Sub application study

EXAMPLE OF STUDIES

Substation Design

- Capacitor bank switching
- Lightning analysis of a PV plant substation

Power Transmission System

- Transformer energization and TRV study in a transmission substation

Distribution and Industrial System

Renewable Energy

- Harmonic analysis of a PV plant

Capacitor bank switching study for a utility located in North America

Background

Substation

The scope of work concerned a Medium Voltage/High Voltage 100MVA substation located at the end of a relatively long radial transmission line. A load increase was forecasted for the next years and it was planned to replace the actual capacitor bank by a 50MVAR unit to provide the additional power without a severe voltage dip.

Realization

The first step of the study consisted in modelling the substation using EMTP. Capacitor banks, current-limiting reactors and circuit breakers are modeled using ideal devices. In order to get the appropriate level of details required by capacitor bank switching analysis, bus work inductances are derived from typical values found in IEEE C37.012-2005. Surge arresters are modeled using the IEEE arrester model for fast front surge. Incoming transmission lines are modeled using a frequency-dependent model built using the power frequency impedance data and the EMTP Line Rebuild tool. The upstream network model is validated by comparing the load-flow results calculated by EMTP with the results provided by the client.

Four scenarios are analyzed using EMTP:

Capacitor bank energization. For the first two cases, a time-domain statistical analysis is performed to evaluate the distribution of overvoltages across the capacitor bank. It consisted in running 500 simulations with different closing times of the capacitor bank breaker. In the first analysis, the natural Gaussian distribution of closing time around the zero-voltage crossing is modeled. In the second analysis, a uniform distribution of closing time on one power cycle is added to take into account that the CB closes at any time during the power frequency cycle. The simulated overvoltages are compared with the withstand level.

Outrush Transient. The capacitor bank is already energized and operating in the steady state and a fault occurs on the 115kV bus. This discharge of the capacitor bank into the fault is called outrush current and can cause important stress on the circuit breaker. The product of the inrush current peak and transient inrush current frequency is compared with the limit defined in IEEE C37.06.

Transient Recovery Voltage (TRV). The current-limiting inductor reduces the severity outrush current but also increases Transient Recovery Voltage of the circuit breaker protecting the capacitor bank. To assess the capability of the capacitor bank circuit-breaker to open after a fault between the inductor and the capacitor bank, the TRV is analyzed with EMTP. The TRV prospective envelope is derived from IEC 62271-100 and compared with the TRV simulated in EMTP.

Results

Simulation shows that the maximum switching overvoltage is kept below the limits and the system should be safe from the overvoltages originated from the switching of the capacitor bank. If a fault occurs between the capacitor bank and the current-limiting reactor, the TRV analysis confirms that the circuit breaker is able to open without any problem.

In the case of 3-phase fault on the 115kV bus, there was a small hazard of circuit breaker damage. The product of the inrush current peak and transient inrush current frequency was slightly higher the IEEE limit, which is an unnecessarily restrictive criterion.

apacitor-bank-switching-

Overvoltages (top) and in inrush current (bottom) during the capacitor bank switching

Lightning surge analysis of a photovoltaic power plant substation located in North America

Background

PANEL-SOLAR

The scope of work concerned a photovoltaic power plant located in North Carolina, U.S.A. This power plant is rated 50 MWac and is in service since 2015.

The utility requested Powersys to verify the insulation coordination against lightning strikes. In particular, it was asked to validate the surge arresters in the original design. The effect of lightning strikes on the incoming transmission line supplying the plant and on the plant site itself are studied.

Realization

Model

The photovoltaic power plant is composed of around ten (10) Inverter Stations (PCS). Each PCS consists of a low-voltage inverter, a 360V/24kV Star-Delta step-up transformer, a current limiting fuse and a Surge Protective Device (SPD) installed on the Delta High Side of each transformer. For this study, small capacitances have been placed on each side of the transformers to model the surge transference effect and chosen in the book “Transients in electrical system. Analysis, Recognition and Mitigation” (J. C. Das). The surges arresters are modelled with the so-called IEEE metal oxide surge arrester model.

The 24 kV/115kV Substation is connected to the upstream network. The incoming 115kV transmission line is modelled in detail, span by span, using frequency-dependent line models which include all conductors mounted on poles, evaluated at lightning frequencies for proper rendering of skin effect. Conductors connecting pole hardware to ground are modelled as distributed parameter lines and the ground resistance of the poles are modelled to include inductive effects. Insulators are modelled as a lumped stray capacitance and shunt with a so-called area-criterion model.

Methodology

The software EMTP has been chosen for this study. A deterministic approach, consisting in specifying the worst amplitude of a lightning strike that can occur over a period of 500 years has been used. In other words, the current amplitude of the most severe lightning strike that can statistically occurs over 500 years is calculated and used to verify if the insulation coordination of the plant ensures the safety of both personal and equipment. Both back-flashover and shielding failure are considered. EMTP simulations are used to evaluate the resulting overvoltage on the MV and LV network and the overvoltage is compared with the insulation levels of all the equipment.

To be valid, simulations took into account:

The behavior of the overhead and underground power lines at the voltage and frequency range of interest,
The surge transference in the HV/MV substation transformer,
The effect of Surge Protective Devices and Surge arresters,
The soil resistivity.

CIGRE-Waveform-of-the-lightning-strike

Results

The study determined that the integrity of all identified equipment of the solar plant will not be compromised by lightning originated overvoltage:

Inverters and switchgear are protected with lightning rod or earthed metallic case. Direct lightning strikes are not susceptible to damage them.
The medium voltage network is entirely underground and composed of 24kV cables. Cables are not likely to be damaged by lightning strikes.
The only source of lightning-originated overvoltage susceptible of damaging the power plant is the stroke on the incoming 115kV transmission line and its transference to the MV network through the substation transformer. This study demonstrates that the risk of having a lightning-originated overvoltage high enough to damage an equipment is less than 2% in a period of 500 years. In that case, the first equipment to fail would be the substation transformer itself.

Substation Design Consulting

EXAMPLE OF STUDIES

Capacitor bank switching study for a utility located in North America

Background

Realization

Results

Lightning surge analysis of a photovoltaic power plant substation located in North America

Background

Realization

Results

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