
There are two parts to the term SCADA [2-5].
Supervisory control indicates that the operator,
residing in the energy control center (ECC), has
the ability to control remote equipment. Data
acquisition indicates that information is
gathered characterizing the state of the remote
equipment and sent to the ECC for monitoring
purposes.
The monitoring equipment is normally located
in the substations and is consolidated in what is
known as the remote terminal unit (RTU).
Generally, the RTUs are equipped with
microprocessors having memory and logic
capability, together with some form of telemetry
to provide the communication link to the ECC.

Relays located within the RTU, on command
from the ECC, open or close selected control
circuits to perform a supervisory action. Such
actions may include, for example, opening or
closing of a circuit breaker or switch, modifying
a transformer tap setting, raising or lowering
generator MW output or terminal voltage,
switching in or out a shunt capacitor or inductor,
and the starting or stopping of a synchronous
condenser.
Information gathered by the RTU and
communicated to the ECC includes both analog
information and status indicators. Analog
information includes, for example, frequency,
voltages, currents, and real and reactive power
flows. Status indicators include alarm signals
(over-temperature, low relay battery voltage,
illegal entry) and whether switches and circuit
breakers are open or closed. Such information is
provided to the ECC through a periodic scan of
all RTUs.

Answer 9.3
the EMS consists of 4 major functions: network model building (including topology processing and state estimation), security assessment, automatic generation control, and dispatch.

These
functions are described in more detail in the
following subsections.
1 Network Model Building
A network model is necessary in order to
determine whether operating conditions are safe
under the existing topology and also under the
event that one or more components fail and are
outaged. The network model must reflect the correct topology and the correct operating
conditions relative to the actual network
conditions. The information available to
construct the network model includes the status
indicators and the analog measurements
available from the SCADA. The result of the
network model builder is a power flow model.

2 Security Assessment
Security assessment determines first, whether the
system is currently residing in an acceptable state
and second, whether the system would respond in
an acceptable manner and reach an acceptable
state following any one of a pre-defined
contingency set. A contingency is the unexpected
failure of a transmission line, transformer, or
generator. Usually, contingencies result from
occurrence of a fault, or short-circuit, to one of
these components. When such a fault occurs, the
protection systems sense the fault and remove the
component, and therefore also the fault, from the
system.
Of course, with one less component, the overall
system is weaker, and undesirable effects may
occur. For example, some remaining circuit may
overload, or some bus may experience an
undervoltage condition. These are called static
security problems.
Dynamic security problems may also occur,
including uncontrollable voltage decline,
generator overspeed (loss of synchronism), or
undamped oscillatory behavior.
Almost all EMS today are capable of
performing static security assessment, because it
only requires a power flow program to do so.
Very few EMS are capable of performing
dynamic security assessment, however, because
the assessment tools are more complex and
computationally intense.

3 Automatic Generation Control
As we have seen already in this course, the
purpose of AGC is to regulate the system
frequency and power interchange between
control areas.
There are two SCADA measurement used by
AGC: total net tie line flow and frequency.
There also exist scheduled values for these two
parameters. The scheduled net tie line flow
depends on the total sales less purchases to other
control areas. The scheduled frequency is
always 60 Hz.

4 Economic Dispatch
Previous to the power industry restructuring, all
ECCs used economic dispatch calculation (EDC) to determine the set point power levels of
all generators in order to supply the demand.
Such a system still exists in some parts of the
country. But in other parts, a market dispatch is
done based on an auction system whereby the
optimization algorithm is similar to EDC except
generator cost-rate curves are replaced by
generator owner bids.

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